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Extracts from Indian Standards for sub-station Equipments - IEC Standard for circuit breakers |
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CIRCUIT -BREAKERS - OBJECTIVE TYPE QUESTIONS
i successful and effective interruption of current
ii undue power loss in the resistor
iii effective fault power dissipation
iv control of rate of rise of restriking voltage.
(b) i and iii
(c) i and iv.
Ans: (c)
Ans: ©
Introduction
Switchgear in a broad sense covers a wide range of equipment connected with switching and protection. A circuit breaker is a switching i.e. current interrupting or making device in switchgear. Th basic requirements of switching in power system practice are two-fold:
1. to permit apparatus and circuits to be conveniently put into or taken out of service
2. to permit appropriate and safe isolation of apparatus and circuits automatically, in a pre-determined time period, when they develop faults.
Devices used for circuit breaking (or
making)
1.
Fuse and iron clad
switches
Fuse is an overcurrent switch in the sense that when the current exceeds a pre-assigned value in a circuit or device, it melts and causes current interruption. The supply is restored only when a healthy one replaces the damaged (melted) fuse in the line. To permit this without any danger of shock to the operator, fuses are connected on the load side of an ironclad switch.
2.
Isolators
An isolator is a switch connected after a circuit breaker. When a circuit or a busbar is taken out of service by tripping the circuit breaker, the isolator is then open circuited and the isolated line is earthed through earth switch so that the trapped line charges are safely conducted to ground.
3.
Circuit breakers
A circuit breaker is a complex circuit -breaking device with the following duties:
· Make or break both normal and abnormal currents
· Appropriately manage the high-energy arc associated with current interruption. The problem has become more acute due to interconnection of power stations resulting in very high fault levels
· Effect current interruption only when it is called upon to do so by the relay circuits. In fact they are required to trip for a minimum of the internal fault current and remain inoperative for a maximum of through fault current
· Rapid and successive automatic breaking and making to aid stable system operation
· 3-pole and single pole auto-relosing arrangement.
In addition to these making and breaking capabilities, a circuit breaker is required to do so under the following typical conditions:
· Short-circuit interruption
· Interruption of small inductive currents
· Capacitor switching
· Interruption of short-line fault
· Asynchronous switching
Classification of circuit breakers
Sketch the constructional details of he following:
1. Air-break circuit breaker or miniature C.B
2. Oil circuit breaker
3. Minimum oil circuit breaker
4. Air blast circuit breaker
5. SF6 circuit breaker
6. Vacuum circuit breaker
· Explain DC and Ac arc interruptions and bring out the difference between the two
· Describe briefly the methods adopted for rapid increase of the dielectric strength in various types of circuit breakers
· Explain the working of a vacuum circuit breaker.
Principles of circuit breaking
DC circuit breaking-effect of decreasing current & increasing arc length
AC circuit breaking-current-zero period-Distortion of AC current wave by arc voltage recovery and restriking voltages: single-frequency & double frequency transients- rate of rise of recovery voltage (RRRV)-control of RRRV-Resistance switching
Current chopping-Interruption of low magnetizing currents-Opening resistors-capacitive current breaking-Switching of capacitor banks and unloaded lines-Interrupting terminal faults & short-line faults
· Explain the phenomenon of AC circuit braking and the factors upon which the restriking voltage depends.
· Explain current chopping and effects of breaking capacitive currents. How are these problems met in practice?
Rated voltage
Rated insulation
Rated frequency
Rated normal current
Rated short circuit breaking current
Rated short circuit making current
Rated opening sequence for auto-reclose CBs
Rated transient recovery voltage for terminal faults (Representation of TRV by 4-parameters and 2- parameters
CB interrupting time-its components in relation to fault clearing time
Single-pole auto reclosing & its effects on system performance
Indoor switchgear
a. Stationary cubicle type
b. Draw-out or truck type
c. SF6 filled switchgear
d. Fuse-switch units
e. Flame proof or explosion proof switchgear
f. Cellular type
g. Corridor switchboard
h. Mimic diagram board
i. Metal-clad switchgear
j. Isolator and earthing switch-vertical break isolator-Double break isolator
· Sketch the layout of a typical sub-station including busbar arrangements
· Define and discuss-rated short circuit breaking current-rated short-circuit making current-Rated TRV and its influence on the breaking & making current
· Explain the need of an isolator in a switching station TOP
For Indian Standards on Underground Cables, see Notes on Underground Cables
LEAD ACID STORAGE STATIONARY BATTERIES [IS: 1651-1991]
TYPE TESTS:
|
S. No. |
Type test |
Purpose |
|
1 |
Verification
of Constructional Requirements |
To verify materials of container, top cover, connector, & positive & negative element (lead/calcium) of a cell |
|
2 |
Verification
of Marking |
To
know the type of cell, make, manufacturing date etc |
|
3 |
Verification
of dimensions |
To
verify dimensions as per rated capacity |
|
4 |
Test
for capacity |
To
confirm the rated capacity of a cell |
|
5 |
Tests
for voltage during discharge |
To
observe cell voltage during AH capacity test as under; a)
Voltage
after 10 min. load – 1.98 V b)
Voltage
after 6 hours load- 1.92 V c)
Voltage
after 10 hours load completed- 1,85 V |
|
6 |
Ampere-hr
and W-hr efficiency test |
a)
AH
capacity shall not be less than 90% b)
WH
capacity shall not be less than 75% |
|
7 |
Test
for loss of capacity on storage |
Shall
be less than 10% |
|
8 |
Endurance
test |
To
apply 13 cycles of 2000 hours (charging & discharging) and to observe
capacity of cell after each cycle discharge. Capacity should exceed 90 % |
Sample Scheme
|
No. of cells in a Lot |
No. of cells as sample |
Permissible defectives |
||||
|
Stage 1 |
Stage 2 |
Stage 1+ stage 2 |
Stage 1 |
Stage 2 |
Stage 1+ stage 2 |
|
|
Up to 50 |
2 |
2 |
4 |
0 |
1 |
1 |
|
51-300 |
3 |
3 |
6 |
0 |
1 |
1 |
|
301-500 |
5 |
5 |
10 |
0 |
2 |
2 |
|
501-1000 |
8 |
8 |
16 |
0 |
2 |
2 |
|
1001 & above |
13 |
13 |
26 |
0 |
3 |
4 |
NICKLE CADMIUM STORAGE STATIONARY BATTERIES[ IS :
10918-1984]
CIRCUIT BREAKERS [IEC: 62271-100] TOP
TYPE TESTS
|
S. No. |
Type test |
Purpose |
|
1 |
Dielectric tests |
To check characteristics of Circuit breaker for the following tests: I)
Dry, wet, power frequency II)
Lightning impulse voltage III)
Switching impulse voltage ( applicable for 420 kV CB and above) |
|
2 |
Radio Interference voltage test |
To determine RIV on CB pole in both close &
open position |
|
3 |
Resistance of the main circuit |
To record circuit resistance during temp. rise test at 200
C |
|
4 |
Temp. rise test |
To ensure capability of contacts to
carry rated normal current within specified temp. rise limits |
|
5 |
Short-time withstand current &
peak withstand current |
To check the ability of the circuit
to carry the maximum rated
short-circuit withstand current ( 2.5 times the RMS value) at 50 Hz in close position during the specified short-term duration
of 1 or 3 sec. |
|
6 |
Mechanical operation test at ambient
temp. |
To check the characteristics of the
breaker for 2000 operations on each
pole ( with multiple circuit
breaker with individual drive) OR
complete assembled breaker
(mechanically gang-operated breaker
with one common drive) |
|
7 |
Short-circuit current making/ breaking tests |
To check ability of the CB to clear
the current on different tests in symmetrical & asymmetrical conditions
as per provisions of IEC |
|
8 |
Capacitive current charging tests: -Line charging current breaking
tests -Cable charging current breaking
tests |
To check withstand capability of
the CB for no load transient lines,
cables , and capacitor banks |
ROUTINE TESTS
|
S. No. |
Routine
test |
Purpose |
|
1 |
Dielectric test on main circuit |
To check the dielectric’s withstand capability of live terminals to
live terminals and to earth in both
close & open condition of CB |
|
2 |
Dielectric test on auxiliary & control circuit |
To check the dielectric’s withstand
capability of auxiliary & control circuit of CB subject to short duration
(60 sec) voltage withstand test for 2
kV . ( For motor or other devices , they are subjected to a dielectric test as per appropriate specs.) |
|
3 |
Design identification test |
Verifies the CB for compliances in terms of language of name plate, identification of aux.
Equipment, colour & quality of paint etc. |
|
4 |
Mechanical operating test |
Includes 1)
5 open-close operations at max.
rated and minimum control voltage 2)
5 close-open operation at rated control voltage 3)
recording opening & closing
times at the rated operating pressure & voltage |
|
5 |
Measurement of resistance of main
circuit |
To record contact resistance of CB
for mechanical operations to be within specified limits |
THE ROUTINE
TESTS ARE TO BE COSIDERED AS ACCEPTANCE TESTS
SAMPLE
CRITERION
It is general practice to select samples equal to 10% of offered lot TOP
CURRENT
TRANSFORMERS [IS: 2705 (Part-1)-1992
|
S. No |
Type test |
Purpose |
|
1 |
Short time current test |
To measure current density of winding w. r. t rated short time current |
|
2 |
Temperature rise test |
To know actual temperature rise in
CT under loading conditions |
|
3 |
Lightning impulse test for CTs for
service in electrically exposed installation |
To know lightning impulse voltage
withstand capacity of insulation |
|
4 |
Switching impulse voltage test for CTs for 420 kV & above |
To know switching impulse voltage
withstand capacity of insulation |
|
5 |
HV power frequency wet withstand
voltage test on outdoor Ct up to & including 245 kV |
To verify power frequency voltage
withstand capacity of external insulation |
|
6 |
Determination of error or other
characteristics according o requirement of designation & accuracy class a) Current error b) Phase displacement error c) Accuracy class d) Burden |
a) To measure actual current ratio against rated ratio b) To measure the phase difference between primary & secondary currents c) To know the actual accuracy against rated d) To measure the impedance of the secondary circuit |
Notes
1.
After the CTs have been subjected to dielectric tests (3, 4, and
5), all routine tests will be carried out.
2.
Type tests may be omitted when the
manufacturer hold certificate of type tests on a similar transformer, which is
acceptable to the purchaser.
Routine Tests
|
S. No. |
Routine
test |
Purpose |
|
1 |
Verification of terminal marking & polarity |
To ensure correct marking & function of CT |
|
2 |
Power frequency dry withstand test on primary winding |
To verify power frequency withstand capacity between primary winding
& earth |
|
3 |
Power frequency dry withstand test on secondary winding |
To verify power frequency withstand capacity between secondary winding
& earth |
|
4 |
·
Over voltage inter-turn test ·
Partial discharge test ·
Current error test ·
Phase displacement error ·
Accuracy class ·
Burden |
·
To ensure inter-turn insulation
strength ·
To ensure insulation strength ·
To measure actual current ratio
against rated ratio ·
To measure phase displacement
between primary & secondary currents ·
To know the actual accuracy against
rated ·
To measure the impedance of the
secondary circuit |
OPTIONAL TESTS
|
S. No. |
Optional test |
Purpose |
|
1 |
Chopped
lightning impulse test as a type test |
To verify
negative polarity lightning impulse voltage withstand capacity of external
insulation |
|
2 |
Measurement of dielectric dissipation factor for oil immersed CT of
72.5 kV & above |
To measure dielectric dissipation factor (tan d) of primary winding between ambient temperature & equipment
temperature under test |
|
3 |
Commissioning tests on new CT up to &including 36 kV |
To measure withstand capacity of primary winding at 80% of rated over
voltage at site |
ACCEPTANCE TESTS
The routine tests are to be considered as acceptance tests
SAMPLE
CRITERIA
It is general practice to select samples equal to 20 % of offered lot
VOLTAGE
TRANSFORMERS (P.T) [IS: 3156 (Part-1)-1978
The tests are similar to CT tests.
For determining the burden, the admittance (mhos) of secondary circuit & the power factor are measured.
Indian
Standards for Isolators, Lightning Arrestors, Switchgear Panels, LV Panels, and
Gas insulated substations
AC
DISCONNECTORS (ISOLATORS)
& EARTHING SWITCHES FOR VOLTGAE >
1000 V [IS: 9921 (Part-4-1985]
TYPE TESTS
|
S. No |
Type test |
Purpose |
|
(A) NORMAL
TYPE TESTS |
||
|
1 |
Dielectric tests |
To verify insulation level |
|
2 |
Temperature rise tests |
To verify that the temperature rise
of any part does not exceed the limit |
|
3 |
Short-time withstand current &
peak withstand current tests |
To prove the capability of the
switches to withstand the stated currents |
|
4 |
Short-circuit making performance of
earthing switches |
To prove the capability of the
switches to carry the short-circuit making current |
|
5 |
Operating & mechanical
endurance tests |
To prove satisfactory
operation & mechanical endurance |
|
(B) TYPE
TESTS ON SPECIAL REQUEST OF USER |
||
|
6 |
Operation under severe ice conditions |
To prove satisfactory operation
under severe snowfall conditions |
|
7 |
Operation at temperature limits |
To prove satisfactory operation
under minimum/ maximum ambient temperature |
|
8 |
Measuring radio interference level |
For rated voltage > 120 kV |
Note: All the test shall be carried out on
complete isolator or earth switch on their operating devices & auxiliary
equipments. Single –pole tests are however permitted in certain cases.
ROUTINE TESTS
|
S. No |
Routine test |
Purpose |
|
1 |
Power frequency voltage withstand test of main circuit |
To verify insulation level including withstand tests of power frequency
voltages on auxiliary equipment |
|
2 |
Voltage test on control & auxiliary circuit |
To verify insulation level |
|
3 |
Resistance measurement of main circuit |
Should not exceed type test value |
|
4 |
Mechanical operation tests |
To ensure that disconnectors/earthing switches comply with operating
conditions at rated, maximum, and minimum supply voltage |
Note: Where isolators/earthing switches
are not assembled at the manufacturer’s works to enable the routine tests to be
made, tests on part assemblies should be subjected to under agreement between
the manufacturer & the user.
The routine tests are to be considered as ACCEPTANCE TESTS
SAMPLE CRITERIA: It is general practice to select samples equal to 10% of offered lot.
METAL OXIDE SURGE ARRESTORS
WITHOUT GAP FOR AC SYSTEM [IEC 60099-4 &IS: 3070 –1993]
TYPE
TESTS [as per IEC]
|
S. No |
Type test |
Purpose |
|
1 |
Insulation withstand test |
Demonstrates the ability of the housing to withstand voltage stresses
during dry /wet conditions |
|
2 |
Residual voltage test |
Demonstrates the protective levels of the arrestor |
|
3 |
Long duration current impulse test |
Demonstrates the ability of he resistor elements to withstand
dielectric & energy stresses without puncture or flashover. |
|
4 |
Operating duty test |
Defines the thermal stability of the arrestor under defined conditions |
|
5 |
Pressure relief test |
Demonstrates the ability of the arrestor housing to withstand
short-circuit currents without violent shattering of the housing under test
conditions |
|
6 |
Test of arrestor disconnectors |
Demonstrates correct operation of the disconnectors |
|
Artificial pollution test (not yet established) |
To show that the internal parts of he arrestor including grading system
are able to withstand pollution without any damage and that the external
insulation does not flashover |
|
|
7 |
Partial discharge test |
Measures the internal partial discharges |
|
Seal leakage test (not yet established) |
Determines the integrity of the arrestor seals |
|
|
8 |
Current distribution test for multi-column arrestor |
Determines the current through each column of parallel resistor when
there is no electrical connection between the columns |
Routine Tests
|
S. No |
Routine
test |
Purpose |
|
1 |
Measurement of reference voltage |
Measured values to be within a range specified by the manufacturer |
|
2 |
Measurement of power frequency voltage on complete arrester |
Measured values to be within a range specified by the manufacturer |
|
3 |
Lightning impulse residual voltage test |
Measured values to be within a range specified by the manufacturer |
|
NOTE: |
Type tests 2. 7 and 8 will be taken as routine tests |
|
SPECIAL TEST
|
S. No |
Special
test |
Purpose |
|
1 |
Special thermal stability test |
Metal oxide arrester temperature or resistive component of current or
power dissipation be monitored during power frequency voltage application to
prove thermal stability |
SAMPLE CRITERIA
[As per clause No. 8.2.1 of IEC: 60099-4:1998]
The acceptance test shall be made
on the nearest lower whole number in the cube root of the number of arrestors
to be supplied.
Indian Standards for LV Panel Boards [IS: 8623-1993]
Ref.: [IS 8623-1993]
|
S. No |
Type test |
Purpose |
|
1 |
Temperature rise test |
To verify the temperature rise of lT panel board including all bus bars and equipments like MCCB, ACB, MCB, Switch disconnectors |
|
2 |
Dielectric properties test |
To verify withstand capacity in HV test |
|
3 |
Verification of the effective connection between the exposed conductive parts of he ASSEMBLY and the protective circuit by inspection or by resistance measurement |
To verify connection between the exposed conductive parts and the protective circuit |
|
4 |
Verification of clearances and creepage distances |
To check for minimum required clearances |
|
5 |
Verification of mechanical operation |
To verify that product is suitable for N number of operations without any problem |
|
6 |
Verification of degree of protection |
To ensure protection against ingress of material as per the various IP code |
|
7 |
Inspection of Assembly including inspection of wiring and, if necessary, electrical operation test |
To check wiring and its proper connections |
|
8 |
Dielectric test |
To verify insulation resistance |
|
9 |
Checking of protective measures and the electrical continuity of the protective circuits |
To check protective measures |
|
10 |
Insulation resistance test |
To verify insulation provided on conductive parts and parts with the body |
|
11 |
Verification of Name plate /rating plate of all inserts |
To confirm use of all inserts as per approved drawing / bill of materials |
Notes:
Select samples equal to 10 % of offered lot. Alternatively, as per sampling criteria given in IS: 2500
|
S. No |
Type test |
Purpose |
|
1 |
Dielectric tests:
|
To verify that complete circuit breaker panel along with auxiliary and control circuits can withstand relevant dielectric tests |
|
2 |
Radio interference voltage test (on one pole of CB during open & closed position |
To check RIV during high voltage |
|
3 |
Temperature rise test |
To record actual temperature rise of breaker panel including all bus bars to be within specified limits |
|
4 |
Measurements of contact pressure & resistance of main circuit |
To verify the contact pressure and resistance as per designed value |
|
5 |
Short-time withstand & peak withstand test |
To verify the short-circuit withstand capacity of C.B Panel |
|
6 |
Mechanical & Environment test |
|
|
Mechanical operation test at ambient air temperature |
To confirm that the product is suitable for N number of operations without any problem |
|
|
Low & high temperature test |
To verify temperature rise of CB & its mechanism to be within specified limit |
|
|
Humidity test |
To check withstanding CB against humidity |
|
|
7 |
Making & Breaking tests |
To verify single phase making/braking capacity of CB |
|
-Single phase making capacity test |
||
|
-Single phase breaking capacity test |
||
|
8 |
Basic short-circuit tests- Duties Nos.1-5 tests |
To verify withstand capacity of CB for various Short-circuit Duties |
|
9 |
Critical current test |
To verify withstanding critical current capacity of CB |
|
Applicable only to CB which have critical current of less than 10% of rated SC breaking current |
||
|
10 |
Single phase SC test |
Applicable to three pole CB |
|
To check adverse effect in operation of CB due unbalanced force produced |
||
|
11 |
Short line fault test |
Applicable to 3 pole CB directly connected to overhead lines |
|
To verify withstanding capacity of CB for various SC duties |
||
|
12 |
Out of phase making/breaking test |
To determine ability of CB to break & make currents during out of phase conditions |
|
13 |
Capacitive current switching tests -Rated line charging breaking current -Rated cable charging breaking current -Rated single capacitor bank breaking current -Rated back to back capacitor bank breaking current -Rated capacitor bank inrush making current |
To check withstanding capacity of CB during line/cable/ capacitor bank charging or breaking current |
|
14 |
Small inductive current switching test |
To check withstanding capacity of CB during inductive current |
|
S. No |
Routine
test |
Purpose |
1
|
Dielectric tests:
|
To
verify that complete circuit breaker panel along with auxiliary and control
circuits can withstand relevant dielectric tests
|
2
|
Mechanical operation test at ambient air temperature |
To confirm that the product is suitable for N number of operations without any problem |
3
|
Visual
& design check
|
|
4
|
Mechanical operation test at ambient air temperature |
To confirm that the product is suitable for N number of operations without any problem |
Notes: