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GUPFC control system consists of the shunt converter control system and series converter control system. This control system based on the vector control approach introduced in [1]. The objective of the control system is to maintain terminal voltage using the shunt converter and injunction of the series voltage vector using the series converter. Depending on the system conditions, the series converter can inject or draw reactive power from the series element. But shunt and series converters do not share reactive power through common DC link. The control function of the shunt converter is divided into two operational modes, as listed below.

1 Power control mode: The shunt converter mainly controls reactive power (VAR) in the system. The reactive power demand decides the gate pulse of the converter, which allows current to flow. Continuous feedback closed-looped system ensures the desired current injection in the system.

2 Voltage control mode: With the help of the droop control method, the voltage regulation can be made automatically at the point of connection with reactive current regulation.

The power flow control system works for the shunt and two series compensators together. The desired active and reactive power flows (P_s1 and Q_S1) are compared with the measured magnitudes (P_s′ and Q′_S) and error is passed through an error amplifier to produce direct and quadrature components for series connected compensating voltages (e_2d and e_2q). The magnitudes of the voltage (E_2dq) at the output of VSC2 and VSC3 are calculated respectively by adding relative phase angle (β₁ and β₂) [18]. The controllable range of active and reactive power flow can easily be determined with open loop control system with rated compensating voltage (E_2dq). The control system is illustrated in Figure 2.11.