VI - HOW TO CALCULATE THE POWER OF CAPACITORS 1) CALCULATION FROM ELECTRICITY BILLS (LV or MV metering EDF "Tarif Vert" rate subscribers) ÉNERGIE RÉACTIVE P + HP
ÉNERGIE RÉACTIVE P + HP
120.000
125.000
au niveau du comptage
TANGENTE phi secondaire primaire 0
P1
PUISSANCES SOUSCRITES P2 P3 P4
P5
525
590
590
590
590
kvar kvar h en franch franchise ise
kvar kvar h en consom consommés més
kvar kvar h à ris ristour tournner
kvar kvar h à fact acturer urer
120.000
96
PUISSANCES RETENUES POUR CALCUL DE PRM P1 P2 P3 P4
P5
70.000
PR 560
Dépassement à facturer
PRM 1
From 1st November 1987, in France, the reactive energy billing limit changed for all "tarif Vert" rate subscribers (LV or MV metering) to: * tg ϕ = 0.4 or cos = 0.928: on the primary winding, * tg ϕ = 0.31 or cos = 0.955: on the secondary winding. For the calculation of the capacitor banks to be installed, proceed using the following method: ■ analyse the 5 electricity bills from November to March, ■ select the month for which the bill is the highest (kvarh to be billed), ■ evaluate the number of hours of operation of the installation every month in day-tariff and peak hours (generally 6 a.m. to 10 p.m. excluding Sundays), ■
calculate the capacitor power Qc to be installed Qc =
kvarh to be billed (monthly) Nb.of working hours (monthly)
* for LV metering, in the calculation of the kvarh to be billed, EDF introduces a fixed rate transformer consumption by applying a coefficient of 0.09 on the secondary winding tg ϕ calculated to obtain the primary winding tg ϕ.
Example Take the subscriber SMITH : . highest reactive energy bill : December 87, . number of kvarh to be billed : 70,000, . monthly number of hours of operation : 350 hours
(day-tariff + peak)
2) CALCULATION FROM MEASURING ELEMENTS READ ON THE HV/LV TRANSFORMER SECONDARY WINDING/ PkW - COS ϕ
Example : Take a plant powered from an 800 kVA HV / LV subscriber station which would like to change the power factor of its installation to : * Cos ϕ = 0.928 (tg ϕ = 0.4) on the primary winding * or Cos ϕ = 0.955 (tg ϕ = 0.31) on the secondary winding with the following readings : • voltage: 400 V three-phase 50 Hz • P = 475 kW • Cos (secondary) = 0.75 (or tg ϕ = 0.88) Qc (bank to be installed) = Pkw (tg ϕ measured - tg ϕ to be obtained)
Qc = 475 (0.88 - 0.31) # 270 kvar Note: the coefficient K = (tg ϕ measured - tg ϕ to be obtained) is obtained easily from the Cos ϕ values using the conversion table on page 9.
3) CALCULATION FOR FUTURE INSTALLATIONS : For future installations, compensation is frequently requested from the commissioning stage. In this case, it is impossible to calculate the bank using conventional methods (electricity bill or measurements on-site). For this type of installation, it is recommended to install a capacitor bank equal to approximately 25% of the nominal power of the corresponding HV / LV transformer.
Example: 1000 kVA transfomer => Q capacitor = 250 kvar Note : this type of ratio corresponds to the following operating conditions: • 1000 kVA transformer • real transformer load = 75% • Cos ϕ of load = 0.80 k = 0.421 • Cos ϕ to be obtained = 0.95 (table on page 9) Qc = 1000 x 75% x 0.80 x 0.421 = 250 kvar
}
4) CALCULATION FOR INDEPENDENT PRODUCERS (SMALL POWER STATIONS) For this type of installation, the independent producer must supply the electricity company with a quantity of reactive energy equal to at least 40% of its active energy production during WINTER day-tariff and peak hours. In this case, the calculation of the capacitor bank should account for: • the on-load reactive consumption of the generator • the on-load consumption of the LV / HV
2003 Conference for Protective Relay Engineers - Texas A&M University April 8-10, 2003, College Station (TX)
Shunt Capacitor Bank Fundamentals and Protection Gustavo Brunello, M.Eng, P.Eng GE Multilin, Canada
[email protected]
Dr. Bogdan Kasztenny
Craig Wester
GE Multilin, Canada GE Multilin, USA
[email protected] [email protected]
ABSTRACT Shunt capacitor banks are used to improve the quality of the electrical supply and the efficient operation of the power system. Studies show that a flat voltage profile on the system can significantly reduce line losses. Shunt capacitor banks are relatively inexpensive and can be easily installed anywhere on the network. This paper reviews principles of shunt capacitor bank design for substation installation and basic protection techniques. The protection of shunt capacitor bank includes: a) protection against internal bank faults and faults that occur inside the capacitor unit; and, b) protection of the bank against system disturbances. Section 2 of the paper describes the capacitor unit and how they are connected for different bank configurations. Section 3 discusses bank designs and grounding connections. Bank protection schemes that initiate a shutdown of the bank in case of faults within the bank that may lead to catastrophic failures are presented in Section 4. The paper does not address the means (fuses) and strategies to protect individual elements or capacitor units, nor the protection of capacitor filter banks. System disturbances and basic capacitor bank control strategies are also discussed. 1. INTRODUCTION Shunt capacitor banks (SCB) are mainly installed to provide capacitive reactive compensation/ power factor correction. The use of SCBs has increased because they are relatively inexpensive, easy and quick to install and can be deployed virtually anywhere in the network. Its installation has other beneficial effects on the system such as: improvement of the voltage at the load, better voltage regulation (if they were adequately designed), reduction of losses and reduction or postponement of investments in transmission. The main disadvantage of SCB is that its reactive power output is proportional to the square of the voltage and consequently when the voltage is low and the system need them most, they are the least efficient. 2. THE CAPACITOR UNIT AND BANK CONFIGURATIONS 2.1 The Capacitor Unit The capacitor unit, Fig. 1, is the building block of a shunt capacitor bank. The capacitor unit is made up of individual capacitor elements, arranged in parallel/ series connected groups, within a steel enclosure. The internal discharge device is a resistor that reduces the unit residual voltage to 50V or less in 5 min. Capacitor units are available in a variety of voltage ratings (240 V to 24940V) and sizes (2.5 kvar to about 1000 kvar).
Shunt Capacitor Bank Fundamentals and Protection
1
Internal Discharge Device Bushing Group of Elements
Element
Case
Fig 1 – The capacitor Unit
2.1.1 Capacitor unit capabilities Relay protection of shunt capacitor banks requires some knowledge of the capabilities and limitations of the capacitor unit and associated electrical equipment including: individual capacitor unit, bank switching devices, fuses, voltage and current sensing devices. Capacitors are intended to be operated at or below their rated voltage and frequency as they are very sensitive to these values; the reactive power generated by a capacitor is proportional to both of them (kVar
Shunt Capacitor Bank Fundamentals and Protection
2
