Generation Protection Final

Generation Protection 2016 Operator Training Seminar

Billy Lee System Operations Administrator Spring 2016

Objectives • In terms of excitation, identify two abnormal operating conditions for generators, unlike other power system components. • Identify the difference between a generator sequential and simultaneous trip. • Identify the relay protection scheme that senses power flowing into a generator from the power system.

• Identify the type of protection relay used in detecting a phasephase stator winding fault. • Given a one – line drawing identify the reverse power relay, frequency relay and differential relay used in generator protection. (Schematic to accompany test question in LMS).

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NERC STANDARD PRC-001-1.1 (ii)

• R1. Each Transmission Operator, Balancing Authority and Generator Operator shall be familiar with the purpose and limitations of Protection System schemes applied in its area.

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August 14, 2003 Northeast Blackout

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Pre-existing conditions

• Critical voltage day • Wide area transfers • Planned Transmission and Generation outages

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East Lake 5 Exciter Failure Causes Trip

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How does this relate to Emergency operations?

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Objective 1

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Abnormal conditions Examples of abnormal conditions are: • over-excitation • over-voltage • loss-of-excitation (Lack of Reactive Support) • unbalanced currents • reverse power • abnormal frequency (Over-Speed)

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Bulk Electric System Emergency • Any abnormal system condition that requires automatic or immediate manual action to prevent or

limit the failure of transmission facilities or generation supply that could adversely affect the reliability of the Bulk Electric System.

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Power System Overview

 Reliably operate the system you have!

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Relay Device Numbers Listed below are relay device numbers typically found on a generator one-line diagram: • 21- Distance (Device) Relay- backup for system and generator zone phase faults. • 24- Volts/Hertz protection • 32- Directional (Reverse Power) relay or anti-motoring protection • 40- Loss of Excitation(Field relay) • 46- Negative Phase Sequence Current • 49- Stator Thermal Protection • 50- Instantaneous Overcurrent • 51G- Time over-current ground relay • 51TN- Backup for ground faults • 51V- Voltage controlled or voltage – restrained time over current relay

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59- Over- voltage protection 59GN- Stator ground fault protection 60- Voltage Balance relay 62B- Breaker Failure timer 64F- Field Ground Detector 78- Loss of Synchronism 81- Frequency Relay (under and over frequency) 86- Hand reset lockout auxiliary relay 87G- Differential Relay – primary phase fault protection 87N- Stator ground fault differential protection 87U- Differential relay for overall generator and transformer protection

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SEL-700G SEL-300G Numerous current, voltage, frequency, distance, power, and out-of-step elements provide comprehensive protection for large, medium, and small generators. Protection includes: Stator Ground fault, current differential, out-of-step, overexcitation, directional power, and event reporting.

https://www.selinc.com/SEL-300G/

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Functional Overview One-Line showing types of Relay devices used for generator protection.

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Objective 2

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Protective actions for generator shutdowns • • • • • • •

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Generator Trip Simultaneous Trip Sequential Trip Manual Turbine Trip Manual Runback and Trip Automatic Runback Manual Runback

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Generator Trip •

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This type of trip opens the generator line breakers and removes the excitation simultaneously but leaves the turbine running at near rated speed. While maintaining rated speed is sometimes not possible for the boiler to operate with full load rejection thus tripping the boiler. An advantage of this type of tripping is if the generator trip can be identified and rectified quickly then resynchronization can happen in a shorter period of time.

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Simultaneous Trip 

A simultaneous trip is acceptable for all generator faults, and generally provides the highest degree of protection for the generator although it does permit a small over-speed and a slight possibility of high over-speed.  This type of trip the turbine valves close, opens the generator line breakers and removes the excitation simultaneously.

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Sequential Trip •

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Sequential tripping, trips the turbine valves closed, then the reverse power relay operates in about 3 seconds and the generator line breakers are opened. When the generator breakers open the excitation is tripped (field breaker trip). This is the preferred tripping for most faults in the turbine or steam generator (boiler). Some schemes have a backup reverse power timing circuit for tripping on reverse power.

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What is the difference between a generator Simultaneous Trip and a Sequential Trip?

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Simultaneous trip trips turbine valves closed, opens the generator breakers and removes excitation simultaneously where sequential trips turbine first then reverse power relay trips generator breakers and excitation.

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Manual Turbine Trip • The Operator may manually trip the turbine, this should cause

Reverse Power relaying to trip the generator breakers and the excitation. • Note: There are no cases for which manually tripping the generator breakers is recommended! • This is because the generator breakers should not normally be tripped until the turbine has tripped and reverse power automatically trips the generator breakers with reverse power relaying.

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Additional Protective actions • Manual Runback and Trip • Manual Runback • Automatic Runback

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Reverse Current Protection • Motoring of a generator will occur when the mass steam flow of the prime mover (turbine) is reduced such that it develops less than no load losses while the generator is still on line. • It is important to note that motoring at synchronous speed will not harm the generator but may damage the turbine though over heating. • Although motoring the generator at less than synchronous speed will cause immediate damage due to induced rotor currents resulting in excessive heating. • Also stator inrush currents may break the stator armature end— winding ties.

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Broken turbine blade Here you see broken turbine blades – the root cause of the fire.

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Motoring Damage causing a hydrogen fire Hydrogen Leak –

Broken seals that allowed hydrogen to get to atmosphere. The shell to the right is the generator shell. The motor is the turning gear.

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Motoring Damage causing a hydrogen fire Here you see Ground Floor Cable Trays – Next to the 4160 switch gear and hydrogen seal oil system. Fortunately the seal oil system didn’t explode.

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Motoring Damage causing a hydrogen fire Here you see Mezzanine to Cable Spreader Room – On the mezzanine near the vital AC and emergency DC.

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Motoring Damage causing a hydrogen fire Here you see Emergency DC – On the mezzanine deck. The breaker handles melted.

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Motoring Damage Motoring without excitation. Here you see rotor tooth damage as a result of localized heating and arcing.

Photograph compliments of National Electric Coil, Columbus OH

Rotor Tooth Tip Damage Due to Motoring 30

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Objectives 3, 4 & 5

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Reverse Power Relay • A Reverse Power (32 Directional) relay is used for generator protection. It senses power flowing into the generator rather than power flowing out to the system. • A typical arrangement of a reverse power protection circuit employs both a Current Transformer (CT) and Voltage Transformer (VT) to power the relay and protect the generator. • Reverse Power relay should be connected to produce a simultaneous trip. • A Breaker Failure protection should be initiated because generator breaker failure may be what caused the reverse power flow condition.

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Reverse Power Relay A 32 device number represents a Directional Power Relay typically used as a reverse power relay protection.

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Source of Motoring Power

G

Reverse Power Relay

32-1

32-1

NONELEC TRIPS

49

64

32-2

86NE Turbine Trip

86NE

32-1

86E

86NE 86 NE

OTHER ELEC TRIPS

A 32 device number represents a Directional Power Relay typically used as a reverse power relay protection.

86 E

86E BKRTrip

86E Field Trip

Figure 3-18 Reverse Power Relays 34

Generator Phase Faults • A stator phase – to – phase fault is any electrical fault between two phases of the armature winding. • This type of fault is very serious because it produces large amounts of current damaging to the winding if allowed to persist. • Also if allowed to grow it will include a ground fault thus causing significant damage to the stator core. • Damage of this kind will result in costly and long repair times. • Phase – to- phase fault in the winding is detected by a (87G) differential relay.

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Phase to Phase Fault Detection

Protected Equipment

Relay

Operating Winding

Restraining Windings

A Differential relay (87) operates by comparing the current going into a phase with the current going out.

Figure 4-01 Phase-to-Phase Fault Detection

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Backup Protection • The need for backup protection is to protect the generator by detecting system faults that have not been isolated as they should have been. • System backup protection as applied to the generator consist of time delay protection for line-to-ground and multiphase fault conditions. • Backup generator protection schemes are used to protect against failure of the primary system protective relaying and subsequent long clearing faults. • Backup protection is usually provided by two types of relays, over-current and distance relays.

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87T Auxiliary Transformer

Backup Overall Differential Protection

87T

Alternate Connection

The generator auxiliary transformer may be included in the differential zone.

Figure 4-10 Generator Phase Fault Backup Overall Differential Scheme 38

Over/Under Frequency Protection • Over frequency can be caused by loss of load on a generator. • Under frequency can be caused by significant addition of load to a generator or sudden reduction of mechanical input power to the generator. • Typically a (81) Frequency relay is used to detect these conditions.

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Under Frequency Protection

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Over Frequency Protection

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Frequency Range

Delay to Trip

Below 60.6 Hz down to and including 60 Hz

No automatic tripping (Continuous operation)

Below 61.6 Hz down to and including 60.6 Hz

Not less than 9 minutes

Below 61.8 Hz down to and including 61.6 Hz

Not less than 30 seconds

61.8 Hz or above

No time delay required

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Review Objectives • In terms of excitation, identify two abnormal operating conditions for generators, unlike other power system components. • Identify the difference between a generator sequential and simultaneous trip. • Identify the relay protection scheme that senses power flowing into a generator from the power system. • Identify the type of protection relay used in detecting a phase-phase stator winding fault. • Given a one – line drawing identify the reverse power relay, frequency relay and differential relay used in generator protection. (Schematic to accompany test question in LMS).

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References • HPC Technical Services Generator Protection G404

• HPC Technical Services Utility Generators: Theory, Controls, Operation, Maintenance & Testing G401 • NERC Glossary of Terms used in NERC Reliability Standards • Southern Illinois University Edwardsville –Generator Control

and Protection • Schweitzer Engineering Laboratories https://www.selinc.com/SEL-700G/

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Questions?

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