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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.
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