This article is about the requirements for the design, manufacture, testing of Power Factor Correction Capacitor Banks of electrical systems rated 480 Volts, 4160 Volts or 13.8 kV. Main keywords for this article are Power Factor Correction Capacitor Banks, Power Capacitors Design Requirements, Capacitor Cells, Capacitor Banks Control Protection, Capacitor Banks Testing.
Power Capacitors References
IEEE (Institute of Electrical and Electronics Engineers)
IEEE Std. 18 Shunt Power Capacitors
National Electrical Manufacturers Association (NEMA)
NEMA CP 1, Shunt Capacitors
National Fire Protection Association (NFPA)
NFPA 70 National Electrical Code.
Power Capacitors Design Requirements
The automatically switched power factor correction capacitor banks shall be furnished by the Manufacturer with the recommendations and instructions. For selection and application of Power Capacitor see following.
Power Factor Correction Capacitor Banks
- The capacitor bank shall be rated for three-phase service for power factor correction at voltage level specified on the electrical one line diagram. The buses shall be braced for the symmetrical current as specified on the electrical one line diagram.
- Power capacitor banks that are installed inside an equipment such as motor control centers or switch gear shall be designed to withstand the voltage rating, electrical short circuit rating and continuous current rating as specified on the electrical one line diagram and/or data sheet of the corresponding equipment.
Enclosures, Busworks and Arrangement
- The enclosure shall be freestanding type utilizing a modular design with provision for expansion. The capacitor cells, contactor, controller and related equipment shall be mounted in the same enclosure to minimize field installation.
- Stand alone enclosures for capacitor banks shall physically consist the following:
a. Enclosure shall be constructed of 11-gauge (3.04 mm) steel
b. Side panels shall be louvered to provide cooling air intake
c. Enclosure shall have front access with removable side and back panels
d. Enclosure door handle(s) with a three-point latch and key lock
e. Four removable lifting eyes mounted on top of the enclosure to properly support the equipment during lifting and installation - Buses shall be designed for 1,000 A per square inch current density, tin plated copper and insulated. All buses shall be braced for the magnitude of momentary current as specified in the one line diagram.
- All flexible power cable shall be stranded copper conductor and installed in accordance with the latest edition of the National Electrical Code, NFPA 70.
- A door mounted, on/off switch to de-energize control power to all door mounted control devices and the indicating lights shall be provided.
Capacitor Cells
- The capacitor elements shall be constructed with polypropylene film dielectric system with self-healing metallized electrodes.
- Furnish capacitor cell with a Pressure Sensitive Interrupter (PSI). PSI protects the cell from rupture by interrupting capacitor current when the internal pressure forces the cover upward, breaking an under cover contact. This provides a visual indication of the cell end of life.
- Provide a discharge resistor on each capacitor cell to discharge the residual voltage to 50 volts or less within one minute after de-energization.
- The capacitor cells shall be impregnated with a biodegradable, non-PCB, NFPA rated Class IIIB manufactured by Dielektrol or approved equal. The flash point of the impregnant shall be at least 220 °C.
- The capacitor cells shall be designed to withstand the duties described in IEEE standard 18 and NEMA standard CP-1.
Capacitor Banks Controls and Protection
- All controls and protection shall be installed indoors.
- The capacitor cells, fuses and switching contactor for each step of capacitor bank shall be mounted on a removable tray for ease of removal and replacement.
- Furnish a door mounted, solid state, multi-step, and adjustable power factor controller. This controller shall automatically switch the steps in and out to provide power factor correction to a desired setpoint. As a minimum, the following shall be provided with the door mounted controller:
a. Manual / Automatic selector switch.
b. Microprocessor based adjustable control.
c. Adjustable target power factor setting from 0.8 inductive (lagging) to 0.95 capacitive (leading)
d. Adjustable switching time delay from 10 to 60 seconds.
e. Fault signaling output relay, if target power factor is not reached.
f. Continuous front panel displays indicating which steps are on.
g. Digital display of actual power factor.
h. No volt release of all output contacts when the supply voltage has been interrupted for 35 millisec. This serves to protect the capacitors after the power as been restored.
i. Rotational switching programs to assure that the steps are used equally. The first step turned on is the first step turned off. - Current limiting fuses on all three phases of each step shall be provided.
- Contactors shall be NEMA rated.
- Provide a voltage transformer of suitable rating for control power and voltage sensing. Protection shall be provided with two primary current limiting fuses and one secondary current limiting fuse.
- Automatically switched capacitors banks to improve power factor shall meet the requirements of the standards listed in the “References” in section 2 above and shall be UL listed or pass the tests performed by a nationally recognized testing laboratory (NRTL).
Capacitor Banks Optional Features
- Provide door mounted indicating lights, one per phase per step to indicate a blown fuse condition.
- Provide door interlock to disconnect power from the main power source when the door is opened. A by-pass mechanism shall be provided to defeat the interlock to be used by qualified personnel.
- All line terminals shall be provided with shroud.
Capacitor Banks Testing and Inspection
The production tests required shall be as follows:
a. All capacitors shall pass a terminal to terminal test at two times the ac RMS rating for a duration of 10 seconds
b. All capacitors shall pass a terminal to case test at rated ac RMS for a duration of 10 seconds
c. All capacitors shall pass a capacitance test at rated voltage
d. All capacitors shall pass a loss determination (power factor) test
e. All capacitors shall pass a discharge resistor test
f. All capacitors shall pass a 100 percent leak test
g. Final equipment checkout of major components to verify operation shall be performed
h. Final electrical test shall include a terminal to case ac withstand test for a duration of 60 seconds
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