MULTILEVEL INVERTER

                                       MULTILEVEL INVERTER
Historical Review

From the late nineteenth century through the middle of the twentieth century, DC-to-AC power conversion was accomplished using rotary converters or motor-generator sets (M-G sets). In the early twentieth century, vacuum tubes and gas filled tubes began to be used as switches in inverter circuits. The most widely used type of tube was the thyratron.

The origins of electromechanical inverters explain the source of the term inverter. Early AC-to-DC converters used an induction or synchronous AC motor direct-connected to a generator (dynamo) so that the generator's commutator reversed its connections at exactly the right moments to produce DC. A later development is the synchronous converter, in which the motor and generator windings are combined into one armature, with slip rings at one end and a commutator at the other and only one field frame. The result with either is AC-in, DC-out. With an M-G set, the DC can be considered to be separately generated from the AC; with a synchronous converter, in a certain sense it can be considered to be "mechanically rectified AC". Given the right auxiliary and control equipment, an M-G set or rotary converter can be "run backwards", converting DC to AC. Hence an Inverter is an inverted converter.

Controlled rectifier and inverters

Since early transistors were not available with sufficient voltage and current ratings for most inverter applications, it was the 1957 introduction of the thyristor or silicon-controlled rectifier (SCR) that initiated the transition to solid state inverter circuits. The commutation requirements of SCRs are a key consideration in SCR circuit designs. SCRs do not turn off or commutate automatically when the gate control signal is shut off. They only turn off when the forward current is reduced to below the minimum holding current, which varies with each kind of SCR, through some external process.

Fig. 4.1. 12-pulse line-commutated inverter circuit

For SCRs connected to an AC power source, commutation occurs naturally every time the polarity of the source voltage reverses. SCRs connected to a DC power source usually require a means of forced commutation that forces the current to zero when commutation is required. The least complicated SCR circuits employ natural commutation rather than forced commutation. With the addition of forced commutation circuits, SCRs have been used in the types of inverter circuits described above.

In applications where inverters transfer power from a DC power source to an AC power source, it is possible to use AC-to-DC controlled rectifier circuits operating in the inversion mode. In the inversion mode, a controlled rectifier circuit operates as a line commutated inverter. This type of operation can be used in HVDC power transmission systems and in regenerative braking operation of motor control systems.

Another type of SCR inverter circuit is the current source input (CSI) inverter. A CSI inverter is the dual of a six-step voltage source inverter. With a current source inverter, the DC power supply is configd as a current source rather than a voltage source. The inverter SCRs is switched in a six-step sequence to direct the current to a three-phase AC load as a stepped current waveform. CSI inverter commutation methods include load commutation and parallel capacitor commutation. With both methods, the input current regulation assists the commutation. With load commutation, the load is a synchronous motor operated at a leading power factor.As they have become available in higher voltage and current ratings, semiconductors such as transistors or IGBTs that can be turned off by means of control signals have become the preferred switching components for use in inverter circuits.

Rectifier and inverter pulse numbers

Rectifier circuits are often classified by the number of current pulses that flow to the DC side of the rectifier per cycle of AC input voltage. A single-phase half-wave rectifier is a one-pulse circuit and a single-phase full-wave rectifier is a two-pulse circuit. A three-phase half-wave rectifier is a three-pulse circuit and a three-phase full-wave rectifier is a six-pulse circuit.

With three-phase rectifiers, two or more rectifiers are sometimes connected in series or parallel to obtain higher voltage or current ratings. The rectifier inputs are supplied from special transformers that provide phase shifted outputs. This has the effect of phase multiplication. Six phases are obtained from two transformers, twelve phases from three transformers and so on. The associated rectifier circuits are 12-pulse rectifiers, 18-pulse rectifiers and so on...

When controlled rectifier circuits are operated in the inversion mode, they would be classified by pulse number also. Rectifier circuits that have a higher pulse number have reduced harmonic content in the AC input current and reduced ripple in the DC output voltage. In the inversion mode, circuits that have a higher pulse number have lower harmonic content in the AC output voltage waveform.

Multilevel Inverter

An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. Static inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries. The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters were made to work in reverse, and thus were "inverted", to convert DC to AC. The inverter performs the opposite function of a rectifier.

Commonly employed multilevel inverter topologies are Diode Clamped, Capacitor Clamped and Cascaded Multilevel inverters.

Cascaded H-Bridges inverter

A single-phase structure of an m-level cascaded inverter is illustrated in Fig. Each separate dc source (SDCS) is connected to a single-phase full-bridge, or H-bridge, inverter. Each inverter level can generate three different voltage outputs, +V_dc, 0, and –V_dc by connecting the dc source to the ac output by different combinations of the four switches, S₁, S₂, S₃, and S₄. To obtain +V_dc, switches S₁ and S₄ are turned on, whereas –V_dc can be obtained by turning on switches S₂ and S₃. By turning on S₁ and S₂ or S₃ and S₄, the output voltage is 0. The ac outputs of each of the different full-bridge inverter levels are connected in series such that the synthesized voltage waveform is the sum of the inverter outputs.

The number of output phase voltage levels m in a cascade inverter is defined by m = 2s+1, where s is the number of separate dc sources. An example phase voltage waveform for an 11-level cascaded H-bridge inverter with 5 SDCSs and 5 full bridges is shown in Fig 4.2.

The phase voltage v_an = v_a1 + v_a2 + v_a3 + v_a4 + v_a5.

For a stepped waveform such as the one depicted in Fig 31.2 with s steps, the Fourier Transform for this waveform follows

  , where n=1,3,5,....

Fig.. 4.2. Single-phase structure of a multilevel cascaded H-bridges inverter

Fig. 4.3. Output phase voltage waveform of an 11-level cascade inverter with 5 separate dc sources

The magnitudes of the Fourier coefficients when normalized with respect to V_dc are as follows:

, where n= 1,3,5,7,.......

The conducting angles, θ₁, θ₂, ..., θ_s, can be chosen such that the voltage total harmonic distortion is a minimum. Generally, these angles are chosen so that predominant lower frequency harmonics, 5th, 7th, 11th, and 13^th, harmonics are eliminated. More detail on harmonic elimination techniques will be presented in the next section.

Multilevel cascaded inverters have been proposed for such applications as static var generation, an interface with renewable energy sources, and for battery-based applications. Three-phase cascaded inverters can be connected in wye, as shown in Fig 4.4, or in delta. P_eng has demonstrated a prototype multilevel cascaded static var generator connected in parallel with the electrical system that could supply or draw reactive current from an electrical system. The inverter could be controlled to either regulate the power factor of the current drawn from the source or the bus voltage of the electrical system where the inverter was connected. P_eng and Joos have also shown that a cascade inverter can be directly connected in series with the electrical system for static var compensation.

Cascaded inverters are ideal for connecting renewable energy sources with an ac grid, because of the need for separate dc sources, which is the case in applications such as photovoltaic’s or fuel cells. Cascaded inverters have also been proposed for use as the main traction drive in electric vehicles, where several batteries or ultra capacitors are well suited to serve as SDCSs. The cascaded inverter could also serve as a rectifier/charger for the batteries of an electric vehicle while the vehicle was connected to an ac supply as shown in Fig 31.3. Additionally, the cascade inverter can act as a rectifier in a vehicle that uses regenerative braking.

The main advantages and disadvantages of multilevel cascaded H-bridge converters are as follows:

Advantages:

·         The number of possible output voltage levels is more than twice the number of dc sources (m = 2s + 1).

·         The series of H-bridges makes for modularized layout and packaging. This will enable the manufacturing process to be done more quickly and cheaply.

Disadvantages:

·         Separate dc sources are required for each of the H-bridges. This will limit its application to products that already have multiple SDCSs readily available.

Fig.4.4. Three-phase wye-connection structure for electric vehicle motor drive and battery charging

Diode-Clamped Multilevel Inverter

The neutral point converter proposed by Nabae, Takahashi, and Akagi in 1981 was essentially a three-level diode-clamped inverter. In the 1990s several researchers published articles that have reported experimental results for four^-, five^-, and six-level diode-clamped converters for such uses as static var compensation, variable speed motor drives, and high-voltage system interconnections.

A three-phase six-level diode-clamped inverter is shown in Fig 31.5. Each of the three phases of the inverter shares a common dc bus, which has been subdivided by five capacitors into six levels. The voltage across each capacitor is V_dc, and the voltage stress across each switching device is limited to V_dc through the clamping diodes.

            Table 3.1 lists the output voltage levels possible for one phase of the inverter with the negative dc rail voltage V₀ as a reference. State condition 1 means the switch is on, and 0 means the switch is off. Each phase has five complementary switch pairs such that turning on one of the switches of the pair requires that the other complementary switch be turned off. The complementary switch pairs for phase leg-A are (S_a1, S_a’1), (S_a2, S_a’2), (S_a3, S_a’3), (S_a4, S_a’4), and (S_a5, S_a’5). Table 31.1 also shows that in a diode-clamped inverter, the switches that are on for a particular phase leg is always adjacent and in series. For a six-level inverter, a set of five switches is on at any given time.

Fig.4.5. Three-phase six-level structure of a diode-clamped inverter

TABLE 4.1

Diode-clamped six-level inverter voltage levels and corresponding switch states

Advantages:

·         All of the phases share a common dc bus, which minimizes the capacitance requirements of the converter. For this reason, a back-to-back topology is not only possible but also practical for uses such as a high-voltage back-to-back inter-connection or an adjustable speed drive.

·         The capacitors can be pre-charged as a group.

·         Efficiency is high for fundamental frequency switching.

Disadvantages:

·         Real power flow is difficult for a single inverter because the intermediate dc levels will tend to overcharge or discharge without precise monitoring and control.

·         The number of clamping diodes required is quadratically related to the number of levels, which can be cumbersome for units with a high number of levels.

 Flying Capacitor Multilevel Inverter

Meynard and Foch introduced a flying-capacitor-based inverter in 1992. The structure of this inverter is similar to that of the diode-clamped inverter except that instead of using clamping diodes, the inverter uses capacitors in their place. The circuit topology of the flying capacitor multilevel inverter is shown in Fig 31.7. This topology has a ladder structure of dc side capacitors, where the voltage on each capacitor differs from that of the next capacitor. The voltage increment between two adjacent capacitor legs gives the size of the voltage steps in the output waveform.

One advantage of the flying-capacitor-based inverter is that it has redundancies for inner voltage levels; in other words, two or more valid switch combinations can synthesize an output voltage. Unlike the diode-clamped inverter, the flying-capacitor inverter does not require all of the switches that are on (conducting) be in a consecutive series. Moreover, the flying-capacitor inverter has phase redundancies, whereas the diode-clamped inverter has only line-line redundancies. 

Fig.4.6. Three-phase six-level structure of a flying capacitor inverter

In addition to the (m-1) dc link capacitors, the m-level flying-capacitor multilevel inverter will require (m-1) × (m-2)/2 auxiliary capacitors per phase if the voltage rating of the capacitors is identical to that of the main switches. One application proposed in the literature for the multilevel flying capacitor is static var generation. The main advantages and disadvantages of multilevel flying capacitor converters are as follows.

Advantages:

·         Phase redundancies are available for balancing the voltage levels of the capacitors.

·         Real and reactive power flow can be controlled.

·         The large number of capacitors enables the inverter to ride through short duration outages and deep voltage sags.

Disadvantages:

·         Control is complicated to track the voltage levels for all of the capacitors. Also, pre-charging all of the capacitors to the same voltage level and startup are complex.

·         Switching utilization and efficiency are poor for real power transmission.

·         The large numbers of capacitors are both more expensive and bulky than clamping diodes in multilevel diode-clamped converters. Packaging is also more difficult in inverters with a high number of levels.

 Modulation Techniques

Modulation techniques for voltage source inverters may be carrier based or carrier-less and open loop or closed loop. These modulation or control techniques for multilevel voltage source inverters are classified in Fig 5. The SPWM technique is considered for study in this paper. It is the simple technique to be implemented. In the SPWM technique, a triangular carrier wave at a high switching frequency is compared with the sinusoidal reference wave at a fundamental output frequency. The SPWM technique is again divided into Alternate Phase Opposition Disposition, Phase Opposition Disposition and In Phase (PH) [12].

Fig.4.7. The generation of switching pulses for power device S1 of the two-level inverter

One triangular carrier wave is compared with a sinusoidal reference wave to generate switching pulses. For power device S4, the complementary of this pulse is to be given.

The control principle of the SPWM is to use several triangular carrier signals keeping only one modulating sinusoidal signal. If an m-level inverter is employed, (m-1) level shifted carriers will be needed. Two and four triangular carrier signals are needed for three- and five-level inverters, respectively. The carriers have the same frequency fc and the same peak-to-peak amplitude Ac. The zero reference is placed in the middle of the carrier set. The modulating signal is a sinusoid of frequency fm and amplitude Am. At every instant, each carrier is compared with the modulating signal. Each comparison switches the switch ‘on’ if the modulating signal is greater than the triangular carrier assigned to that switch. Obviously, the actual driving signals for the power devices can be derived from the results of the modulating–carrier comparison by means of a control logic circuit. Pulses for the lower two devices Sa1 ’ and Sa2  are complementary to these pulses, respectively.

Fig.4.8. Generation of switching pulses for power devices Sa1 and Sa2 of the three-level inverter

Summery

            In this chapter the basic operation of inverter is studied. Mainly these circuits are used to convert DC power into AC. In this conversion, the converted output voltage contains harmonic content. To minimize the harmonics present in the AC side of the inverters, multilevel converter topology is introduced. These multilevel converters are classified according to the connections used. These converters are classified as: Cascaded H-Bridge inverter, Diode clamped multilevel inverter and Flying capacitor multilevel inverter. The circuital arrangement, advantages and disadvantages of these topologies are discussed in detail. And finally sinusoidal PWM technique is discussed.