Why core balance CT is preferred over residual connected CT’s to sense earth fault in 6.6 kV feeders?

Why core balance CT is preferred over residual connected CT’s to sense earth fault in 6.6 kV feeders?
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1 Answer

Answer :

a) To avoid relay mal-operation due to CT saturation

b) Better sensitivity is got.

c) High pickup and TMS avoided in IDMT earth fault relay. 
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Related questions

Where core balance CT’s are used?

Description : Where core balance CT’s are used? 

Last Answer : In earth fault protection used. It senses the zero sequence current.  

Why 2 types of earth fault relays in 6.6 kV side of transformers?

Description : Why 2 types of earth fault relays in 6.6 kV side of transformers?

Last Answer : I – Trips 6.6 kV breakers only. It gives primary protection for 6.6 kV bus bars. I1 – Trips the both HT and LT breakers. It acts as a backup to ref and also acts as backup to bus bar earthfault relay. 

Explain with simple diagram the core balance CT.

Description : Explain with simple diagram the core balance CT.

Last Answer : In core balance CT all the three phases go through the core and the resultant magnetic flux is zero. Because the flux of three phases cancel each other. So the secondary output of CT is zero and ... energised to trip the circuit. Saturation is no problem because the core size is very big.  

Where core balance CT is used?

Description : Where core balance CT is used?

Last Answer : Core balance CT is used in earth fault protection.  

Give the details of switchyard 220 kV CB, Isolator, CT, CVT and lightning arrestor.

Description : Give the details of switchyard 220 kV CB, Isolator, CT, CVT and lightning arrestor.  

Last Answer : 220 kV SF6 Circuit Breaker 1. Make - ABB 2. Air pressure blocking a. Close Block - 17.3 bar. b. Open block - 16.7 bar. c. Auto reclose block - 19 bar. 3. SF6 pressure block a. Alarm - 5. ... , storage and erection. Rated voltage : 216 kV rms. Operating voltage : 184 kV rms. 

To protect the power transformer (Y-Y with neutral grounded) against fault, what type of connection do the CTs have ?

Description : To protect the power transformer (Y-Y with neutral grounded) against fault, what type of connection do the CTs have ?

Last Answer : To protect the power transformer (Y-Y with neutral grounded) against fault, what type of connection do the CTs have ? Δ-Δ connection

Why resistance grounding preferred for less than 33 kV and more than 415 V?

Description : Why resistance grounding preferred for less than 33 kV and more than 415 V?

Last Answer : a) To limit the earth fault current for equipment safety else, high short ckt forces dislocate in windings/bus bars etc, b) Over voltage due to arcing ground reduced c) Permits earth fault protection (not possible in ungrounded system)

In replacing a defective transformer with a new one, which of the following actions must be carried out to insure proper operation of the equipment it serves? A. The secondary leads must be grounded for 10 minutes to remove static charges. B. The transformer connections must be made as before with regard to the indicated polarity. C. The iron core of the transformer must be flashed to pre-magnetize it. D. The iron core must be grounded for 10 minutes to remove any residual magnetism.

Description : In replacing a defective transformer with a new one, which of the following actions must be carried out to insure proper operation of the equipment it serves? A. The secondary leads must be grounded ... magnetize it. D. The iron core must be grounded for 10 minutes to remove any residual magnetism.

Last Answer : Answer: B

For LV applications (below 1 kV), ______________ cables are used. (A) Paper insulated. (B) Plastic. (C) Single core cables. (D) Oil filled.

Description : For LV applications (below 1 kV), ______________ cables are used. (A) Paper insulated. (B) Plastic. (C) Single core cables. (D) Oil filled.

Last Answer : Ans: C For low voltage applications single core cables are suitable.

Before servicing the device indicated as 'A' in panels #1 and #3 of the illustration, the device labeled 'CT' must __________. EL-0003 A. have the disconnected leads taped to prevent short circuiting. B. only be connected to multimeter on the ammeter setting C. be short circuited D. have one lead grounded to discharge static electricity for the prevention of damage to electronic components

Description : Before servicing the device indicated as 'A' in panels #1 and #3 of the illustration, the device labeled 'CT' must __________. EL-0003 A. have the disconnected leads taped to prevent ... D. have one lead grounded to discharge static electricity for the prevention of damage to electronic components

Last Answer : Answer: C

What is advantage of cast resin CT’s?

Description : What is advantage of cast resin CT’s? 

Last Answer : Can withstand bursting forces under short ckt, protect damages against external causes impervious to moisture.  

What is the various design of CT's in switchyard?

Description : What is the various design of CT's in switchyard?

Last Answer : Bus coupler CT's- live tank design 2000-1000A/1A All other CT's- dead tank design 800-600-400A/1A - lines and GT. 125A/1A – SUT

Why delta – delta CT’s are used for star – star transformer differential protection?

Description : Why delta – delta CT’s are used for star – star transformer differential protection?

Last Answer : Say primary neutral is not solidly earthed. Then for any earthfault on secondary terminal, the primary current distribution is so for external fault, the differential is likely to operate if sequence current ... into relay. The 2:1:1 distribution is possibly only for core type or delta tertiary.

What is the advantage of housing CT’s with in bushings?

Description : What is the advantage of housing CT’s with in bushings? 

Last Answer : Bushing acts as a primary insulator for the CT.  

Mention the initial commissioning checks on CT’s, PT’s and relays.

Description : Mention the initial commissioning checks on CT’s, PT’s and relays.

Last Answer : Commissioning checks on CT a) Terminal marking correctness. b) Polarity of terminals. c) Insulation resistance between primary and secondary. d) Insulation resistance between primary to earth and secondary ... h) Burden check of relay. i) Primary injection test. j) Secondary injection test. 

If the fault current is 6000 A, the relay plug setting is 150% and the CT ratio is 400/5, the plug

Description : If the fault current is 6000 A, the relay plug setting is 150% and the CT ratio is 400/5, the plug setting multiplier will be

Last Answer : If the fault current is 6000 A, the relay plug setting is 150% and the CT ratio is 400/5, the plug setting multiplier will be 10

If the fault current is 2000 A, the relay setting is 50% and CT ratio is 400:5, the plug setting multiplier will be

Description : If the fault current is 2000 A, the relay setting is 50% and CT ratio is 400:5, the plug setting multiplier will be

Last Answer : If the fault current is 2000 A, the relay setting is 50% and CT ratio is 400:5, the plug setting multiplier will be 10 A

Purpose of Over current & earth fault relay; type- CAG

Description : Purpose of Over current & earth fault relay; type- CAG  

Last Answer : This relays are designed for instantaneous phase or earth fault protection and instantaneous high set over current protection.; A standard hinged-armature unit forms the basic movement for this relay. It consists an ... to the end of the core while the other arm to operate a set of contacts.  

Purpose of Directional inverse Over current & earth fault relay; type- CDD

Description : Purpose of Directional inverse Over current & earth fault relay; type- CDD

Last Answer : Directional phase or earth fault protection of ring-mains, parallel transformers, transformer feeders, parallel feeders etc., employing the time graded principle.; This relay comprises an inductive ... induction disc unit. The directional unit is normally provided with voltage polarising coils. 

Why earth switches are provided in 220 kV bays?

Description : Why earth switches are provided in 220 kV bays?  

Last Answer : When bay CB trips, both end (station and grid) CB will trip. The earth switches are provided because the grid will always be alive so to prevent any shocks to the operator or maintenance personnel who is working on the line or bay due to accidental energizing of the bus.  

A 20-MVA, 6.6-kV, 3-phase alternator is connected to a 3-phase transmission line. The per unit

Description : A 20-MVA, 6.6-kV, 3-phase alternator is connected to a 3-phase transmission line. The per unit positive-sequence, negative-sequence and zero-sequence impedances of the alternator are j0.1, j0.1 and j0.04 respectively. The neutral of ... the fault is  (A) 513.8 V (B) 889.9 V (C) 1112.0 V (D) 642.2 V

Last Answer : A 20-MVA, 6.6-kV, 3-phase alternator is connected to a 3-phase transmission line. The per unit positive-sequence, negative-sequence and zero-sequence impedances of the alternator are j0.1, ... line. The voltage of the alternator neutral with respect to ground during the fault is 642.2 V

how can I get rid of wasps that have taken over my hummingbird feeders?

Description : how can I get rid of wasps that have taken over my hummingbird feeders?

Last Answer : when you take one at a time down to clean & fill them, put a little vaseline on each spot where the hummingbirds will feed. The wasps can't land and will go somewhere else.........

A single core cable is designed to work for a single phase system with a peak voltage of 220 kV. The maximum

Description : A single core cable is designed to work for a single phase system with a peak voltage of 220 kV. The maximum permissible stress in the dielectric is not to exceed 20 kV/mm. For economical size, the overall core diameter of the cable is (A) 28.2 mm (B) 26.4 mm (C) 24.4 mm (D) 22.0 mm

Last Answer : A single core cable is designed to work for a single phase system with a peak voltage of 220 kV. The maximum permissible stress in the dielectric is not to exceed 20 kV/mm. For economical size, the overall core diameter of the cable is 22.0 mm

relay is preferred for phase fault on short transmission line. (a) Induction type (b) Reactance (c) Impedance (d) None of the above

Description : relay is preferred for phase fault on short transmission line. (a) Induction type (b) Reactance (c) Impedance (d) None of the above

Last Answer : (b) Reactance

Which relay is preferred for phase fault on short transmission line?

Last Answer : Reactance relay is preferred for phase fault on short transmission line.

What are the types of cables used in 6.6 kV and 415 V system voltages?

Description : What are the types of cables used in 6.6 kV and 415 V system voltages? 

Last Answer : In 6.6 kV system AC (unearthed) grade having stranded aluminium conductors cables are used. Their insulation's are as follows. 1. XLPE - Crossed linked polyethylene insulation. 2. FRLS PVC - Fire ... V grade copper or aluminium stranded cables are used. Their insulations are HR PVC and FS type. 

What are the interlocks between 415 V and 6.6 kV switch gear?

Description : What are the interlocks between 415 V and 6.6 kV switch gear? 

Last Answer : 6.6 kV Breaker closed cannot rack in or out. Service lever cannot move while breaker closed. Breaker cannot be closed in in-between position. 415 V Breaker door cannot be opened when breaker is ... . Breaker cannot be closed in in-between position. Breaker closed cannot be rack in or out. 

What is the difference between lockout of 6.6 kV and 415 V CB’s?

Description : What is the difference between lockout of 6.6 kV and 415 V CB’s?

Last Answer : Voltage levels. Manual / auto reset. Lockout relays of 415 V breakers are all Electro-magnetic type. There is no mechanical latch. But in 6.6 kV it is of mechanical latch type relays.

What is the distribution of DC control supply in CL IV, III, II- 415V and 6.6 kV?

Description : What is the distribution of DC control supply in CL IV, III, II- 415V and 6.6 kV?

Last Answer : Closing coil and Trip coil 2 supply from one source. Trip coil 1 supply from one source. Protections supply from one source. 

How 6.6 kV-bus supply was chosen?

Description : How 6.6 kV-bus supply was chosen?

Last Answer : 11 kV was rejected in view of the high insulation cost with 11 kV motors. 3.3 kV was rejected, since max motor size with 3.3 kV bus is limited to 2 MW. But we are having the motors having rating more than 2 MW, which cannot suit to 3.3 kV bus. 6.6 kV bus we can start upto 5 mw size motor. 

What is the design basis of 6.6 kV aluminium bus bars?

Description : What is the design basis of 6.6 kV aluminium bus bars?  

Last Answer : a) Temperature rises not exceed 90 ºC. b) Withstand short ckt stresses. c) Take care of thermal expansion. 

During unit operation can we have one UT feeding both unit 6.6 kV loads?

Description : During unit operation can we have one UT feeding both unit 6.6 kV loads? 

Last Answer : No, logically prevented.  

Why 6.6 kV transformer is resistance grounded by 10 ohms and current limited to 400 A?

Description : Why 6.6 kV transformer is resistance grounded by 10 ohms and current limited to 400 A?

Last Answer : a) To reduce burning and melting in faulted switchgear or machine. b) To reduce mechanical stresses in equipment. c) To reduce the electrical hazards by stray ground fault currents in the ground ... screen in 6.6 kV XLPE (cross-linked polyethylene). Therefore the cost of the cable decreases. 

What is the difference in action of lock out pressure signal on 6.6 kV and 220 kV breakers?

Description : What is the difference in action of lock out pressure signal on 6.6 kV and 220 kV breakers?

Last Answer : When lock out signal comes to 6.6 kV breakers the breaker will trip. Where as in case of 220 kV breakers the breaker will not trip. If the breaker is open it will be open only and can not be closed. If it is in closed condition it will be closed.

What type breakers are provided in 6.6 kV buses?

Description : What type breakers are provided in 6.6 kV buses?

Last Answer : ABB. Make, SF6 gas, 1250A and 2000A capacity circuit breakers are provided in 6.6 kV buses. 

List some important loads to 6.6 kV buses.

Description : List some important loads to 6.6 kV buses. 

Last Answer : a. Auxiliary transformers. b. PHT motors. c. BFP motors.  d. CEP motors. e. CCW motors. f. Chiller motors. g. Pressuring pump motors. 

What are the main sources of power supply to 6.6 kV buses?

Description : What are the main sources of power supply to 6.6 kV buses? 

Last Answer : a. Unit transformer which steps down the generated voltage to 6.6 kV from the generator. b. Start up transformer, which steps down the grid voltage to 6.6 kV. 

What is the minimum voltage required for starting of 6.6 kV motors?

Description : What is the minimum voltage required for starting of 6.6 kV motors?

Last Answer : Minimum 80% of rated voltage.

The voltage at the two ends of a transmission line are 132 KV and its reactance is 40 ohm. The Capacity of the line is (A) 435.6 MW (B) 217.8 MW (C) 251.5 MW (D) 500 MW

Description : The voltage at the two ends of a transmission line are 132 KV and its reactance is 40 ohm. The Capacity of the line is (A) 435.6 MW (B) 217.8 MW (C) 251.5 MW (D) 500 MW

Last Answer : (A) 435.6 MW

The hook-on AC volt-ammeter consists essentially of a split-core and a rectifiertype instrument connected to the secondary winding of a _____________. A. potential transformer B. control transformer C. current transformer D. reactance transformer

Description : The hook-on AC volt-ammeter consists essentially of a split-core and a rectifiertype instrument connected to the secondary winding of a _____________. A. potential transformer B. control transformer C. current transformer D. reactance transformer

Last Answer : Answer: C

If a transformer is connected to a DC source, the transformer will overload at the ____________. A. contacts B. primary coil C. secondary coil D. core

Description : If a transformer is connected to a DC source, the transformer will overload at the ____________. A. contacts B. primary coil C. secondary coil D. core

Last Answer : Answer: B

When earth fault occurs: a) voltage potential at the earth mat increases due to grounding b) voltage potential at the earth mat decreases due to grounding c) voltage potential at the earth mat remains zero irrespective of fault d) none of the above

Description : When earth fault occurs: a) voltage potential at the earth mat increases due to grounding b) voltage potential at the earth mat decreases due to grounding c) voltage potential at the earth mat remains zero irrespective of fault d) none of the above

Last Answer : Ans: A

How generator earth fault relay works?

Description : How generator earth fault relay works? 

Last Answer : 100% earth fault relay works on the principle involving monitoring of neutral side and line side components of 3rd harmonic voltages produced by generator in service. Since the machine is grounded with ... be detected by VL3 neutral displacement module, which is tuned to find frequency.   

Will rotor earth fault relay operate for earth fault in main exciter winding or RCU Diode Bridge also?

Description : Will rotor earth fault relay operate for earth fault in main exciter winding or RCU Diode Bridge also? 

Last Answer : Yes. 

Why not high resistance for earth fault than using grounding transformer & resistor 0.45 ohms?

Description : Why not high resistance for earth fault than using grounding transformer & resistor 0.45 ohms?

Last Answer : It is mechanically unwide. Difficult to manufacture.  

Why earth fault current of generator should not be reduced below 5A?

Description : Why earth fault current of generator should not be reduced below 5A? 

Last Answer : To limit over voltage due to neutral shift.  

at the terminal of a three-phase 6.6Kv alternator a load R=200 ohm is connected between two-phase and the third phase is opened. Z1=z2=5 ohm Zo=2 find out the fault current .

Description : at the terminal of a three-phase 6.6Kv alternator a load R=200 ohm is connected between two-phase and the third phase is opened. Z1=z2=5 ohm Zo=2 find out the fault current .

Last Answer : 32A

Why CT operated over load relay is using for loads of high acceleration time upto 30 seconds? How it getting back?

Description : Why CT operated over load relay is using for loads of high acceleration time upto 30 seconds? How it getting back?

Last Answer : The saturable current transformers linearly transforms the current upto twice the set current, but above this value the transformer core gets saturated and the secondary current is proportionally less. Thus ... overload. When current reduces the core gets de-saturated, as material design is such. 

What is over voltage interturn test for CT?

Description : What is over voltage interturn test for CT?

Last Answer : With secondary open, pass rated current in primary for 1 min. Then check secondary for insulation. 

A generator is connected through a 20 MVA, 13.8/138 kV step down transformer, to a transmission line.

Description : A generator is connected through a 20 MVA, 13.8/138 kV step down transformer, to a transmission line. At the receiving end of the line a load is supplied through a step down transformer of 10 MVA, 138/69 kV rating. A 0.72 ... load (in per unit) in generator will be (A) 36 (B) 1.44 (C) 0.72 (D) 0.18

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