TARGET POWER FACTOR

How to size a capacitor bank ?

CALCULATIONS AND TABLES

Calculation and selection of required capacitor rating

Qc = P * {tan [acos (pf1)] - tan [ acos (pf2)]}

Qc = required capacitor output (kVAr)  
pf1 = actual power factor
pf2 = target power factor
P = real power (kW)  

The table below shows the values for typical power factors in accordance with the above formula

 

0.7 0.75 0.8 0.85 0.9 0.92 0.94 0.96 0.98 1
Actual Power Factor
0,40 1.27 1.41 1.54 1.67 1.81 1.87 1.93 2 2.09 2.29
0,45 0.96 1.1 1.23 1.36 1.5 1.56 1.62 1.69 1.78 1.98
0.5 0.71 0.85 0.98 1.11 1.25 1.31 1.37 1.44 1.53 1.73
0,55 0.5 0.64 0.77 0.9 1.03 1.09 1.16 1.23 1.32 1.52
0,60 0.31 0.45 0.58 0.71 0.85 0.91 0.97 1.04 1.13 1.33
0,65 0.15 0.29 0.42 0.55 0.68 0.74 0.81 0.88 0.97 1.17
0,70 0 0.14 0.27 0.4 0.54 0.59 0.66 0.73 0.82 1.02
0.75 0 0.13 0.26 0.4 0.46 0.52 0.59 0.68 0.88
0,80 0 0.13 0.27 0.32 0.39 0.46 0.55 0.75
0.85 0 0.14 0.19 0.26 0.33 0.42 0.62
0,90 0 0.06 0.12 0.19 0.28 0.48


The required capacitor output may be calculated as follows:
select the factor (matching point of actual and target power factor) k
calculate the required capacitor rating with the formula:

Qc = k * P

Example:
actual power factor = 0.70, target power factor = 0.96, real power = P = 500kW,

Qc = k * P = 0.73 * 500kW = 365 kVAR

FAQ on power factor and capacitor bank:

1. How reactive power is generated ?
2. Effects of Poor Power factor
3. Type of power factor improvement methods
4. Benefits of power factor correction
5. Key components of a typical capacitor bank
6. What is harmonic ?
7. Harmful effects of harmonic
8. Protection of capacitor against harmonic
9. Reduction of Harmonic
10. How to size a capacitor bank ?

 

What is harmonic ?

Harmonic are disturbances created by loads such as:
. Thyristor controlled drives
. Uninterruptable power supplies (UPS)
. Arc furnaces
. Frequency drives
. Energy saving lamps
. Electronic equipments (e.g. personal computers, TV, ...)

Harmful effects of harmonic

. Tripping of circuit breakers and fuses.
. Overloading of transformers, capacitors and other electrical distribution equipment.
. Additional losses.
. Excessive current in the neutral line.
. Malfunction of computers and other electronic equipments.
. Serial and Parallel Resonance.
. Measurement errors of energy counters.

Protection of capacitor against harmonic

Capacitor can be protect from overloading due to harmonic using detuned reactor.
A detuned capacitor bank consists of a series circuit of capacitor(s) and a specific filter circuit reactor. The resonance frequency of a detuned bank is heavily detuned, means not close to any existing harmonic.

Reduction of Harmonic

Harmonic can be reduced by tuned filter
A tuned capacitor bank consists of a series circuit of capacitor(s) and specific filter circuit reactor, similar to a detuned unit. The difference is the resonance frequency. Tuned filters are tuned very close to harmonic frequencies. The respective harmonic will be absorded by the filters which tuned to that particular order. Hence, capacitors and reactors used for tuned filtering must be specially made to withstand heavy harmonic current.

How reactive power is generated ?

Every electric load that works with magnetic fields (motors, chokes, transformers, inductive heating, arc-welding generators) produces a varying degree of electrical lag, what is called inductance. This lag of inductive loads maintains the current sense (eg positive) for a time even though the negativegoing voltage tries to reverse it. This phase shift between current and voltage is maintained, current and voltage having opposite signs. During this time, negative power or energy is produced and fed back into the network. When current and voltage have the same sign again, the same amount of energy is again needed to build up the magnetic fields in inductive loads. This magnetic reversal energy is called reactive power. In alternating voltage networks (50/60 Hz) such a process repeats 50 or 60 times a second. So an obvious solution is to briefly store the magnetic reversal energy in capacitors and relieve the network (supply line) of this reactive energy. For this reason, automatic reactive power compensation systems (detuned/conventional) are installed for larger loads like factory plants. Such systems consist of a group of capacitor units that can be cut in and cut out and which are driven and switched by a power factor controller as determined by a current transformer.

 

Benefits of power factor correction

1 Power factor correction reduces the reactive power in a system. Power consumption and thus power costs drop in proportion.

2 Effective installation use An improved power factor means that an electrical installation works more economically (higher effective power for the same apparent power).

3 Improved voltage quality

4 Fewer voltage drops

5 Optimum cable dimensioning Cable cross-section can be reduced with improvement of power factor (less current). In existing installations for instance, extra or higher power can be transmitted.

6 Smaller transmission losses The transmission and switching devices carry less current, ie only the effective power, meaning that the ohmic losses in the leads are reduced.

Key components of a typical capacitor bank

PFC controller
Modern PFC controllers are microprocessorized. The microprocessor analyzes the signal from a current transformer and produces switching commands to control the contactors that add or remove capacitor stages. Intelligent control by microprocessorized PFC controllers ensures an even utilization of capacitor steps, minimized number of switching operations and optimized life cycle.

Fuse/MCCB
An HRC fuse or MCCB acts as a safety device for shortcircuit protection.

Capacitor contactor
Contactors are electromechanical switching elements used to switch capacitors or reactors and capacitors in standard or detuned PFC systems. The switching operation can be performed by mechanical contacts or an electronic switch (semiconductor). The latter solution is preferable if fast switching is required for a sensitive load for example.

Reactor
(compensation and filtering) Power distribution networks are increasingly subjected to harmonic pollution from modern power electronic devices, socalled nonlinear loads, eg drives, uninterruptible power supplies, electronic ballasts. Harmonics are dangerous for capacitors connected in the PFC circuit, especially if the capacitors operate at resonant frequency. The series connection of reactor and capacitor to detune the series resonant frequency (the capacitor's resonant frequency) helps to prevent capacitor damage. Critical frequencies are the 5th and 7th harmonics (250 and 350 Hz). Detuned capacitor banks also decrease the harmonic distortion level and clean the network.

Capacitor
Power factor correction capacitors produce the necessary leading reactive power to compensate the lagging reactive power. PFC capacitors should be capable of withstanding high inrush currents caused by switching operations (> 100 * IN). If capacitors are connected in parallel, ie as banks, the inrush current will increase (> 150 * IN) because the charging current comes from the grid as well as from capacitors parallel to the switched one.

Effects of Poor Power factor

Poor power factor results in a higher energy consumption and costs, a less power distributed via the network, a power loss in the network, a higher transformer losses, a increased voltage drop in power distribution networks.

Type of power factor improvement methods

Power factor improvement can be achieved by a compensation of reactive power with capacitors, a active compensation – using semiconductors, a overexcited synchronous machine (motor/generator).

For capacitive compensation, It can be;
A individual or fixed compensation - each reactive power producer is individually compensated, or
A group compensation - reactive power producers connected as a group and compensated as a whole.Usually, a central or automatic compensation by a PFC system at a central point.

 

General

Awareness of the necessity of power quality is increasing, and power factor correction (PFC) will be implemented on a growing scale in future. Enhancing power quality – improvement of power factor – saves costs and is a fast return on investment.In power distribution, in low- and medium-voltage networks, PFC focuses on the power flow (cos .) and the optimization of voltage stability by generating reactive power – to improve voltage quality and reliability at distribution level.