Equipment to improve power factor
From Electrical Installation Guide
Compensation at LV
At low voltage, compensation is provided by:
- Fixed-value capacitor
- Equipment providing automatic regulation, or banks which allow continuous adjustment according to requirements, as loading of the installation changes
Note: When the installed reactive power of compensation exceeds 800 kvar, and the load is continuous and stable, it is often found to be economically advantageous to install capacitor banks at the medium voltage level.
(see Fig. L11)
Compensation can be carried out by a fixed value of capacitance in favourable circumstances
This arrangement employs one or more capacitor(s) to form a constant level of compensation. Control may be:
- Manual: by circuit-breaker or load-break switch
- Semi-automatic: by contactor
- Direct connection to an appliance and switched with it
These capacitors are applied:
- At the terminals of inductive devices (motors and transformers)
- At busbars supplying numerous small motors and inductive appliance for which individual compensation would be too costly
- In cases where the level of load is reasonably constant
Automatic capacitor banks
(see Fig. L12)
Compensation is more-commonly effected by means of an automatically-controlled stepped bank of capacitors
This kind of equipment provides automatic control of compensation, maintaining the power factor within close limits around a selected level. Such equipment is applied at points in an installation where the active-power and/or reactive-power variations are relatively large, for example:
- At the busbars of a general power distribution board
- At the terminals of a heavily-loaded feeder cable
The principles of, and reasons, for using automatic compensation
Automatically-regulated banks of capacitors allow an immediate adaptation of compensation to match the level of load
A bank of capacitors is divided into a number of sections, each of which is controlled by a contactor. Closure of a contactor switches its section into parallel operation with other sections already in service. The size of the bank can therefore be increased or decreased in steps, by the closure and opening of the controlling contactors.
A control relay monitors the power factor of the controlled circuit(s) and is arranged to close and open appropriate contactors to maintain a reasonably constant system power factor (within the tolerance imposed by the size of each step of compensation). The current transformer for the monitoring relay must evidently be placed on one phase of the incoming cable which supplies the circuit(s) being controlled, as shown in Figure L13.
Power factor correction equipment including static contactors (thyristors) instead of usual contactors is particularly suitable for a certain number of installations using equipment with fast cycle and/or sensitive to transient surges.
The advantages of static contactors are:
- Immediate response to all power factor fluctuation (response time as low as 40 ms according to regulator option)
- Unlimited number of operations
- Elimination of transient phenomena on the network on capacitor switching
- Fully silent operation
By closely matching compensation to that required by the load, the possibility of producing overvoltages at times of low load will be avoided, thereby preventing an overvoltage condition, and possible damage to appliances and equipment.
Overvoltages due to excessive reactive compensation depend partly on the value of source impedance.