Size and Location of Capacitor in Electrical System-(Part1)
February 6, 2013 5 Comments
Type of Capacitor Bank as per Its Application:
(1) Fixed type capacitor banks
- The reactive power supplied by the fixed capacitor bank is constant irrespective of any variations in the power factor and the load of the receivers.
- These capacitor banks are switched on either manually (circuit breaker/ switch) or semi automatically by a remote-controlled contactor.
- This arrangement uses one or more capacitor to provide a constant level of compensation.
- These capacitors are applied at the terminals of inductive loads (mainly motors), at bus bars
- Manual ON/OFF operation.
- Not meet the require kvar under varying loads.
- Penalty by electricity authority.
- Power factor also varies as a function of the load requirements so it is difficult to maintain a consistent power factor by use of Fixed Compensation i.e. fixed capacitors.
- Fixed Capacitor may provide leading power factor under light load conditions, Due to This result in over voltages, saturation of transformers, mal-operation of diesel generating sets, penalties by electric supply authorities.
- Where the load factor is reasonably constant.
- Electrical installations with constant load operating 24 hours a day
- Reactive compensation of transformers.
- Individual compensation of motors.
- Where the kvar rating of the capacitors is less than, or equal to 15% of the supply transformer rating, a fixed value of compensation is appropriate.
- Size of Fixed Capacitor bank Qc ≤ 15% kVA transformer
(2) Automatic type capacitor banks
- The reactive power supplied by the capacitor bank can be adjusted according to variations in the power factor and the load of the receivers.
- These capacitor banks are made up of a combination of capacitor steps (step = capacitor + contactor) connected in parallel. Switching on and off of all or part of the capacitor bank is controlled by an integrated power factor controller.
- The equipment is applied at points in an installation where the active-power or reactive power Variations are relatively large, for example:
- At the bus bars of a main distribution switch-board,
- At the terminals of a heavily-loaded feeder cable.
- Where the kvar rating of the capacitors is less than, or equal to 15% of the supply transformer rating, a fixed value of compensation is appropriate. Above the 15% level, it is advisable to install an automatically-controlled bank of capacitors.
- Control is usually provided by contactors. For compensation of highly fluctuating loads, fast and highly repetitive connection of capacitors is necessary, and static switches must be used.
Types of APFC:
- Automatic Power Factor correction equipment is divided into three major categories:
- (1)Standard = Capacitor + Fuse + Contactor + Controller
- (2)De tuned = Capacitor + De tuning Reactor + Fuse + Contactor + Controller
- (3)Filtered = Capacitor + Filter Reactor + Fuse + Contactor + Controller.
- Consistently high power factor under fluctuating loads.
- Prevention of leading power factor.
- Eliminate power factor penalty.
- Lower energy consumption by reducing losses.
- Continuously sense and monitor load.
- Automatically switch on/off relevant capacitors steps for consistent power factor.
- Ensures easy user interface.
- Automatically variation, without manual intervention, the compensation to suit the load requirements.
- Variable load electrical installations.
- Compensation of main LV distribution boards or major outgoing lines.
- Above the 15% level, it is advisable to install an automatically-controlled bank of capacitors.
- Size of Automatic Capacitor bank Qc > 15% kVA transformer.
|Individual capacitors||Most technically efficient, most flexible||Higher installation & maintenance cost|
|Fixed bank||Most economical, fewer installations||Less flexible, requires switches and/or circuit breakers|
|Automatic bank||Best for variable loads, prevents over voltages, low installation cost||Higher equipment cost|
|Combination||Most practical for larger numbers of motors||Least flexible|
Type of Capacitor as per Construction:
(1) Standard duty Capacitor:
- Construction: Rectangular & Cylindrical(Resin filled / Resin coated-Dry)
- Steady inductive load.
- Non linear up to 10%.
- For Agriculture duty.
- Construction: Rectangular & Cylindrical (Resin filled / Resin coated-Dry/oil/gas)
- Suitable for fluctuating load.
- Non linear up to 20%.
- Suitable for APFC Panel.
- Harmonic filtering
(3) LT Capacitor:
- Suitable for fluctuating load.
- Non linear up to 20%.
- Suitable for APFC Panel & Harmonic filter application.
Selecting Size of Capacitor Bank:
- The size of the inductive load is large enough to select the minimum size of capacitors that is practical. For HT capacitors the minimum ratings that are practical are as follows:
|System Voltage||Minimum rating of capacitor bank|
|3.3 KV , 6.6KV||75 Kvar|
|11 KV||200 Kvar|
|22 KV||400 Kvar|
|33 KV||600 Kvar|
- Unit sizes lower than above is not practical and economical to manufacture.
- When capacitors are connected directly across motors it must be ensured that the rated current of the capacitor bank should not exceed 90% of the no-load current of the motor to avoid self-excitation of the motor and also over compensation.
- Precaution must be taken to ensure the live parts of the equipment to be compensated should not be handled for 10 minutes (in case of HT equipment) after disconnection of supply.
- Crane motors or like, where the motors can be rotated by mechanical load and motors with electrical braking systems, should never be compensated by capacitors directly across motor terminals.
- For direct compensation across transformers the capacitor rating should not exceed 90 % of the no-load KVA of the motor.
Selection of Capacitor as per Non Liner Load:
- For power Factor correction it is need to first decide which Type of capacitor is used.
- Selection of Capacitor is depending upon many factor i.e. operating life, Number of Operation, Peak Inrush current withstand capacity.
- For selection of Capacitor we have to calculate Total Non Liner Load like UPS, Rectifier, Arc/Induction Furnace, AC/DC Drives, Computer, CFL Blubs, and CNC Machines.
- Calculation of Non liner Load, Example: Transformer Rating 1MVA,Non Liner Load 100KVA
- % of non Liner Load= (Non Liner Load/Transformer Capacity) x100 = (100/1000) x100=10%.
- According to Non Linear Load Select Capacitor as per Following Table.
|% Non Liner Load||Type of Capacitor|
|Up to 15%||Heavy Duty|
|Up to 20%||Super Heavy Duty|
|Up to 25%||Capacitor +Reactor (Detuned)|
Configuration of Capacitor:
- Power factor correction capacitor banks can be configured in the following ways:
- (1) Delta connected Bank.
- (2) Star-Solidly Grounded Bank.
- (3) Star-Ungrounded Bank.
(1) Star-Solidly Grounded
- Initial cost of the bank may be lower since the neutral does not have to be insulated from ground.
- Capacitor switch recovery voltages are reduced
- High inrush currents may occur in the station ground system.
- The grounded-Star arrangement provides a low-impedance fault path which may require revision to the existing system ground protection scheme.
- Typically not applied to ungrounded systems. When applied to resistance-grounded systems, difficulty in coordination between capacitor fuses and upstream ground protection relays (consider coordination of 40 A fuses with a 400 A grounded system).
- Application: Typical for smaller installations (since auxiliary equipment is not required)
- Industrial and commercial capacitor banks are normally connected ungrounded Star, with paralleled units to make up the total kvar. It is recommended that a minimum of 4 paralleled units to be applied to limit the over voltage on the remaining units when one is removed from the circuit. If only one unit is needed to make the total kvar, the units in the other phases will not be overloaded if it fails.
- In industrial or commercial power systems the capacitors are not grounded for a variety of reasons. Industrial systems are often resistance grounded. A grounded Star connection on the capacitor bank would provide a path for zero sequence currents and the possibility of a false operation of ground fault relays. Also, the protective relay scheme would be sensitive to system line-to-ground voltage Unbalance, which could also result in false relay tripping.
- In Industrial and Commercial.
(3) Delta-connected Banks
- Delta-connected banks are generally used only at distributions voltages and are configured with a Single series group of capacitors rated at line-to-line voltage.
- With only one series group of units no overvoltage occurs across the remaining capacitor units from the isolation of a faulted capacitor unit. Therefore, unbalance detection is not required for protection and they are not treated further in this paper.
- In Distribution System.
Effect of series and Parallel Connection of capacitor:
- This is the most popular method of connection. The capacitor is connected in parallel to the unit.
- The voltage rating of the capacitor is usually the same as or a little higher than the system voltage.
- This method of connection is not much common. Even though the voltage regulation is much high in this method,
- It has many disadvantages. One is that because of the series connection, in a short circuit condition the capacitor should be able to withstand the high current.
- The other is that due to the series connection due to the inductivity of the line there can be a resonance occurring at a certain capacitive value. This will lead to very low impedance and may cause very high currents to flow through the lines.