What is Reactive Power is the question, that every one can not give clear answer. Reactive Power is nothing but Power required to charging Capacitors and building magnetic field in inductors. Pure Capacitors and Inductor does not consume Active Power.
Pure Capacitor in shunt generates Reactive power and Pure Inductors in Series consumes Reactive Power. So, Capacitors and Inductors are reactive components.
Reactive Power = √3*(V ph to ph)*(I)*Sin Ø
Some thing about Reactive Power is:-
Reactive Power is in quadrature with active power.
Reactive Power requirement of the transmission line, Transformers, Motors is fulfill by installing shunt capacitor at load end or at sub-stations.
For extra high voltage lines, in low load condition reactive power control of line can be done by installing shunt reactors at the EHV sub-stations.
Voltage of the system is very much affected by reactive power.
The reactive power supplied to an A.C. circuit is the product of the voltage and the reactive (Watt-less) component of the current, this reactive current component being in quadrature with the voltage. For a single phase circuit.
Q = VI Sin Ø for single phase circuit
Q = √3 VI Sin Ø for three phase circuit
Inductive loads are “SINKS” for Reactive Power (Q+ve)
Transformer
Heavily loaded transmission line
Induction motor
Under excited synchronous machine This results in lagging power factor
Capacitive loads are “SOURCES” of Reactive Power (Q-ve)
Shunt Capacitor
Lightly loaded transmission line/cable
Over excited synchronous machine
TYPES OF COMPENSATION
There are two types of compensations
SERIES COMPENSATION
SHUNT COMPENSATION
SERIES COMPENSATION
Series Capacitors in power system are used since long for improvement of transmission capacity of an existing line, improvement in system stability and improvement of voltage profile in distribution network.
Series capacitors are connected in series with the line in order to reduce the effective reactance of the line.
Series capacitor in distribution voltage level (upto 33 kV) is a simple pole mounted design, named MINICAP.
A MINICAP mainly consists of capacitors, load break switch, Spark gap, Isolators and relays for protection.
SHUNT COMPENSATION
Shunt compensation is employed for off-setting the reactive power generated by over head lines and inductive loads which results into premature saturation of power system sources e.g. Generators, transformers and transmission lines. Shunt capacitors alleviate the excess loading of system and enable further active loads to be drawn from the same system. Further, the reduction in line current results into reduction of system losses. The system also benefits in the form of improved voltage profile.
Improvement of Power factor with Shunt Capacitor Bank.
The function of a shunt capacitor applied as a single unit or in groups of units is to supply lagging kilovars to the system at the point where they are connected. A shunt capacitor has the same effect as an overexcited synchronous condenser, generator or motor. It supplies the kind of kilovars or current to counteract the out of phase component of current required by an induction motor as illustrated in fig.
Advantage of Improved Power Factor
Reduce lagging component of circuit current.
Increase voltage level at the load.
Improves voltage regulation if the capacitor units are properly switched.
Reduce I2 R power loss in the system because of reduction in current.
Reduce I2 X Kilovar loss in the system because of reduction in current.
Increase power factor of the system.
Decrease loading on the system.
Reduction in switch gear rating.
Reduction in conductor / bus bar size.
Reduction in line load.
Above advantages can be easily understood by following figure.
Power Factor Vector Relationship
KVA = KW ÷ PF
KW = KVA x PF
PF = KW ÷ kVA
– KVA: Total Power required for a given load
– KW: Working Power required to produce work
– KVAR: Reactive Power needed to generate magnetic fields for inductive loads such as motors
– Power Factor: The relationship of real power (KW) and total power (KVA) consumed
• Cosine of angle shown
• Percentage or decimal expression
Mostly Following types of capacitor banks are used :
For 11 KV: 2.1 MVAR to 3.0 MVAR
For 66 KV: 10 MVAR
For 132 KV: 20 MVAR
Capacitor cells having rating of 65 KVAR,75 KVAR, 110 KVAR,200 KVAR, and applying series parallel combination required MVAR as per our system requirement is achieved.
Type of capacitor Cell.
External fuse (Use generally 11 KV Capacitor Bank)
Internal fuse (Use generally 66 & 132 KV Capacitor Bank)
It is recommended that capacitors bank must be balanced in all three phase, otherwise it badly effects in performance on capacitor bank and system also. If any-one cell failed in any phase, then one cell of equivalent KVAR capacity of failed cell should be removed in such a way that all three phase capacitance must be equal.
Calculation for available capacity of Capacitor Bank Can be calculated from the following equation .
For example 14.4 MVAR capacitor bank rated voltage 76 KV installed at 66 KV System available capacity is 10.85 MVAR as shown under.
Calculation for working capacity of Capacitor Bank Can be calculated from the above equation, in which capacity of removed cell deduct from installed capacity. If 9 nos of 200 KVAR cell removed from bank then,
Calculation of Capacitance value from the rating of the unit
C= (KVAr x 10^9) /(2f (Vn)2
Where,
kVAr = Reactive Kilo Volts Amperes of the Capacitor unit
f = Frequency , Hz
C = Capacitance of the unit , μ Farad
Vn = Voltage of the unit, volts
Calculation of unit Capacitance after element failure
Cn = (1/(1+K))/C0
Where,
K = n / (Pi – n) Si
n = Number of failed elements
Si = Number of series groups
Pi = Number of parallel elements per series group
Cn = Capacitance of the unit after the failure of “n” elements
Co = Original / rated Capacitance
SELECTION OF BANK AND ITS SIZE
In any system where compensation is required Bank size has to be decided first. To mains this parameters considered are Total load of the S/S where compensation is required existing power factor & target power factor.
Once the above data is available then bank size can be decided by following formula,
KVAR required = Q = P.{tan (cos”‘ X) – tan(cos’l Y)}.
Where,
P = Total Load of the plant in KW where compensation is required .
X = Existing Power factor
Y = Targeted Power factor
For Example :
For a S/S with load of 10 MW, Existing PF of 0.82 lag & Targeted power factor of 0.95 lag
KVAR required = 10000 *(tan (cos-‘ 0.82)-tan(cos-1 0.95)}
= 10000 {tan 34.92-tan18.19}
= 10000 (0.6981-0.3285)
= 3986 KVAR
Application and setting of Relays
1. Over Current and Earth fault Relay.
Over current and Earth fault relays are used to protect capacitor bank against possible faults :As per relevant 1S, Capacitor bank shall be capable of taking a overload of 30 % than normal current. Hence relay range will be decided considering the above factor.
2. Over Voltage Relay
As already discussed capacitor bank should trip whenever system voltage exceeds 110% of rated voltage. But Bank shouldn’t trip due to momentary fluctuations. To achieve this over voltage relay will be set at 110 % of rated voltage with a time delay of one second.
3. Under Voltage Relay
To ensure tripping of Capacitor bank during power failure Under voltage relay will be used and recommended setting is 90% of rated voltage. However wherever low voltage problem is persisting relay setting can be reduced to 70% of rated voltage with an time delay of one second.
4. Unbalance Current / Neutral Displacement Relay.
Whenever Capacitor bank is connected in Double star Configuration Unbalance Current relay will be used. Whereas if bank is connected in single star Neutral displacement will be used. Setting for these relays will be decided based on the design parameters. Whenever Capacitor bank is connected in Double star Configuration Unbalance Current relay will be used. Whereas if bank is connected in single star Neutral displacement will be used. Setting for these relays will be decided based on the design parameters.
OPERATION AND MAINTENANCE
01. Don’t switch on capacitor bank under light load condition. This can result in leading power factor which is not desirable.
02. Check breaker healthiness periodically including SF- 6 gas pressure & Trip coil healthiness.
03. Check tightness of connections-especially in capacitor bank to avoid local heating and failure of brazing in bushing studs due to that.
04. Check relay operation periodically.
05. Clean all bushings and insulators periodically.
06. Check earthing periodically.
07. Give a coat of paint to units and reactors once in two years, this will avoid corrosion of unit body and will increase unit life.
08 Whenever minor leak is observed in unit arrest immediately by applying M-Seal or by soft soldering.
09. Never touch capacitor units until and unless bank is switched Off & Discharged.
10. Always ensure healthiness of DC control.
11. Apply petroleum Jelly to isolator contacts periodically & check alignment.
12. Apply grease liberally to moving components inside breaker mechanism.
TROUBLE SHOOTING
In the unfortunate event of tripping of capacitor bank on failure of any unit observe the following.
Whenever bank trips on over current / earth fault, please check capacitor bank & other equipments for possible external fault. ‘Isolate the fault if observed. If any bird fault is observed please ensure healthiness of units before charging once again.
Whenever bank trips on Unbalance I Neutral displacement relay operation, it indicates failure of some elements in one OR more units.
To find out the faulty unit / units disconnect the bank from bus , discharge &: earth the bank.
Disconnect the star points in both stars. Measure capacitance of three limbs of each star. Wherever difference in capacitance is observed measure individual unit capacitance in that limb. Change the defective unit with the healthy one and restore the connections.
Whenever there is a alarm stating GAS Pressure low , immediately switch off bank and top up SF-6 gas in breaker. This will avoid blocking of tripping after further reduction in gas pressure.
If the unbalance current value exceed 50% of the tripping value, the capacitance of all the units in the bank shall be checked and those whose value deviate by more than 10% from the original value must be replaced.
CORRECTIVE ACTION AFTER RELAY OPERTION
Unbalanced protection
When the capacitor bank has been disconnected by the unbalance protection, the capacitance of all the units shall be checked and the faulty units shall be replaced with units having capacitance as close as possible to that of the faulty unit. The unbalance voltage/current shall be checked after recharging the bank. ‘
Overload protection
In case of tripping by overload protection due to temporary over voltages/ over current, the bank may be put into service when the disturbance has been eliminated,
NOTE: AFTER TRIPPING, THE BANK SHALL BE RE-ENERGISED ONLY AFTER 5 MINUTES SO THAT RESIDUAL VOLTAGE ACROSS TERMINALS IS LESS THAN 10% OF THE RATED VOLTAGE.
Dos & Dont Capacitor Bank :
Dos
Check that capacitor bank current is same in all the three phase.
Jumper should not be hot & if found hot it must be attended on prior base.
Earth cell before working on capacitor bank.
Provide HT para tape on bare studs of capacitor cells, Reactors, RVT etc. to avoid bird fault.
Destroy failed cell buried in pit.
Dont:
Do not run capacitor bank on leading PF.
Capacitor bank must not be made on while bus load is less than capacitor bank current.
The period between capacitor bank off and on must be more than 10 minutes (Discharging time for capacitor cell is nearly 10 minutes).
Do not take frequent operation of capacitor bank as capacitor bank breaker has to cut capacitive voltage and ampere.