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State Space Analysis and Duty Cycle Control of a
Switched Reactance based Center-Point-Clamped
Reactive Power Compensator
Abstract:
A new approach for static reactive power compensation has been presented in
this paper. Conventional thyristor controlled Static VAR Compensators (SVC) have
inherent disadvantages like slow response times and poor harmonic performance as
these FACTS devices are based on slow switching power electronics devices.
Alternately, Pulse Width Modulated (PWM) dc-ac inverters and direct ac-ac
converter structures offer higher bandwidth and push the spectral content to higher
switching frequencies that are easier to filter. However, the application space of this
approach is limited by the low voltage blocking capability of power devices
employed in these converters. A center-point-clamped ac-ac direct power converter
has been reported recently in literature which operates on the principle of neutralpoint-clamped dc-ac inverter. By clamping the grid voltage to its mid-point, the
center-point clamped converter structure reduces voltage stress on the bidirectional
switches by 50%. Compared to the conventional two level and multilevel dc-ac
inverters, the proposed compensator based on direct ac-ac conversion has a simpler
structure and control. The operating principle as well as dynamic analysis for the
proposed VAR compensation approach has been presented in the paper. A feedback
controller has been designed for closed loop control. Simulation results presented in
the paper verify that proposed converter offers better control of reactive power,
retrofit capability, and reduced voltage stress on the bidirectional switches.
Furthermore, it has been shown that leading and lagging reactive compensation can
be accomplished with a smooth control of the reactance through duty cycle
modulation.
Existing System:
 In the early 20th century, in order to improve the power quality of the grid,
reactive power compensation would be provided to the grid using synchronous
condensers.
 In the later years, advances in power semiconductors have facilitated the use of
power semiconductor device based reactive power compensators to maintain the
power quality as well as voltage profile of the grid within acceptable tolerances.
 But the employment of the VAR compensator topologies that have been reported
in the literature remains challenging due to non -availability of power
semiconductor devices rated at utility scale voltage and power levels.
 The topology of Center-Point-Clamped AC-AC Reactive Power Compensator
that has been introduced in this paper uses power semiconductor devices rated at
half the utility scale voltages.
Block diagram:
Circuit Diagram:
(Switched Reactance based reactive power compensator)
Disadvantage:
 Slow Response Time
 Poor Harmonic performance.
Proposed System:
 The proposed VAR compensator offers a unique methodology to clamp the input
voltage at half the magnitude of source voltage, thereby enabling the use of low
voltage power electronic switches to operate with higher system voltages and
improving the power transfer efficiency of the VAR compensator.
 The switching pattern is easier to implement than multilevel converter based
VAR compensator topologies.
 Furthermore, dynamic analysis of this converter has enabled us to model a
feedback controller for better controllability and superior output power quality.
 Simulation results presented in the paper verify the operation of the reactive
power compensator in injecting or absorbing reactive power to or from the
transmission line depending on the duty cycle control.
 Also, it is observed that the line current THD is maintained at less than 5%.
Advantage:
 Quick response time
 Good harmonic performance
Circuit Diagram:
(proposed Center-Point-Clamped Reactive Power Compensator)
References:
[1] G. Venkataramanan, B. Johnson, A. Sundaram, “An AC-AC Power
Converter for Custom Power Applications,” IEEE Trans. on Power Delivery,
vol. 11, No. 3, July 1996, pp. 1666-1671.
[2] L. Gyugyi, R.A. Otto, T.H. Putman, "Principles and Applications of Static,
Thyristor-Controlled Shunt Compensators", IEEE Trans. PAS, Vol. PAS-97,
No. 5, Sept/Oct, 1978.
[3] L. Gyugyi, “Power electronics in electric utilities: Static VAr
compensators,” Proceedings of the IEEE, vol. 16, no. 4, pp. 483493, Apr.1988.
[4] Dixon, J.; Moran, L.; Rodriguez, J.; Domke, R., “Reactive Power
Compensation Technologies: State-of-the-Art Review,” Proceedings of the
IEEE, vol.93, no.12, pp.2144, 2164, Dec. 2005.
[5] H. Jin, G. Goós, and L. Lopes, “An efficient switched-reactor based static
VAr compensator,” IEEE Trans. Ind. Appl., vol. 30, no.4, pp. 997–1005,
Jul./Aug. 1994.