Generator Differential Protection

SUDHIR KUMAR SRIVASTAV

Additional General Manager-RAPDRP

NTPC LIMITED, NEW DELHI

sudhirksri1@gmail.com

Generator Differential Protection

An electrical fault between phases of Generator winding causes heavy flow of fault current inside the generator, resulting highly extensive damage to machine.  The damage may be in coil, in core or in both. Any damage to the core of generator is most severe condition.

To avoid above damage, we uses Generator Differential Protection, which act on differential current between Neutral & Phase current of Generator, as shown in diagram. If a fault occurs inside the stator winding between CT1 & CT2, a distinct difference will be there between the current at the neutral end & phase terminal ends of the particular winding. This difference is detected by Differential relay (87G).

The current entering and leaving the protected object are determined by CT and compared by relays by means of a circuit as shown in diagram.

During normal condition any current flowing in CT1 will flow out from CT2 & there will not be any difference between CT1 & CT2 currents. Also there polarity will be same, so differential current through relay 87 G will be zero & relay will not operate.

A fault inside the protected zone is fed from either one side or both sides depending upon the current source present, thus producing a difference current in the differential circuit. If this differential current exceeds a set percentage (normally 10%) of the normal current flowing in the protected object, the relay pick up and initiate tripping of the generator, thus protecting generator from severe damage. Balancing resistances are used to avoid mal-operation of relay during transient conditions.

Test Procedures:

1.      Relay Test: Remove the relay from cabinet. Test its accuracy of operation through secondary current injection test at 25%, 50%, 75%, 100% & 125% of rated current.

2.      CT test: Following tests to be conducted on CT:

Ø  CT polarity test: To check about proper connection.

Ø  CT ratio test: Through primary injection method & secondary current to measure at relay terminal.

Ø  CT saturation test

Ø  CT secondary circuit insulation resistance test.

Ø  CT secondary one end earth connection check.

3.      Online protection test: Short any two phase of Generator bus duct between CT1 & CT2 as shown in drawing. Roll the turbine, slightly increase the excitation till fault set current & let the relay operate. This test to be done in strict supervision of system expert, with proper care & monitoring, to avoid any damage to machine.

Generator Protection

SUDHIR KUMAR SRIVASTAV

Additional General Manager-RAPDRP

NTPC LIMITED, NEW DELHI

sudhirksri1@gmail.com

Generator & Generator Protection

Generator:

Generator is a devise which converts mechanical energy into electrical energy.

•         Mechanical energy is drawn by turbine.

•         Electrical energy is sent out to user through transmission & distribution system.

•         Generated voltage = 4.44 * flux * Frequency * No. of turns

Generator Components:

Stator: It is stationary part of Generator. It’s winding is connected to Power transformer to step up the voltage for transmission of generated power at high voltage. For a constant power output when voltage is increased, current is reduced (I=P/V), resulting less transmission losses (heat loss=I²R).

Rotor: It is coupled with turbine & rotates at turbine speed.

Excitation system: It is connected to rotor winding and generates rotating magnetic field at the speed of turbine. When this rotating magnetic field cuts stator winding, an E.M.F. is generated, resulting in output of electrical energy at a particular voltage.

Generated Voltage:

·         Normally, Generated voltage in power plant is 15 KV to 25 KV, depending on the capacity.

·         Limitation on high voltage generation is flux density & insulation thickness.

·         Generator is directly connected to GT (To step up the voltage for transmission) and to UAT (To step down the voltage for unit auxiliary power consumption).

·         Output power from Generator is stepped up to 400KV or 220 KV by Generator Transformer for transmitting the power to distribution utilities.

• Advantages of high voltage transmission:

o   Flow of current is less, resulting reduction in cu loss (Heat loss).

o   Less current requires less diameter of current carrying conductor, resulting less weight. Also supports (pole/towers) required for conductor will cost less.

• Unit Auxiliary Transformer (UAT) is directly connected to output of Generator & stepped down the voltage to 6.6 KV or 11 KV for running of unit auxiliary equipments.
• This 6.6 KV system is again stepped down to 0.4 KV for running of plant services equipments / lighting.

Generator Protection:

Task of protective system:

•         Detects abnormal condition or defect.

•         Alarm the operating staff.

•         Unload and/or trip the machine.

Requirement of Protective device:

•         Selectivity                                         

•         Safety against fault tripping

•         Reliability

•         Sensitivity

•         Tripping Time

Type of Generator Protection:

1.    Differential Protection:

•         Generator Differential

•         UAT Differential

•         Overhead Line Differential

•         GT differential

•         Overall Differential

2.    Stator Earth Fault Protection:

1. Stator Earth Fault
2. Stator Stand by Earth Fault

3.    Rotor Earth Fault protection

4.    Stator inter turn fault protection

5.    Negative phase sequence protection

6.    Generator back-up impedance protection

7.    Loss of excitation protection

8.    Pole slipping

9.    Over voltage protection

10. Over fluxing protection

11. Low Forward Power Protection

12. Reverse Power Protection

13. Generator LBB protection

14. GT Protection:

                      i.        Buchholz Protection

                    ii.        PRV Protection

                   iii.        WTI / OTI

15. UAT Protection

16. Bus Bar Protection