Fundamentals of Generator Protection

Generator Protection Relay | Source: SEL

The core of an electrical power system is the generator. There are power units based on steam, gas, naphtha, water power, diesel engine drive, and windmills. The range of size extends from a few hundred KVA (or even less) for engine-driven and hydro sets up to turbine driven sets exceeding 500MVA in rating. 

Generating units are the source of the power system and their security against any adverse conditions is most important in the system. The generator protection must ensure a fast and selective detection of any fault in order to minimize their dangerous effects.

Protection of passive elements like transmission lines and transformers is relatively simple which involves isolation of faulty element from the system, whereas protection of generators involves tripping of generator field breaker, generator breaker, and turbine.

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Generator Protections are broadly classified into three types:

CLASS A: This covers all electrical protections for faults within the generating unit in which generator field breaker, generator breaker and turbine should be tripped. 

CLASS B: This covers all mechanical protections of the turbine in which turbine will be tripped first and following this generator will trip on reverse power/low forward power protections. 

CLASS C: This covers electrical protection for faults in the system in which the generator will be unloaded by tripping of generator breaker only. The unit will come to house load operation and the UAT will be in service. Various protections of this class are: 

• 220 kV (HV side of Generator Transformer) busbar protection. 
• Generator Transformer HV side breaker pole discrepancy. 
• Generator negative phase sequence protection 
• Generator Transformer over current / Earth fault protection 
• Reverse power protection without turbine trip.

Generator Differential Protection (87 G):

It is unit-type protection, covering the stator winding for phase-to-phase faults due to the breakdown of insulation between stator phase windings. This relay is not sensitive to single- line to earth faults as the earth fault current is limited due to the high neutral earthing resistance. 

If CTs of identical ratios are used on the neutral and line side of the generator, an operating current setting of 20% can be adopted. It is instantaneous in operation and it trips the generator breaker (Class – A) to eliminate the system in – feed to the fault along with field breaker and turbines. 

For all machines of ratings 10 MVA and above, this protection shall be provided.

Generator Differential Protection

Generator – Transformer Differential Protection (87T):

This is similar to Generator Differential Protection, which covers from the generator terminals up to the HV breaker of the generator transformer. . Sometimes this relay is not provided where Generator and Generator Transformer Overall Differential relay (87O) is provided. 

87G & 87T functions should have the features of through fault restraint, magnetizing inrush restraint.

Generator Rotor | Source: Mitsubishi Heavy Industries

Rotor earth fault protection:

This protection shall be provided for machines of all sizes. This protection shall be connected for alarm and the operator may take the machine at the earliest opportunity after the first earth fault has occurred. This protection will have a sensitive voltage function operating on a bridge measurement basis with auxiliary equipment. It will have two levels, one for alarm and one for the trip. 

The settings adopted in general are: 

• For alarm: 25 KJ Ohm, 1.0 Sec. 
• For trip: 5 K Ohm, 0.5 Sec. 

Rotor Earth Fault Relay

Generator & Generator Transformer Overall Differential Protection (87O)

Besides generator differential and generator transformer differential, an overall differential relay can be provided between generator neutral side CTs and generator transformer Hv side CTs (and HV side CTs of UAT if provided) covering both generator and generator transformer. 

The principle of operation of the above relay is similar to any differential relay and it is also termed a unit differential relay. 

Overall Differential Relaying

Backup impedance Protection (21G):

This operates for phase faults in the unit, in the HV yard, or in the adjacent transmission lines, with a suitable time delay. It operates as a backup when the corresponding main protection fails. 

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Negative phase sequence protection (46 G):

It safeguards the generator rotor against overheating caused by the induced double frequency (100 Hz) currents when negative phase sequence currents are present in the stator. The negative phase sequence current(I2) can appear due to unbalanced single-phase loads or transmission line unsymmetrical faults. 

It should be set according to the Negative Phase Sequence capability of the generator. 

• I2**2 xt = 30 for Thermal Units 
• I2**2 xt = 40 for Hydro Units 

Gas Turbine | Source: General Electric

The alarm stage can be set at 50% of continuous withstand capability of the machine with a time delay of 3 to 5 Sec.

Negative Phase Sequence

Voltage restrained overcurrent protection (51 / 27 G):

This will operate when the fault current from the generator terminals becomes low due to excitation system characteristic with under-voltage criteria. It operates as backup protection for system faults with suitable time delays. 

Generator overloads protection (51G):

It is used as an additional check of the stator winding temperature high protection. The relay can be connected 

• For alarm with a setting of 110%. 
• For a trip with a setting of 125% with due time delay.

Generator Stator Earth Fault Protection (64G):

The high neutral earthing resistance arrangement limits the generator earth fault current, minimizing the damage to core laminations. Although a single-phase earth fault is not critical, it requires clearance within a short time due to: 

• It may develop into a phase to phase fault 
• If a second earth fault occurs the current is not longer limited by the earthing resistor. 
• Fire may result from earth fault arc

95% Stator earth fault protection (64G1):

It is an overvoltage relay monitoring the voltage developed across the secondary of the neutral grounding transformer in case of ground faults. It covers the generator, LV winding of generator transformer, and HV winding of UAT. A pickup voltage setting of 5% is adopted with a time delay setting of about 1.0 Sec. For all machines of ratings, 10 MVA and above this shall be provided.

Stator Earth Fault Relay

100% stator earth fault protection (64G2):

This is a 3rd harmonic U/V relay. It protects 100% of the stator winding. During the machine running condition, there will be a certain third harmonic voltage at the neutral side of the generator. This 3rd harmonic voltage will come down when a stator earth fault occurs causing this relay to operate. This shall have voltage check or current check unit, to prevent faulty operation of the relay at generator stands still or during the machine running down period.

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Loss of Excitation (40G):

In case of loss of excitation, the generator goes out of synchronism and starts running asynchronously at a speed higher than the system, absorbing reactive power from the system. Under these conditions, the stator end regions and part of the rotor get over heated. 

This protection shall have: 

• Mho characteristic lying in 3rd and 4th quadrants of impedance diagram with adjustable reach and offset. 
• An under-voltage and/or overcurrent relay as an additional check. 
• A timer with an adjustable range of 1-10 Seconds

Recommended Settings:

• Diameter of Mho circle =Xd 
• Off set of Mho circuit from the origin = xd1 /2
• Time delay = 1 Sec. 
• Under voltage relay = 110 – 115% of generator rated current 

Low Forward Power Relay (37G):

In thermal machines, when the steam flow through the turbine is interrupted by closing the ESVs or the governor valves, the remaining steam in the turbine generates (low) power and the machine enters to motoring conditions drawing power from the system. This protection detects low forward power conditions of the generator and trips generator breaker after a time delay, avoiding motoring of generator. The low forward power relay will be provided with a ‘turbine trip’ interlock in thermal machines. A setting of 0.5% of rated active power of generator with a time delay of 2.0 Sec. shall be adopted. 

Reverse Power relay (32G):

Reverse power protection shall be used for all types of generators. When the input to the turbine is interrupted the machine enters into motoring condition drawing power from the system. Reverse power relay protects the generators from motoring conditions. In thermal machines, reverse power condition appears subsequent to low forward power condition.

For reverse power relay, a setting of 0.5% of rated active power of generator with 2 stage timer as given below. 

• Stage – I: - With turbine trip interlock, a time delay of 2 Sec. shall be adopted. 
• Stage – II:- Without ‘ turbine trip’ interlock, a time delay of about 20 Sec. can be adopted to avoid unnecessary tripping of the unit during system disturbance causing a sudden rise in frequency or power swing conditions. 

Pole Slip Relay (98 G):

The pole slipping relay is designed to protect synchronous generators against the possibility of the machine running an unstable region of the ‘power angle curve’ which would result in power oscillations and pole slip. Pole slipping of generators with respect to the system leading to an increase in rotor angular position beyond the generator transient stability limits. 

Some of the causes for pole slipping are as follows.

• Large network disturbance 
• Faults on the network close to the generator. 
• Loss of generator field. 
• Operating the generator in an excessive under excited mode. 
• Loss of evacuation.

Generator Under Frequency Protection (81 G): 

The Under Frequency Protection: 

• Prevents the steam turbine and generator from exceeding the permissible operating time at reduced frequencies.
• Ensures that the generating unit is separated from the network at a preset value of frequency. - Prevent over fluxing (v/f) of the generator (large over fluxing for short times). 
• The stator under frequency relay measures the frequency of the stator terminal voltage.

Setting Recommendations:

• For Alarm: 48.0 Hz, 2.0 Sec. time delay. 
• For Trip: 47.5 Hz, 1.0 Sec. (or) As recommended by Generator Manufacturers. 

Generator Overvoltage Protection (59 G): 

An overvoltage on the terminals of the generator can damage the insulator of the generator, bus ducting, breakers, generator transformer, and auxiliary equipment. Hence overvoltage protection should be provided for machines of all sizes. 

Settings recommendations:

• Stage-I : Over voltage pickup = 1.15 x Un Time delay = 10 Sec.
• State-II : Over voltage pickup = 1.3 x Un Time delay = 0.5 Sec.

Standby Earth Fault Protection (51 NGT): 

This relay monitors the current in the generator transformer neutral. It can detect earth faults in the Transformer HV side or in the adjacent network.

Setting recommendations:

As this relay pickup for faults in the system, it has to be time graded with the transmission lines emanating from that generating station. Normally IDMT relay is provided 

• Operating Current Setting = 20% In 
• Operating Time = 1.5 to 2.0 Sec. (or) Greater than (max.) Zone-3 time of adjacent Transmission Lines. 

Suggested Protective Schemes for Generating Units

Reference: 

Title: Handbook for Protection Engineers

Author: Mohammed Fasil

Document: PDF | pages 21-34 | Download