Industrial and Commercial Power SystemsTopic 6

PROTECTIONS

The University of New South Wales

School of Electrical Engineeringand Telecommunications

1

FUNCTION OFELECTRICAL PROTECTION SYSTEMS

Current overload

Short circuit current effects

Earth fault current (and possible arcing)

Excessive thermal heating (by overload or limitation of thermal dissipation)

Voltage excursions outside operation limits

Unbalanced 3-phase currents and voltages

Frequency variations

Loss of synchronism of motors

Problems:

Protection against personnel injury

Protection against equipment damage

Coordination and proper discrimination of protection operation

3 primary aims of electrical protection:

Reliability - dependable and secure

Fast action if necessary with speed dependent on fault magnitude

Selectivity (discrimination) in its isolation of circuitry

Over-current protection

Requirements

We focus only on

Current monitoring – current transformer (CT) acts as current source to protection relay (burden).

Timing of operation – protection relay or microprocessor controller provides tripping signal to C/B after a suitable time (short for high fault currents, longer for low currents)

Circuit interruption – by fuse or circuit breaker actuated by relay

Functions required (for over-current protection):

Small installations use miniature circuit breakers [MCBs] or moulded case circuit breakers [MCCBs] with current sensor and timer incorporated within breaker housing.

In large supply systems, current sensor, timing relay, and interrupter are separate items.

2

ZONES OF PROTECTION

Divide network into smaller areas, each zone served by its own protection device.

Overlap of zones for greater reliability

Zoning example:TransformerMotors / generatorsOverhead linesFeeders and final circuits

Zones of protection indicated by dashed lines enclosing power-system components in each zone.

Differential Relay

Protection for three-phase transformer

3

PRIMARY AND BACKUPPROTECTION

To improve reliability

Used in some situations

Radial networks normally have series arranged protective elements

4

PROTECTION RELAYS

Use with some sensors (eg. CT)

Relay compares sensor output with some predetermined upper limit which is the allowable normal operating limit.

When limit exceeded, relay will then operate, e.g. trip C/B after some appropriate time delay.

well-defined current-time characteristic. A specific current will cause relay to trip and thus operate C/B at a well-defined time after fault initiation.

in general, follow inverse I-t relationship, with high fault currents causing tripping in shortest times .

have facilities to change operating times and tripping currents over a wide range of values.

used to grade operating time of series circuit protection to achieve proper discrimination of operation for a particular application.

Classification of relay types:

Function: protection, monitoring, control, auxiliary

Type: electromagnetic, electronic/computer-based, non-electric (e.g. thermal, pressure, mechanical etc.)

Parameters sensed: current, voltage, frequency, power, temperature, pressure, velocity.

concerned only with current sensing relays of electromagnetic, electronic and microprocessor computer-based types

49 Thermal relay

50 Instantaneous overcurrent relay

51 Time-delay overcurrent relay

64 Earth fault protection relay

67 Directional overcurrent relay

79 Reclosing relay

Numbering scheme for common protection relays:

5

OVERCURRENT PROTECTIONBY

CURRENT TRANSFORMERSAND RELAYS

Discrimination is important in large scale high power high voltage systems.

Protection requires much greater flexibility in operating times.

Achieved by use of a separate CT as fault current sensor, feeding a separate protection relay which is then used to trip a CB after a time determined by relay operating

characteristics.

Current-time characteristics:

Inverse IDMT (Inverse definite minimum time)

Very inverse IDMT

Extremely inverse IDMT

Instantaneous operation

Long time earth fault

Definite time operation relay (e.g 2, 4, 8s etc)

Unit protection:

Discrimination achieves minimal disruption but requires delayed response time for through fault current.

If fault occurs within an item such as a transformer, protection should also be able to operate very quickly in response to such an internal fault.

Unit operation provides protection only for faults within a particular item of equipment.

Use 2 CTs as sensors at input and output. Any difference (differential current) will indicate a fault within unit. This can be used to operate an instantaneous trip relay.

CT requirements:

Design of protection CTs is different from metering CTs. More difficult:

wide dynamic range to detect high fault current response to high-frequency currents

Protection CTs have dynamic range from 1 – 25 p.u. of rated current over which good (though not necessarily high) accuracy must be maintained

Differential use: rely on two CTs being identical in characteristics. often not so even for same design.

Plug Setting (PS):

Standard overcurrent relays are usually rated at either 1A or 5 A tripping current.

discrete settings in terms of rated trip current.

50, 75, 100, 125, 150, 175, and 200%

Example: can set trip current between 2.5A and 10A for standard 5A relay

Earth fault relays have different settings: 20, 30, 40, 50, 60, 70, and 80%

Time multiplier setting (TMS):

Standard operating time can be adjusted continuously in the range 0.1 to 1.

Example: Standard IDMT, operating time = 3s at M=10 If set TMS=0.2 then operating time = 0.6s

Thus can use same relay type but vary PS and TMS for different protection operations.

6

DETERMINATION OFRELAY SETTING

In idealised situation pickup occur if PSM just exceeds 1 no tripping even if PSM only slightly less than 1

Inaccuracies and mechanical delays in relay operation in practice

When designing protection: To ensure operation when fault present, choose a

minimum PSM of 1.3 To ensure non-operation under normal operational

conditions, choose PSM <0.8

Thus, PSM between 0.8 and 1.3, under normal system operation, should be avoided.

Basic time/current characteristics of IDMT relays.

Time grading between relays in series.

EXAMPLES

11kVbusbar

Transformer33/11kV 20MVA

circuitbreaker

feeders

current transformer[1000/5]

TransformerIDMT relay

CT[400/5] Feeder

IDMT relayPS=125%TMS=0.3

11kVbusbar

Transformer33/11kV 20MVA

circuitbreaker

feeders

current transformer[1000/5]

TransformerIDMT relay

CT[400/5] Feeder

IDMT relayPS=125%TMS=0.3

Feeder relay:

Fault current 5000A → 5000 x 5/400 = 62.5A to relay

Rated relay current = 5A, PS = 125%→ pick-up current = 1.25 x 5 = 6.25 A.

Thus, PSM = 62.5/6.25 = 10

from relay curve → tripping time = 3s for PSM=10 at TMS=1

time multiplier=0.3 → actual operating time 0.3x3 = 0.9s

Add 0.5s margin to allow for uncertainties in operation time.

Thus, need transformer protection relay to operate after 1.4s at 5000A fault current.

Transformer relay:

(1) First find PS for normal load conditions

transformer rated current = 1050A (on 11kV side)

allow overload of 1.3 pu (=1365A) without tripping relay

Thus, at this current PSM should be <1.0, say 0.9 to ensure non-operation at normal loads up to 1.3 pu

At 1365A, actual relay current = 1365 x 5/1000 = 6.8 A

For a rated relay operating current of 5A, this gives a multiple of 6.8/5 = 1.36 times relay operating rating.

Nearest PS is 150%. This gives PSM = 6.8/1.5x5 = 0.91.

Close enough to 0.9 to ensure non-operationso choose PS = 150%.

Transformer relay:

(2) Determine operating time at specified fault current level.

For 5000A fault, relay current = 5000 x 5/1000 = 25 A

Thus, fault current PSM = 25/1.5x5 = 3.33

From IDMT curve at PSM = 3.33, relay operating time = 5.7s at TMS=1.

Need operating time of 1.4s for proper discrimination.

Thus, TMS of transformer relay 1.4/5.7 = 0.246 (say 0.25)

Thus, required settings of transformer relay are:

PS: 150%TMS: 0.25

Another example:

why ?

Time-current curves with discrimination.

AS/NZS 60898.1:2004Electrical accessories – circuit breakers for overcurrent protection for household and similar installations.

Conventional tripping current = current value that causes CB to trip within the conventional time

Conventional time is 1h for CB with rated current <64A or 2h for above

Conventional tripping current of CB is 1.45 times its rated current.

Conventional non-tripping current of CB is 1.13 times its rated current.

Preferred values of rated current:6, 8, 10, 13, 16, 20, 25 32, 40, 50, 63, 80, 100, 125A

Standard values of rated short-circuit capacity:1500A, 3000A, 4500A, 6000A, 10kA

Type B – magnetic trip settings 3 to 5 times rated current. For constant load not subject to high inrush current, e.g. resistive loads

Type C – trip settings 5 to 10 times rated current. Suitable for general purpose, most common

Type D – trip settings 10 to 20 (or even 50) times rated current. Used mostly for highly inductive loads, eg. motors

3 types of MCBs:

Type B: If 3 IN, opening time not less than 0.1s. If 5 IN, will trip in less than 0.1s

0.4 s for final sub-circuits that supply socket outlets (<64A), or hand-held class 1 equipment, or portable equipment for manual movement during use

5 s for other circuits including sub-mains and final sub-circuits supplying fixed or stationery equipment

Maximum disconnection times

or earth leakage circuit breakers (ELCB)

prevent electrocution when current ‘leaks’ through body to general ground mass earth

provide protection by tripping when earth leakage current exceeds limit

30mA 0.3s RCDs required for circuits supplying lighting and socket outlets in

domestic installations socket outlet circuits in residential sections of other

electrical installations

problem of nuisance tripping

Residual current devices (RCD)

RCD level of sensitivity

. Type 1: rated tripping current < 10mAmainly for protection of single appliancesand in various hospital situations

. Type 2: rated tripping current between 10mA - 30mAprotect final sub-circuits where a group ofappliances require protection against directcontact (resulting in ventricular fibrillation).

. Type 3: rated tripping current > 30mAprotect heavier equipment where protectionagainst indirect contact only is required.