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DESIGN CALCULATION FOR 6.6kV CAPACITOR BANKS Doc No: (4)DOC-006
CONTENT 1.0 General........................................................................................................................3 2.0 Applicable Technical Standards................................................................................3 3.0 Site Conditions............................................................................................................3 4.0 General Power Supply Details...................................................................................3 5.0 Design Features...........................................................................................................4 6.0 Sizing of Capacitor Banks..........................................................................................4 7.0 Results.........................................................................................................................6
1.0
General
1.1
The purpose of this document is to provide project design calculation for sizing of the 6.6kV capacitor banks for M/S UTCL, Katni Unit.
1.2
This document describes the 6.6kV capacitor sizing calculation as per the various 6.6kV motor data sheets and sizing the capacitor for the 7.2kV system (considering 10% over voltage), sizing the series reactor for the required 7.2kV capacitor bank system
2.0
Applicable Technical Standards The following standards shall be applicable for design, manufacture and testing of the 6.6kV Capacitor Bank systems.
3.0
IEC: 60871-1
Shunt Capacitors for AC Power Systems
IEC: 60289
Reactors
IEC: 60252
AC Motor Capacitors
Site Conditions Altitude
<1000
Temperature
50 deg C Max - 84% Min. - 40%
Humidity Atmosphere 4.0
M Design Max / Min %
Highly dusty with fine cement dust
General Power Supply Details The available MV and LT power supply is as given below: MV Power supply
:
3 Phase, 6.6 kV, 50 Hz
Voltage variation
+/- 10%
Frequency variation
:
+/- 5%,
Control Supply
:
230V AC Normal Supply for heaters
110 V DC protection units, auxiliary relays and signalling lamps
110V DC for Breaker Closing and tripping for 6.6kV and 230V AC for 415V system.
230 VAC supply for spring charging, electronic modules & Contactors etc.
Fault Levels 1)
33kV side
:
26.2kA for 1 second
2)
6.6kV side
:
40kA for 1 second
3)
415V side
:
50kA for 1 second
5.0
Design Features The offered Capacitor shall be rated for 7.2 kV systems. HT capacitors shall be considered across each HT motors to improve the power factor to 0.95 lag.
6.0 6.1
The capacitors will be connected at the 6.6kV outgoing terminals of motor feeders.
Each capacitor bank shall be provided with HRC fuses for protection.
The capacitors shall be all polypropylene and low loss type.
Internal resistors shall be provided to ensure discharging of capacitors to a safe level as per IEC guidelines.
The capacitor Banks along with the reactors shall be housed in a CRCA enclosure.
Sizing of Capacitor Banks Design Basis The basis for the selection of the kVAR for a capacitor bank is:
6.2
Sizing the capacitor bank as per the data furnished in the motors data sheets
The continuous over voltage is considered as 10% i.e. 7.26kV.
Calculation of the kVAR rating for 1850kW Ball Mill Main Drive: Motor Data: Rating of Motor: 1850kW System voltage: 6.6kV Max Continuous system voltage: 7.26kV Power factor of the motor when running at 75% of full load
: 0.78
Efficiency of the motor when running at 75% of full load : 94.5% (Power factor and efficiency of the motor are considered at 75% of full load, assuming that the motor will be loaded at an average of 75%) Target Power factor: 0.95 lag (To improve the power factor from 0.78 to 0.95, after adding the capacitor bank in the motor circuit) Required kVAR of the Capacitor Bank to achieve the target power factor = kVAR = kW x (Tan φi – Tan φd) Where φi = Cos-1 Pfi
φd
=
= Initial Power Factor Angle
Cos-1 Pfd = Target Power Factor Angle
So, required kVAR 6.3
= 0.75*1850*(tan (cos-1 (0.78)) - tan (cos-1 (0.95))) = 657.1kVAR
Calculation of the kVAR rating for 1850kW Ball Mill Motor at 7.26kV: From the above calculation, the required kVAR is 657 kVAR. This calculated 657 kVAR is at rated voltage of 6.6kV.
Equivalent impedance Xeq to achieve 657kVAR at 6.6kV, Xeq
= = =
(kV)2 / MVAR (6.6)2 / 0.657 66.3 Ω / Phase
Assume a series reactor of 6.0% of capacitive impedance. i.e. Series Reactance XL = 0.06*XC Equivalent impedance Xeq = XC - XL = XC – 0.06*XC = XC (1-0.06) Hence XC=Xeq/ (1-0.06) Capacitive Reactance, XC = 70.532 Ω / Phase The capacitance C is derived as: C
= = =
1/(2 *π* f * XC) 45.11 X 10-6 Farad 45.11µF
Inductive Reactance XL is derived as: XL
= = =
0.06*XC 0.06 *70.532 4.232 Ω / Phase
Inductance L of the series reactor is derived as: L
= XL /(2 *π* f) = 13.465 X 10-3 Henry = 13.465mH The current flowing through series combination of capacitor & reactor is, If
= = =
(Vph with O/V)/ Xeq 7.26*1000/(1.732*66.3) 63.22Amps
Voltage drop across reactor VL
= = =
√3 * If * XL √3 * 63.22 * 4.232 463.39V
Voltage appearing across the capacitor bank with a 6% series reactor and a continuous over voltage (COV) of 10% will be: VC
= = =
VN (with COV) + VL 6.6*1.1 + 0.463 7.723 kV
where VN :nominal rated voltage of the system
Hence, if at 6.6kV the compensation required for 657 kVAR, Then compensation at 7.723 kV would be Qc
= = =
(VC) 2 *1000/ XC (7.723) 2 *1000/ 70.532 845.64 kVAR
Rating of Capacitor bank selected is 846 kVAR at 7.723 kV (three phases)
6.4
kVAR rating of the Reactor The reactor is 6% of the capacitor bank value. i.e., KVAR of the reactor = 0.06 * kVAR of capacitor = 0.06*845.64 = 50.74 kVAR. Hence the rated kVAR of the reactor is 50.74 kVAR
6.5
The effective capacitive reactive power Qc is < 0.9*I (no load)*6.6*1.732 (846-51=795)
< 0.9*80*6.6*1.732 (=823)
Hence the selected kVAR is suitable to avoid the motor self-excitation.
7.0
Results The sizing results of the capacitor banks with series reactors for the HV motors are as below: 1) Ball Mill Motor – 846kVAR @ 7.723kV with a 6% series reactor.
