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The resulting equation of GUPFC is considered using Equations (2.10) to (2.14) for finding out factors on which injected power depends. The resulting equation has the form given below:

(2.15)

Equation (2.15) is true only if ash, ase, msh, mse, XT remain constant. It can be seen that series injected power by the GUPFC is controlled by DC voltage (V_dc) and phase angle difference between V₁ and V_se1 (α). This means the sub-system voltage control depends on two quantities (i.e. V_dc and α).

2.5 Active GUPFC

The use of two SSSCs in Generalized Unified Power Flow Controller (GUPFC) is aimed at controlling active as well as reactive power. In the proposed method, GUPFC installed for the same purpose by employing active sources like fuel cells to the common DC bus instead of BESS. The system shown in Figure 2.8 where the fuel cell is embedded with GUPFC in sub-system to ensure uninterrupted supply under all fault conditions.

The heavily loaded feeders get relaxation due to impedance control and voltage profile improvement after implementation of the proposed method. The dependability of generating stations on grid power for supplying auxiliaries will be considerably reduced with improved power quality at load side with or without grid connectivity when fed with active GUPFC based sub-system [17]. Figure 2.9 shows the general outline of the proposed system.

In reasonable working conditions as sub-system sources float, the main grid supplies power to sub-system loads based on demand. Whenever there is a grid disturbance and the main grid is unable to provide additional power to the sub-system, it has to separate from the main grid at a point of common coupling (PCC). The sub-system continues supplying power to the auxiliary system of generating stations. Figure 2.10 shows the condition when subsystem source i.e., DG embedded GUPFC fulfils the demand of auxiliaries.

      The proposed method is an application for generating stations. In this research work, thermal generating stations considered as it has the highest installed capacity in India [3]. Due to the grid disturbance, a generating station, usually, experiences shortfall of power for running auxiliaries. This difficulty is overcome by using the energy from the sub-system. However, the stable operation of the sub-system is also an issue of concern when it has a renewable, nature dependent source. The use of fuel cells can provide an uninterrupted and stable power supply if installed with proper electronic converters.

Figure 2.8 Proposed system.

Figure 2.9 Proposed system with grid connection.

Figure 2.10 Proposed system without grid connection.

      2.5.1 Active GUPFC Control System

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.

       2.5.1.1 Series Converter

Figure 2.12 shows basic control system strategy for series converters. It corrects the magnitude of the load voltage by providing corrected magnitude and angle compared with reference value. The Space Vector Pulse Width Modulation (SVPWM) helps to transfer phase voltage reference to modulation time delay cycle. The closed loop system monitors V₁ to apply appropriate corrections in the system.

      Considering power demand calculation as:

      Where

      p = Active power demand

      q = Reactive power demand

Figure 2.11 Basic control of GUPFC compensator logic.

Figure 2.12 Basic control of series compensator logic.

v₁

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