In this section we will examine in detail the single-phase full-bridge VSC. Its power circuit is shown in Figure 6.26. It consists of two identical legs like the half-bridge single-phase converter (Figure 6.23) discussed in Section 6.3.1. Specifically, there are four switching elements (S1, S2, S3, S4), four antiparallel diodes (D1, D2, D3, D4) and a DC bus voltage source Vdc that can be a single capacitor. The other leg provides the return path for the current this time and the DC bus mid-point does not need to be available to connect the load. The output voltage v0 appears across the two points A and B as shown in Figure 6.26.
The control restriction discussed for the single-phase half-bridge topology (Figure 6.23) applies to this converter as well. Clearly the control signals for the switch pairs (S1, S2) and (S3, S4) must be complementary to avoid any bridge destruction due to shoot through of infinite current (at least theoretically).
There are two control methods for this topology. The first one treats the switches (S1, S4) and (S2, S3) as a pair. This means that they are turned on and off at the same time and for the same duration. For square-wave operation the switches S1 and S4 are on for half of the period. For the other half, the pair of S2, S3 is turned on. Like the single-phase half-bridge VSC, the direction of the output current i0 determines the conduction state of each semiconductor.
When the two switches S1 and S4 are turned on, the voltage at the output is equal to the DC bus voltage Vdc. Similarly, when the switches S2 and S3 are turned on the output voltage is equal to - Vdc. Such circuit operation is illustrated in Figure 6.27.
In the first case, when the direction of the output current io is positive as shown in Figure 6.26, the current flows through switches S1 and S4 and the power is transferred from the DC side to the AC one (t4 < t < t5). When the current becomes negative, although the switches S1 and S4 are turned on, the diodes D1 and D4 conduct the current and return power back to the DC bus from the AC side (t3 < t < t4). For the other half of the period, when the switches S2 and S3 are turned on and the current is positive, the diodes D2 and D3 conduct (t1 < t < t2). In this
Fig. 6.26 Single-phase full-bridge VSC.
Fig. 6.27 Key waveforms of the single-phase full-bridge VSC circuit operation. (a) output voltage V0 = VAB; (b) output current i0; (c) input DC bus current id; (d) harmonic spectrum of the output voltage V0 = VAB; (e) harmonic spectrum of the output current i0; and (f) harmonic spectrum of the input DC bus current id.
instance, power is transferred also back to the DC side from the AC side. Finally, when the current is negative, the switches S2 and S3 carry the current and assist the converter to transfer power from the DC bus to the AC side (t2 < t < t3). In summary, there are four distinct modes of operation for this converter when the control method shown in Figure 6.27 is employed (two inverter modes and two rectifier modes). Simply said, at all times two switches are turned on and the legs are controlled in a synchronized way.
The output voltage v0 = vAB is shown in Figure 6.27(a). The output current i0 and the input DC current id are also plotted in Figures 6.27(b) and (c) respectively. Similarly, like the case of the half-bridge topology, the square-wave generated across the AC side includes all odd harmonics and being a single-phase system, the third harmonic is also present (Figure 6.27(d)). These harmonics when reflected back to the DC side source include all even harmonics (Figure 6.27(f).
Fig. 6.28 Quadrants of operation of the single-phase full-bridge VSC.
The fundamental component of the output voltage v0 waveform has an amplitude value of
And its various harmonics are given by
where h is the order of the harmonic.
The converter is capable of operating in all four quadrants of voltage and current as shown in Figure 6.28. The various modes and their relationship to the switching and/or conduction state of the semiconductors are also summarized in Table 6.4 for further clarity. The phase relationship between the AC output voltage and AC output current does not have to be fixed and the converter can provide real and reactive power at all leading and lagging power factors. However, the converter itself cannot control the output voltage if the DC bus voltage Vdc remains constant. There is a need to adjust the level of the DC bus voltage if one wants to control the rms value of the output voltage v0.
There is however a way to control the rms value of the fundamental component of the output voltage as well as the harmonic content of the fixed waveform shown in Figure 6.27(a). In this method, the control signals of the two legs are not
Table 6.4 Modes of operation of the single-phase full-bridge VSC
previous Single-phase half-bridge VSC
- Voltage-source converters or else voltage-source inverters (VSIs): the DC bus input is a voltage source (typically a capacitor) and its current through can be either positive or negative. This allows power flow between the DC and AC sides to be bidirectional through the reversal of the direction of the current.
- Current-source converters (CSCs) or else current-source inverters (CSIs): the DC bus input is a current source (typically an inductor in series with a voltage source, i.e. a capacitor) and its voltage across can be either positive or negative. This also allows the power flow between the DC and AC sides to be bidirectional through the reversal of the polarity of the voltage.
The current has a fundamental-frequency component iAC which leads the supply voltage by π/2 radians. Its amplitude îAC is given by
next The thyristor-switched capacitor (TSC)
b The values apply to both phase and line currents, except that triples harmonics do not appear in the line currents. Balanced conditions are assumed.
With both 6-pulse and 12-pulse TCR compensators, the need for filters and their frequency responses must be evaluated with due regard to the possibility of unbalanced operation. The influence of other capacitor balks and sources of harmonic currents in the electrical neighbourhood of the compensator must also be taken into account. For this purpose, several software packages are available and some examples with a specific one will be provided later.
The 12-pulse connection has the further advantage that if one half is faulted the other may be able to continue to operate normally. The control system must take into account the 30° phase shift between the two TCRs, and must be designed to ensure accurate harmonic cancellation. A variant of the 12-pulse TCR uses two separate transformers instead of one with two secondaries.
Maintain voltage at or near a constant level
- under slowly varying conditions due to load changes
- to correct voltage changes caused by unexpected events (e.g. load rejections, generator and line outages)
- to reduce voltage flicker caused by rapidly fluctuating loads (e.g. arc furnaces).
Improve power system stability
- by supporting the voltage at key points (e.g. the mid-point of a long line)
- by helping to improve swing damping.
Improve power factor
Correct phase unbalance
In recent years, reports have shown that improvements in the performance of the semiconductors can be achieved by replacing silicon with the following:
- silicon carbide (SiC)
- semiconducting diamond
- gallium arsenide.
The first group of devices is the most promising technology (Palmour et al., 1997).
These new power semiconductor materials offer a number of interesting characteristics, which can be summarized as follows:
- large band gap
- high carrier mobility
- high electrical and thermal conductivity.
Due to the characteristics mentioned above, this new class of power device offers a number of positive attributes such as:
- high power capability
- operation at high frequencies
- relatively low voltage drop when conducting
- operation at high junction temperatures.
Such devices will be able to operate at temperatures up to approximately 600°C. It is anticipated that this technology will probably offer semiconductors with characteristics closer to the desired ones discussed in the previous section.
Another important development is associated with the matrix converter (direct AC-AC conversion without a DC-link stage). For this converter bidirectional self- commutated devices are needed to build the converter. At the moment research efforts show some promising results (Heinke et al., 2000). However, a commercial product is probably not going to be available before the next decade or so.
Fig. 5.17 A lossless snubber circuit for an inverter leg.
Progress in semiconductor devices achieved over the last twenty years and anticipated developments and improvements promise an exciting new era in power electronic systems. Snubberless operation of fully controlled semiconductors at high values of current and voltage and their rates of change will be realizable in the near future. New emerging applications of these semiconductors in areas such as Power Transmission and Distribution and High Voltage Industrial Motor Drives will be possible. The thyristor will remain the only component for certain applications, due to its unmatched characteristics. However, expected improvements of the GTO and IGBT technology and emerging new devices may replace it sooner than later. New applications and use of improved semiconductors may be possible. The next ten to twenty years will therefore see design and use of power electronic systems towards the 'silicon only' with 'no impedance' reactive power compensators and a totally electronically controlled power system as will be discussed later.
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