PSMA Consulting - Power System Studies - Static Synchronous Compensators (STATCOM)

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.

Principle of Operation of STATCOM

The schematic diagram of STATCOM is shown in Fig. 1. It consists of the VSC, coupling transformer, and DC energy storage element (optional). The VSC is connected to the transmission system through a coupling transformer. The DC capacitors can be used to stabilize the controlled DC voltage which is needed for the operation of VSC. The DC energy storage element provides the facility to exchange the active power with the transmission system. In Fig. 1, EBus (Et) is the bus voltage of the transmission system, ESTATCOM (Es) is the 3-phase output voltage of the STATCOM, and ISTATCOM (Iac) is the injected AC current of the STATCOM, and EL is the voltage across the transformer coupling reactor. The vector diagram for active and reactive power flow scenarios are demonstrated in Fig. 2.

Fig. 1. Schematic of STATCOM.

Fig. 2. Vector diagram for power flow scenarios.

Reactive Power (Q) Flow Scenario

The reactive power flow (Q) scenarios 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 AC current (Iac) flows from the VSC to the AC system. In this case, the injected AC current (Iac) is 90° leading to the bus voltage (Et) of the system (Et). So, the STATCOM can supply capacitive reactive power to the AC system. The vector diagram for reactive power (Q) release scenario is shown in Fig. 2(a).
If ES < Et then the AC current (Iac) flows from the AC system to the VSC. In this case, the injected AC current (Iac) is 90° lagging to the bus voltage (Et) of the system (Et). So, the STATCOM can absorb inductive reactive power from the AC system. The vector diagram for reactive power (Q) absorption scenario is shown in Fig. 2(b).
If ES = Et then the reactive power exchange is zero. So, the STATCOM is in a floating state.
In most cases, the STATCOM can be configured without a DC energy storage element to control the reactive power (both capacitive and inductive range) only.

Active Power (P) Flow Scenario

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

The VSC can exchange the active power (P) with the transmission system if the energy storage element is connected to the DC side of the converter.
The phase shift between the VSC output voltage (ES) and the transmission bus 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 (ES) leads to the phase angle of the transmission bus voltage (Et), then the VSC can supply the real power from its DC energy storage to the AC system. The vector diagram for active power (P) release scenario is shown in Fig. 2(c).
If the phase angle of the VSC output voltage (ES) lags the phase angle of the transmission bus voltage (Et), then the VSC can absorb the real power from the AC transmission system. The vector diagram for reactive power (P) release scenario is shown in Fig. 2(d).

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. In the capacitive region, the maximum transient current is determined by the maximum current turn-off capability of the VSC switches. In the inductive region, the maximum transient current is determined by the maximum allowable junction temperature of the VSC switches.

Fig. 3. V-I characteristics of STATCOM.

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.