MEASURING INSTRUMENTS

a) Voltmeter: - Voltage in an AC circuit is measured by voltmeter. The voltmeter is connected across the load or winding. For high voltage, voltage transformer is necessary to step down the voltage for measurement. Voltmeter is connected across the secondary circuit of PT. Voltmeter can be replaced on line by removing fuses or keeping voltmeter selector switch in OFF position.

b) Ammeter: - Current in a circuit is measured by ammeter connected in series of current path. If current is high, suitable current transformer (CT) is necessary to step down current for measurement. Ammeter is connected in series of secondary circuit of CT. Ammeter can be replaced by shorting CT secondary wires or keeping ammeter selector switch in OFF position.

c) Energy Meter: -The Power in electrical circuit is measured by energy meter.

Energy is the total power consumed over a certain period and is measured in kilowatt-hour (KWH). One kilowatt-hour is equal to the energy consumed when power is utilized at the rate of one kilowatt for one hour. The term ‘unit’ used for expressing consumption of electrical energy is equal to one kilowatt-hour, and all tariffs for energy consumption are based on this unit. A registering mechanism in the energy meter indicates the total energy consumption. Energy meters will record correctly, if connections are made with due care to the polarity and the terminal markings. Energy meters can be changed or replaced while in service by use of T.T.B. (Test terminal block). In TTB, CT secondary can be shorted during removal of Meter (avoiding open circuit of CT secondary) & PT supply can be made OFF by disconnecting type arrangement or by removing fuses. Energy meter records Import / Export energy parameters.

Import parameters are displayed by arrow in direction and Export parameters in direction.

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POWER TRIANGLE

Apparent Power Reactive Power KVA (S) KVAr (Q)

φ

Active Power KW (P)

Power Factor = Cos φ = (Active Power) / (Apparent Power)

Active Power: The actual amount of power that produces the effective work is called active or real power. It is measured in Watts.

Reactive Power: The power drawn by reactive load such as Capacitors and Inductors in a system is called reactive power. It is measured in VAr (Volt amp reactive).

Apparent Power: The total power demanded by the load is the product of current and voltage. This power is referred as apparent power. It is measured in VA (Volt amp).

Multiplication Factor of Energy Meter:

M.F. = [(Feeder CTR x PTR) / (Meter CTR x PTR)]

Case 1: - a) Feeder CTR = 600/1 A, Feeder PTR = 33000/110 V b) Meter CTR = 300/1 A, Meter PTR = 33000/110 V [(600/1) x (33000/110)]

M.F. of Energy Meter = --- = 2 [(300/1) x (33000/110)]

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A Typical SECURE Energy Meter connection is shown below

R

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d) Earth Tester: - Resistance of the earth pit ‘E’ in following figure can be measured directly with the help of an ‘earth tester’

Earth Tester

E P C Electrode Potential Current

(Earth Pit) Electrode Electrode

E is the earth pit electrode under measurement; P & C are two auxiliary electrodes of 15-20 mm diameter and 40 cm long bars. The electrode C1 is planted at a distance of approx. 25 metres from E and P1 is fixed centrally between E and C1.

One reading of Pit resistance is taken by rotating handle of earth tester. Two more readings are taken by shifting P1 a distance of 3 metres on either side of its central position. The value is the resistance of Electrode E to the earth.

e) Insulation Tester (Megger): - Insulation resistance between an insulated conductor (part) and earth is checked by megger. Phase conductor is connected to the terminal marked ‘Line’ on the megger and the terminal marked ‘Earth’ is connected to the earth continuity conductor or an efficient earth. The handle is turned to indicate a steady reading on the instrument. A megger, with its handle being turned gently, should record zero when its two leads are touched together, and read infinity when its leads are held apart.

P1 P2

C1 C2

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f) Oil Tester (BDV Tester): - Dielectric breakdown strength of transformer oil is one of the most reliable tests for proving the condition of oil. Oil sampling is done by taking due care. The glass bottle into which oil is drawn should be perfectly clean, clear, transparent and dry. It should then be thoroughly rinsed with oil known to be good. The sample of oil should drawn preferably be drawn from the bottom of the transformer tank. As water is heavier than oil, it settles down at the bottom. The first sample or two may be thrown away if it contains sludge or droplets of water.

The gap between two electrodes is to be maintained / checked at 2.5 mm by gauge and the test cup is cleaned properly. The cup is then filled with the sample oil to be tested up to 1 cm above the electrodes. The cup top should then be covered with a clean glass plate and allowed to rest for at least 5 minutes so that all air bubbles may disappear. Any bubbles still standing on the surface may be removed with a clean glass rod. Use thin rubber gloves if you can, so that the sweat on your fingers may not cause any contamination of the oil. Carry out test as per procedure until there is positive and final breakdown of the oil. The test is carried out for six times on the same sample after a gap of at least 5 minutes. The average of all six readings is the dielectric strength of oil under test.

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Some Important Numbers used with their meanings 2: Time Delay Relay or Timer

21: Distance Protection Relay 27: Under Voltage Relay

49: Winding Temperature Indicator

50/51: IDMT Over Current Relay with Instantaneous element 50/51N: IDMT Earth Fault Relay with Instantaneous element 52: AC Circuit Breaker

59: Over Voltage Relay

62: Pole Discrepancy Relay with timer 63: Gas Operated Relay (Buchholz Relay) 64R: Restricted Earth Fault Relay

67: Directional Over Current Relay 67N: Directional Earth Fault Relay 75: P.T. selection Relay

80: DC Supervision Relay

86: Master Trip / Locking Out Relay 87: Differential Relay

89: Line Switch / Isolator (Electrically Operated) 94: Anti-pumping Relay (For Breaker Control) 95: Trip Circuit Supervision Relay

96: Gas Pressure Relay (For Breaker Control)

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Trouble Shooting Works: -

S/No. Probable Trouble Cause / Works to be attended

CT Circuits

1 Noise in CT - Open circuit of secondary circuit

- Loose connections in secondary

circuit

- Ammeter switch fault causing open circuit in CT secondary

2 Ammeter is not recording - Ammeter may be faulty

- Ammeter switch is not making contact

PT Circuits

1 Voltmeter not showing correct reading

- Check fuses

- If above is OK, voltmeter may be faulty

- Loose connections in PT circuit 2 Energy meters recording on

lesser side - CT secondary circuit may be in shorting position for one or two phases

- PT circuit fuse may be blown - Loose connections

- Energy meter may be faulty D.C. Protection Circuits

1 Non working of trip healthy

indication - Fusing of bulb or bulb may be fitted loose

- Loose connections

- Resistance may be open circuited - Misalignment of auxiliary contacts of breaker

- D.C. fuse may be loose or blown off - Trip coil is open

- Push button may be faulty

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S/No. Probable Trouble Cause / Works to be attended D.C. Protection Circuits

- Check the circuit as per circuit diagram

- No free movement of plunger of close coil

- Mechanical trouble in breaker - Spring might not have been charged

- Non resetting of Master / Trip relay - Non closing from remote end, if Local / Remote switch on local position

- Closing switch or Push button may

be faulty

- Check the circuit as per circuit diagram

4 Tripping of breaker without Indication

- Due to shorting of D.C Positive - D.C. leakage

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S/No. Probable Trouble Cause / Works to be attended D.C. Protection Circuits

5 Mal-operation of Relay - Defect in relay or setting, If relay is defective, relay needs to be replaced - Wiring connection problem

6 Relay Flag not resetting - Mechanical defect in the flag mechanism

7 Spring charging motor does not

start - Either loose fitting of fuse and link or blowing of fuse

- Aux. relay provided may not working

- Sealing (hold on) supply getting to the aux. relay through ‘accept’ push button might have disconnected due to faulty ‘accept’ push button

2 Continuous ringing of Bell - D.C. leakage

- Disturbance in aux. relay contacts adjustment

3 Non resetting of Bell - Accept push button faulty - D.C. leakage

- Aux. relay faulty

4 Non resetting of Indication - Reset push button faulty - D.C. leakage

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S/No. Probable Trouble Cause / Works to be attended Indication Circuits

1 Lamp not indicating for breaker ON-OFF position

- Either loose fitting of fuse and link or blowing of fuse

- Lamp may be loose fitted or fusing of lamps

- Loose connections

- Defective aux. switch contacts of breaker

2 Semaphore not working - Either loose fitting of fuse and link or blowing of fuse

- Semaphore coil might have burnt

- Loose connections

- Defective aux. switch contacts of breaker or Isolator/Earth switch, as applicable

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Safety Electrical Clearances: -

Most of the equipments in a substation are provided with sufficient insulation from breaking down. There is a need for assurance that the breakdown or flashover will not occur to the operating personnel and some safe distance is to be maintained.

Clearances are broadly categorized as below.

a) Phase to earth clearance: - Equipment phase to earth clearance depends on the type of insulation material used. While equipment bushings take care of external clearances, the insulating material inside the equipment like oil, SF6, and vacuum take care of earth clearance internally.

b) Phase to phase clearance: - It is the clearance between two conductors charged electrically. Sufficient phase-to-phase clearance has to be provided in air to prevent flash over & breakdown of air insulation. This clearance is one of the factors in deciding bay width in substations.

c) Section clearance: - This clearance is required from point of safety to operating personnel. It is distance between two sections of a substation that enables a person to work on one section of a substation, in a safe manner, that the phase to earth clearance is maintained between the live point and the approach of the working personnel with sufficient margin.

d) Ground clearance: - It is a distance between ground level and bottom of any insulator in an outdoor substation.

Standard Safe Clearances

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Indian Electricity Rule – 77