Florida Reliability Coordinating Council
Relay Class
John White
System Protection Manager
Ray Dawson
System Protection Technician
Introduction
• The reasons for protection
• Safety of the public and employees.
• Reliability of power supply to the
customers.
• Prevent damage to equipment.
• What kind of equipment is
protected: generators, transformers,
transmission lines, breakers and
distribution lines.
1
Types of faults
• Three phase, three phase to ground,
phase to phase, phase to phase to ground
and single phase to ground.
• Three phase faults have the highest fault
current.
• Single phase faults have the lowest fault
current.
• The fault current is determined by the
impedance of the fault path.
• Fault paths closer to the source will have
less impedance.
• Faults caused by trees will have higher
impedance.
Equipment used
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CT current transformer
PT potential transformer
Transducers
Time overcurrent relay/51
Instantaneous Over current relay/50
Undervoltage relay/27
Recloser relay/79
Under frequency relay/81
2
CT current transformer
• The secondary winding has a standard
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rating of 5 amp. It’s indicated as 400/200/5
the 400 and 200 are the primary current
which gives us an 80 to 1 and 40 to 1 ratio
respectably.
It is important that CTs be accurate at fault
levels which may be 10 times normal load
current. Standard classifications of CTs
allow 10% error for current flow up to 20
times rated value. They are marked “Class
C ###“. The higher the number the higher
current that can flow though the CT and
still be 10% accurate.
Never open a live CT circuit without first
shorting the secondary side. Without the
secondary side shorted very high voltage
will be seen on the secondary circuit.
Potential Transformers PT’s
• The secondary voltage is usually
69V for relaying and 120V for
metering.
• There is no problem with error
readings as during a fault the
voltage will drop.
• At above 115KV coupling capacitors
are used. This is called CCVT. They
are connected in series causing a
voltage drop across each cap.
• PT’s should never be shorted.
3
Transducers
• Measure voltages and currents and
converts them to a 0 to 1 milliamp dc
output.
• A scaling is used in SCADA
(supervisory control and data
acquisition) to convert the milliamp
signal to a numerical number.
Time Over current relay/51
• Operate on the electromagnetic induction
principle. The protective relay is
essentially a small AC motor. Using
shaded pole produce two fluxes that are at
different phase angles.
• When current is passed through the
electromagnet coil,a magnetic field is
produced, which applies a torque to the
disc. When the magnetic field becomes
strong enough to overcome friction in the
damping magnet, bearing and tension in
the spiral spring, the disc rotates to move
the moving contact. Eventually the moving
and fixed contacts close, completing the
trip coil circuit to the breaker.
4
Time Over current relay/51
• The disc rotates at a speed directly
proportional to the amount of flux
induced into it by the electromagnet.
• The time dial provides adjustment for
how long it takes the moving contact and
stationary contact to make contact. With
a higher number on the time dial, this
increases traveling distance and contact
time.
• The spiral spring is use to set minimum
pickup, resets the disc in normal
conditions or after a trip and provides a
temporary path for the DC trip current.
The spiral spring is not designed to carry
the trip current for very long. A seal-in
bypass is used.
5
Time Over current relay/51
• The seal in coil is in series with the
contacts on the spiral spring. The
seal in coil contacts are in parallel
with the spiral spring contacts. The
seal in contacts shunt the current
away from the spiral spring contacts.
• The seal in coil also has a flag that
drops to indicate which relay had the
overcurrent condition.
6
Instantaneous Over current relays/50
• Instantaneous relays have no time
delay. With a high level fault the
instantaneous unit will operate
before time overcurrent relay.
• It’s a simple clapper relay with a
core screw on top to adjust the
pickup range.
• The contacts are two moving contact
on a bridge and two stationary
contacts. The contacts are directly
across the trip circuit for the
breaker.
Undervoltage relays/27
• The construction of a undervoltage relay
is very similar to an overcurrent relay. The
main different is the operating coil. An
overcurrent relay’s operating coil is
wound with a few turns of heavy wire. A
voltage relay’s operating coil is wound
with many turns of fine wire.
• The disc rotates in the counterclockwise
direction.
• The contact closing torque for an
undervoltage relay is provided by the
tension in the spiral spring.
• A time delay is the same as for an over
current relay.
7
DEVICES USED AT OUC
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21 Distance relay
25 Synchronizing-check device
27 Undervoltage relay
43 Manual transfer or selector device
50 Instantaneous overcurrent relay
51 Time overcurrent relay
52 AC circuit breaker
57 Grounding device
59 Overvoltage relay
63 Gas pressure relay
64 Ground protective relay
67 AC directional overcurrent relay
69 Permissive control device
74 Alarm relay
79 Reclosing relay
81 Frequency relay
86 Locking-out relay
87 Differential protective relay
The right man for the job
Mr. Bill Douglas
8
OUC SYSTEM
OUC Protective Relay Statistics
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60 Transmission Lines
4000 Protective Relays
36 Substations
14 Generators
400 Breakers
200 Transformers
200 Feeder Breakers
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OUC Protective Relay
main schemes
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Protects Transmission Lines
Protects Distribution Lines
Protects Transformers
Protects Generators
Protects Busses
Protects Reactors
10
Stanton Substation
11
S**t Happens
12
13
FAULT
CHARACTERISTICS
Test Question # 11
TYPES of FAULTS :
•Single Phase to Ground
•Two Phase to Ground
•Three Phase to Ground
•Phase to Phase
•Phase to Phase to Phase
Impedance Diagram
FAULT
Voltage= 22800 volts Load Z (one leg)= 70 ohms I= 300 amps
Test Question # 12: 22800 volts / 3 ohms of fault impedance= 7600 amps.
14
Ground current possibly damages generator before 50 G relay
operates.
Ground Fault Current circulates from the load to the generator
stator unimpeded.
15
Resistance, Reactance, Impedance
„
Resistance-(R)
The opposition to the flow of
current in an electrical circuit.
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Reactance-(X)
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Impedance- (Z)Combination of Resistance and
A measure of opposition to a
sinusoidal current made up of Capacitance and inductance
Reactance that opposes flow of current.
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Test Question # 13
Test Question #14: Reactor/Resistor limits fault current
to about 10 amps preventing damage to stator.
16
•Utility standard of Delta Wye configuration for GSU
transformer prevents fault current from circulating through
stator.
•GSU boosts voltage for ease of transmission- 20kV to
230kV.
Voltages and Currents are 120 degrees between phases.
During fault Voltage A-n collapses and Current A-n increases dramatically.
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Disturbance recording
VA
•
Part of System Protections
responsibilities is to analyze fault
data acquired from the
microprocessor based protective
relays such as the examples
shown.
VB
VC
IA
IB
IC
R1 Trip Brkr 757
R2 Trip Brkr 759
R3 SCADA RS1 Op
R6 757 RI N/A
R7 759 RI N/A
R9 757 BF Init
File: J:\System Protection\Faults and oscillographys\SECA 6,20,03\SECA,6,20,03.
Generator Protection
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OUC uses high speed generator schemes that isolates the fault in
the Generator in approximately 2 cycles clearance (.033
seconds).
Generator Relay
Generator
GSU
Transformer
150,000 volt
Breaker
507
To OUC
system
509
150,000 volt
Breaker
FAULT
18
GENERATOR
PROTECTION
Test Question # 15
TYPES of Generator Faults :
•Stator Fault
•Motoring by loss of Prime Mover
•Rotor Ground
•Excitation Failure
•Over-voltage/ Over-excitation
•Turbine Trip
•87G- Generator
Differential
•87T- GSU Transformer
Differential
•87ST- Reserve
Auxiliary Differential.
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Test Question #16:
VAR= Volt Ampere
Reactive
•40- Loss of Field Relay
•81- Under frequency
Relay
•59-Under Voltage
Relay.
With loss of field condition, Generator cannot
produce VARS, lowering terminal voltage and
overheating of the stator.
•46- Negative Sequence
Relay
20
•59G- Generator Ground
Relay
•27G- Third harmonic
supervision Relay
Test Question #
17:
Third Harmonic is
at 180 HZ
TRANSFORMER
PROTECTION
Test Question # 18
Types of Transformer Faults :
•Winding to Winding Fault
•Winding to Ground Fault
•Core Movement
•Bushing Failure
•Arrestor Failure
•Through Fault
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Transformer Protection
• Relay Set 1 and 2 are high speed current differential schemes isolates
the faulted transformer. Approximately 2 cycle clearance (33
milliseconds)
• KBCH digital Current Differential relays for RS1.
• T60 GE microprocessor Current Differential for RS2
• Also uses Backup Overcurrent Protection in RS2
Differential relay
507
SWGR
10
XFMR
509
FAULT
Transformer Damage Curve
Test Question # 19:
Transformer damaged in 50
seconds at 20,000 Amps
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Transformer Differential Relay
General Electric
T60 UR Differential
relay
Primary Current Flow
Normal Operation
23
Fault Current Flow
Fault Condition
Transformer Protection
• OUC uses a high speed current differential
scheme that isolates faulted substation
transformers in approximately 2 cycles
clearance (.033 seconds)
Differential relay
150,000 volt
Breaker
switchgear
To customers
13,000 volt
Breaker
507
10
509
FAULT
transformer
150,000 volt
Breaker
24
Test Question #20:
51 G Ground Relay located in ground leg
of the low side of the transformer
Test Question # 21:
87T – Transformer Differential Relay
63 – Sudden Pressure Relay
51- Over-current Back Up Relay low side
50/51 - Over-current Back Up Relay high side
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TRANSMISSION LINE
PROTECTION
Test Question # 22
Types of Transmission Line Faults :
•Single Phase to Ground
•Two Phase to Ground
•Three Phase to Ground
•Phase to Phase
•Phase to Phase to Phase
•Arrestor Failure
•Through Fault
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Transmission Line Protection
• Relay Set 1 is high speed current differential schemes isolates
the faulted segment of a transmission line. Approximately 2 cycle
clearance (33 milliseconds)
• L90 digital Current Differential relays used by OUC.
• Also uses Direct Transfer Trip for Breaker Failure
Fiber communication
L90
L90
501
507
FAULT
503
509
Transmission Line Protection
• OUC uses high speed current differential
schemes that isolates the faulted segment
of a transmission line in approximately 2
cycles clearance (.033 seconds).
•The relays communicate over the OUC fiber
Optic Network
Protective Relay
501
Fiber communication
Transmission Breaker
Protective Relay
Transmission Breaker
507
Transmission Breaker
509
Transmission Line
503
Transmission Breaker
FAULT
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Transmission Line Protection
• Relay Set 2 is high speed directional impedance relay that isolates the
faulted segment of a transmission line using Permissive Overeach
Transfer Trip
Test Question # 23:
Zone 1 set at 95% of transmission line
Zone 2 set at 125% of transmission line
T1
T1
P442
Zone 1 95 % of line
501
503
P442
507
Zone 2 125 % of line FAULT
509
Directional distance relay
Remote Substation
Substation
Length of Line = 10. 7 miles
Line Impedance – Positive sequence - .20 + j 1.11
Zero sequence
- 1.97 + j 3.55
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Under Load Conditions no Trip
outside of zone.
During fault, impedance moves into trip zone
For short transmission lines we use Reactance Relays that operate
at 90 degrees between voltage and current.
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Line Current Differential Relay
General Electric L90 UR Relay
Line Current Differential Relay
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Fault
Line Current Differential Relay Trip Conditions
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Description of Substation Automation
INTERNET
OUC
CUSTOMER
CORPORATE NETWORK
OUC
INTRANET
Firewall
CORPORATE
USER
PERSHING OPERATIONS CONTROL CENTER
REMOTE
(EMERGENCY)
CONTROL ROOM
Firewall
ENERGY
MANAGEMENT
SYSTEM
TRANSMISSION
SYSTEM
WIDE AREA NETWORK
BACKUP ENERGY
MANAGEMENT
SYSTEM
(ALL SUBSTATIONS)
MODEM
LINE
SUBSTATION
FUTURE
SYSTEMS
RELAYS
SUBSTATION
AUTOMATION
SYSTEM
MODEM
LINE
PROGRAMMABLE
LOGIC
CONTROLLERS
REMOTE TERMINAL
UNITS
Comparison of Electro-Mechanical to Microprocessor-Based Protective
Relays
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Advantages of Electromechanical
•Lower Cost
Test Question # 24:
•Easier to Test
Disadvantages of Electromechanical
•Need three individual relays
•Maintain more often
•Moving Parts
•Usually performs only one protective function
•Slower operating than Micro-Processor Based
Advantages of Micro-Processor Based
•Performs hundreds of protective functions
•Only one relay needed for all phases
•Faster operating than Electromechanical
•Self-Monitoring
Test Question # 25:
•Takes up less space
•Longer maintenance cycle
Disadvantages Micro-Processor Based
•Higher Cost
•Complex training for testing
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How a Micro-Processor Based Relay works.
Test Question # 26:
It takes an analog signal from PT’s and CT’s and converts
it into digital data for analysis.
How a Micro-Processor Based Relay works.
Analog data under Sine Wave is
converted to digital data and
analyzed by the processor
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How a Micro-Processor Based Relay works.
The microprocessor in the relay analyzes millions of bits
of data per second and determines if the amplitude is
sufficient to trip. All in 18 milliseconds or less.
Converted into Digital samples
Analog data
Customer Feeder Protection
• OUC uses a high speed microprocessor feeder
relay that isolates faulted customer feeder in
approximately 3 cycles clearance (.050 seconds)
Main relay
Feeder relay
150,000 volt
Breaker
13,000 volt
Breaker
13,000 volt
Breaker
11
501
10
503
switchgear
FAULT
transformer
150,000 volt
Breaker
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