Page 127 - Power Electronic Control in Electrical Systems

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Power electronic control in electrical systems 115

3. If y cR is in quadrature with the current angle then it controls active power flow
performing as a variable series impedance compensator.
4. At any other value of y cR , it performs as a combination of a phase shifter and a
variable series impedance compensator. This is in addition to being a voltage
regulator by suitable control of jV cR j.

In addition to providing a supporting role in the active power exchange that takes
place between the series converter and the AC system, the shunt converter may also
generate or absorb reactive power in order to provide independent voltage magnitude
regulation at its point of connection with the AC system.
The UPFC power flow model presented in this chapter uses the equivalent circuit
shown in Figure 4.6(b), which consists of a shunt connected voltage source, a series
connected voltage source and an active power constraint equation which links the
two voltage sources
V vR ˆjV vR j(cos y vR ‡ j sin y vR ) (4:9)

V cR ˆjV cR j(cos y cR ‡ j sin y cR ) (4:10)

Ref V vR I ‡ V cR I gˆ 0 (4:11)

vR m
These equations are adjusted in a coordinated fashion using Newton's algorithm to
satisfy the specified control requirements. Similarly to the shunt and series voltage
sources used to represent the STATCOM and the DVR, respectively, the voltage
sources used in the UPFC application would also have limits. For the shunt con-
verter the voltage magnitude and phase angle limits are: V vR min V vR V vR max and
0 y vR 2p. The corresponding limits for the series converter are: V cR min V cR
V cR max and 0 y cR 2p.

4.2.7 The HVDC-Light
The HVDC-Light comprises two VSCs, one operating as a rectifier and the other as
an inverter. The two converters are connected either back-to-back or joined together
by a DC cable, depending on the application. Its main function is to transmit
constant DC power from the rectifier to the inverter station, with high controllabil-
ity. The schematic representation of the HVDC light and its equivalent circuit are
shown in Figure 4.7. A fuller schematic representation is shown in Figure 1.13.
One VSC controls DC voltage and the other the transmission of active power
through the DC link. Assuming lossless converters, the active power flow entering
the DC system must equal the active power reaching the AC system at the inverter
end minus the transmission losses in the DC cable. During normal operation, both
converters have independent reactive power control.
The power flow model for the back-to-back HVDC light may be based on the use
of one voltage source for the rectifier and one voltage source for the inverter linked
together by a constrained power equation
V vR1 ˆjV vR1 j(cos y vR1 ‡ j sin y vR1 ) (4:12)