Size and Location of Capacitor in Electrical System-(Part 2)

SIZE AND LOCATION OF CAPACITOR IN ELECTRICAL SYSTEM-(PART 2)

SIZE OF CIRCUIT BREAKER, FUSE AND CONDUCTOR OF CAPACITOR BANK:

(A) THERMAL AND MAGNETIC SETTING OF A CIRCUIT BREAKER:

(1) SIZE OF CIRCUIT BREAKER:

1.3 to 1.5x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors * 1.31×In for Heavy Duty/Energy Capacitors with 5.6% Detuned Reactor(Tuning Factor 4.3) * 1.19×In for Heavy Duty/Energy Capacitors with 7% Detuned Reactor(Tuning Factor 3.8) * 1.12×In for Heavy Duty/Energy Capacitors with 14% Detuned Reactor(Tuning Factor 2.7) * Note: Restrictions in Thermal settings of system with Detuned reactors are due to limitation of IMP (Maximum Permissible current) of the Detuned reactor.

(2) THERMAL SETTING OF CIRCUIT BREAKER:

1.5x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors

(3) MAGNETIC SETTING OF CIRCUIT BREAKER:

5 to10 x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors * Example :150kvar,400v, 50Hz Capacitor * Us = 400V, Qs = 150kvar,Un = 400V, Qn = 150kvar * In = 150000/400√3 = 216A * Circuit Breaker Rating = 216 x 1.5 = 324A * Select a 400A Circuit Breaker. * Circuit Breaker thermal setting = 216 x 1.5 = 324 Amp * Conclusion:- Select a Circuit Breaker of 400A with Thermal Setting at 324A and * Magnetic Setting ( Short Circuit ) at 3240A

(B) FUSE SELECTION

The rating must be chosen to allow the thermal protection to be set to: * 1.5 to 2.0 x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors. * 1.35×In for Heavy Duty/Energy Capacitors with 5.7% Detuned Reactor (Tuning Factor 4.3) * 1.2×In for Heavy Duty/Energy Capacitors with 7% Detuned Reactor (Tuning Factor 3.8) * 1.15×In for Heavy Duty/Energy Capacitors with 14% Detuned Reactor(Tuning Factor 2.7) * For Star-solidly grounded systems: Fuse > = 135% of rated capacitor current (includes overvoltage, capacitor tolerances, and harmonics). * For Star -ungrounded systems: Fuse > = 125% of rated capacitor current (includes overvoltage, capacitor tolerances, and harmonics). * Care should be taken when using NEMA Type T and K tin links which are rated 150%. In this case, the divide the fuse rating by 1.50. * Example 1: 150kvar,400v, 50Hz Capacitor * Us = 400V; Qs = 150kvar, Un = 400V; Qn = 150kvar. * Capacitor Current =150×1000/400 =375 Amp * To determine line current, we must divide the 375 amps by √ 3 * In (Line Current) = 375/√3 = 216A * HRC Fuse Rating = 216 x1.65 = 356A to * HRC Fuse Rating = 216 x 2.0 = 432A so Select Fuse Size 400 Amp

PROBLEMS WITH FUSING OF SMALL UNGROUNDED BANKS

Example: 12.47 kV, 1500 Kvar Capacitor bank made of three 3 No’s of 500 Kvar single-phase units. * Nominal Capacitor Current=1500/1.732×12.47=69.44 amp * Size of Fuse=1.5×69.44 =104 Amp= 100 Amp Fuse. * If a capacitor fails, we say that It may approximately take 3x line current. (3 x 69.44 A = 208.32 A). * It will take a 100 A fuse approximately 500 seconds to clear this fault (3 x 69.44 A = 208.32 A). The capacitor case will rupture long before the fuse clears the fault. * The solution is using smaller units with individual fusing. Consider 5 No’s of 100 kVAR capacitors per phase, each with a 25 A fuse. The clear time for a 25 A fuse @ 208.32 A is below the published capacitor rupture curve.

Size of capacitor circuit conductors should be at least 135% of the rated capacitor current in accordance with NEC Article 460.8 (2005 Edition).

SIZE OF CAPACITOR FOR TRANSFORMER NO-LOAD COMPENSATION.

FIXED COMPENSATION

The transformer works on the principle of Mutual Induction. The transformer will consume reactive power for magnetizing purpose. Following size of Capacitor Bank is required to reduce reactive component (No Load Losses) of Transformer.

Selection of capacitor for transformer no-load compensation

KVA Rating of the Transformer Kvar Required for compensation Up to and including 315 KVA 5% of KVA Transformer Rating 315 to 1000 KVA 6% of KVA Transformer Rating Above 1000 KVA 8% of KVA Transformer Rating

SIZING OF CAPACITOR FOR MOTOR COMPENSATION:

The capacitor provides a local source of reactive current. With respect to inductive motor load, this reactive power is the magnetizing or “no load current“which the motor requires to operate. * A capacitor is properly sized when its full load current rating is 90% of the no-load current of the motor. This 90% rating avoids over correction and the accompanying problems such as over voltages.

(1)IF NO-LOAD CURRENT IS KNOWN:

The most accurate method of selecting a capacitor is to take the no load current of the motor, and multiply by 0.90 (90%). * Example: Size a capacitor for a 100HP, 460V 3-phase motor which has a full load current of 124 amps and a no-load current of 37 amps. * Size of Capacitor =No load amps(37 Amp) X 90% = 33 Kvar

IF THE NO LOAD CURRENT …