Single-phase half-bridge VSC

Let us consider first the simplest and basic solid-state DC-AC converter, namely the single-phase half-bridge VSC. Figure 6.23 shows the power circuit. It consists of two switching devices (S1 and S2) with two antiparallel diodes (D1 and D1) to accommodate the return of the current to the DC bus when required. This happens when the load power factor is other than unity. In order to generate a mid-point (0) to connect the return path of the load, two equal value capacitors (C1 and C2) are connected in series across the DC input. The result is that the voltage Vdc is split into two equal sources across each capacitor with voltage of Vdc/2. The assumption here is that the value of the capacitors is sufficiently large to ensure a stiff DC voltage source. This simply means that their voltage potential remains unchanged during the operation of the circuit. This also means that the potential of the mid-point (0) is constant with respect to both positive and negative DC bus rails at all times (Vdc/2) and - Vdc/2 respectively).
Single-phase half-bridge VSC
Fig. 6.22 Single-phase half-bridge VSC.
Let us now examine the operation of this circuit. It can be explained in combination with Figure 6.24. The two control signals for turning on and off the switches S1 and S2 are complementary to avoid destruction of the bridge. This would happen due to the throughput of high current coming from the low impedance DC voltage sources, if both switches were turned on simultaneously. When the switch S1 is turned on (t3 < t < t5), the output voltage v0 = vAO is equal to the voltage Vdc/2 of the capacitor C1. The mode of operation of the switching block (S1 and D1) is then controlled by the polarity of the output current i0. If the output current is positive, with respect to the direction shown in Figure 6.23, then the current is flowing through switch S1 (t4 < t < t5, Figure 6.24). If the output current is negative, the diode D1 is conducting, although switch S1 is turned on (t3 < t < t4). Similarly, if the switch S2 is turned on (t1 < t < t3), the output voltage is equal to the voltage Vdc/2 of the capacitor C2 with the polarity appearing negative this time. The output current i0 once again determines the conduction state of the switch and diode. If the output current is positive, the diode D2 is conducting (t1 < t < t2). If the output current is negative, the current flows through switch S2 (t2 < t < t3). Such states of switches and diodes are clearly marked in the waveforms of Figure 6.24 for the various time intervals. The modes of operation of the half-bridge single-phase VSC are also summarized in Table 6.3.
Figure 6.24(a) shows the output voltage waveform v0 = vA0 generated by the converter operation as previously explained. Due to the square-wave generated by the converter, the output voltage waveform is rich in harmonics. Specifically, as shown in Figure 6.24(c) all odd harmonics are present in the spectrum of the output voltage. The fact that the converter cannot control the rms value of the output voltage waveform at fundamental frequency is also a limitation. A separate arrangement must be made to vary the DC bus voltage Vdc in order to vary and control the output voltage v0.
Key waveforms of the single-phase half-bridge VSC circuit operation
Fig. 6.24 Key waveforms of the single-phase half-bridge VSC circuit operation. (a) output voltage V0 = VA0; (b) output current i0; and (c) harmonic spectrum of the output voltage V0 = VA0.
Single-phase half-bridge VSC
The amplitude of the fundamental component of the output voltage square-wave v0 shown in Figure 6.24(a) can be expressed using Fourier series as follows
{({\hat V_0})_1} = {({\hat V_{A0}})_1} = \frac{{4.{V_{dc}}}}{{1.\pi }}                 (6.18)
The amplitude of all the other harmonics is given by
{({\hat V_0})_h} = {({\hat V_{A0}})_h} = \frac{{4.{V_{dc}}}}{{2.\pi .h}} = \frac{{{{({{\hat V}_0})}_1}}}{h}{\rm{        }}h = 3,5,7,9,......           (6.19)
where h is the order of the harmonic.
quadrants of operation of the single-phase half-bridge VSC
Fig. 6.25 quadrants of operation of the single-phase half-bridge VSC.
The converter discussed here operates in all four quadrants of output voltage and current as shown in Figure 6.25. There are two distinct modes of operation associated with the transfer of power from the DC to the AC side. When the power flows from the DC bus to the AC side, the converter operates as an inverter. The switches S1 and S2 perform this function. In the case that the power is negative, which means power is returned back to the DC bus from the AC side, the converter operates as a rectifier. The diodes D1 and D1 perform this function.
The capability of the converter to operate in all four quadrants (Figure 6.25)
means that there is no restriction in the phase relationship between the AC output voltage and the AC output current. The converter can therefore be used to exchange leading or lagging reactive power. If the load is purely resistive and no filter is attached to the output the diodes do not take part in the operation of the converter and only real power is transferred from the DC side to the AC one. Under any other power factor, the converter operates in a sequence of modes between a rectifier and an inverter. The magnitude and angle of the AC output voltage with respect to the AC output current control in an independent manner the real and reactive power exchange between the DC and AC sides.
This converter is also the basic building block of any other switch-mode VSC.
Specifically, the combination of the switching blocks (S1 and the antiparallel diode D1) and (S2 and D2) can be used as a leg to build three-phase and other types of converters with parallel connected legs and other topologies. These types of converters will be described later.
previous Voltage-source converters (VSCs) and derived controllers
next Single-phase full-bridge VSC
Share this article :


Post a Comment

Please wait for approval of your comment .......