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
Mention NFPA-110 alarms and protection – mostly pertain to the engine, oil pressure, temperature, etc.

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
Explain you will use the terms alternator and generator interchangeably

4. Under & Over Voltage Protection
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
The AVR will normally maintain the voltage within specified limits. Therefore a sustained under voltage could indicate a severe overload condition or the loss of a generator in a paralleled system. Inverse time relationship, with built in – adjustable time delays.
A typical UV default setting is 85% of nominal voltage for 10 seconds.
A typical OV default setting is 110% of nominal voltage for 10 seconds.

5. Reverse Power Protection
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
The magnitude of motoring power varies depending upon the type of prime mover used. For instance, a diesel engine typically has a max motoring power value of 25%

6. Loss of Field Protection
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

7. Phase Balance Current Protection
Unbalanced loads
Unbalanced system faults
Open conductors
Unbalanced I2 currents induce 2X system frequency currents in the rotor causing overheating
Device 46
NEGATIVE SEQUENCE CURRENTS
Also, Used to verify the generator set phase sequence matches the bus phase sequence prior to allowing the paralleling breaker to close. Genset will shut down in the event of a is-match.

8. Backup Overcurrent Protection
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
Overload and short circuit protection are often provided by a molded case circuit breaker for small LV generators and a multi-function generator protection relay such as a Schweitzer 300G or a GE Multilin 489.

9. Ground Overcurrent Protection
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
Again this functionality is often proved by a MCCB for LV applications where the NEC requires GFP.

10. Device 60 Voltage Balance Relay
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
Typically a MV application

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. Temperature Protection
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
Typically not used in LV machines or at least not small kVA sizes below 2000kVA. More common in MV alternators.

13. IEEE Recommended Protection Schemes
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
I will not discuss large generators or medium sized machine larger than 3 MW

14. Small Generators – 1000kVA Device 51V – Backup overcurrent
Device 51G - GFP
Device 32 – Reverse Power
Device 40 – Loss of Field
Device 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 Differential
Device 46 – Negative phase sequence for paralleling or utility paralleling
I have no experience with machines larger than 3 mW, so my comments will be limited to 3MW and smaller machines.

16. My Opinion – 3mW and less GENSET AM SW VM KW KWH PF 40 32 GOV AVR 51VHZ 27 81 59
SURGE SUPPRESSORS 46
25C 25 86 SS
UL listed utility grade generator protection relay
SWITCHGEAR
TRIP
CLOSE
My Opinion – 3mW and less
Recommend your synchronizer be both voltage and frequency matching and not just frequency matching.
The IEEE does not mention under frequency protection (81) in their list! When the output frequency cannot be maintained. Time delay adjustable (0-20 seconds) and (0-10 Hz) below the nominal governor setting and similar settings for over frequency.
A few more subtle, but important protective features:
BREAKER FAIL TO OPEN WARNING – breaker aux contact can fail, so the control monitors the status of the breaker.
BUS & GENERATOR SET INPUT PT CALIBRATION ERROR – sort of a device 60 on steroids. Monitors and compares bus PT voltage against the genset PT voltage and warns if there are different values when the paralleling breaker is closed.
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 shall be permitted to be protected against overload by the generator overload devices) required by
Let’s talk about the NEC. Notice it doesn’t say much about over current protection in general. A Device 51 pretty much covers it. There is a wide gap between the NEC and the IEEE recommendations.

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
Major Points:
Describe time overcurrent curve and how to interpret what it is saying (how much current for how long before trip)
Point out thermal damage curve, noting that in this case can carry 10 times rated current for 1 second, 3 times for 10 seconds, but 1 times line never crosses the red line
Fully rated conductor is protected by alternator protection
Thermal damage curve doesn’t indicate point of alternator failure—it indicates when its life is significantly shortened
Don’t want survive a fault today, and have a sudden failure under normal conditions tomorrow
In general, molded case breakers with thermal-magnetic trip will not protect an alternator
This is where it gets a little technical, but is the guts of what the AHJ needs to know to evaluate that the genset is protected. Generally the AHJ depends on the CSE to come up with the proper protection, but the AHJ needs to verify that the design is suitable.
On the x axis is current (rated at 1.0 for this picture). It is a log-log curve. The y axis is time (also a log curve).
What does thermal damage mean for a generator? It is NOT when it has failed completely and/or is smoking. It IS when its life has been shortened unacceptably. The reason for this is the day after the generator has withstood a short circuit condition, we don’t want it to fail unexpectedly under NORMAL conditions.
How do we know if the generator has been damaged (represented by the redline)? Most generator have class H insulation and is allowed to reach an internal temp of 300 degree C. At higher temps than this the insulation is considered to have been damaged, because it’s life will be very short if it has not failed completely. Unfortunately, you don’t know if the generator has reached 300 degrees C. This is the trouble with whole thing. A good example is what happens at your home when a cb trips. What do you do? Most likely you just reset the cb. The reality is that the cb has been damaged and should be replaced, because it could fail in the future, but most people don’t do this. Look at the chart to see that with 3x rated current it will take about 10 secs for the generator to be damaged. At 10x rated current, the generator will be damaged in about 1 sec.
UL 2200 does NOT currently address thermal damage, if the question is asked, so it does NOT take care of this problem.
So this (red line) is the thermal damage curve of the generator. You can see at 750 A you have never crossed the line, but blue line (cb trip curve) both is to the left (nuisance trip) and to the right (no protection.)
The main thing is to recognize that the most common protection is a molded case breaker with a thermal magnetic trip. To verify protection you need to overlay the protection trip curve on the generator damage curve. If the protection trips sooner than damage occurs at all current levels, you know you are protected. The bad news is that about 90% of generators are protected with a molded case cb with thermal-magnetic trip, which means they do NOT meet the requirements for protection. We will hit the red line before you hit the blue curve under nearly all overload conditions. You can select a cb that has an electronic trip that will work, but you would need to do an analysis to make sure the generator is protected:
The power conductor feeders are sized to the cb and not the generator. So in the case of many circuit breakers, there is NOTHING protecting the generator.
From our prospective, we have been building microprocessors since 1994 that measures the current out of the generator. So for the past ten years we have built the protection into the generator already so we didn’t have to worry about what protection was on the generator and whether a fully rated feeder was properly protected.
Now, with this information in hand, the AHJ can tell that he needs an alternator thermal damage curve for the generator, and a protective device time/overcurrent curve to verify if the alternator is protected. Thermal damage curve must be to the right of the trip curve.

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

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
Cummins MV alternators are provided with current sensing CT’s mounted on the wye side of the alternator. This means that the CT’s will sense any fault on the alternator, including faults inside the machine.

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.
FIGURE 1: Differential Zone encompasses the alternator through the paralleling breaker
Differential (87) relaying is recommended for use by the IEEE Red Book (ANSI/IEEE Standard Chapter 6, Section ) When properly installed and adjusted, it will provide a high degree of sensitivity to faults in the portion of a distribution circuit monitored by the device. The implementation in generator set applications is simple: Three current transformers (CT’s) are placed in the individual phases at the “wye” side of the alternator winding, with 3 additional CT’s placed at the load side of the paralleling breaker. This provides a zone of protection covering the alternator, feeder wiring, and paralleling breaker. The two sets of CT’s are monitored for current flow, and when the current flow at the alternator does not match the current flow at the breaker, it is assumed that a fault in the circuit is causing the imbalance between the two sets of CT’s, and shutdown of the generator set can be initiated. This design is has value because it monitors not only the conductors in the circuit, but also the winding of the alternator itself.

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
For a 3-phase fault condition, the alternator exciter must provide considerable excitation power to the field of the machine to support the high current level on all three phases. Many generator sets are provided with “series boost” or “permanent magnet generator” (PMG) excitation support systems to provide the additional field current to drive these high current levels. Without these devices, alternator output current on a 3-phase bolted fault would rapidly decay to near zero current levels. With series boost or PMG systems alternators are generally designed to provide 3 times rated current for approximately 10 seconds on a 3-phase bolted fault, so that there is sufficient time for downstream devices to clear before the generator set must be shut down to prevent damage to the alternator. The figure below shows the output current on flow on an alternator with an excitation support system, when subjected to a 3-phase fault.

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

30. Single Phase Fault
On a single-phase fault condition the performance of the machine is considerably different. Generally it takes less excitation power to operate the generator set with a single-phase fault than it does to operate it at full load. This means that when a single phase fault occurs, the output current on the faulted phase does not decay as rapidly, and can often be maintained at 6-8 times rated current. This obviously can damage the alternator much more quickly than a fault that results in decaying current levels. The slide shows a comparison of the time required to reach damaging temperature levels in an alternator with various fault conditions.
An often-ignored phenomenon is what will happen during a fault condition on an unfaulted phase. Several different conditions could exist. If a machine has single phase voltage sensing and a shunt type excitation system and the fault occurs on the sensed phase, the machine with voltage and current on all phases will collapse when the fault occurs. There may or may not be sufficient current flow to allow downstream devices isolate the faulted portion of the system.