The D-STATCOM is basically one of the custom power devices. It is nothing but a STATCOM but used at the Distribution level. The D-STATCOM is a voltage or current source inverter based custom power device connected in shunt with the power system. It is connected near the load at the distribution systems. The key component of the D-STATCOM is a power VSC that is based on high power electronics technologies. Basically, the D-STATCOM system is comprised of three main parts: a VSC, a set of coupling reactors and a controller. The basic principle of a D-STATCOM installed in a power system is the generation of a controllable ac voltage source by a voltage source converter (VSC) connected to a dc capacitor (energy storage device). The ac voltage source, in general, appears behind a transformer leakage reactance. The active and reactive power transfer between the power system and the D-STATCOM is caused by the voltage difference across this reactance. The D-STATCOM is connected in shunt with the power networks at customer side, where the voltage-quality problem is a concern. All required voltages and currents are measured and are fed into the controller to be compared with the commands. The controller then performs feedback control and outputs a set of switching signals to drive the main semiconductor switches (IGBTs, which are used at the distribution level) of the power converter accordingly. The ac voltage control is achieved by firing angle control. Ideally the output voltage of the VSC is in phase with the bus voltage. In steady state, the dc side capacitance is maintained at a fixed voltage and there is no real power exchange, except for losses.
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| Figure: Basic structure of DSTATCOM in distribution system |
OPERATION OF DSTATCOM:
DSTATCOM consists of an inverter, dc link capacitance C that providing the dc voltage for inverter, coupling inductance L used for current filter and reactive power exchange between D-STATCOM and power system and a control unit to generate PWM signals for the switches of inverter. Rdc and R respectively represents switching losses in inverter and winding resistance of coupling inductance. Exchange of reactive power between distribution system and D-STATCOM is achieved by regulating amplitude of the inverter output voltage Vi. The D-STATCOM operation is illustrated by the phasor diagrams shown in Figure 2.
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| Figure 2: Phasor diagrams for operation modes of D-STATCOM |
If output voltage of D-STATCOM Vi is equal to AC system voltage Vs, exchange reactive power between D-STATCOM and gird will be zero and D-STATCOM operates in standby mode (Figure 1(a)).
If output voltage of D-STATCOM Vi is greater than ac system voltage Vs, D-STATCOM generate a capacitive reactive power (Figure 1(b)) and finally if output voltage of D-STATCOM Vi is lower than ac system voltage Vs, DSTATCOM absorbed an inductive reactive power (Figure 1(c)).
Reactive and active power that generated or (absorbed) by D-STATCOM respectively is given,
Q=VsX(Vs−ViCosδ)
P=VsViXSinδ
Where X is reactance of coupling inductance and δ is phase angle between fundamental voltages of D-STATCOM and AC grid.