Introduction To Protective Relays

This training is applicable to protective relaying and System Monitoring tasks associated with

NERC Standards PRC-001 and TOP-006

Learning Objectives

Upon completion of the training, the participant will be able to:

1. Recognize the importance of batteries in protective relaying.

2. Identify the 3 basic purposes of protective relays

3. Differentiate between Primary and Backup relaying.

4. Distinguish what determines the zones of protection verses tripping zones.

Applicable NERC Standards

• PRC-001 System Protection Coordination• 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.

• TOP-006 Monitoring System Conditions• R3. Each Reliability Coordinator, Transmission Operator, and Balancing

Authority shall provide appropriate technical information concerning protective relays to their operating personnel.

Station Battery

NERC’s definition of a Protection System includes:

Station dc supply associated with protective functions

(including batteries, chargers, and non-battery based dc supply)

Associated Alarms

Loss of AC to Battery Charger

Low Battery Voltage

Loss of DC

Battery Chargers

• Convert an AC source to DC, and maintain adequate charge on the batteries.

• Do NOT have the capacity to carry the full station DC load.

Protective Relay Principles

• Cannot prevent faults• Proactive – pilot wire relays; temperature relays• Reactive – relays that initiate action after fault occurs

• Use inputs from Monitoring devices• Current, Voltage, Temperature, Pressure

• Initiates corrective action as needed to remove fault• Minimize Damage from the fault by quick interruption

• The quicker the fault is removed, the less damage.• Isolate Only the Faulted Area (Zone).• Maintain Service to Other Areas (Zones) During and/or after Isolation of the Fault.

Protective Relay Principles

• Designed & utilized to protect against Faults.• But could potentially trip due to loading in excess Facility Ratings or

System Operating Limits (SOLs). • It is incumbent on the System Operator to alleviate these conditions real

time and Day ahead planners pre-contingency.• Settings determined with system normal

• During System Restoration / Islanding events• May not have enough fault current available to reach trip values.• Have Switching Personnel monitor ammeters when closing devices.• When closing by supervisory control, monitor ammeters on SCADA.

G5

G4 G2 G3

G1

G6

30Ω

30Ω

10Ω

70Ω

40Ω20Ω

25Ω Fault Current25,000 Amps

Fault Current2,000 Amps

G5

G4 G2 G3

G1

G6

30Ω

30Ω

10Ω

70Ω

40Ω20Ω

25Ω

Black Start

Reduced available fault current during blackouts can affect relay operation

Network verses Radial

• Radial • One source - fault interrupting devices are in series with lateral feeds to

customers.• Distribution Circuits are typically radial.

• Network• Dual or Multiple Sources• The Transmission system is largely a Transmission Network.• Relay coordination becomes more complicated and more expensive.

• Magnitude of current and Time Delay• Direction of current• Communication channels

Primary verses Backup Relays

• Primary relays are normally expected to operate first and trip breakers when faults occur within the zone they protect. • Instantaneous on Transmission and EHV (>500kV) circuits• Sub-transmission (<100kV) may or may not be instantaneous

• Depending on relaying used and location of fault

• Backup relays operate to clear around a CB that fails to interrupt a fault within a specific time period.• Local Backup (7 – 15 cycles)

• Breaker Failure or Transfer Trip relays at the local station• Remote Backup (20 – 30 cycles)

• Time Delay or Zone 2 relays at remote stations

Zones of Protection

• Defined by Current Transformers (CTs) that sense current flow into the zone.• Each zone will have unique targets

• All primary equipment is included in at least one zone of protection.• Overlapping ensures no equipment is left unprotected.

A B C D

RR R RRR R R

Elm

Ash Oak

Various Zones of Protection

• Overlapping occurs even when a CB is not present.• Transmission bushing has 2 CTs

LOR Tripped By Numerous Relays

• Transformer and Bus differential zones shown.• Both zones will still trip the same LOR.

• LOR (lock-out relay) trips Distribution CBs,

Circuit Switcher XT1 and MOAB X1

R R

RR

R

LORFault in Bus Zone of Protection

Bus Targets and LOR

Fault in Transformer Zone of Protection

Transformer Targets and LOR

Fault in Trans Circuit Zone of Protection (Includes Transformer Surge Arresters )

Line Targets

Potential Sources for Relays

• Some relays require a voltage/potential input in addition to the current input, to monitor the zone determined by the associated CTs.• Do NOT define the zone of protection• Should be attached close to equipment they protect

• Sources of Relay Potential• Potential Transformer (PT) – Most accurate• Coupling Capacitor Potential Device (CCPD)• Coupling Capacitor Voltage Transformer (CCVT)• Resistive Potential Device

Fundamentals of Relay Protection Potential for Relaying (cont.)

Coupling Capacitor Potential Device. (CCPD)

Coupling Capacitor Voltage Transformer (CCVT)

• Uses a series capacitor voltage divider principle.• Typically used for relay potential 138 kV and above.• CCVT is more accurate and has more capacity.

Potential for Relaying (cont.)

•Cascading of Capacitor cans•All cans intact, each group has same

impedance and voltage divides equally.• If one can fails in a group, the impedance of

that group increases.• That group then has more of the voltage.

• More stress on cans in the group.•As other cans fail, more voltage (and more

stress) is on cans in the group, and cascading can occur.

RELAY RPD

Overcurrent Relays

• Can be Instantaneous (50) or time-delayed (51)• Can be non-directional or directional.

• Non-directional used on radial circuits• Directional used on Network circuits

• Ground Relay• Sees only imbalance current.• Usually set lower than phase relay• Ground target only on some faults

• Phase relays• Phase relays must be set above load current. • Use Undervoltage scheme where load approaches available fault current

Φ Φ Φ

G

Phase Relays

Ground Relay

A

B

C

CB

Fault

Undervoltage/OverCurrent Scheme

• A special scheme in which the over-current relays can’t operate unless low voltage indicates a fault.• The Voltage Relay keeps the over-current relay coils

shorted for normal voltage• Removes the short to place the over-current relay coils

in service when voltage is depressed due to fault conditions.

• Cheaper than adding Electro-Mechanical impedance relay

UV Relay

Trip Coil

Tripping Relay

Relay

Coil

Instantaneous Clearing of FaultOnly in Middle 80%

• Fault between Breaker “A” and point “X”• INST target at Breaker “A” and Time target at “B”

• Fault between points “A” & “B”

A CB

INST

TOC

INST

TOC

INST

TOC

XFault

A CB

INST

TOC

INST

TOC

INST

TOC

X

Y

Fault

Differential Relaying

• Operate when the power into a protected zone does NOT equal the power out of the protected zone. • Basically - CTs algebraically add for paths into and out of the zone to

cancel at the relay operate coil.• The differential relay is preset to operate instantaneously when the

difference that is seen by the relay exceeds its trip setting.• Bus Zones (87B)• Transformer Zones (87T)• Generator Zones (87G)

Differential Relaying – External Fault

• Current into the Zone equals the current leaving the Zone• Secondary current sums to zero at the operate coil

10A10A

87B

A B600/5600/5

1200A

1200A

0A

BUS

Fault

Differential Relaying – Internal Fault

• Current into the zone does not equal the current leaving the zone• Secondary current combines and goes through the operate coil of the

relay

10A10A

87B

A B600/5600/5

1200A

1200A

20A

BUSFault

Differential Relays Trip Lockout Relays

• The 87T trips the lockout relay which in turn trips the associated breakers.

87T

A B

138 kV

69 kV

1200/5600/5

600A 1200A

5A 5A

10A

Fault

5A 5A

86T/87XT

Lockout Relays

• When reset, burning light monitors the trip coil• Older LORs, the light was wired separately from the relay• When tripped, the light will go off, and a target flag will show

• The second light will burn when a relay is sending trip to the Lockout Relay• Older LORs do not have the Standing Trip light

Trip Ckt OK

Tripped Position

Standing Trip

Reset Position

Lockout Relays

• Never hold lockout relay in reset position; or try to reset if standing trip light is on

Trip Contacts

Trip Contacts

Coil

LatchCoil Contact

Impedance (Distance) Relays

• Sub-transmission circuits have high impedances • Instantaneous overcurrent relays can be set for proper coordination.

• Impedances of circuits operated at 138kV and above are much lower• Coordination with overcurrent relays is more difficult

• Impedance (Distance) relays use the secondary current and secondary voltage during a fault to calculate the impedance to the fault.• Since the impedance per mile of the circuit is known, the impedance to the fault

can be used to estimate the distance to the fault. • If impedance to the fault is within the Zone 1 setting, an

instantaneous trip occurs.

Impedance (Distance) Relays

• Zone 1 is instantaneous• Typical setting is 80-90%

• Zone 2 has up to a 40-cycle delay• Typical setting is 120-150%

• Zone 3 has up to a 90-cycle delay• Typical setting is 200%

Zone 2 150%

Zone 3

Zone 1 90%

21

DC

A B

50 miles

75 miles

FE

25 miles

HG

Impedance (Distance) Relays

• Instantaneous clearing only in middle 80% of circuit

Zone 2

A

21

B C

Zone 1

Zone 2

Zone 1

80%Source Source

Z2

Z1

21

Zone 2

A

21

B C

Zone 1

Zone 2

Zone 1

80%Source Source

Z1

Z1

21

Directional Comparison Carrier Blocking

• Impedance relays (Z1, Z2, Z3) remain in service, and function as Backup Relays

• The Directional Comparison Carrier Blocking scheme uses a Zone-3 impedance relay with no time delay

• Communication channel used only to transmit a blocking signal for external faults

Relays of CB B will send blocking signal to prevent CB A from tripping for a fault on circuit C - D.

Z-3 Instantaneous

Z-3 Instantaneous

DA B C

21A

21B

E

Directional Comparison Carrier Blocking

• When fault current is detected• A blocking signal is transmitted ONLY for external faults

• No signal is transmitted for internal faults• Trip occurs when a fault is detected and no blocking signal is received

Wave Traps

Reverse looking Carrier Start element

Forward looking tripping element

Directional Comparison Carrier Blocking

• Multiple circuits (Fault on G-H)• Carrier Relays for CBs _________________see fault into the circuit; and therefore

will not transmit• Carrier Relays for CBs _______________ see fault into the bus; and therefore will

transmit• CBs G and H trip because their respective relays see fault current into the circuit

and no blocking signal was received.

A BDC

FE

HG

A,D,E,G,H

B,C,F

Phase Comparison

• Phase comparison systems use only current for fault location (i.e. internal or external)• Very desirable on lines with variable impedance, such as a line with switched series

capacitor or series reactor compensation.• Upon fault detection, the comparer logic relay compares the current

at each terminal.• If the phase angle and magnitude are within a preset comparison

window, no tripping will occur.• If the angle reverses at either end (signifying a 180o power reversal,

(which is indicative of an internal fault), the comparer will initiate trip of the breaker.

Phase Comparison External Fault

• The over-current fault detector relays see fault current but neither comparer sees a difference in phase angle.• No trip occurs for Breakers 1 or 2

Phase Comparison Internal Fault

• The over-current fault detector relays see fault current. • Comparers see a 180o difference in phase angles.• Trip occurs for Breakers 1 & 2

Transfer Trip Schemes

• Direct Transfer Trip (DTT)• Also used for Breaker failure to trip remote breakers, lines terminated by transformers, and

with shunt reactors.• Direct Under-reach Transfer Trip (DUTT)• Permissive Under-Reach Transfer Trip (PUTT)• Permissive Over-Reach Transfer Trip (POTT)

• Most common TT scheme used for line protection• Guard signal is transmitted constantly to check integrity of the Transfer Trip

Channel.• When a trip signal is needed, the signal is shifted from the Guard frequency to the Trip

frequency.• Used as backup to the two redundant primary relays on EHV

Direct Under-Reach Transfer Trip

• When under-reaching Zone 1 element detects a fault:• Trips local CB instantaneously, and• Sends Direct Transfer Trip signal to trip remote CBs

• Upon receipt of Direct Transfer Trip signal, CBs trip instantaneously with no other condition necessary

• Transfer trip signal important when fault is beyond Z1

TTR

Z1

Permissive Under-reach Transfer Trip

• When under-reaching Zone 1 element detects a fault:• Trips local CBs instantaneously, and• Sends permissive Transfer Trip signal to relays at remote terminal.

• If relays at remote terminal see only a Zone 2 fault:• The permissive signal will bypass the time delay and allow the CBs at the remote terminal to trip

instantaneously.

• For faults in the middle 80%• CBs at both terminals will trip instantaneously by Zone 1• Will also receive TT signal

• For faults beyond Zone 1 reach of one terminal• Permissive Transfer Trip signal becomes very important

Permissive Under-reach Transfer Trip

• For a fault close to CB 2• Permissive Transfer Trip will set up instantaneous tripping of CB 1

Z1

Z2TTR

Permissive Over-reach Transfer Trip

• More common as line protection scheme than other TT schemes.

• If the overreaching Distance Relay sees a fault, a permissive TT signal is sent to the other end.

• To trip instantaneously:• The overreaching Distance Relay must see a fault, and• Receive a permissive transfer trip signal from the opposite terminal.

Permissive Over-reach Transfer Trip

• For a fault at any point on the circuit• Fault is seen by over-reaching element at both terminals• Both terminals receive the permissive signal• Both terminals trip instantaneously

Over-Reach Zone of CB 1

• Over-Reach Zone of CB 2

Distance Relay

Distance Relay

Zone Designation Comparison

• GE and SEL both use Zone 2 to designate the Pilot tripping zone (60-cycle delay)• Traditionally this zone was designated Zone 3

Function Traditional GE SEL

Inst trip zone Zone 1 Z1 Z1P

30~ delay zone Zone 2 Z3 Z4P

60~ delay & Pilot zone Zone 3 Z2 Z2P

Pilot Block (Reverse looking) No Target Z4 Z3P

SEL 421 Targets

• Targets can be labelled as each region desires

Summary

• Standards require Operators to have knowledge of relay systems• Importance of DC system in Protection systems• Digital relays emulate control and reclose switch positions• Principles of relay protections

• Can’t prevent faults• Three main purposes

• Quick interruption of faults• For permanent faults, isolate only the faulted zone• Use reclosing relays to restore as much of the system as possible after a fault

• Transmission relays set for fault current• During blackout restoration, relay protection is compromised

Summary

• Primary and Backup relays• Zones of Protection

• Targets indicate in which zone the fault was• Zones overlap at a CB

• Potential sources for relays• Overcurrent relays• Differential relays• Lockout Relays• Impedance (Distance) Relays

• Zones 1, 2 and 3

Questions?