Friday, March 06, 2015

What are the Common Generator Problems and It's Protection



Introduction

Generator protection is very important in power plant operation. The protection of generators involves the consideration of more possible abnormal operating conditions than the protection of any other system element. In unattended power stations, automatic protection against all harmful abnormal conditions should be provided.

Problem

Over Excitation
When the ratio of the voltage to frequency (volts/Hz) exceeds a set value for a given generator, severe overheating could occur due to saturation of the magnetic core of the generator and the subsequent inducement of stray flux in components not designed to carry flux.Such over-excitation most often occurs during start-up or shutdown while the unit is operating at reduced frequency, or during a complete load rejection, which leaves transmission lines connected to the generating station.

A volts/Hz relay, with an inverse time characteristic that matches the capabilities of the protected equipment and with definite time setpoints, is used to protect the generator from over excitation.

Loss-Of-Synchronism Protection
When two areas of power systems, or two interconnecting systems, lose synchronism, there will be large variations in voltages and currents throughout the systems. The voltages will be maximum and the currents minimum, when the systems are in phase. The voltages will be minimum and the currents maximum, when the systems are 180 degrees, out of phase.


Negative Phase Sequence or Unbalanced Currents
Unbalanced faults and other system conditions can cause unbalanced three phase currents in the generator. The negative sequence components of these currents cause double frequency currents in the rotor that can lead to overheating and damage.

Over Voltage

Generator over voltage may occur during a load rejection or excitation control failure. In the case of hydroelectric or gas turbine driven generators, upon load rejection, the generator may speed up and the voltage can reach high levels without necessarily exceeding the generator’s V/Hz limit.

The voltage regulating equipment often provides this protection. If it is not, it should be provided by an AC overvoltage relay. This relay should have a time delay unit with pickup at about 110% of the rated voltage. It should also have an instantaneous unit with pickup at about 130% to 150% of the rated voltage. It is not generally required with steam turbine driven generators.

Under Voltage

An under voltage condition is a decrease in the rms AC voltage, to less than 90% at the power frequency for a duration, longer than 1 minute. The term "brownout" is often used to describe sustained periods of under voltage initiated by the utility to reduce power demand. Under voltages result from events which are the reverse of those causing over voltages.


Reversal of Power

For generators operating with another generator, it is imperative that the power direction be supervised. If the prime mover fails, the alternator operates as a motor and drives the prime mover. A relay detects the reversal of power direction and switches off the alternator. Power losses and damage to the prime mover are avoided.

Dead Generator Energization Protection
If a dead generator is accidentally energized, while on turning gear, it will start and behave as an induction motor. During the time when the generator is accelerating, very high currents are induced in the rotor and it may be damaged very quickly.


Over Frequency
Faults in the system can result in a system breakup into islands, which leaves an imbalance between available generation and the load. This results in an excess of power for the connected loads. Excess power results in an over frequency condition with a possible overvoltage from reduced load demands.

Full or partial load rejection can lead to overspeed of the generator, therefore, over frequency operation. In general, over frequency operation does not pose any serious overheating problem unless the rated power and about 105% voltage are exceeded. Control action can be taken to reduce the generator speed and frequency to normal, without tripping the generator.

Under Frequency
When insufficient power is being generated for the connected load, under frequency results with a heavy load demand. The drop in voltage causes the voltage regulator to increase the excitation, which results in overheating in both the stator and rotor. At the same time, more power is being demanded with the generator less able to supply it at the reduced frequency.

Prolonged operation of a generator, at reduced frequencies, can cause particular problems for gas or steam turbine generators, which are susceptible to damage from operations outside of their normal frequency band. The turbine is more restrictive than the generator, at reduced frequencies, because of possible mechanical resonance in many stages of the turbine. If the generator speed is close to the natural frequency of any of the blades, there will be an increase in vibration, which can lead to cracking of the blade structure.

While load shedding is the primary protection against generator overloading, under frequency relays should be provided to provide additional protection.

Stator Ground Fault
Although a single field-ground fault will not affect the operation of a generator or produce any immediate damaging effects, the first ground fault establishes a ground reference, thereby making a second ground fault more likely. This will increase the stress to ground at other points in the field. A second ground fault will cause extensive damage by:
  • Shorting out parts of the field winding
  • Causing high unit vibrations
  • Causing rotor heating from unbalanced currents
  • Arc damage at the points of the fault

Protection

Ground Fault Protection

One of the main causes of ground fault is insulation failure. The zero sequence impedance of a generator is usually lower than the positive or negative sequence impedance, therefore, for a solidly grounded generator, the single phase to ground fault current is greater than the three-phase fault current. Generators are usually grounded through an impedance, to limit the ground fault current.

The fault current available for sensing a phase to ground fault, on an impedance grounded generator, can be very small compared to phase-to-phase faults. Depending on the location of the fault and the method of grounding the generator, separate ground fault protection is usually provided.

Stator Overheating Protection

This problem is caused by overloading or by failure of the cooling system. Overheating because of short-circuited laminations is very localized and it is just a matter of chance whether it can be detected before serious damage is done.

The practice is to embed resistance temperature-detector coils (RTDs), or thermocouples in the slots with the stator windings of generators larger than 500 to 1000 kVA. Fig. 9 shows the bridge circuits employed with RTDs. Enough of these detectors are located at different places in the windings so that an indication can be obtained of the temperature conditions throughout the stator.

Several of the detectors that give the highest temperature indication are selected for use with a temperature indicator or recorder, usually having alarm contacts. The detector giving the highest indication may be arranged to operate a temperature relay to sound a alarm.


Overspeed

Overspeed protection is recommended for all prime mover driven generators. The overspeed element should be responsive to machine speed by mechanical, or equivalent electrical connection. If it is electrical, the overspeed element should not be adversely affected by generator voltage.

The overspeed element may be furnished as part of the prime mover, or its speed governor, or of the generator. It should operate the speed governor, or whatever other shutdown means is provided to shutdown the prime mover. It should also trip the generator circuit breaker. This is to prevent over frequency operation of the generator itself from the AC system.

The overspeed element should be adjusted to operate about 3% to 5% above the full load rejection speed.

Phase Fault Protection

Phase faults, in a generator stator winding, can cause thermal damage to insulation, windings, and the core, and mechanical shock to shafts and couplings. Trapped flux within the machine can cause fault current to flow for many seconds after the generator is tripped and the field is disconnected.

Primary protection, for generator phase-phase faults, is best provided by a differential relay. Differential relays will detect phase-phase faults, three phase faults, and double phase-to-ground faults. With low-impedance grounding of the generator, some single phase to ground faults can also be detected.

Automatic Bus Switchover

A type of an automatic bus switchover unit, shown in Fig. 10, operates in the following manner.

Normal Utility Power Mode
Under normal circumstances, when utility power is available, the utility power runs through the transfer switch control contactors, the power is connected to the distribution panel and then to the electrical loads. A battery charger installed, in the transfer switch control, is powered by the utility to keep the starting battery, in the generator set, charged.

Power Outage Occurs
When the utility power voltage fails to less than 85% of its normal value, or it fails entirely, the standby power system will automatically go through a start sequence. The transfer switch control circuitry constantly monitors the power quality from both the utility source and the generator set. When the transfer switch control circuitry senses the unacceptable utility power, the control waits for 3 seconds and then sends a signal to start the generator set engine. If the utility power returns before the 3 seconds has passed, the generator set will not be signaled to start.

When the start signal is received and providing the manual/auto switch is set to auto, the engine starts, reaches the proper operating speed and AC power is available, at the generator set. The transfer switch control circuitry senses this, waits for the 3 seconds and will then transfer the generator set power to the transfer switch contactors. The sequence of operation usually occurs in less than 10 seconds from the time the power outage occurred to the time when generator set power is connected.

The transfer switch includes a manually operated handle. If the transfer circuitry does not cause the automatic transfer to generated power, the manual/auto switch can be moved to the manual position and the handle then used to transfer from utility power to emergency power, or visa-versa.

Utility Power Returns
When the utility power comes back on, the transfer switch control circuitry senses this and will watch for acceptable voltage levels, for a period of 5 minutes. After this
5-minute period and the voltage levels have been stable, the control will signal the transfer switch contactors to re-transfer the load back to utility power source and then disconnect the generator set source. At this point, the generator set is “off-line” and will be operated automatically another 5 minutes, to allow it to properly cool down. After this cool down cycle, the generator set will be automatically shutdown and reset to standby mode.


Grounding System

A ground is defined as a reference point of zero voltage potential, which is usually an actual connection to the ground of the earth. The need for grounding is very important in that an open ground condition could present severe safety problems to anyone operating the power generation equipment. Grounding assures that any person who touches any of the metal parts will not receive a high voltage electrical shock. The conductor, which is used for this purpose, is either a bare wire or a green insulated wire.


Lightning Arrestors

Lightning arrestors are used to cause the conduction to ground of excessively high voltages that are caused by lightning strikes or other system problems. Power lines and associated equipment could become inoperable when struck by lightning. They are designed to operate rapidly and repeatedly, if necessary. Lightning arrestors are connected to transformers or the insides of switchgear.

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