Static Synchronous Compensator (STATCOM)

• A Static Synchronous Compensator (STATCOM) can be operated as a shunt-connected static VAR compensator whose capacitive or inductive output current can be controlled independently of the AC system voltage.

• STATCOM controls the reactive power generation and absorption using power electronic converters, for example, voltage source converters (VSC). In STATCOM, the AC capacitor banks and shunt reactors are not required for controlling the reactive power.

• The main applications of STATCOM are voltage control, VAR compensation, dynamic & transient stability, voltage stability and damping oscillations.

1. STATCOM Operation

• The schematic diagram of STATCOM is shown in Fig. 1. It consists of the voltage source converter, coupling transformer, and DC energy storage element (optional). The VSC is connected to the utility bus through a coupling transformer. The DC capacitors can be used to stabilize the controlled DC voltage which is needed for the operation of VSC. In Fig. 1, EUtility (Et) is the bus voltage of the utility or system, ESTATCOM (Es) is the 3-phase output voltage of the STATCOM, and ISTATCOM (Iq) is the injected reactive current of the STATCOM. Fig. 2 shows the equivalent circuit of the STATCOM (adjustable voltage source behind the reactance) and power flow scenario of the AC system. 

1.1. Reactive Power (Q) Flow Scenario

The conditions for reactive power flow (Q) are given below:

• The reactive power flows from a higher voltage magnitude to a lower voltage magnitude.  The exchange of reactive power between the VSC and AC system can be achieved by varying the amplitude of the 3-phase output voltage of the STATCOM (ES).

• If ES > Et then the current (Iq) flows from the VSC to the AC system. So, the STATCOM can generate capacitive reactive power for the AC system.

• If ES < Et then the current (Iq) flows from the AC system to the VSC. So, the STATCOM can absorb inductive reactive power from the AC system.

• If ES = Et then the reactive power exchange is zero. So, the STATCOM is in a floating state.

1.2. Active Power (P) Flow Scenario

The conditions for real or active power flow (P) are given below:

• The phase shift between the VSC output voltage (ES) and the AC system voltage (Et) can control the real power exchange between the converter and the AC system. The active power flows from a leading angle to a lagging angle.

• If the phase angle of the VSC output voltage leads to the phase angle of the AC system voltage, then the VSC can supply the real power from its DC energy storage to the AC system.

• If the phase angle of the VSC output voltage lags the phase angle of the AC system voltage, then the VSC can absorb the real power from the AC system.

• However, the STATCOM may be operated to supply only reactive power (both capacitive and inductive).

1.3. V-I Characteristics of STATCOM

• Fig. 3 shows the V-I characteristics of the STATCOM. It shows that the STATCOM can be operated to supply both the capacitive and inductive reactive power. Also, the STATCOM can provide full capacitive reactive power for any system voltage even for 0.2 per unit.

References

• Mathur, R. Mohan, and Rajiv K. Varma, Thyristor-based FACTS controllers for electrical transmission systems, 2002.

• Narain G. Hingorani, Laszlo Gyugyi, “Understanding FACTS concepts and technology of flexible AC transmission systems”, 1999.