Make your own free website on Tripod.com

CIRCUIT-BREAKERS/ SUBSTATIONS

 

Introduction

 Devices used for circuit breaking( or making)

 Classification of circuit breakers

  Principles of circuit breaking

 Ratings of circuit breakers

 Substations

 Extracts from Indian Standards for sub-station Equipments    - IEC Standard for circuit breakers

 OBJECTIVE TYPE QUESTIONS

Indian Standards for CTs/ PTs

Indian Standards for Isolators (AC Disconnectors)

IEC & Indian Standards for Lightning Arrestors,

Indian Standards for Switchgear Panels, LV /HV Panels, and IEC standards for   Gas- insulated substations

 

 HOME takes you to the start page after you have read these  Topics. Start page has links to other topics.

 

CIRCUIT -BREAKERS - OBJECTIVE TYPE QUESTIONS

 

  1. Which of the following results when a resistance is placed across the contacts of a circuit breaker

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.

  1. all four above

(b) i and iii

(c) i and iv.

Ans: (c)

  1. The circuit breaker rating relevant to transient stability is
  1. rated voltage
  2. rated operating time
  3. rated breaking current

Ans: ©

TOP

 

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?

Ratings of circuit breakers

 

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

 

Substations

 

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

 

Extracts from Indian Standards for sub-station Equipments

 

 

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 %

 

ACCEPTANCE TESTS   Type test Nos. 2, 3, 4, and 5 will be used as the Acceptance tests.

 

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    

 

   

                                           Indian Standards for CTs/ PTs

 

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

 

TOP 

 

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.

TOP

 

 

 

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.

TOP  

 

   Indian Standards for LV Panel Boards    [IS: 8623-1993]

 

 

There is no specific Indian Standard for the tests to be performed. However the manufacturer shall carry out following tests on the low voltage panel board and ask for type/routine tests of bought out items like switch fuse units, meter, fuses, copper strip, indicating instruments etc.

Ref.: [IS 8623-1993]

 

TYPE TEST

 

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:

 

SAMPLING CRITERIA

 

Select samples equal to 10 % of offered lot. Alternatively, as per sampling criteria given in IS: 2500

 

 

TOP 

 

SWITCHGEAR PANELS WITH HV AC CIRCUIT BREAKER [IS: 13118-1991]

 

TYPE TESTS

 

S. No

Type test

Purpose

1

Dielectric tests:

  • 1.1 Lightning & switching impulse voltage test
  • 1.2 Power frequency voltage test
  • 1.3 Artificial pollution test
  • 1.4 Partial discharge test
  • 1.5 Tests on auxiliary & control circuits

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

 

 

ROUTINE TESTS

 

 

S. No

Routine test

Purpose

1

Dielectric tests:

 

  • Dry power frequency test
  • Voltage withstand tests on auxiliary  & control circuits

 

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:

 

SAMPLING CRITERIA

 

Select samples equal to 10 % of offered lot.

 

Indian standards for Gas- insulated substations   [IEC: 517 and IEC-694]

 

 

 

 

 

 

 

TOP