What is power factor?Methods to improve the power factor?
Power factor and method to improve power factor DisadvantagesLow Power Factor Why to improve the power factor? cost of electricity
Power factor and method to improve power factor:
Definition of power factor:
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| power factor |
It's the ratio between the true power (real power) doing useful work and the apparent power of the supply system in a circuit in which both voltage and current are in step, the power factor is 100% or UNITY.
For certain technical reasons, such as the inductive effect of a Motor or other apparatus, the current may lag behind the voltage. In such a case, only a part of the current becomes available for doing useful work, and is referred a lagging power factor.
The Power Factor is an indicator of the quality of design and management of an electrical installation. It relies on two very basic notions: active and apparent power. The active power P (kW) is the real power transmitted to loads such as motors,lamps, heaters, and computers. The electrical active power is transformed into mechanical power, heat or light.
In a circuit where the applied r.m.s. voltage is Vrms and the circulating r.m.s. current is Irms, the apparent power S (kVA) is the product: Vrms X Irms. The apparent power is the basis for electrical equipment rating.
The Power Factor λ is the ratio of the active power P (kW) to the apparent power (S)
(kVA):
λ = P(kW) / S(kVA)
The load may be a single power-consuming item, or a number of items (for example an entire installation).
The value of power factor will range from 0 to 1.Many supply authorities, therefore penalize the consumer for a bad power factor, or give a rebate for a satisfactory power factor which allows a better employment of their distribution system
Why to improve the power factor?
Improvement of the power factor of an installation presents several technical and economic advantages, notably in the reduction of electricity bills.
Reduction in the cost of electricity:
Good management in the consumption of reactive energy brings economic advantages.These notes are based on an actual tariff structure commonly designed to encourage consumers to minimize their consumption of reactive energy.The installation of power-factor correction equipment on installations permits the consumer to reduce his electricity bill by maintaining the level of reactive-power consumption below a value contractually agreed with the power supply authority.In this particular tariff, reactive energy is billed according to the tanϕ criterion.As noted:
tanϕ = Q (kvar) / P (kW)
The same ratio applies to energies:
tanϕ = Q (kvarh) / P (kWh)
Against the financial advantages of reduced billing, the consumer must balance the cost of purchasing, installing and maintaining the power factor correction equipment and controlling switch gear, automatic control equipment (where stepped levels of compensation are required) together with the additional kWh consumed by the losses of the equipment, etc.
It may be found that it is more economic to provide partial compensation only, and that paying for some of the reactive energy consumed is less expensive than providing 100% compensation.
Technical/economic optimization:
A high power factor allows the optimization of the components of an installation.
Overating of certain equipment can be avoided, but to achieve the best results, the correction should be effected as close to the individual inductive items as possible.
Reduction of cable size:
Tables shows the required increase in the size of cables as the power factor is reduced from unity to 0.4, for the same active power transmitted. Reduction of losses (P, kW) in cables
Tables shows the required increase in the size of cables as the power factor is reduced from unity to 0.4, for the same active power transmitted. Reduction of losses (P, kW) in cables
| Multiplying factor for the cross-sectional area of the cable core(s) | 1 | 1.25 | 1.67 | 2.5 |
|---|---|---|---|---|
| cosφ | 1 | 0.8 | 0.6 | 0.4 |
Losses in cables are proportional to the current squared, and are measured by the kWh meter of the installation.
Reduction of the total current in a conductor by 10% for example, will reduce the losses by almost 20%.
Reduction of voltage drop Power factor correction equipment reduce or even cancel completely the (inductive) reactive current in upstream conductors, thereby reducing or eliminating voltage
drops.
Note: Over compensation will produce a voltage rise at the equipment level.
Increase in available power:
By improving the power factor of a load supplied from a transformer, the current through the transformer will be reduced, thereby allowing more load to be added.
In practice, it may be less expensive to improve the power factor, than to replace the transformer by a larger unit.
If P & VL are constant, the load current lL ,is inversely proportional to power factor.
Disadvantages of Low Power Factor:
power equation
If P & VL are constant, the load current lL ,is inversely proportional to power factor.
Lower the power factor, higher the current and vice versa.
Hence the current for a given load supplied at constant voltage, will higher at lower power factor, and low at high power factor.
The high current due to poor power factor
The undesirable effect of operating a low load at a low power factor is due to the large current required for a low power factor.
The important disadvantages of low power factor are:
Higher current is required by the equipment, due to which the economic cost of the equipment is increased.
Higher current is required by the equipment, due to which the economic cost of the equipment is increased.
At low power factor, the current is high which gives rise to high copper losses in the system and therefore the efficiency of the system is reduced.
Higher current produced a large voltage drop in the apparatus. This results in the poor voltage regulation.
Since both the capital and running cost are increased, the operation of the system at low power factor (whether it is lagging or leading) is uneconomical from the supplier’s point of view.
How to improve the power factor?
Theoretical principle of power factor improvement:
An inductive load having a low power factor requires the generators and transmission/distribution systems to pass reactive current (lagging the system voltage by 90 degrees) with associated power losses and exaggerated voltage drops.
If a bank of shunt capacitors is added to the load, its (capacitive) reactive current will take the same path through the power system as that of the load reactive current.
Since, this capacitive current Ic (which leads the system voltage by 90 degrees) is in direct phase opposition to the load reactive current (IL).
The two components flowing through the same path will cancel each other, such that if the capacitor bank is sufficiently large and Ic = IL, there will be no reactive current flow in the system upstream of the capacitors.
This is indicated in Figure (a) and (b) which show the flow of the reactive
components of current only.
In this figure:
 |
| Reactive current components only flow pattern |
This is indicated in Figure (a) and (b) which show the flow of the reactive
components of current only.
In this figure:
R :-represents the active-power elements of the load
L :-represents the (inductive) reactive-power elements of the load
C :-represents the (capacitive) reactive-power elements of the power-factor correction
equipment (i.e. capacitors).
By using what equipment low power factor can improve ?
1) Compensation at LV
At low voltage, compensation is provided by:
- Fixed-value capacitor
- Equipment providing automatic regulation, or banks which allow continues adjustment according to requirements, as loading of the installation changes
2) Fixed capacitors
- Manual: by circuit-breaker or load-break switch
- Semi-automatic: by contactor
- Direct connection to an appliance and switched with it
- 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
3)Automatic capacitor banks:
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 bus bars of a general power distribution board
- At the terminals of a heavily-loaded feeder cable
Reasons for using automatic power factor compensation (APFC) :
A bank of capacitors is divided into a number of sections, each of which is controlled by 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.
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
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