Alternator Protection for Emergency Standby Engine Generators

1. Alternator Protection for Emergency Standby Engine Generators Kenneth L. Box P.E. Regional Sales Manager – Power Electronics Cummins Power Generation

2. Engine Generators • Control • Monitoring & Alarms • Engine Protection • System Protection – Paralleling Applications • Alternator Protection

3. IEEE/ANSI Standards 141 & 242 • Recommended Practice for Protection & Coordination of Industrial & Commercial Power Systems • Recommended Practice for Electric Power Distribution for Industrial Plants

4. Protects against a severe overload condition (27) Initiates the starting of an emergency standby genset (27) Load shed shut down in the event of AVR failure (27) Protect against dangerous over-voltages (59) Backup to internal V/Hz limiters Commonly combined 27/59 Devices 27 59 Under & Over Voltage Protection

5. Provides backup protection for the prime mover. It detects reverse power flow (kW) should the prime mover lose it’s input energy without tripping its generator feeder breaker Prevents motoring, drawing real power from the system Device 32 Reverse Power Protection

6. Senses when the generator’s excitation system has been lost. Important for paralleling generator applications or when paralleling with the utility. When generator loses excitation it will steal excitation from other gensets & quickly overheat the rotor due to induced slip-frequency currents Reverse VAR protection Device 40 Loss of Field Protection

7. Unbalanced loads Unbalanced system faults Open conductors Unbalanced I2currents induce 2X system frequency currents in the rotor causing overheating Device 46 Phase Balance Current Protection

8. The function of generator backup protection is to disconnect the generator if a system has not been cleared by the primary protective device Time delays Device 51V Backup Overcurrent Protection

9. Provides backup protection for all ground relays in the system at the generator voltage level Provides protection against internal generator ground faults Commonly provided as GF alarm. Device 51G Ground Overcurrent Protection

10. Monitors the availability of PT voltage. Blocks improper operation of protective relays and control devices in the event of a blown PT fuse Device 60 Voltage Balance Relay

11. Differential Protection • For rapid detection of generator Φ to Φ or Φ-G faults. • When NGR’s are used, 87G should be used. • Used for protection of larger generators • Zone protection Device 87

12. Resistance temperature detectors are used to sense winding temperatures. A long term monitoring philosophy that is not readily detected by other protective devices RTD’s Temperature Protection

13. SMALL MACHINES Up to 1000kVA, 600V maximum MEDIUM MACHINES 1000kW to 12,500 kVA regardless of voltage LARGE MACHINES Up to 50,000 kVA regardless of voltage Any recommendation based entirely on machine size is not entirely adequate. The importance of the machine to the system or process it serves & the reliability required are the important factors IEEE Recommended Protection Schemes

14. Small Generators – 1000kVA • Device 51V – Backup overcurrent • Device 51G - GFP • Device 32 – Reverse Power • Device 40 – Loss of Field • Device 87 - Differential

15. Medium Size Generators – 1 to 12.5 mVA • Device 51V – Backup overcurrent • Device 51G - GFP • Device 32 – Reverse Power • Device 40 – Loss of Field • Device 87 - Differential • Device 46 – Negative phase sequence for paralleling or utility paralleling

16. GENSET KW KWH PF 40 32 GOV AVR 51V AM SW SURGE SUPPRESSORS VM HZ 27 81 59 SW 86 SS UL listed utility grade generator protection relay VM HZ 46 25C 25 SW SWITCHGEAR TRIP CLOSE My Opinion – 3mW and less TRIP

17. NFPA70 - NEC • 445.12(A)Overload Protection • Generators, except AC generator exciters, shall be protected from overloads by inherent design, circuit breakers, fuses, or other acceptable overcurrent protective means suitable for the conditions of use. • 240.15(A) • Overcurrent Device Required. A fuse or an overcurrent trip unit of a circuit breaker shall be connected in series with each ungrounded conductor. A combination of a current transformer and overcurrent relay shall be considered equivalent to an overcurrent trip unit. • 240.21(G) Conductors from Generator Terminals • Conductors from generator terminals that meet the size requirements of 445.13 shall be permitted to be protected against overload by the generator overload devices) required by 445.12

18. Is the Alternator Protected? • Generator is required to be protected • Generator conductors are assumed protected by same device protecting the genset. • Most common protection is molded case breaker with thermal/magnetic trip • 100% rated thermal magnetic breakers don’t fully protect alternator • Generator Protective Relay provides the best protection & superior coordination for downstream devices

19. Time Current 100% Rated Electronic Trip Breaker is an Improvement

20. Some Generator Mfrs offer self contained alternator protection • Is it UL listed as a generator protection relay? • Does it provide O/L protection for the alternator and O/L and short circuit protection for the feeder? • Can it protect its transfer switch on the emergency side?

21. Differential Protection (87) • Rarely selected for LV machines smaller than 1.5 mW. • How do you mount the CT’s? • Cost vs. benefit?

22. Differential Protection (87) • The value of differential protection is that it is very fast in detecting faults in a circuit. • High current levels that pass through both sets of CT’s will not cause a trip on common events like motor starting, or even on downstream faults that are intended to be cleared by other means. • The high speed of operation for faults sensed within the operating zone makes it possible limit damage inside an alternator stator when a fault inside the machine occurs. • The device would also operate on a feeder fault, but in general, once a fault is sensed in a feeder, the feeder will be replaced,

23. Differential Protection (87) • A key point to remember is that differential relays don’t prevent damage, they LIMIT damage. • If a relay is properly operating it won’t trip until there is actually a line to ground fault somewhere in its zone of protection. • By limiting the duration of a fault, it is often possible to limit damage, but there is STILL damage. • Eventually, you will have to deal with it. • Some mfrs. have high speed internal single phase protection

24. Differential Protection (87) • The protective devices selected for a specific application should always be selected based on an understanding of the balance between reliability and protection. • The more protection used in the system the lower the reliability, because of the higher probability of failing the system due to a nuisance trip.

25. Recommendations • Use the IEEE Recommended protection schemes with a dose of common sense. • Always carefully consider the balance of protection versus reliability, especially when the protection is for equipment that is operating for very few hours. • With some mfrs. the alternator current sensing function monitors faults inside the machine. When the machine incorporates protection for the alternator from overcurrent conditions based on an I2t function, and regulates single phase faults differential protection is optional. • On 15kV class machines, the alternator stator is expensive enough that it would probably be repaired rather than replaced, so it will make more sense to try to limit damage in the machine and have it repaired, in the general case. • In cases where it is decided to use differential protection, it is desirable to minimize the zone of protection and use properly sized and matched CT’s so that the probability of nuisance tripping is reduced. Since the generator set provides overcurrent protection from the alternator “out”, differential protection can be applied with matched CT’s provided and mounted at the wye side and alternator output, preferably in the terminal cabinet. The differential relay can be mounted in the vicinity of the generator set or in the switchgear.

26. Recommendations • A good standardized design is superior to an optimized custom design. • Custom designs breed custom problems

27. Questions?

28. 3Φ Fault – Current Regulation Peak Current: IR/X”d Regulates at 3X Rated Shuts down before damage

29. 1Φ Fault – Current Regulation NOTE: THIS CURVE SHOWS FAULT CLEARED BEFORE SHUTDOWN.

30. Single Phase Fault