ch6 power system slide09

8/14/2019 Ch6 Power System Slide09

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BEE3133

Electrical Power SystemsChapter 6: System Protection

Rahmatul Hidayah Salimin


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Introduction

System Protection: the equipment use

to detect and isolate the faulty section

from the system automatically.


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Introduction

Short circuit occur when equipmentinsulation fails due to system overvoltagescaused by:

Lightning or switching surges Flashover line-line (caused by wind) Flashover to tree

Insulation contamination by dirt/salt Mechanical failure

Cable insulation failure

Natural causes Tower/pole or conductor falls Objects fall on conductors


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Introduction

Short circuit currents can be several ordersof magnitude larger than normal operatingcurrents

If it is allowed to persist, may cause: Damage to the equipment due to heavy currents,

unbalanced current, or low voltage produces bythe short circuit

Fire and explosion effect equipment/people

Disruption of service in the entire power systemarea


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Introduction

Careful design, operation and

maintenance of system protection can

minimize the occurrence of shortcircuit but cannot eliminate them.


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Fault Currents and Voltages


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Function of System Protection

Cause the prompt removal from service of anyelements of power system when it suffers ashortcircuit, or when it start to operate in any abnormal

manner that might cause damage or otherwiseinterfere with the effective operation of the rest ofthe system.

Provide indication of the locationand type of failureso that the data can be used to assist in expediting

repair and analyzing the effectiveness of fault-prevention and mitigation features.


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Function of System Protection

Why do we need system protection:

Detect fault

Isolate faulted component

Restore faulted component

Aims:

Continued supply for rest of system Protect faulted part from damage


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Types of Protection

A Fuses For LV Systems, Distribution Feeders and

Transformers, VTs, Auxiliary Supplies

B - Over current and earth fault Widely used in All Power Systems

Non-Directional Directional


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Types of Protection

C - Differential For Distribution Feeders, Busbars,

Transformers, Generators etc

High Impedance

Low Impedance

Restricted E/F

Biased

Pilot Wire


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Types of Protection

D - Distance For Transmission and Sub-transmission Lines

and Distribution Feeders, Also used as back-up protection for

transformers and generators withoutsignaling with signaling to provide unitprotection e.g.:

Time-stepped distance protection Phase comparison for transmission lines Directional comparison for transmission lines


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Types of Protection

E - Miscellaneous:

Under and over voltage

Under and over frequency A special relay for generators, transformers, motors

etc.

Control relays: auto-reclose, tap change control, etc.

Tripping and auxiliary relays


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Design Criteria/Characteristics

Simplicity

Economy

Speed

Sensitivity

Selectivity

Reliability


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Reliability Operate dependably and in healthy operating

condition when fault conditions occur, even after

remaining idle for months or years. Selectivity

Clearly discriminate between normal andabnormal system condition to avoid unnecessary,false trips.

Sensitivity Ability to distinguish the fault condition, although

the different between fault and normal conditionis small.


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Speed Fault at any point in the system must be

detected and isolated rapidly to minimize fault

duration and equipment damage. Any intentionaltime delays should be precise.

Economy Provide maximum protection at minimum cost

Simplicity Minimize protection equipment and circuitry
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Economic Factor

Total cost should take account of :

Relays, schemes and associated panels and panel wiring

Setting studies

Commissioning

CTs and VTs

Maintenance and repairs to relays

Damage repair if protection fails to operate

Lost revenue if protection operates unnecessarily


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Economic Factor

The cost of protection is equivalent to an insurance policy

against damage to plant, and loss of supply and customer

goodwill. Acceptable cost is based on a balance of economics and

technical factors. Cost of protection should be balanced

against the cost of potential hazards.

There is an economic limit on what can be spent.

MINIMUM COST :Must ensure that all faulty equipment is

isolated by protection.


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Relationship between reliability of supply, its

value and cost to the consumer


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System Protection Components

Transducer / Instrument Transformer

Relay

Circuit Breaker


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Function:

Transducers/Instrument Transformers Provide low current and voltage, standardized levels suitable for

the relays operation. Relays

Discriminate between normal operating and fault conditions.

When current exceed a specified value relay will be operated andcause the trip coil of CB to be energized/open their contact.

Circuit Breakers Open the line


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System Protection Flow

RelayTransducer Fault

OccurCircuit

Breaker

FaultClear

voltage or current rise from normal condition

voltage/current is reduced to match with relay rating

activate circuit breaker

circuit isolation


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Zones of Protection

For fault anyway within the zone, the

protection system responsible to

isolate everything within the zone fromthe rest of the system.

Isolation done by CB

Must isolate only the faulty equipmentor section


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Zones of Protection

Zones are defined for:

Generators

Transformers

Buses

Transmission and distribution lines

Motors


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Zones of Protection


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Zones of Protection

Characteristics:

Zones are overlapped.

Circuit breakers are located in the overlapregions.

For a fault anywhere in a zone, all circuit

breakers in that zone open to isolate thefault.
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Overlapped of Protection

No blind spot:

Neighboring zones are overlapped to avoid

the possibility of unprotected areas

Use overlapping CTs:

Isolation done by CB. Thus, it must be

inserted in each overlap region to identifythe boundary of protective zones.


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Overlapped of Protection

Overlap accomplish by having 2 sets of

instrument transformers and relays for each

CB. Achieved by the arrangement of CT and CB.


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Primary & Back-up Protection

Primary protection is the protection

provided by each zone to its elements.

However, some component of a zoneprotection scheme fail to operate.

Back-up protection is provided which

take over only in the event of primaryprotection failure.


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Example

a) Consider the power system shown below, with the

generating source beyond buses 1, 3 and 4. What

are the zones of protection in which the system

should be divided? Which circuit breakers will openfor faults at P1 and P2?

1

2

3

4

P1

B

P2A C


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Fault at P1 = A, B, C

Fault at P2 = A, B, C,D, E

1

2

3

4

P1

B

P2A C

D

E


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Example

a) If three circuits breakers are added at the tap

point 2, how would the zones of protection be

modified? Which circuit breakers will operate for

fault at P1 and P2 under these conditions?1

2

3

4

P1

B

P2A C


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1

23

4

P1

B

P2A C

D

E

H

F G

Fault at P1 = A, F

Fault at P2 = C,D,E,G


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Zone Discrimination

A system as shown with relays and breakers marked.A single fault has resulted in the operation ofbreakers B

1, B

2, B

3and B

4.Identify the location of the

fault Answer:

Fault in the overlap zone at breaker B2 as shown


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Back-up Protection

1.Duplicate Primary

Provide primary protection when the primary-relaying equipment is out of service for maintenance

or repair Disconnect when primary relaying operates correctly

Operate with sufficient time delay (coordinationtime delay) if primary not operate

When short circuit occur, both primary and back-upstart to operate, but if primary is operate, then theback-up will reset.


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Back-up Protection

2.Remote Back-up

located outside boundary of Zone of Protection


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Example

Fault Primary Back-up

K C, D, E A, B, F


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Example

Fault Primary Back-up

Line E, F C, D, E, F, G, H A, B, I, J


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Transducers

Also known as Instrument Transformer

Use to reduce abnormal current & voltagelevels and transmit input signals to therelays of a protection system.

Why do we need transducer: The lower level input to the relays ensures that

the physical hardware used to construct therelays will be small & cheap

The personnel who work with the relays will beworking in a safe environment.


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Transducers

Current and Voltage Transformers

Correct connection of CTs and VTs to the

protection is important directional,distance, phase comparison and

differential protections.

Earth CT and VT circuits at one point only;


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VT and CT Schematic


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Voltage Transformers

VT is considered to be sufficiently accurate.

It is generally modeled as an ideal transformer.

VT secondary connected to voltage-sensingdevice with infinite impedance.


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Types of VTs

Electromagnetic VT

Capacitive VT

Busbar VTs

Special consideration needed when used for line protection

LV application(12 kV or lower)

Industry standard transformer with a primary winding at a

system voltage and secondary winding at 67 V(line-to-neutral) and

116 V(line-to-line).


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Voltage/Potential

Transformer

(VT/PT)


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HV and EHV

Capacitor-coupled VT (CVT)

C1 & C2 are adjusted, so that a few kVs ofvoltage is obtains across C

2

Then, stepped down by T

VTs must be fused or protected by MCB.


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VT ratios:

ratio of the high voltage/secondary

voltage1:1 2:1 2.5:1 4:1

5:1 20:1 40:1 60:1

80:1 100:1 200:1 300:1400:1 600:1 800:1 1000:1

2000:1 3000:1 4500:1


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Current Transformers

CT is an instrument transformer that is used

to supply a reduced value of current to

meters, protective relays, and other

instruments.

The primary winding consist of a single turn

which is the power conductor itself.

CT secondary is connected to a current-sensing device with zero impedance.


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CTs ratio(secondary current rating is 5A)

50:5 100:5 150:5 200:5

250:5 300:5 400:5 450:5500:5 600:5 800:5 900:5

1000:5 1200:5

CTs also available with the secondary ratingof 1A


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Reclosers and Fuses

Automatic reclosers are commonly used fordistribution circuit protection.

Recloser: self-controlled device for automatically

interrupting and reclosing an AC circuit with presetsequence of openings and reclosures

Have built-in control to clear temporary faults andrestores service with momentary outages.

Disadvantages: increase hazard when circuit is physically contacted by

people.

Recloser should be locked out during live-line maintenance.


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Reclosers and Fuses

1. An upstream fuse/relay

has detected a fault

2. Downstream systemisolated by fuse or

breaker

3. Automatic re-closing

after delay successful iffault not permanent


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Relays

Discriminate between normal operatingand fault conditions.

Type of Relays Magnitude Relay

Directional Relay

Distance/Ratio Relay

Differential Relay

Pilot Relay


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Magnitude Relays

Also called as Overcurrent Relay

Response to the magnitude of input quantities ie.current.

Energize CB trip coil when the fault current magnitudeexceeds a predetermined value or trips when a currentrises above a set point (pick-up current).

If it is less than the set point value, the relay remainsopen, blocking the trip coil.

Time-delay Overcurrent Relay also have the same

operating method but with an intentional time-delay.


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Directional Relays

Responds to fault only in one direction, either to theleft or to the right of its location

Operation depends upon the direction (lead or lag) ofthe fault current with respect to a reference voltage.

The directional element of these relays checks thephase angle between the current and voltage of onephase, and allows the overcurrent unit to operate ifthis phase angle indicates current in the reversedirection.


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Ratio Relays

Operate for certain relations between the

magnitudes of voltage, current and the phase angle

between them.

Measures the distance between the relay locationand the point of fault, in term of impedance,

reactance and admittance.

Respond to the ratio of two phasor quantities as

example Voltage and Current (Z = V/R) Also called impedance or distance relay


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Differential Relays

Respond to the vector difference between two currents within

the zone protection determined by the location of CTs.

Not suitable for transmission-line protection because the

terminals of a line are separated by too great a distance to

interconnect the CT secondaries.

For the protection of generators, transformers, buses,

Most differential-relay applications are of the current-

differential type.


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Differential Relays

Fault occur at X

Suppose that current flows through the primary circuit either toa load or to a short circuit located at X.

If the two current transformers have the same ratio, and are

properly connected, their secondary currents will merelycirculate between the two CTs as shown by the arrows, and nocurrent will flow through the differential relay.

Relay


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A flow on one side only, or even some currentflowing out of one side while a larger currententers the other side, will cause a differentialcurrent.

In other words, the differential-relay currentwill be proportional to the vector differencebetween the currents entering and leaving theprotected circuit; and, if the differentialcurrent exceeds the relays pickup value, therelay

Relay

Differential Relays


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Differential Relays

When a short circuit develop anywhere betweenthe two CTs.

If current flows to the short circuit from bothsides as shown, the sum of the CT secondarycurrents will flow through the differential relay.

It is not necessary that short-circuit currentflow to the fault from both sides to causesecondary current to flow through the

differential relay.

Relay


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Pilot Relays

The term pilot means that between the

ends of the transmission line there is an

interconnecting channel of some sort overwhich information can be conveyed.

Use communicated information from

remote sites as input signals.


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Pilot Relays

Transmitting fault signals from a remote zone

boundary to relays at the terminals of a long

TL

Pilot relaying provides primary protection only;back-up protection must be provided by

supplementary relaying.

Type : wire pilot, carrier-current pilot and

microwave pilot.


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Pilot Relays

1 2

A B C

Station 1 consist of meter for reading

voltage, current and power factor.

Distance relay, tell the different between

fault at A (middle) and B (end) by knowingthe impedance characteristic per unit length

of the line.

ZA ZB


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Pilot Relays

1 2

A B C

Could not possibly distinguish between fault Band C because impedance would be so small-

Mistake in tripping CB for fault B or C

Solution- indication from station B, when the

phase angle of the current at S-B(with respectto current A) is different by approximately 180o

from it value for fault in the line section AB.


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Pilot Relays

(with respect to current A) is

different by approximately 180o

from it value for fault in the line

section

(with respect to current A) isnot different in degree from it

value for fault in the line

section

1 2

B CA

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