PRESENTED BY
SRIPATHI RAJU

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Hybrid Reactive Power Compensation System: Advanced Control Strategy Development
Abstract
• Electric Arc Furnaces are a major cause of line disturbance problems, in terms of flickers and of harmonic pollution.
• Various solutions had been proposed and applied, with the aim to compensate or control line disturbance phenomena.
• Optimal solutions can be found by using series and shunt devices,
• but system cost, reliability and efficiency become main issues.
• In the paper, authors propose a hybrid control and compensation
• system that allows an optimal tradeoff in performance and system cost, while providing high efficiency and reliability.
• The solution is a thyristor based series controller coupled with a high performance shunt compensator. The use of Phase Control Thyristor and the fast changing characteristics of furnace load impose the development of an open loop advanced control strategy based on a load estimator and on the prediction of instantaneous active and reactive power associated with partialized waveforms.
 Power associated with harmonics is also
computed on line, in order to achieve better performances under extreme load conditions and in order to minimize the power required by the expensive shunt compensator effectiveness is shown by extensive computer simulations
 Melting mass resulting in an abrupt impedance change. Due to this effect, reactive power drawn by the mains has a steep variation, resulting in a considerable inductive drop at the Point of Common Coupling (PCC).
 Flickers are generated by low frequency variations in load characteristics. To reduce flickers phenomenon, the instantaneous reactive power drawn from the line must be as constant as possible.
 Active power stabilization is not essential, because associated voltage line variations are reduced.
 Elimination of the harmonics, generated by the furnace or the compensation device switching actions, is needed in order to reduce interference with other devices on Medium Voltage (MV) lines, especially drives and other sensitive equipment.
SHUNT TYPE COMPENSATORS
• hunt type VAR compensator systems had been proposed
• by Gyugyi and Otto [1] in 1978. Such compensators operate
• by means of thyristors valves coupled with inductors and
• capacitors. An arbitrary reactive power can be drawn from the
• line by choosing the proper firing angle for thyristors,
• compensating load variations. Phase control operated by
• thyristor introduces harmonics difficult to be damped and the
• overall performance is reduced because the valves operate with a half-period delay.
SCR BASED SERIES CONTROL
 Series control capability of performing a true control on load supply and not just a compensation of disturbances is a clear and relevant advantage.
 This is relevant when the furnace operates in short circuit. In such a worst case condition, the slow electrodes actuators action must be performed, while a shunt compensator has to bear the full reactive power unbalance.
 SERIES CONTROL STRATEGY
 SHUNT COMPENSATOR
LOAD PARAMETERS ESTIMATION
 Reliable R and L estimation is needed in order to allow a
 correct computation of load power at different firing angles.
 The main problem is related to the fact that voltage supplied to
 the load is partialized by PCTs and the estimator dynamic
 response must be as fast as possible.
SHUNT COMPENSATOR POWER LIMITATION
 Shunt compensator apparent power is related to furnace
 active power and to phase control angle, in a way not easy to
 predict. Incorrect operation or extreme load conditions can
 lead to an VSI overload. Of course, compensator control can
 protect VSI from dangerous overloading, but in this case
 harmonic cancellation operation is impaired. A tradeoff
 among power required from compensator, harmonic
 cancellation and reactive power control must be found.
HYBRID COMPENSATION SYSTEM OVERVIEW
 Shunt compensator main duties are canceling current harmonics generated by phase control action, removing transients due to slow PCT action and control delays.
CONCLUSIONS
 Active compensators are becoming a mandatory solution,
 when high power distorting non-linear loads are connected to the mains, because the increased customer sensitivity imposes
a high power quality demand.
 In this paper, the authors propose an advanced hybrid reactive power compensation system that provides an optimal trade-off between cost and performances, thanks to the exploitation of combined semiconductor technologies
(Thyristors and IGBTs) and to the action performed by a new devised control strategy. Results show that the main design goal of minimizing VSI size has been achieved, while maintaining constant reactive power and cancelling current harmonics.
 While the main concept validity has been proved by
 extensive simulation activity, a critical point has been focused
 in the load parameters estimator. A further research work will
 take in account a more detailed furnace model, to test system
 behaviour under more realistic conditions.