A transmission stability improvement using unified power flow controller

A transmission stability improvement  using unified power flow controller

                                                              ABSTRACT

 The necessity to deliver cost effective energy in the power market has become a major concern in this emerging technology era. Therefore, establishing a desired power condition at the given points are best achieved using power controllers such as the well-known High Voltage Direct Current (HVDC) and Flexible Alternating Current Transmission System (FACTS) devices. High Voltage Direct Current (HVDC) is used to transmit large amounts of power over long distances. The factors to be considered are Cost, Technical Performance and Reliability. A Flexible Alternating Current Transmission System (FACTS) is a system composed of static equipment used for the AC transmission of electrical energy. It is meant to enhance controllability and increase power transfer capability of the network. It is generally power electronics based system. A Unified Power Flow Controller (or UPFC) is a FACTS device for providing fast-acting reactive power compensation on high-voltage electricity transmission networks. The UPFC is a versatile controller which can be used to control active and reactive power flows in a transmission line. The focus of this paper is to identify the improved Power Transmission Capability through control scheme and comprehensive analysis for a Unified Power Flow Controller (UPFC) on the basis of theory, computer simulation. The conventional control scheme cannot attenuate the power fluctuation, and so the time constant of damping is independent of active- and reactive-power feedback gains integrated in its control circuit. The model was analysed for different types of faults at different locations, keeping the location of UPFC fixed at the receiving end of the line, With the addition of UPFC, the magnitude of fault current and oscillations of excitation voltage reduces. Series and Shunt parts of UPFC provide series and shunt injected voltage at certain different angles.

Index Terms –Flexible ac transmission system (FACTS), High-voltage dc transmission (HVDC), FACTS devices, Power transfer controllability, PWM, Faults in HVDC System.

                                                                                          

CHAPTER 1

I.INTRODUCTION

The rapid development of power systems generated by increased demand for electric energy initially in industrialized countries and subsequently in emerging countries led to different technical problems in the systems, e.g., stability limitations and voltage problems. However, breaking Innovations in semiconductor technology then enabled the manufacture of powerful thermistor’s and, later of new elements such as the gate turn-off thermistor’s (GTO) and insulated gate bipolar transistors (IGBT). Development based on these semiconductor devices first established high-voltage dc transmission (HVDC) technology as an alternative to long distance ac transmission. HVDC technology, in turn, has provided the basis for the development of flexible ac Transmission system (FACTS) equipment which can solve problems in ac transmission. As a result of deregulation, however, Operational problems arise which create additional requirements for load flow control and needs for ancillary services in the system. This paper summarizes Flexible ac transmission system (FACTS),High-Voltage DC Transmission (HVDC), FACTS devices, Power transfer controllability, Fault in HVDC System are discussed in this paper to explain how greater performance of power network transmission with various line reactance can be achieved.[1,2].

(a) Reduced maintenance

(b) Better availability

(c) Greater reliability

(d) Increased power

(e) Reduced losses

(f) Cost-effectiveness

During the state of power exchange in interconnected lines to a substation under variable or constant power, the HVDC converters comprehends the power conversion and later stabilizes the voltage through the lines giving a breakeven margin in the power transmission. The first large scale Thyristor’s for HVDC were developed decades ago. HVDC became a conventional technology in the area of back to- back and two- terminal long-distance and submarine cable schemes [3]. However, only few multi terminal schemes have been realized up to now. However, further multi terminal HVDC schemes are planned in the future (Fig. 1). The main application area for HVDC is the interconnection between systems which cannot be interconnected by AC because of different operating frequencies or different frequency controls. This type of interconnection is mainly represented by back-to-back stations or long-distance transmissions when a large amount of power, produced by a hydropower plant, for instance, has to be transmitted by overhead line or by submarine cable. HVDC schemes to increase power transmission capability inside of a system have been used only in a few cases in the past. However, more frequent use of such HVDC applications can be expected in the future to full-fill the requirements in deregulated.[4,6]. Fig 1 various types of HVDC Connections Static var compensators control only one of the three important parameters (voltage, impedance, phase angle) determining the power flow in ac power systems: the amplitude of the voltage at selected terminals of the transmission line. Theoretical considerations and recent system studies (1) indicate that high utilization of a complex, Interconnected ac power system, meeting the desired objectives for availability and operating flexibility, may also require the real time control of the line impedance and the phase angle. Hingorani (2) proposed the concept of flexible ac transmission systems or FACTS, which includes the use of high power electronics, advanced control centres, and communication links, to increase the usable power transmission capacity to its thermal limit. [5].

When using carrier based Pulse Width Modulation (PWM), its switching frequency has to be increased (typically, 33 times fundamental frequency even higher [17], which cause considerable power losses. It reduces the total efficiency and economy of the UPFC-HVDC project. And they are also the Impediments for equipment aimed at the green, renewable Sector. Therefore, with regard to PWM technology suited for UPFC-HVDC, how to reduce switching frequency and possess good harmonics performance, excellent transient control capability simultaneously become critical. And this is exactly the aim of the paper. The paper presents an innovative hybrid PWM technology, which comprises a combination of a first PWM with a first switching pattern and a second PWM with a second switching pattern. Hence during a first mode of operation, which may be a steady-state operation, the converter is controlled by the first PWM and during a second mode of operation, which may be a transient operation, the converter is controlled by the second PWM. An intelligent detection function which enables the modulation and the corresponding control system will smoothly switch from the first PWM to the second PWM and vice-versa when a disturbance causing a transient occurs. The development of FACTS-devices has started with the growing capabilities of power electronic components. Devices for high power levels have been made available in converters for high and even highest voltage levels. The overall starting points are network elements influencing the reactive power or the impedance of a part of the power system. The series devices are compensating reactive power. With their influence on the effective impedance on the line they have an influence on stability and power flow. The UPFC provides power flow control together with independent voltage control [7, 8].

CHAPTER 2

HIGH VOLTAGE DC TRANSMISSION SYSTEM

       Over long distances bulk power transfer can be carried out by a high voltage direct current (HVDC) connection cheaper than by a long distance AC transmission line. HVDC transmission can also be used where an AC transmission scheme could not (e.g. through very long cables or across borders where the two AC systems are not synchronized or operating at the same frequency). However, in order to achieve these long distance transmission links, power convertor equipment is required, which is a possible point of failure and any interruption in delivered power can be costly. It is therefore of critical importance to design a HVDC scheme for a given availability.

The HVDC technology is a high power electronics technology used in electric power systems. It is an efficient and flexible method to transmit large amounts of electric power over long distances by overhead transmission lines or underground/submarine cables. It can also be used to interconnect asynchronous power systems

The fundamental process that occurs in an HVDC system is the conversion of electrical current from AC to DC (rectifier) at the transmitting end and from DC to AC (inverter) at the receiving end.

There are three ways of achieving conversion

1.      Natural commutated converters

2.      Capacitor Commutated Converters

3.      Forced Commutated Converters

2.1 NATURAL COMMUTATED CONVERTERS: (NCC)

  NCC are most used in the HVDC systems as of today. The component that enables this conversion process is the thyristor, which is a controllable semiconductor that can carry very high currents (4000 A) and is able to block very high voltages (up to 10 kV). By means of connecting the thyristors in series it is possible to build up a thyristor valve, which is able to operate at very high voltages (several hundred of kV).The thyristor valve is operated at net frequency (50 Hz or 60 Hz) and by means of a control angle it is possible to change the DC voltage level of the bridge..

2.2 CAPACITOR COMMUTATED CONVERTERS: (CCC)

  An improvement in the Thyristor-based Commutation, the CCC concept is characterized by the use of commutation capacitors inserted in series between the converter transformers and the Thyristor valves.  The commutation capacitors improve the commutation failure performance of the converters when connected to weak networks.

2.3 FORCED COMMUTATED CONVERTERS: (FCC)

   This type of converters introduces a spectrum of advantages, e.g. feed of passive networks (without generation), independent control of active and reactive power, power quality. The valves of these converters are built up with semiconductors with the ability not only to turn-on but also to turn-off. They are known as VSC (Voltage Source Converters). a new type of HVDC has become available. It makes use of the more advanced semiconductor technology instead of Thyristor’s for power conversion between AC and DC. The semiconductors used are insulated gate bipolar transistors (IGBTs), and the converters are voltage source converters (VSCs) which operate with high switching frequencies (1-2 kHz) utilizing pulse width modulation (PWM).

2.4 CONFIGURATIONS OF HVDC:

There are different types of HVDC systems

 Such as:

Mono-polar HVDC system

In the mono-polar configuration, two converters are connected by a single pole line and a positive or a negative DC voltage is used. In Fig 2.1 there is only one Insulated transmission conductor installed and the ground or sea provides the path for the return current.

Fig 2.1: Mono polar HVDC system

Bipolar HVDC system:

This is the most commonly used configuration of HVDC transmission systems. The bipolar configuration shown in Fig 2.2 uses two insulated conductors as Positive and negative poles. The two poles can be operated independently if both Neutrals are grounded. The bipolar configuration increases the power transfer capacity.

Under normal operation, the currents flowing in both poles are identical and there is no ground current. In case of failure of one pole power transmission can continue in the other pole which increases the reliability. Most overhead line HVDC transmission systems use the bipolar configuration.

                                                          Fig2.2: Bipolar HVDC system

Homo-polar HVDC system:

               In the homo polar configuration as shown in Fig2.3 two or more conductors have the negative polarity and can be operated with ground or a metallic return. With two Poles operated in parallel, the Homo-polar configuration reduces the insulation costs. However, the large earth return current is the major disadvantage.

Fig2.3: Homo-polar HVDC system

2.5 MULTI-TERMINAL HVDC SYSTEM:

               In the multi terminal configuration, three or more HVDC converter stations are geographically separated and interconnected through transmission lines or cables. The System can be either parallel, where all converter stations are connected to the same voltage as shown in Fig(b).  or series multi-terminal system, where one or more converter stations are connected in series in one or both poles as shown in Fig. (c). A hybrid multi-terminal system contains a combination of parallel and series connections of converter stations

Fig 2.4:  multi-terminal HVDC system

 2.6 voltage-source converter:

A voltage-source converter is connected on its ac-voltage side to a three-phase electric power network via a transformer and on its dc-voltage side to capacitor equipment. The transformer has on its secondary side a first, a second, and a third phase winding, each one with a first and a second winding terminal. Resistor equipment is arranged at the transformer for limiting the current through the converter when connecting the transformer to the power network. The resistor equipment includes a first resistor, connected to the first winding terminal of the second phase winding, and switching equipment is adapted, in an initial position, to block current through the phase windings, in a transition position to form a current path which includes at least the first and the second phase windings and, in series therewith, the first resistor, which current path, when the converter is connected to the transformer, closes through the converter and the capacitor equipment, and, in an operating position, to interconnect all the first winding terminals for forming the common neutral point. 

In VSC HVDC, Pulse Width Modulation (PWM) is used for generation of the fundamental voltage. Using PWM, the magnitude and phase of the voltage can be controlled freely and almost instantaneously within certain limits. This allows independent and very fast control of active and reactive power flows. PWM VSC is therefore a close to ideal component in the transmission network. From a system point of view, it acts as a zero inertia motor or generator that can control active and reactive power almost instantaneously. Furthermore, it does not contribute to the short-circuit power, as the AC current can be controlled.

Voltage Source Converter based on IGBT technology:

The modular low voltage power electronic platform is called Power-Pack. It is a power electronics building block (PEBB) with three integrated Insulated Gate Bipolar Transistor (IGBT) modules. Each IGBT module consists of six switches forming three phase legs. Various configurations are possible. For example three individual three-phase bridges on one PEBB, one three phase bridge plus chopper(s) etc. The Power-Pack is easily adaptable for different applications.

The IGBT modules used are one Power Pak as it is used for the SVR. It consists of one three-phase bridge (the three terminals at the right hand side), which provides the input to the DC link (one IGBT module is used for it) and one output in form of one single phase H-bridge (the two terminals to the left) acting as the booster converter. For the latter two IGBT modules are used with three paralleled phase legs per output terminal. By paralleling such PEBBs adaptation to various ratings is possible.

GTO/IGBT (Thyristor based HVDC):     

         Normal Thyristor’s (silicon controlled rectifiers) are not fully controllable switches (a "fully controllable switch" can be turned on and off at will.) Thyristor’s can only be turned ON and cannot be turned OFF. Thyristor’s are switched ON by a gate signal, but even after the gate signal is de-asserted (removed), the Thyristor remains in the ON-state until any turn-off condition occurs (which can be the application of a reverse voltage to the terminals, or when the current flowing through (forward current) falls below a certain threshold value known as the holding current.) Thus, a Thyristor behaves like a normal semiconductor diode after it is turned on or "fired".

The GTO can be turned-on by a gate signal, and can also be turned-off by a gate signal of negative polarity.

Turn on is accomplished by a positive current pulse between the gate and cathode terminals. As the gate-cathode behaves like PN junction, there will be some relatively small voltage between the terminals. The turn on phenomenon in GTO is however, not as reliable as an SCR (Thyristor) and small positive gate current must be maintained even after turn on to improve reliability.

Turn off is accomplished by a negative voltage pulse between the gate and cathode terminals. Some of the forward current (about one third to one fifth) is "stolen" and used to induce a cathode-gate voltage which in turn induces the forward current to fall and the GTO will switch off (transitioning to the 'blocking' state.)

GTO Thyristor’s suffer from long switch off times, whereby after the forward current falls, there is a long tail time where residual current continues to flow until all remaining charge from the device is taken away. This restricts the maximum switching frequency to approx. 1 kHz.

It may however be noted that the turn off time of a comparable SCR is ten times that of a GTO.  Thus switching frequency of GTO is much better than SCR.

Gate turn-off (GTO) thyristors are used not only to turn on the main current but also to turn it off, provided with a gate drive circuit. Unlike conventional thyristors, they have no commutation circuit, downsizing application systems while improving efficiency. They are the most suitable for high-current, high speed switching applications, such as inverters and chopper circuits.

Bipolar devices made with SiC offer 20-50X lower switching losses as compared to conventional semiconductors. A rough estimate of the switching power losses as a function of switching frequency is shown in Figure 4. Another very significant property of SiC bipolar devices is their lower differential on-state voltage drop than similarly rated Si bipolar device, even with order of magnitude smaller carrier lifetimes in the drift region.

This property allows high voltage (>20 kV) to be far more reliable and thermally stable as compared to those made with Silicon. The switching losses and the temperature stability of bipolar power devices depend on the physics of operation of the device.

 The two major categories of bipolar power devices are: (a) single injecting junction devices (for example BJT and IGBT); and (b) double injecting junction devices (like Thyristor-based GTO/MTO/JCT/FCT and PIN diodes). In a power BJT, most of the minority carrier charge resides in the low doped collector layer, and hence its operation has been approximated as an IGBT. The limited gain of a BJT will make the following analysis less relevant for lower voltage devices.

  Silicon carbide has been projected to have tremendous potential for high voltage solid-state power devices with very high voltage and current ratings because of its electrical and physical properties. The rapid development of the technology for producing high quality single crystal [SiC] wafers and thin films presents the opportunity to fabricate solid- state devices with power-temperature capability far greater than devices currently available. This capability is ideally suited to the applications of power conditioning in new more- electric or all-electric military and commercial vehicles.

These applications require switches and amplifiers capable of large currents with relatively low voltage drops. One of the most pervasive power devices in silicon is the Insulated Gate Bipolar Transistor (IGBT). However, these devices are limited in their operating temperature and their achievable power ratings compared to that possible with [SiC]. Because of the nearly ideal combination of characteristics of these devices, we propose to demonstrate the first 4H-SiC Insulated Gate Bipolar Transistor in this Phase I effort. Both n-channel and p-channel SiC IGBT devices will be investigated. The targeted current and voltage rating for the Phase I IGBT will be a >200 Volt, 200 mA device, that can operate at 350 C.

12-Pulse Converters:

The basic design for practically all HVDC converters is the 12-pulse double bridge converter which is shown in Figure below. The converter consists of two 6-pulse bridge converters connected in series on the DC side. One of them is connected to the AC side by a YY-transformer, the other by a YD transformer. The AC currents from each 6-pulse converter will then be phase shifted 30°. This will reduce the harmonic content in the total current drawn from the grid, and leave only the characteristic harmonics of order 12 m±1, m=1,2,3..., or the 11th, 13th, 23th, 25th etc. harmonic. The non-characteristic harmonics will still be present, but considerably reduced. Thus the need for filtering is substantially reduced, compared to 6-pulse converters. The 12-pulse converter is usually built up of 12 thyristor valves. Each valve consists of the necessary number of thyristors in series to withstand the required blocking voltage with sufficient margin. Normally there is only one string of thyristors in each valve, no parallel connection. Four valves are built together in series to form a quadruple valve and three quadruple valves,

Fig 2.5:  12-pulse converter.

Fig 2.6: two 12 pulse converter

Main elements of a HVDC converter station with one bipolar consisting of two 12-pulse converter unit.

Together with converter transformer, controls and protection equipment constitute a converter. The converter transformers are usually three winding transformers with the windings in Yy d N-connection. There can be one three-phase or three single phase transformers, according to local circumstances. In order to optimize the relationship between AC- and DC voltage the converter transformers are equipped with tap changers.

HVDC converter stations                

An HVDC converter station  is normally built up of one or two 12-pulse converters as described above, depending on the system being mono- or bipolar. In some cases each pole of a bipolar system consists of two converters in series to increase the voltage and power rating of the transmission. It is not common to connect converters directly in parallel in one pole. The poles are normally as independent as possible to improve the reliability of the system, and each pole is equipped with a DC reactor and DC filters. Additionally the converter station consists of some jointly used equipment.

This can be the connection to the earth electrode, which normally is situated some distance away from the converter station area, AC filters and equipment for supply of the necessary reactive power.

Fig 2.7: Mono-polar HVDC transmissions Voltage in station B according to reversed polarity convention.

CHAPTER 3

BASIC CONTROL PRINCIPLES

3.1 CONTROL SYSTEM MODEL:

The control model mainly consists of measurements and generation of firing signals for both the rectifier and inverter. The PLO is used to build the firing signals. The output signal of the PLO is a ramp, synchronized to the phase-A commutating bus line-to-ground voltage, which is used to generate the firing signal for Valve 1. The ramps for other valves are generated by adding 60 to the Valve 1 ramp. As a result, an equidistant pulse is realized. The actual firing time is calculated by comparing the order to the value of the ramp and using interpolation technique. At the same time, if the valve is pulsed but its voltage is still less than the forward voltage drop, this model has a logic to delay firing until the voltage is exactly equal to the forward voltage drop. The firing pulse is maintained across each valve for 120.

The measurement circuits use zero-crossing information from commutating bus voltages and valve switching times and then convert this time difference to an angle (using measured PLO frequency). Firing angle (in seconds) is the time when valve turns on minus the zero crossing time for valve.

Extinction angle (in seconds) for valve is the time at which the commutation bus voltage for valve crosses zero (negative to   positive) minus the time valve turns off. The control schemes for both rectifier and inverter of the CIGRÉ HVDC system are available in the example file in PSCAD/EMTDC Version 4.0.1.

Following are the controllers used in the control schemes:

• Extinction Angle Controller;

• dc Current Controller;

• Voltage Dependent Current Limiter (VDCOL).

 Rectifier Control: The rectifier control system uses Constant Current Control (CCC) technique. The reference for current limit is obtained from the inverter side. This is done to ensure the protection of the converter in situations when inverter side does not have sufficient dc voltage support (due to a fault) or does not have sufficient load requirement (load rejection). The reference current used in rectifier control depends on the dc voltage available at the inverter side. Dc current on the rectifier side is measured using proper transducers and passed through necessary filters before they are compared to produce the error signal. The error signal is then passed through a PI controller, which produces the necessary firing angle order. The firing circuit uses this information to generate the equidistant pulses for the valves using the technique described earlier.

Inverter Control: The Extinction Angle Control or control and current control have been implemented on the inverter side. The CCC with Voltage Dependent Current Order Limiter (VDCOL) has been used here through PI controllers. The reference limit for the current control is obtained through a comparison of the external reference (selected by the operator or load requirement) and VDCOL (implemented through lookup table) output. The measured current is then subtracted from the reference limit to produce an error signal that is sent to the PI controller to produce the required angle order.

 The control uses another PI controller to produce gamma angle order for the inverter. The two angle orders are compared, and the minimum of the two is used to calculate the firing instant.

3.2  SIMULINK CONTROL SYSTEM MODEL:

The control blocks available in SIMULINK have been used to emulate the control algorithm described above Section, and enough care has been taken. Some control parameters required conversion to their proper values due to differences in units. The rectifier side uses current control with a reference obtained from the inverter VDCOL output (implemented through a lookup table), and the inverter control has both current control and control operating in parallel, and the lower output of the two is used to generate the firing pulses. The angle is not provided directly from the converter valve data. It needed to be implemented through measurements taken from valve data. The control block diagrams are shown in Fig.

Fig 3.1: Simulink Model for sag, swell and 7^th harmonic

Fig 3.2:  Simulink model for pi controller

Fig 3.3: Addition of sag and swell

3.3 DC transmission control

The current flowing in the DC transmission line shown in Figure below is determined by the DC voltage difference between station A and station B. Using the notation shown in the figure, where rd represents the total resistance of the line, we get for the DC current is

                 In rectifier operation the firing angle α should not be decreased below a certain minimum value αmin, normally 3°-5° in order to make sure that there really is a positive voltage across the valve at the firing instant. In inverter operation the extinction angle should never decrease below a certain minimum value γ_min, normally 17°-19° otherwise the risk of commutation failures becomes too high. On the other hand, both α and γ should be as low as possible to keep the necessary nominal rating of the equipment to a minimum. Low values of α and γ also decrease the consumption of reactive power and the harmonic distortion in the AC networks.

To achieve this, most HVDC systems are controlled to maintain γ = γ_min in normal operation. The DC voltage level is controlled by the transformer tap changer in inverter station B. The DC current is controlled by varying the DC voltage in rectifier station A, and thereby the voltage difference between A and B. Due to the small DC resistances in such a system, only a small voltage difference is required, and small variations in rectifier voltage gives large variations in current and transmitted power. The DC current through a converter cannot change the direction of flow. So the only way to change the direction of power flow through a DC transmission line is to reverse the voltage of the line. But the sign of the voltage difference has to be kept constantly positive to keep the current flowing. To keep the firing angle ‘α’ as low as possible, the transformers tap changer in rectifier station A is operated to keep ‘α’ on an operating value which gives only the necessary margin to αmin to be able to control the current.

 3.4 CONVERTER CURRENT/VOLTAGE CHARACTERISTICS:

The resistive voltage drop in converter and transformer, as well as the non-current voltage drop in the thyristor valves are often disregarded in practical analysis, as they are normally in the magnitude of 0.5 % of the normal operating voltage. The commutation voltage drop, however, has to be taken into account because its magnitude 5 to 10 % of the normal operating voltage. The direct voltage Ud from a 6-pulse bridge converter can then be expressed by

Where α is the firing angle, If the converter is operating as inverter it is more convenient to operate with extinction angle γ instead of firing angle α. The extinction angle is defined as the angle between the end of commutation to the next zero crossing of the commutation voltage. Firing angle α, commutation angle μ and extinction angle γ are related by

                                      

In inverter mode, the direct voltage from the inverter can be written as

                      

The current/voltage characteristics expressed in above are shown for normal values of id and dxN. In order to create a characteristic diagram for the complete transmission, it is usual to define positive voltage in inverter operation in the opposite direction compared to rectifier operation.

It is clear that to operate both converters on a constant firing/extinction angle principle is like leaving them without control. This will not give a stable point of operation, as both characteristics have approximately the same slope. Small differences appear due to variations in transformer data and voltage drop along the line. To gain the best possible control the characteristics should cross at as close to a right angle as possible. This means that one of the characteristics should preferably be constant current. This can only be achieved by a current controller.

If the current/voltage diagram of the rectifier is combined with a constant current controller characteristic we get the steady state diagram in Figure below for converter station A. A similar diagram can be drawn for converter station B. If we apply the reversed polarity convention for the inverter and combine the diagrams for station A and station B we get the diagram in Figure below in normal operation, the rectifier will be operating in current control mode with the firing angle            

            Fig 3.4: Steady state ud/id diagram for converter station A   Steady state ud/id diagram for converter station A&B

The inverter has a slightly lower current command than the rectifier and tries to decrease the current by increasing the counter voltage, but cannot decrease γ beyond γ_min. Thus we get the operating point A. We assume that the characteristic for station B is referred to station A i.e. it is corrected for the voltage drop along the transmission line. This voltage drop is in the magnitude of 1-5 % of the rated DC voltage.

If the AC voltage at the rectifier station drops, due to some external disturbance, the voltage difference is reduced and the DC current starts to sink. The current controller in the rectifier station starts to reduce the firing angle α, but soon meets the limit αmin, so the current cannot be upheld. When the current sinks below the current command of the inverter, the inverter control reduces the counter voltage to keep the current at the inverter current command, until a new stable operating point B is reached. If the current command at station A is decreased below that of station B, station A will see a current that is too high and start to increase the firing angle ‘α’, to reduce the voltage. Station B will see a diminishing current and try to keep it up by increasing the extinction angle γ to reduce the counter voltage. Finally station A meets the γ_min limit and cannot reduce the voltage any further and the new operating point will be at point C. Here the voltage has been reversed to negative while the current is still positive, that is the power flow has been reversed. Station A is operating as inverter and station B as rectifier. The difference between the current commands of the rectifier and the inverter is called the current margin. It is possible to change the power flow in the transmission simply by changing the sign of the current margin, but in practice it is desirable to do this in more controllable ways. Therefore the inverter is normally equipped with a αmin limitation in the range of 95°-105°. To avoid current fluctuations between operating points A and B at small voltage variations the corner of the inverter characteristic is often cut off. Finally, it is not desirable to operate the transmission with high currents at low voltages, and most HVDC controls are equipped with voltage dependent current command limitation.

3.3 MASTER CONTROL SYSTEM:

The controls described above are basic and fairly standardized and similar for all HVDC converter stations. The master control, however, is usually system specific and individually designed. Depending on the requirements of the transmission, the control can be designed for constant current or constant power transmitted, or it can be designed to help stabilizing the frequency in one of the AC networks by varying the amount of active power transmitted. The control systems are normally identical in both converter systems in a transmission, but the master control is only active in the station selected to act as the master station, which controls the current command. The calculated current command is transmitted by a communication system to the slave converter station, where the pre-designed current margin is added if the slave is to act as rectifier, subtracted if it is to act as inverter. In order to synchronize the two converters and assure that they operate with same current command (apart from the current margin), a tele-communications channel is required.

Should the telecommunications system fail for any reason, the current commands to both converters are frozen, thus allowing the transmission to stay in operation. Special fail-safe techniques are applied to ensure that the telecommunications system is fault-free. The requirements for the telecommunications system are especially high if the transmission is required to have a fast control of the transmitted power, and the time delay in processing and transmitting these signals will influence the dynamics of the total control system.

Conclusion of Different HVAC-HVDC

In order to examine the behavior of the losses in combined transmission and not in order to provide the best economical solutions for real case projects. Thus, most of the configurations are overrated, increasing the initial investment cost and consequently the energy transmission cost. The small number of different configurations analyzed provides a limited set of results, from which specific conclusions can be drawn regarding the energy transmission cost. Nevertheless, the same approach, as for the individual HVACHVDC systems, is followed in order to evaluate the energy availability and the energy transmission cost.

CHAPTER 4

UNIFIED POWER FLOW CONTROLLER

4.1 INTRODUCTION:

Flexible AC Transmission Systems, called FACTS, got in the recent years a well-known term for higher controllability in power systems by means of power electronic devices. Several FACTS-devices have been introduced for various applications worldwide. A number of new types of devices are in the stage of being introduced in practice.

In most of the applications the controllability is used to avoid cost intensive or landscape requiring extensions of power systems, for instance like upgrades or additions of substations and power lines. FACTS-devices provide a better adaptation to varying operational conditions and improve the usage of existing installations. The basic applications of FACTS-devices are:

• Power flow control,

• Increase of transmission capability,

• Voltage control,

• Reactive power compensation,

• Stability improvement,

• Power quality improvement,

• Power conditioning,

• Flicker mitigation,

• Interconnection of renewable and distributed generation and storages.

Operational limits of transmission line:

Figure 4.1 shows the basic idea of FACTS for transmission systems. The usage of lines for active power transmission should be ideally up to the thermal limits. Voltage and stability limits shall be shifted with the means of the several different FACTS devices. It can be seen that with growing line length, the opportunity for FACTS devices gets more and more important.

The influence of FACTS-devices is achieved through switched or controlled shunt compensation, series compensation or phase shift control. The devices work electrically as fast current, voltage or impedance controllers. The power electronic allows very short reaction times down to far below one second.

Fig 4.1: operational limits of transmission lines for different levels

The development of FACTS-devices has started with the growing capabilities of power electronic components. Devices for high power levels have been made available in converters for high and even highest voltage levels. The overall starting points are network elements influencing the reactive power or the impedance of a part of the power system. Figure 4.2 shows a number of basic devices separated into the conventional ones and the FACTS-devices.

For the FACTS side the taxonomy in terms of 'dynamic' and 'static' needs some explanation. The term 'dynamic' is used to express the fast controllability of FACTS-devices provided by the power electronics. This is one of the main differentiation factors from the conventional devices. The term 'static' means that the devices have no moving parts like mechanical switches to perform the dynamic controllability. Therefore most of the FACTS-devices can equally be static and dynamic.

Fig 4.2:  overview of major FACTS- Devices

The left column in Figure 4.2 contains the conventional devices build out of fixed or mechanically switch able components like resistance, inductance or capacitance together with transformers. The FACTS-devices contain these elements as well but use additional power electronic valves or converters to switch the elements in smaller steps or with switching patterns within a cycle of the alternating current. The left column of FACTS-devices uses Thyristor valves or converters. These valves or converters are well known since several years. They have low losses because of their low switching frequency of once a cycle in the converters or the usage of the Thyristor’s to simply bridge impedances in the valves.

The right column of FACTS-devices contains more advanced technology of voltage source converters based today mainly on Insulated Gate Bipolar Transistors (IGBT) or Insulated Gate Commutated Thyristor’s (IGCT). Voltage Source Converters provide a free controllable voltage in magnitude and phase due to a pulse width modulation of the IGBTs or IGCTs. High modulation frequencies allow to get low harmonics in the output signal and even to compensate disturbances coming from the network. The disadvantage is that with an increasing switching frequency, the losses are increasing as well. Therefore special designs of the converters are required to compensate this transmission line.

4.2 SHUNT DEVICES:

The most used FACTS-device is the SVC or the version with Voltage Source Converter called STATCOM. These shunt devices are operating as reactive power compensators. The main applications in transmission, distribution and industrial networks are:

• Reduction of unwanted reactive power flows and therefore reduced network losses.

• Keeping of contractual power exchanges with balanced reactive power.

• Compensation of consumers and improvement of power quality especially with huge demand fluctuations like industrial machines, metal melting plants, railway or underground train systems.

• Compensation of Thyristor converters e.g. in conventional HVDC lines.

• Improvement of static or transient stability.

Almost half of the SVC and more than half of the STATCOMs are used for industrial applications. Industry as well as commercial and domestic groups of users require power quality. Flickering lamps are no longer accepted, nor are interruptions of industrial processes due to insufficient power quality. Railway or underground systems with huge load variations require SVCs or STATCOMs.

Static VAR compensator (svc):

Electrical loads both generate and absorb reactive power. Since the transmitted load varies considerably from one hour to another, the reactive power balance in a grid varies as well. The result can be unacceptable voltage amplitude variations or even a voltage depression, at the extreme a voltage collapse.

A rapidly operating Static VAR Compensator (SVC) can continuously provide the reactive power required to control dynamic voltage oscillations under various system conditions and thereby improve the power system transmission and distribution stability.

Applications of the SVC systems in transmission systems:

Ø  To increase active power transfer capacity and transient stability margin

Ø  To damp power oscillations

Ø  To achieve effective voltage control

In addition, SVCs are also used in

1. Transmission systems

Ø  To reduce temporary over voltages

Ø  To damp sub synchronous resonances

Ø  To damp power oscillations in interconnected power systems

2. Traction systems

Ø  To balance loads

Ø  To improve power factor

Ø  To improve voltage regulation

3. HVDC systems

Ø  To provide reactive power to ac–dc converters

4. Arc furnaces

Ø  To reduce voltage variations and associated light flicker

Installing an SVC at one or more suitable points in the network can increase transfer capability and reduce losses while maintaining a smooth voltage profile under different network conditions. In addition an SVC can mitigate active power oscillations through voltage amplitude modulation.

SVC installations consist of a number of building blocks. The most important is the Thyristor valve, i.e. stack assemblies of series connected anti-parallel Thyristors to provide controllability. Air core reactors and high voltage AC capacitors are the reactive power elements used together with the Thyristor valves. The step up connection of this equipment to the transmission voltage is achieved through a power transformer.

Fig 4.3: SVC building blocks and voltage / current characteristic

In principle the SVC consists of Thyristor Switched Capacitors (TSC) and Thyristor Switched or Controlled Reactors (TSR / TCR). The coordinated control of a combination of these branches varies the reactive power as shown in Figure. The first commercial SVC was installed in 1972 for an electric arc furnace. On transmission level the first SVC was used in 1979. Since then it is widely used and the most accepted FACTS-device.

 SVC using a TCR and an fc:

In this arrangement, two or more FC (fixed capacitor) banks are connected to a TCR (thyristor controlled reactor) through a step-down transformer. The rating of the reactor is chosen larger than the rating of the capacitor by an amount to provide the maximum lagging VARs that have to be absorbed from the system. By changing the firing angle of the thyristor controlling the reactor from 90° to 180°, the reactive power can be varied over the entire range from maximum lagging VARS to leading VARS that can be absorbed from the system by this compensator.

Fig 4.4: SVC of the FC/TCR type:

The main disadvantage of this configuration is the significant harmonics that will be generated because of the partial conduction of the large reactor under normal sinusoidal steady-state operating condition when the SVC is absorbing zero MVAR. These harmonics are filtered in the following manner. Triplex harmonics are canceled by arranging the TCR and the secondary windings of the step-down transformer in delta connection. The capacitor banks with the help of series reactors are tuned to filter fifth, seventh, and other higher-order harmonics as a high-pass filter. Further losses are high due to the circulating current between the reactor and capacitor banks.

Fig 4.5 Comparison of the loss characteristics of TSC–TCR, TCR–FC compensators and synchronous condenser

These SVCs do not have a short-time overload capability because the reactors are usually of the air-core type. In applications requiring overload capability, TCR must be designed for short-time overloading, or separate thyristor-switched overload reactors must be employed.

 SVC using a TCR and TSC:

This compensator overcomes two major shortcomings of the earlier compensators by reducing losses under operating conditions and better performance under large system disturbances. In view of the smaller rating of each capacitor bank, the rating of the reactor bank will be 1/n times the maximum output of the SVC, thus reducing the harmonics generated by the reactor. In those situations where harmonics have to be reduced further, a small amount of FCs tuned as filters may be connected in parallel with the TCR.

Fig 4.6: SVC of combined TSC and TCR type

When large disturbances occur in a power system due to load rejection, there is a possibility for large voltage transients because of oscillatory interaction between system and the SVC capacitor bank or the parallel. The LC circuit of SVC,in the FC compensator. In the TSC–TCR scheme, due to the flexibility of rapid switching of capacitor banks without appreciable disturbance to the power system, oscillations can be avoided, and hence the transients in the system can also be avoided. The capital cost of this SVC is higher than that of the earlier one due to the increased number of capacitor switches and increased control complexity.

STATCOM:

In 1999 the first SVC with Voltage Source Converter called STATCOM (Static VAR Compensator) went into operation. The STATCOM has a characteristic similar to the synchronous condenser, but as an electronic device it has no inertia and is superior to the synchronous condenser in several ways, such as better dynamics, a lower investment cost and lower operating and maintenance costs. A STATCOM is built with Thyristors with turn-off capability like GTO or today IGCT or with more and more IGBTs. The static line between the current limitations has a certain steepness determining the control characteristic for the voltage.

The advantage of a STATCOM is that the reactive power provision is independent from the actual voltage on the connection point. This can be seen in the diagram for the maximum currents being independent of the voltage in comparison to the SVC. This means, that even during most severe contingencies, the STATCOM keeps its full capability.

In the distributed energy sector the usage of Voltage Source Converters for grid interconnection is common practice today. The next step in STATCOM development is the combination with energy storages on the DC-side. The performance for power quality and balanced network operation can be improved much more with the combination of active and reactive power.

Fig 4.7: STATCOM structure and voltage / current characteristic

STATCOMs are based on Voltage Sourced Converter (VSC) topology and utilize either Gate-Turn-off Thyristors (GTO) or Isolated Gate Bipolar Transistors (IGBT) devices.          The STATCOM is a very fast acting, electronic equivalent of a synchronous condenser. If the STATCOM voltage, Vs, (which is proportional to the dc bus voltage Vc) is larger than bus voltage, Es, then leading or capacitive VARS are produced. If Vs is smaller than Es, then lagging or inductive VARS is produced.

Fig 4.8:  6 Pulses STATCOM

The three phases STATCOM makes use of the fact that on a three phase, fundamental frequency, steady state basis, and the instantaneous power entering a purely reactive device must be zero. The reactive power in each phase is supplied by circulating the instantaneous real power between the phases. This is achieved by firing the GTO/diode switches in a manner that maintains the phase difference between the ac bus voltage ES and the STATCOM generated voltage VS. Ideally it is possible to construct a device based on circulating instantaneous power which has no energy storage device (ie no dc capacitor).

A practical STATCOM requires some amount of energy storage to accommodate harmonic power and ac system unbalances, when the instantaneous real power is non-zero. The maximum energy storage required for the STATCOM is much less than for a TCR/TSC type of SVC compensator of comparable rating.

Fig 4.9: STATCOM Equivalent circuit

Several different control techniques can be used for the firing control of the STATCOM. Fundamental switching of the GTO/diode once per cycle can be used. This approach will minimize switching losses, but will generally utilize more complex transformer topologies. As an alternative, Pulse Width Modulated (PWM) techniques, which turn on and off the GTO or IGBT switch more than once per cycle, can be used. This approach allows for simpler transformer topologies at the expense of higher switching losses.

The 6 Pulse STATCOM using fundamental switching will of course produce the 6 N+1 harmonics. There are a variety of methods to decrease the harmonics. These methods include the basic 12 pulse configuration with parallel star / delta transformer connections, a complete elimination of 5th and 7th harmonic current using series connection of star/star and star/delta transformers and a quasi-12 pulse method with a single star-star transformer, and two secondary windings, using control of firing angle to produce a 30° phase shift between the two 6 pulse bridges. This method can be extended to produce a 24 pulse and a 48 pulse STATCOM, thus eliminating harmonics even further. Another possible approach for harmonic cancellation is a multi-level configuration which allows for more than one switching element per level and therefore more than one switching in each bridge arm. The ac voltage derived has a staircase effect, dependent on the number of levels. This staircase voltage can be controlled to eliminate harmonics.

4.3 SERIES DEVICES:

Series devices have been further developed from fixed or mechanically switched compensations to the Thyristor Controlled Series Compensation (TCSC) or even Voltage Source Converter based devices.

The main applications are:

Ø  Reduction of series voltage decline in magnitude and angle over a  power line,

Ø  Reduction of voltage fluctuations within defined limits during changing power transmissions,

Ø  Improvement of system damping resp. damping of oscillations,

Ø  Limitation of short circuit currents in networks or substations,

Ø  Avoidance of loop flows representation. power flow adjustments.

TCSC:

Thyristor Controlled Series Capacitors (TCSC) addresses specific dynamical problems in transmission systems. Firstly it increases damping when large electrical systems are interconnected. Secondly it can overcome the problem of Sub Synchronous Resonance (SSR), a phenomenon that involves an interaction between large thermal generating units and series compensated transmission systems.

The TCSC's high speed switching capability provides a mechanism for controlling line power flow, which permits increased loading of existing transmission lines, and allows for rapid readjustment of line power flow in response to various contingencies. The TCSC also can regulate steady-state power flow within its rating limits.as shown in fig 4.10

From a principal technology point of view, the TCSC resembles the conventional series capacitor. All the power equipment is located on an isolated steel platform, including the Thyristor valve that is used to control the behavior of the main capacitor bank. Likewise the control and protection is located on ground potential together with other auxiliary systems. Figure shows the principle setup of a TCSC and its operational diagram. The firing angle and the thermal limits of the Thyristors determine the boundaries of the operational diagram.

Fig 4.10:  a model of TCR

Advantages

Ø  Continuous control of desired compensation level

Ø  Direct smooth control of power flow within the network

Ø  Improved capacitor bank protection

Ø  Local mitigation of sub synchronous resonance (SSR). This permits higher levels of compensation in networks where interactions with turbine-generator torsional vibrations or with other control or measuring systems are of concern.

Ø  Damping of electromechanical (0.5-2 Hz) power oscillations which often arise between areas in a large interconnected power network. These oscillations are due to the dynamics of inter area power transfer and often exhibit poor damping when the aggregate power transfer over a corridor is high relative to the transmission strength.

SSSC:

            SSSC emulates like a variable inductor or capacitor in series with a transmission line and it imitates inductance or capacitive reactance in turn to regulate effective line reactance between two ends. Series controller in general control current injection.

4.4 SHUNT AND SERIES DEVICES:

Dynamic power flow controller: A new device in the area of power flow control is the Dynamic Power Flow Controller (DFC). The DFC is a hybrid device between a Phase Shifting Transformer (PST) and switched series compensation.

A functional single line diagram of the Dynamic Flow Controller is shown in Figure 4.12. The Dynamic Flow Controller consists of the following components:

Ø  a standard phase shifting transformer with tap-changer (PST)

Ø  series-connected Thyristor Switched Capacitors and Reactors (TSC / TSR)

Ø  A mechanically switched shunt capacitor (MSC). (This is optional depending on the system reactive power requirements)

Fig 4.11:  Principle Configuration of DFC

Based on the system requirements, a DFC might consist of a number of series TSC or TSR. The mechanically switched shunt capacitor (MSC) will provide voltage support in case of overload and other conditions. Normally the reactance of reactors and the capacitors are selected based on a binary basis to result in a desired stepped reactance variation. If a higher power flow resolution is needed, a reactance equivalent to the half of the smallest one can be added.

The switching of series reactors occurs at zero current to avoid any harmonics. However, in general, the principle of phase-angle control used in TCSC can be applied for a continuous control as well. The operation of a DFC is based on the following rules:

Ø  TSC / TSR are switched when a fast response is required.

Ø  The decrease of overload and work in stressed situations is handled by the TSC / TSR.

Ø  The switching of the PST tap-changer should be minimized particularly for the currents higher than normal loading.

Ø  The total reactive power consumption of the device can be optimized by the operation of the MSC, tap changer, switched capacities and reactors.

In order to visualize the steady state operating range of the DFC, we assume an inductance in parallel representing parallel transmission paths. The overall control objective in steady state would be to control the distribution of power flow between the branch with the DFC and the parallel path. This control is accomplished by control of the injected series voltage.

The PST (assuming a quadrature booster) will inject a voltage in quadrature with the node voltage. The controllable reactance will inject a voltage in quadrature with the throughput current. Assuming that the power flow has a load factor close to one, the two parts of the series voltage will be close to collinear. However, in terms of speed of control, influence on reactive power balance and effectiveness at high/low loading the two parts of the series voltage has quite different characteristics. The steady state control range for loadings up to rated current is illustrated in Figure 4.12, where the x-axis corresponds to the throughput current and the y-axis corresponds to the injected series voltage.

Fig 4.12:  Operational diagram of a DFC

Operation in the first and third quadrants corresponds to reduction of power through the DFC, whereas operation in the second and fourth quadrants corresponds to increasing the power flow through the DFC. The slope of the line passing through the origin (at which the tap is at zero and TSC / TSR are bypassed) depends on the short circuit reactance of the PST.

Starting at rated current (2 kA) the short circuit reactance by itself provides an injected voltage (approximately 20 kV in this case). If more inductance is switched in and/or the tap is increased, the series voltage increases and the current through the DFC decreases (and the flow on parallel branches increases). The operating point moves along lines parallel to the arrows in the figure. The slope of these arrows depends on the size of the parallel reactance. The maximum series voltage in the first quadrant is obtained when all inductive steps are switched in and the tap is at its maximum.

Now, assuming maximum tap and inductance, if the throughput current decreases (due e.g. to changing loading of the system) the series voltage will decrease. At zero current, it will not matter whether the TSC / TSR steps are in or out, they will not contribute to the series voltage. Consequently, the series voltage at zero current corresponds to rated PST series voltage. Next, moving into the second quadrant, the operating range will be limited by the line corresponding to maximum tap and the capacitive step being switched in (and the inductive steps by-passed). In this case, the capacitive step is approximately as large as the short circuit reactance of the PST, giving an almost constant maximum voltage in the second quadrant.

SSSC:

            SSSC emulates like a variable inductor or capacitor in series with a transmission line and it imitates inductance or capacitive reactance in turn to regulate effective line reactance between two ends. Series controller in general control current injection.

4.5 UNIFIED POWER FLOW CONTROLLER:

Introduction:

            It Can be a standalone controller as STATCOM and SSSC. This type of controller is a reactive compensator with the exception of producing its own losses.it is also recognized as “unified” controller and requires small amount of power for DC circuit exchange occurring between the shunt-series converter.

“The UPFC is a combination of a static compensator and static series compensation. It acts as a shunt compensating and a phase shifting device simultaneously.”

Block diagram of UPFC:

Fig 4.13: Principle configuration of an UPFC

 The UPFC consists of a shunt and a series transformer, which are connected via two voltage source converters with a common DC-capacitor. The DC-circuit allows the active power exchange between shunt and series transformer to control the phase shift of the series voltage. This setup, as shown in Figure 5.1, provides the full controllability for voltage and power flow. The series converter needs to be protected with a Thyristor bridge. Due to the high efforts for the Voltage Source Converters and the protection, an UPFC is getting quite expensive, which limits the practical applications where the voltage and power flow control is required simultaneously.

4.6 OPERATING PRINCIPLE OF UPFC:

The basic components of the UPFC are two voltage source inverters (VSIs) sharing a common dc storage capacitor, and connected to the power system through coupling transformers. One VSI is connected to in shunt to the transmission system via a shunt transformer, while the other one is connected in series through a series transformer.

A basic UPFC functional scheme is shown in fig.5.2

Fig 4.14: a model of UPFC

The series inverter is controlled to inject a symmetrical three phase voltage system (VSE), of controllable magnitude and phase angle in series with the line to control active and reactive power flows on the transmission line. So, this inverter will exchange active and reactive power with the line. The reactive power is electronically provided by the series inverter, and the active power is transmitted to the dc terminals. The shunt inverter is operated in such a way as to demand this dc terminal power (positive or negative) from the line keeping the voltage across the storage capacitor Vdc constant. So, the net real power absorbed from the line by the UPFC is equal only to the losses of the inverters and their transformers. The remaining capacity of the shunt inverter can be used to exchange reactive power with the line so to provide a voltage regulation at the connection point.

The two VSI’s can work independently of each other by separating the dc side. So in that case, the shunt inverter is operating as a STATCOM that generates or absorbs reactive power to regulate the voltage magnitude at the connection point. Instead, the series inverter is operating as SSSC (static synchronous series compensator) that generates or absorbs reactive power to regulate the current flow, and hence the power low on the transmission line.

The UPFC has many possible operating modes. In particular, the shunt inverter is operating in such a way to inject a controllable current, ish into the transmission line. The shunt inverter can be controlled in two different modes:

VAR Control Mode: The reference input is an inductive or capacitive VAR request. The shunt inverter control translates the VAR reference into a corresponding shunt current request and adjusts gating of the inverter to establish the desired current. For this mode of control a feedback signal representing the dc bus voltage, Vdc, is also required.

Automatic Voltage Control Mode: The shunt inverter reactive current is automatically regulated to maintain the transmission line voltage at the point of connection to a reference value. For this mode of control, voltage feedback signals are obtained from the sending end bus feeding the shunt coupling transformer.

The series inverter controls the magnitude and angle of the voltage injected in series with the line to influence the power flow on the line. The actual value of the injected voltage can be obtained in several ways.

Direct Voltage Injection Mode: The reference inputs are directly the magnitude and phase angle of the series voltage.

Phase Angle Shifter Emulation mode: The reference input is phase displacement between the sending end voltage and the receiving end voltage. Line Impedance Emulation mode: The reference input is an impedance value to insert in series with the line impedance

Automatic Power Flow Control Mode: The reference inputs are values of P and Q to maintain on the transmission line despite system changes.

CHAPTER 6

 SIMULATION RESULTS

5.1 UNBALANCING PHASE VOLTAGES:

In Fig. 6, Shows HVDC system with UPFC the real power Output in the line is controlled to obtain steady-state condition when system harmonics is introduced. The weak power Transmission normally occurring in long transmission lines was studied using MATALB. The diagram given in Fig. 6 shows the computational layout of HVDC which is simulated for damping system harmonics and rectification as well as with power inversion in its converters. Simulation of HVDC System carried out using MATLAB / SIMULINK with UPFC and Simulation results was presented to create oscillations with the line current and power waveforms during the power transmission.

Fig 6.2 to Fig 6.10 shows the simulation results of HVDC system when three phase, Line to Ground and double line ground with and without UPFC. From the simulations results , it is observed that when different types of faults i.e. three phase ., Line to Ground and Double Line to ground occurs the system are having more oscillations and system takes more time to reach the steady state operation.. By using UPFC the system reduces oscillation and thereby enhanced the power transfer capability of HVDC system.

Fig 6.1: Simulation Result of HVDC systems

5.2 THREE-PHASE FAULT OCCURS ON INVERTER:

In Fig. 6.1, fault is created in phase A of the rectifier bus at t=0.03sec, it results in unbalancing of the phase voltages and generates harmonic oscillations in DC voltages and currents. The DC voltages and currents of the rectifier are distorted and attain peak values up to 0.9 per unit and 0.016 per unit respectively at time t=0.12sec.

Fig.6.2 Simulation Result HVDC system when three phase fault occurs.

In Fig 6.2, it is observed that a 3-phase fault is created in the inverter side of HVDC system. The PWM controller activates and clears the fault. The fault clearing can be seen first by a straight line of ‘0’ voltage between t=0.03sec to t=0.08sec. Before the fault a V_abc=0.17pu and I_abc=0.15pu. After the fault is cleared at t=0.3sec, the recovery is slow and there are oscillations in DC voltage and current of the magnitude 0.13pu and 0.1pu respectively. The rectifier DC voltage and current oscillate and settles to the prefault values in about 3 cycles after the fault is cleared on Inverter.

5.3 THREE-PHASE FAULT OCCURS ON INVERTER WITH UPFC:

Fig 6.3: Simulation Result HVDC system when three phase fault occurs on Inverter with UPSC.

From Fig 6.3, it is observed that different types of faults i.e., three phase, line to ground and double line to ground is created in the inverter side of HVDC system at t=0.15 sec. When these faults occur in the system, it takes more time to reach the steady state operation. The PWM controller activates and clears the fault. Further, with the addition of UPFC the system reduces oscillations and get pure sinusoidal waveform at voltage Vabc=0.9 p. u and current Iabc=0.95 p.u at time t=0.15 sec.

5. 4 STEADY STATE OPERATION OF HVDC SYSTEM ON RECTIFIER SIDE:

Fig 6.4:  Simulation Result for steady state operation of HVDC system on rectifier side.

At the rectifier side, when the fault is applied at time t=0.03sec, voltage and current magnitudes are of the order of 1pu and 1.5pu respectively and alpha angle is equal to 7 degrees which is shown in Fig 9.If alpha angle is changed to higher value the system takes longer time to reach steady state .If alpha value increases, current value decreases. The waveforms obtained at rectifier side are same for different types of faults.

5. 5    STEADY STATE OPERATION OF HVDC SYSTEM ON INVERTER SIDE

Fig 6.5: Simulation Result for steady state operation of HVDC system on Inverter side

At the inverter side, when the fault is applied at time t=0.02sec,voltage and current magnitudes are of the order of 0.03pu and 0.8pu respectively and extension angle is equal to 180 degrees which is shown in Fig . 10. The waveforms obtained at inverter side are same for different types of faults.

5. 6    LINE AND ACTIVE AND REACTIVE POWER OF HVDC SYSTEM

Fig 6.6:  Simulation Result for line active and reactive powers of HVDC system

In Fig 6.6, when a fault is created at time t=0.21sec, the active and reactive power is maintained at 800KW and 400KVAR respectively from time t=0sec to t=0.21sec.At time t=0.27sec both active and reactive power attain stability and becomes steady state. It is observed that no power fluctuations occur in P and Q after t=0.27sec.By trial and error, the integral gain is set to be 5, so that the steady state errors are reduced for P and Q.

5. 7    LINE-TO-GROUND FAULT OCCURS ON INVERTER SIDE

Fig   6.7: Simulation Result HVDC system when Line to Ground fault occurs on Inverter side.

In Fig 6.7, it is observed that a Line to Ground fault is created in the inverter side of HVDC system at time t=0.025sec. The PWM controller activates and clears the fault. Before the fault a Vabc=0.14pu and Iabc=0.013pu. After the fault is cleared at t=0.08sec, the recovery is slow and there are oscillations in DC voltage and current of the magnitude 0.2pu and 0.05pu respectively.

5.8  LINE-TO-GROUND FAULT OCCURS ON INVERTER SIDE WITH UPFC:

Fig 6.8:  Simulation Result HVDC system when Line to Ground fault with UPfC

From Fig 6.8,it is observed that different types of faults i.e., three phase, line to ground and double line to ground is created in the inverter side of HVDC system at t=0.15 sec. When these faults occur in the system, it takes more time to reach the steady state operation. The PWM controller activates and clears the fault. Further, with the addition of UPFC the system reduces oscillations and get pure sinusoidal waveform at voltage Vabc=0.9 p. u and current Iabc=0.95 p.u at time t=0.15 sec.

5. 9    DOUBLE-TO-GROUND FAULT OCCURS ON INVERTER SIDE  

Fig.6.9 Simulation Result HVDC system when Double Line to Ground fault occurs on Inverter side.

In Fig 14, it is observed that a Double Line to Ground fault is created in the inverter side of HVDC system at time t=0.02sec. The PWM controller activates and clears the fault. Before the fault a Vabc=0.17pu and Iabc=0.15pu. After the fault is cleared at t=0.33sec, the recovery is slow and there are oscillations in DC voltage and current of the magnitude 0.33pu and 0.1pu respectively.

5.10   DOUBLE-TO-GROUND FAULT OCCURS ON INVERTER SIDE WITH UPFC:

Fig 6.10: Simulation Result HVDC system when Double Line to Ground fault with UPfC

CHAPTER 6

CONCLUSION

6.1 CONCLUSIONS

According to results that UPFC improves the system performance under the transient and the normal conditions. However, it can control the power flow in the transmission line, effectively. With the addition of UPFC, the magnitude of fault current reduces and oscillations of excitation voltage also reduce. The "current margin" is essential to prevent misfire of the Thyristor valves. DC filters and AC filters can not only eliminate the harmonic effects but also reduce the total harmonic distortion (THD) as well. The current waveform in the case of a conventional controller has a lot of crests and dents, suffers from prolonged oscillations, whereas by using PWM controller, DC current fast returns to its nominal value.

The overshoot in case of the PWM controller is slightly less than conventional controllers. It is more economical for the HVDC transmission system to transfer more power as the power factor is almost near to unity and the energy loss is low UPFC, however, has shown its flexibility in easing line congestion and promoting a more controllable flow in the lines. HVDC can be very useful for long transmission lines.

6.2 SCOPE FOR THE FUTURE WORK:

It is more recommended in networks or interconnected lines that have high variation of power demands and complicated network connections with different power frequencies. UPFC in general is good for promoting line load-ability and pool through interconnected network buses more effectively. UPFC can be very useful for deregulated energy market as an alternative choice for more power generation to the load area.

REFERENCES:

[1]        Lee Wei Sheng, Ahmad Razani and Neelakantan Prabhakaran, Senior Member,         IEEE “Control of High Voltage Direct Current (HVDC) Bridges for Power Transmission Systems” Proceedings of 2010 IEEE Student Conference on Research and Development (SCOReD 2010), 13 - 14 Dec 2010, Putrajaya, Malaysia.

[2]     E.M. Yap, M. Al-Dabbagh and P.C Thum, “Using UPFC Controller in Mitigating Line Congestion for Cost-efficient Power Delivery, “submitted at the Tencon 2005, IEEE conference, May 2005.

                     

[3]        J.W. Evan, “Interface between automation and Substation,” Electric Power substations engineering, J.D. Macdonald, Ed. USA: CRC Press,2003, pp. 6-1 (Chapter 6).

[4]        X.-P. Zhang, "Multiterminal Voltage-Sourced Converter Based HVDC Models for Power Flow Analysis", IEEE Transactions on Power Systems, vol. 18, no. 4, 2004, pp.1877-1884.

[5]        D J Hanson, C Horwill, B D Gemmell, D R Monkhouse, "ATATCOM-Based Reloadable SVC Project in the UK for National rid", in Proc. 2002 IEEE PES Winter Power Meeting, New York City, 7- 31 January 2002. uip_2.pd.

[6]        2012 International Conference on Computer Communication and Informatics (ICCCI 2012), Jan. 10 – 12, 2012, Coimbatore, INDIA