Cutler-Hammer - Eaton Canada

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B1-6 Surge Protection Medium-Voltage Distribution Equipment Metal-Clad Switchgear – Drawout Vacuum Breakers (VCP-W) Application and Technical Data VacClad-W metal-clad switchgear is applied over a broad range of circuits, and is one of the many types of equipment in the total system. The distribution system can be subject to voltage transients caused by lighting or switching surges. Recognizing this phenomenon, the industry has developed standards to provide guidelines for application of electrical equipment, which should be used in the design of distribution systems independent of the breaker interrupting medium. These standards are: ANSI C62 – Guides and Standards for Surge Protection IEEE 242 – Buff Book IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems IEEE 141 – Red Book Recommended Practice for Electric Power Distribution for Industrial Plants ANSI C37.20.2 Metal-Clad Switchgear In general, if the BIL of the system is equal to the BIL of VacClad-W metal-clad switchgear, no protection is required against switching surges; however, standard BIL dry-type transformers and rotating apparatus rarely meet this criteria. For circuits exposed to lightning, protection is recommended in line with standard practices. In a wide range of applications, not all circuits require surge protection. Therefore, VacClad-W metal-clad switchgear does not include any surge protection as standard. The user exercises the options as to the type of protection deemed necessary, depending on the individual circuit characteristics and cost considerations. The following recommendations are outlined to provide guidelines of minimum surge protection for metal-clad switchgear and the associated system equipment: 1. Lightning – Provide standard lightning protection. 2. Switching surge protection: a. Liquid-filled transformer — no surge protection required. b. Dry-type transformers: 38 kV - 170 kV BIL — no surge protection required. 27 kV - 125 kV BIL — no surge protection required. 15 kV - 95 kV BIL — no surge protection required. 7.5 kV - 95 kV BIL — no surge protection required. 5 kV - 60 kV BIL — no surge protection required. Turn-to-turn insulation protection: Surge capacitors may be required on some systems where a steep rate of rise is expected which may damage turn-to-turn insulation. For all other voltage/BIL ratings for dry-type transformers, surge protection (arresters or capacitors) is recommended at the transformer terminals, in line with established practices. Metal oxide surge arresters can be supplied in VacClad-W switchgear as an alternate to above. c. Motors — Surge capacitors at the motor terminals (and surge arresters where appropriate). d. Generators — Surge capacitors and station class surge arresters at machine terminals. e. Switching overhead lines and underground cables — No surge protection required. f. Capacitor switching — No surge protection required. g. Shunt reactor switching — Threephase 15 kV dry-type reactors less than 9 MVA require surge protection at the reactor’s terminals. These application guidelines for VacClad-W metal-clad switchgear were established after extensive analysis of medium-voltage power systems. Surge Arresters The modern metal-oxide surge arresters are recommended because this latest advance in arrester design ensures better performance and high reliability of surge protection schemes. Manufacturer’s technical data must be consulted for correct application of a given type of arresters. Notice that published arrester MCOV (maximum continuous operating voltage) ratings are based on 40-45°C ambient temperature range. In general, the following guidelines are recommended for arrester selections, when installed inside the VCP-W metal-clad switchgear: a. Solidly Grounded System – Minimum arrester MCOV rating should be equal to 1.05 x VLL/(1.732 x T), where VLL is nominal line-to-line service voltage, 1.05 factor allows for +5% voltage variation above the nominal voltage according to ANSI C84.1, and T is derating factor to allow for operation at 55°C switchgear ambient, which should be obtained from the arrester manufacturer for the type of arrester under consideration. Typical values of T are: 0.946 to 1.0. b. Low Resistance Grounded Systems (systems grounded through resistor rated 10 seconds) – Arrester 10-second MCOV capability at 60°C, which is obtained from manufacturer’s data, should be equal to 1.05 VLL, where VLL is nominal line-to-line Cutler-Hammer May 2001 service voltage, and 1.05 factor allows for +5% voltage variation above the nominal voltage. c. Ungrounded Systems or Systems Grounded through impedance other than 10-second resistor – The arrester minimum MCOV rating should be equal to 1.05 VLL/T, where VLL and T are as defined above. Surge Capacitors Metal-oxide surge arresters limit the magnitude of prospective surge overvoltage, but are ineffective in controlling its rate of rise. Specially designed capacitors with low internal inductance are used to limit the rate of rise of this surge overvoltage to protect turn-to-turn insulation. Recommended values for surge capacitors are: 0.5 µf on 5 kV and 7.5 kV, 0.25 µf on 15 kV, and 0.13 µf on 24 kV and higher systems. Arc Furnace Transformer Protection For arc furnace transformer protection, recommended RC network and arresters are shown below. For values of Cs and Rs, see Table 5. The resistor is important to limit capacitive current in the event of a re-ignition. This is especially important when large power-factor correction capacitor banks are used. Transformer Arc Furnace Main Bus VCP-W Vacuum Circuit Breaker R S C S Surge Arrester Resistor-Capacitor Protection for Arc Furnace Instrument Transformers Instrument transformers are used to protect personnel and secondary devices from high voltage and permit use of reasonable insulation levels for relays, meters and instruments. The secondaries of standard instrument transformers are rated at 5 amperes and/or 120 volts, 60 Hertz. Voltage Transformers Selection of the ratio for voltage transformers is seldom a question since the primary rating should be equal to or higher than the system line-to-line voltage. The number of potential transformers per set and their connection is determined by the type of system and the relaying and metering required.
Cutler-Hammer May 2001 The 3-phase, 3-wire system with 2-element watt-hour meters would require a set of two line-to-line voltage transformers. If line-to-ground potential is also required for a directional ground relay, then a set of three line-to-ground voltage transformers could be used to provide both line-to-line potential for the 2-element watt-hour meter and line-to-ground potential for the ground relay. Ground detection lights or relays for the ungrounded system requires three line-toground voltage transformers and a separate set is usually recommended for this purpose. The 3-phase, 4-wire, solidly grounded system usually requires three line-to-ground voltage transformers for 2 1 /2- or 3-element metering. Where synchronizing of generators or systems is involved, it is recommended that only line-to-line potential be used. Current Transformers The current transformer ratio is generally selected so that the maximum load current will read about 70 percent full scale on a standard 5-ampere coil ammeter. Therefore, the current transformer primary rating should be 140 to 150 percent of the maximum load current. Maximum system fault current can sometimes influence the current transformer ratio selection since the connected secondary devices have published one-second ratings. The zero-sequence current transformer is used for sensitive ground fault relaying or self-balancing primary current type machine differential protection. The zero-sequence current transformer is available with a nominal ratio of 50/5 or 100/5 and available opening size for power cables of 7.25 inches, special zero-sequence transformers with larger windows are also available. The minimum number of current transformers for circuit relaying and instruments is three current transformers, one for each phase or two-phase connected current transformers and one zero-sequence current transformer. Separate sets of current transformers are required for differential relays. The minimum pickup of a ground relay in the residual of three-phase connected current transformers is primarily determined by the current transformer ratio. The relay pickup can be reduced by adding one residual connected auxiliary current transformer. This connection is very desirable on main incoming and tie circuits of low resistance grounded circuits. Standard accuracy current transformers are normally more than adequate for most standard applications of microprocessor-based ➀ Cable surge impedance 37Ω. ➁ For solidly grounded 4160 volt system only or any type 2400 volt system. Medium-Voltage Distribution Equipment Metal-Clad Switchgear – Drawout Vacuum Breakers (VCP-W) Table 6: Standard Voltage Transformer, 60 Hertz LL = Line-to-line connection. LG = Line-to-ground connection. B1-7 Rating- Volts 2400 4200 4800 7200 8400 10800 12000 14400 15600 18000 21000 24000 27000 36000 Ratio 20-1 35-1 40-1 60-1 70-1 90-1 100-1 120-1 130-1 150-1 175-1 200-1 225-1 300-1 Switchgear Voltage Transformer – ANSI Accuracy kV Class kV BIL Maximum Number Per Set and Connection 5 60 2LL or 3LG 7.5 and 15 Application and Technical Data Table 5: Characteristics of Transformers for Arc Furnaces and Protection Arc Furnace Transformer R-C Protection Voltage kV MVA Continuous Current Short Circuit Magnetizing Current Cap. Cs RES. Rs Ohm 50 Ft. Cable➀ 100 Ft. Cable➀ 1000 Ft. Cable➀ A Z% % A Microf. ohm watt ohm watt ohm watt 25.5 50 1130 2.4 0.155 1.75 0.018 15.47 0.15 21.87 0.21 69.16 0.66 23 35 878 3.5 0.16 1.4 0.016 16.41 0.10 23.21 0.14 73.39 0.45 13.2 30 1312 3 0.796 10.4 0.204 4.56 1.54 6.45 2.18 20.40 6.90 13.2 50 2000 2.4 0.765 15.30 0.326 3.60 3.13 5.10 4.43 16.12 14.00 95 2LL or 3LG 27 125 2LL or 3LG 38 170 2LL or 3LG Standard Ratio’s 20,➁ 35, 40 35, 40, 60, 70, 100, 120 90, 100, 120, 130, 150, 175, 200, 225 Table 7: Current Transformers, 55°C Ambient CT Ratio (MR= Multi-Ratio) 50:5 75:5 100:5 150:5 200:5 250:5 300:5 400:5 500:5 600:5 800:5 1000:5 1200:5 1500:5 2000:5 2500:5 3000:5 4000:5 600:5 MR 1200:5 MR 2000:5 MR 3000:5 MR 50:5 Zero Seq 100:5 Zero Seq Burdens at 120 Volts Burdens at 69.3 Volts Thermal W, X, Y Z M ZZ W, X Y M Z Rating 55°C Conn. 0.3 1.2 0.3 LL LG LG➂ 0.3 0.3 0.3 0.6 0.3 0.3 0.3 1.2 LL LG LG➂ 0.3 0.3 0.3 1.2 0.3 0.3 0.3 1.2 LL LG LG➂ 175, 300 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 LL LG LG➂ Metering Accuracy Classification Relaying Accuracy Classification at 60 Hz Standard Burden B 0.1 1.2 1.2 1.2 0.6 0.6 0.6 0.6 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 at 60 Hz Standard Burden B 0.5 2.4 2.4 2.4 2.4 2.4 2.4 1.2 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 ➂ For solidly grounded system only. ➃ Not listed in C37.20.2. at 60 Hz Standard Burden B 1.8 2.4 2.4 2.4 2.4 1.2 0.3 0.3 0.3 0.3 0.3 0.3 0.3 2.4 0.3 0.3 0.3 Minimum Accuracy Required per IEEE C37.20.2 C10 C10 C10 C20 C20 C20 C20 C50 ➃ C50 C100 ➃ C100 C100 C100 ➃ C100 C100 ➃➃➃➃ Standard Accuracy Supplied in VCP-W Switchgear C10 C20 C20 C20 C20 C20 C50 C50 C50 C100 C100 C100 C200 C200 C200 C200 C200 C200 C100 C200 C200 C200 C20 C20 Voltamp 700 400 700 1000 550 1000 1000 550 1000 1000 550 1000 Optional High Accuracy Available in VCP-W Switchgear — — — C50 C50 C50 C100 C100 C100 C200 C200 C200 C400 C400 C400 C400 C400 C400 C200 C400 C400 C400
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Cutler-Hammer - Eaton Canada

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