The UPFC has many possible operating modes. In particular, the shunt inverter is operating in such a way to inject a controllable current, Ish into the transmission line. The shunt inverter can be controlled in two different modes:
1) VAR CONTROL MODE:
The reference input is an inductive or capacitive VAR request. The shunt inverter control translates the VAR reference into a corresponding shunt current request and adjusts gating of the inverter to establish the desired current. For this mode of control a feedback signal representing the dc bus voltage, Vdc, is also required.
2) AUTOMATIC VOLTAGE CONTROL MODE:
The shunt inverter reactive current is automatically regulated to maintain the transmission line voltage at the point of connection to a reference value. For this mode of control, voltage feedback signals are obtained from the sending end bus feeding the shunt coupling transformer.
The series inverter controls the magnitude and angle of the voltage injected in series with the line to influence the power flow on the line. The actual value of the injected voltage can be obtained in several ways.
1) DIRECT VOLTAGE INJECTION MODE:
The reference inputs are directly the magnitude and phase angle of the series voltage.
2) PHASE ANGLE SHIFTER EMULATION MODE:
The reference input is phase displacement between the sending end voltage and the receiving end voltage.
3) LINE IMPEDANCE EMULATION MODE:
The reference input is an impedance value to insert in series with the line impedance.
4) AUTOMATIC POWER FLOW CONTROL MODE:
The reference inputs are values of P and Q to maintain on the transmission line despite system changes.