THYRISTOR CONTROLLED AUTOMATIC

POWER FACTOR CORRECTION

BACKGROUND

The power factor correction of electrical loads is a common problem to almost all the industrial sector. Most of the loads draw a certain quantity of active power together with reactive power from the mains. The Active power (kW) actually performs the work whereas the reactive power (KVAr) could be interpreted as an undesirable burden on the supply. The increased flow of reactive power has following disadvantages,

Overloading of power Source i.e. Transformer or Alternator.
Higher voltage drops throughout the system.
Additional heating and loss of energy due to increased cable (I²R) loses.
Increase in the rating of Switch - Gears, Cables and other protective devices. e. Higher KVA demand. f. Levy of penalty.

It is therefore essential that all efforts should be made to reduce the flow of reactive Power in order to optimize system performance and cost savings.

POWER FACTOR

The power factor (cos Φ) of a load is defined has the ratio of Active power to Apparent power, i.e. P.F (Cos Φ) = kW / kVA.

CLOSER THE POWER FACTOR TO UNITY, THE LESS REACTIVE POWER IS DRAWN FROM THE SUPPLY SURCE.

METHODS OF POWER FACTOR IMPROVEMENT: -

The power factor can be improved by switching ON/OFF the capacitor banks in 3 different manners, viz.

1] Manual Switching

In this method, constant monitoring of the load pattern is required, so as to enable the operator to switch ON/OFF the capacitor banks.

2] Contactor Logic

In this method, the capacitor banks are switched ON/OFF through elector mechanical contractors. This method is most unsuitable where variable load conditions exists and has several limitations as can be seen from the comparison given below.

3] Thyristor Switched Capacitor Banks, based on ZERO VOLTAGE Logic

This method is perfectly suitable even in load conditions and has absolutely on limitations whatsoever. Being totally solid state, there is no wear & tear of the system. Detailed write – up and its comparison with contractor – logic is mentioned for your study.

COMPARISON BETWEEN

CONTACTOR-LOGIC AND THYRISTOR SWITCHED APFC UNITS

FEATURES	CONTACTOR-LOGIC	THYRISTOR SWITCHED
Switching Method	Random Switching	Switching at Zero Voltage
Switching Surges	Very high	Negligible
Stress Factor	High stress on	Low stress on thyristor
Response Time	Very sluggish (in min.)	Very fast (in m.sec.)
ON/OFF switching	Limited	Unlimited
Type of switch	Electro- mechanical More wear &tear Short life	Totally SOLID STATE No wear & tear Long life
Maintenance Costs	Very high	Almost negligible

POWER FACTOR COMPENSATION WITH THTRISTOR SWITCHES

In certain installations having large and fast fluctuating loads, the power factor cannot be corrected with electromechanically driven capacitor stages. In such cases, the traditional system is not fast enough to follow the reactive power demands of the load. Hence, it becomes absolutely necessary to use capacitor banks switched by thyristor switches, based on the rough and well proven thyristor technology. The Thyristorised switching solution has the following advantages:

1. Absence of ON- Switching surges

Each capacitor step is connected at ZERO VOLTAGE across the switch, even when the capacitor is totally or partially charged. This eliminates the switching surges and avoids interference in electronic equipment supplied by the same low voltage line.

2. Absence of turn – OFF surges

The capacitor disconnection takes place at zero current. Thus there are no side effects because of current interruption in parallel inductive loads.

3 Unlimited ON/OFF Speed

Because of the above advantages the thyristor switches may be operated at very fast rate. In fact this thyristor switch itself can respond with in one cycle if required by the load.

4 Immediate response to the reactive power demands

This response cannot be achieved with the conventional electromechanical switching technique. The thyristor switching is the only solution to compensate the power factor of systems with loads like, Welding machines, Cranes, Lifts and other inductive loads having frequent and short load cycles.

5 Low wear and tear of the capacitors and the switches

The overall life of the equipment is considerably increased with respect to the conventional system, due to the elimination of the surges and absence of moving mechanical parts.

THYRSTOR CONTROLLED AUTOMATIC P. F. CORRECTION UNIT

- BASED ON ZERO VOLTAGE SWITCHING TECHIQUE:

OUR TECHNOLOGY:

The controller will evaluate the reactive loads by taking instantaneous measurement in all four quadrants of voltage and current. These values are digitized in the microprocessor and are continuously evaluated to provide the active and reactive components. The value of cos Φ is calculated from these two values.

The value of each capacitor step will be evaluated by the controller and stored in its memory permanently, even if the voltage supply to the controller fails.

The controller also has the facility to record a defective capacitor step in its memory, and such defective capacitor steps (due to fuse failure) will be excluded from the regulation process, and can be brought back into the system after rectification of the fault in that capacitor step.

STANDARD FEATURES OF THE CONTROLLER:

Automation self-adjustment to any capacitor step value.
Allows manual switching of each capacitor steps.
Digital indication of power factor preset parameters and specified installation data.
No- volt release feature to immediately disconnect all capacitor in the even of power failure for more than 35msecs.
Digital setting of individual parameters including target power factor Switching time, step limit etc.
Elimination of defective capacitors and their indication.
Digital reading facility of switching cycles per capacitor steps.
Digital encoded reading of capacitor sizes.
Visual display of harmonic overload alarm.
Visual display of target cos Φ alarm.
Stage indication of steps switched in, as well as total banks in operation.
Easy plug-in back panel terminal connection.
Protection against capacitor over-current, due to over-voltage, short-circuit, system resonance and presence of higher value of harmonics in the system.
Digital display of voltages, Currents, kW, kV Ar, KWh, and Frequency.

TECHNICAL SPECIFICATIONS:

1	Voltage	380-415 Volts, 50Hz
2	Current	5A or 1A Selectable through DIP switch
3	Target P. F. range	0.90Lag…Unity…0.90lead (also indicated on Digital Display)
4	Switching time	Programmable
5	Temperature Range	0 to 50 ° C.