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Saturday, 28 April 2012

Substation Equipments

Substation equipments
Transformer: transformers in distribution s, step down the primary voltage to lower values suitable for distribution purpose. Three phase oil cooled in delta/star are used. Sometimes 3-winding transformers are also used so that a different supply voltage may be obtained, from tertiary.
Circuit breakers: whenever a fault occurs, the relays sense the fault and send trip signal to the circuit breaker. The circuit breakers disconnect and isolate the faulty section. Oil circuit breakers, air blast circuit breakers, and sulphur hexafluoride circuit breakers are used in power systems
Isolators: whenever maintenance or repair work is  to be carried out on an equipment in a substation , it is disconnected from the supply by isolators. They are normally operated on no load. Isolators are generally interlocked with circuit breakers and earthing switches.
CT/PT: a current transformer steps down the line current for the purpose of relaying and metering. Generally single turn primary current transformers are used. The line or conductor passes through the core and acts as primary. The secondary, having large no. of turns, is wound on the core. The instrument or relay is connected to the secondary. The secondary rated current is either 1A or 5A. Normally 1A is used for metering and 5A is used for protection.
A voltage or potential transformer is used to step down the system voltage to a low value for metering and relaying. It may be an iron core type or capacitive voltage transformer (CVT), the CVT being more commonly used. The rated secondary voltages are 110V or 230V line to line.
Bus bar: the incoming and outgoing circuits are connected to bus bars. These may be either rigid aluminium tubular bus bars supported on post insulators or flexible ACSR stranded conductors bus bars supported from two end by strain insulators.
Protective relays: wherever a fault occurs the protective relay operates and sends a trip signal to circuit breakers. The relays are housed in panel in the control room.
Time graded over current protection is used for feeders. Distance protection and carrier current protection are used for transmission line. Differential protection is used for transformer protection.
Surge arrester: surge arrestor protects the substation equipment from lighting and switching surges. Present day power system invariably uses valve type arrestors.
Earthing switch: the earthing switch is connected between the line conductor and earth and is mounted on the frame of of isolator. Normally it is in open position. When the line is disconnected the earthing switch is closed to discharge the trapped charges to earth.
Station earthing system: the function of station earthing system is to provide a low resistance path for flow of earth fault current for safety of equipment and personnel.
Shunt capacitors: they supply the reactive power to the loads and thus help in improving the power factor. They are switched on during pick load conditions and switched off during opp-peak condition. Shunt capacitor banks are connected in star or delta across the tertiary winding of the transformers.
Station battery and charging current: a substation needs DC supply for protection and control purposes. This supply is obtained from batteries. 110Vdc supply is used for medium size substations while 240V supply is used for large substations. The batteries are equipped with charging equipment.
Typical Limiting Values of Sub Station Electrical Equipments.
1. Transformer / Reactor:

Sr. No.
Equipment / test data
Permissible limits
Reference
A)
Transformer oil


a) BDV


-At the time of first charging
600 kV (Gap – 2.5 mm) – Minimum
IS – 1866
-During O&M
50 kV (Gap – 2.5 mm) – Minimum
IS – 1867
b) Moisture content

IS – 1868
-At the time of first charging
15 PPM (Max.)
IS – 1869
-During O&M
25 PPM (Max.)
IS – 1870
c) Resistivity at 90 degree C
0.1-1012 Ohm-CM (Min.)
IS – 1871
d) Acidity
0.2 mg KOH/gm (Max.)
IS – 1872
e) IFT at 27 degree C
0.018 N/M (Min.)
IS – 1873
f) Tan delta at 90 degree C
0.20 (Max.)
IS – 1874
g) Flash point
126 Deg. C (Min.)
IS – 1875
B)
Vibration level for reactors
200 Microns (Peak to Peak)
IS – 1876

60 Microns (Average)
IS – 1877
C)
Tan delta for bushing at 20 Deg. C
0.007*
IEC – 137
D)
Capacitance for bushing
+ 5% variation
IEC – 138
E)
IR value for winding
1000 M-Ohm By 5.0/10.0 kV Megger
IEC – 139
F)
Tan delta for windings at 20 Deg. C
0.007*
IEEE/C57.12.90.1980
G)
Contact resistance of bushing terminal connectors
10 M. Ohm / Connector
NGC.UK Recommendations
H)
Turret Neutral CT ratio errors
39
IS – 2705

 2. Circuit Breakers 

Sr. No.
Equipment / test data
Permissible limits
Reference
A)
Dew point of SF6 gas
Dew point values as per Annexure – II

B)
Dew point of operating air
-45 Deg. C at ATM. Pressure

C)
CB Operating timings
400 kV               220KV

a) Closing time (Max.)
150 MS              200MS

b) Trip time (Max.)
25 MS                35MS

c) Close/trip time, Pole discrepancy


– Phase to Phase (Max.)
3.33 MS            3.33MS

-Break to break (Max.) of same phase
2.5 MS              2.5MS

D)
PIR time


BHEL make
12-16 MS
Manufacturers Recommendations



ABB make
8-12 MS
Manufacturers Recommendations
NGEF make
8-12 MS
Manufacturers Recommendations
M&G make
8-12 MS
Manufacturers Recommendations
TELK make
8-12 MS
Manufacturers Recommendations
ABB make (HVDC)
8-12 MS
Manufacturers Recommendations
E)
PIR opening time prior to opening of main contacts (ABB, CGL, NGEF make CBs)
5 MS (Min.) at rated pressure
Manufacturers Recommendations
F)
Pir and main contacts overlap time [BHEL, M&G, ABB (imported) make CBs]
6 MS (Min.) at rated pressure
Manufacturers Recommendations
G)
Tan delta of grading capacitors
0.007 at 20 Deg. C

H)
Capacitance of grading capacitors
Within + 10% / -5% of the rated value
IEC 359
I)
Contact resistance of CB
150 M. Ohm

J)
Contact resistance of CB terminal connector
10 M. Ohm per connector
NGC, UK recommendations
K)
IR value:


1. Phase – earth
1000 M Ohm (Min.) by 5.0 / 10.0 kV Megger

2. Across open contacts
1000 M Ohm (Min.) by 5.0/10.0 kV Megger

3. Control cables
50 M Ohm (Min.) by 0.5 kV Megger

L)
Pressure switch settings


-SF6 gas pressure switches
Within + 0.1 Bar of set value

-Operating air pr. Switches
Within + 0.1 Bar of set value

-Operating oil pr. Switches
Within + 0.1 Bar of set value

M)
BDV of oil used for MOCB


-At the time of filling
40 kV at 2.5 mm Gap (Min.)
Mfgs. Recommendation
-During O&M
20 kV at 2.5 mm Gap. (Min.)
Mfgs. Recommendation

 3. Current Transformer

Sr. No.
Equipment / test data
Permissible limits
Reference
A)
IR value


1. Primary – earth
1000 M – Ohm (Min.) by 5.0/10.0 kV Megger

2. Secondary – earth
50 M – Ohm (Min.) by 0.5 kV Megger

3. Control cables
50 M-Ohm (Min.) by 0.5 kV Megger

B)
Tan delta value
0.007* at 20 Deg. C

C)
Terminal Connector
10 M-Ohm per connector
NGC, UK Recommendations
D)
CT ratio errors
+ 3% -Protection cores
IS – 2705
D)
CT ratio errors
+ 1% -Metering cores
IS – 2706

 4. Capacitive Voltage

Sr. No.
Equipment / test data
Permissible limits
Reference
A)
Tan Delta
0.007* at 20 Deg. C

B)
Capacitance
Within +10%/-5% of the rated value
IEC – 358
C
Contact resistance of terminal connector
10 M-Ohm per connector
NGC, UK Recommendations
D)
IR Value
IR Value

1. Primary – earth
1000 M – Ohm (Min.) by 5.0/10.0 kV Megger

2. Secondary – earth
50 M – Ohm (Min.) by 0.5 kV Megger

3. Control cables
50 M-Ohm (Min.) by 0.5 kV Megger

E)
EMU tank oil parameters
EMU tank oil parameters

a) BDV (Min.)
30 kV (Gap. –2.5 mm)
IS – 1866
b) Moisture content (Max.)
35 ppm
-do-
c) Resistivity at 90 Deg. C
0.1 – 1012 Ohm. – CM
-do-
d) Acidity
0.5 mg kOH /gm (Max.)
-do-
e) IFT at 27 Deg. C
0.018 N/M (Min.)
-do-
f) Tan delta at 90 Deg. C
1.0 Max.
-do-
g) Flash point
125 Deg. C (Min.)
-do-
F)
CVT voltage ratio errors
+ 5% protection cores
IEEE/C93.1.1990
F)
CVT voltage ratio errors
+ 0.5% metering cores
IEC 186

 5. Isolators

Sr. No.
Equipment / test data
Permissible limits
Reference
A)
Contact resistance
300 M-Ohm. (Max.)

B)
Contact resistance of terminal connector
10 M – Ohm per connector
NGC, UK Recommendations
C)
IR value


1. Phase – earth
1000 M – Ohm (Min.) by 5.0/10.0 kV Megger
NGC, UK Recommendations
2. Across open contacts
1000 M – Ohm (Min.) by 5.0/10.0 kV Megger
NGC, UK Recommendations
3. Control cables
50 M-Ohm (Min.) by 0.5 kV Megger


 6. Surge Arrester

Sr. No.
Equipment / test data
Permissible limits
Reference
A)
Leakage current
500 M-Amp. (Resistive)
Hitachi, Japan Recom.
B)
IR value
1000 M-Ohm. (Min.)
Hitachi, Japan Recom.

 7. Miscellaneous

Sr. No.
Equipment / test data
Permissible limits
Reference
A)
Station earth resistance
1.0 Ohm (Max.)

B)
Thermo vision scanning


Temp. up to 15 Deg. C (above ambient)
Normal

Temp. above 15-50 Deg. C (above ambient)
Alert

Temp. above 50 Deg. C (above ambient)
To be immediately attended

C)
Terminal connectors – Contact resistance
10 M-Ohm per connector
HGC, UK Recommendations
D)
IR values


1. All electrical motors
50 M-Ohm (Min.) by 0.5 kV Megger

2. Control cables
50 M-Ohm (Min) by 0.5 kV Megger

3. Lt. Transformers
100 M.-Ohm (Min.) by Megger

4. Lt. Switchgears
100 M – Ohm (Min.) by 0.5 kV Megger


 8. Batteries

Sr. No.
Equipment / test data
Permissible limits
Reference
A)
Terminal connector resistance
10 M – Ohm + 20%
ANSI/IEEE – 450 1989
B)
Specific gravity
1200 + 5 GM/L at 27 Deg. C


 Temperature Correction Factor for Tan Delta Measurement


 Sr. No.
 Oil temperature Deg. C
 Correction factor(K)
 1
 10
 0.8
 2
 15
 0.9
 3
 20
 1.0
 4
 25
 1.12
 5
 30
 1.25
 6
 35
 1.40
 7
 40
 1.55
 8
 45
 1.75
 9
 50
 1.95
 10
 55
 2.18
 11
 60
 2.42
 12
 65
 2.70
 13
 70
 3.00


If Tan Delta of bushing/winding/CVT/CT is measured at oil temperature T Deg. C. Then Tan Delta at 20 Deg. C shall be as given below:
Tan Delta at 20 Deg. C = Tan Delta at Temp T Deg. C / Factor K.
Dew Point Limits for SF6 Gas in EHV Circuit Breakers

Sr. No
Make of C.B
Dew point at rated Pr. Deg. C
 Corresponding dew point at Atmospheric .Pr.
 Remarks
1
BHEL 
 -15
 -36
 At the time of commissioning
 -7
 -29
 During O&M
 -5
 -27
 Critical
2
M&G
 –
 -39
 At the time of commissioning

 -32
 During O&M
3
CGL
 -15
 -35
 At the time of commissioning
 -10
 -31
 During O&M
4
ABB
 -15
 -35
 At the time of commissioning
 -5
 -26
During O&M
5
NGEF
 -15
 -36
At the time of commissioning
 -7
 -29
 During O&M

 Note: Dew point of SF6 gas varies with pressure at which measurement is carried out. So it is to be ensured that if measurement is done at pressure other than atmospheric pressure, it needs to be converted to the atmospheric pressure.

8 comments:

  1. Informative blog indeed about Capacitor Voltage Transformer and its manufacturing process.

    ReplyDelete
  2. Hi, nice interesting informative blog. We also manufacture and supply Servo Voltage Stabilizers & Isolation and CVT Transformers.
    Thanks for sharing.

    ReplyDelete
    Replies
    1. thanks for comment. could you send mail on servo voltage stabilizer to amieclub@gmail.com

      Delete
  3. Valuable information and excellent design you got here! I would like to thank you for sharing your thoughts into the stuff you post!!

    Substation Commissioning

    ReplyDelete
  4. The substations are where the voltages are increased to high values by using step up transformers, and after the transmission, they are again stepped down for distribution.
    Transformer Manufacturer in India

    ReplyDelete
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