Define knee point voltage of a CT.

Define knee point voltage of a CT.
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1 Answer

Answer :

When the primary of a CT is open circuited and supply (variable) of system frequency is given to secondary, then a 10% increase in voltage constitutes 50% increase in current. That voltage is the knee point voltage. At this point the core is saturated and a little increase in voltage constitutes a great increase in current. kpv decides the opening range of the CT. Above kpv the ratio of transformer will not be applicable.

kpv = RCT + RLEADS + RRELAY  
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Related questions

Define knee point voltage.

Description : Define knee point voltage.

Last Answer : The voltage applied to secondary of CT keeping the primary open at which 10% increase in voltage causes 50% increase in excitation current. 

What is meant by knee point voltage?

Description : What is meant by knee point voltage?

Last Answer : Knee point voltage is the voltage at which a CT gets saturated. It is one of the important factors taken into consideration for selecting a current transformer.

What is Knee point voltage?

Description : What is Knee point voltage? 

Last Answer : EMF applied to secondary of current transformer (CT) which, when increased by 10% voltage causes the excitation current to increase by 50%. 

Draw the circuit diagram for finding out the knee point voltage and explain the procedure.

Description : Draw the circuit diagram for finding out the knee point voltage and explain the procedure.

Last Answer : Connect the circuit as shown. O/P of variac should be zero. Increase it to 5 Volts and take down the value of current from the ammeter. Now increase the voltage by 10% (5 + 10% = ... point. Above this they cannot detect the fault correctly. Measuring CT operate in the ankle region.   

What is meant by knee point voltage?

Description : What is meant by knee point voltage?

Last Answer : Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.

Can you tell from a CT scan what the amount of damage is to a knee?

Description : Can you tell from a CT scan what the amount of damage is to a knee?

Last Answer : MRI’s are much better. When I dislocated my knee, I got two CT scans, but had to eventually go in and get an MRI to examine the full extent of the injury. The doctor will have to decide what can be seen from your CT, but I wouldn’t be surprised if you need an MRI.

Knee CT scan?

Description : Knee CT scan?

Last Answer : DefinitionA computed tomography (CT) scan of the knee is an imaging method that uses x-rays to create detailed pictures of the knee.Alternative NamesCAT scan - knee; Computed axial tomography scan - knee; ... slices. These images can be stored, viewed on a monitor, or printed on film. Three-dime

But is it advisable to keep the secondary of CT in open conditions? Will not induce very high voltage?

Description : But is it advisable to keep the secondary of CT in open conditions? Will not induce very high voltage?

Last Answer : If the CT has only one secondary winding, we should keep it always short cktd for safety, but if the CT has multiple secondary, then if one secondary voltage is kept limited by suitable loading, then the other secondary voltage is eventually limited proportionately. 

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. 

Define: i) Knee voltage ii) Reverse saturation current

Description : Define: i) Knee voltage ii) Reverse saturation current 

Last Answer : i) Knee voltage : The applied forward voltage, at which the PN junction starts conducting and current starts increasing exponentially is called knee voltage. It is 0.7V for Si and 0. ... semiconductor diode caused by diffusion of minority carriers from the neutral regions to the depletion region.

What is the operating point in the Magnetising characteristic of protection CT & measuring CT?

Description : What is the operating point in the Magnetising characteristic of protection CT & measuring CT? 

Last Answer : Protection CT – Operation at ankle point only. Measuring CT – Operation from ankle to knee point 

What is knee point?

Description : What is knee point?

Last Answer : Knee point is the region, where 10% increase in flux causes 50% increase in exciting ampere-turns. 

What is the output voltage of a CT when it is set on electrical zero?

Description : What is the output voltage of a CT when it is set on electrical zero?

Last Answer : Zero or minimum voltage.

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Description : What is the amplitude and voltage induced into the rotor when the CX is turned 90º while the CT remains on electrical zero?

Last Answer : The voltage is maximum and in phase with the ac excitation voltage to the CX.

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

Difference between Current Transformer (CT) and Potential Transformer (PT)

Description : Difference between Current Transformer (CT) and Potential Transformer (PT)

Last Answer : Current Transformers (CT) and Potential Transformers (PT) are used to measure the current and voltage in a circuit of the order of hundreds of amperes and volts respectively. A CT has large number of ... and flux density vary over a wide range whereas in PT, they vary over a limited range only

What are the errors in CT?

Description : What are the errors in CT?

Last Answer : (a) Ratio error Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100 The value of transformation ratio is not equal to the turns ratio. (b) Phase angle error: Phase angle =180/ [(ImCos -I1Sin )/nIs]

Where wound type CT used and where not used?

Description : Where wound type CT used and where not used?

Last Answer : Used where low CT ratio req. Not used where high short ckt. Current exists.

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. 

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?

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How CT is connecting in ckt?

Description : How CT is connecting in ckt? 

Last Answer : If the primary of CT is delta connected load the CT will be in star connection and vice versa. This is because to have square root 3 time compensation. 

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Description : Why ICT are used?(INTERPOSING CT)

Last Answer : To correct the system primary CT errors in case of high current faults out side CT zone (ICT’s primary CT is 800/1, but in fault current may go to thousands of amps. This ICT will take care of those errors. a) Matching the ratios. b) Matching the phase angle differences. 

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

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

Last 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. 

What are the checks on CT & PT?

Description : What are the checks on CT & PT?

Last Answer : a) Polarity checks b) Insulation checks c) Ratio checks d) Knee point voltage (only for PS class CT)- magnetising characteristic test. 

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Description : Where CT secondary of 1A we are using?

Last Answer : For long distance current transmission, to reduce the IR drop.  

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Description : What is the purpose of CT & PT?

Last Answer : For transformation of current, voltage to a lower level for the purpose of Measurement, Protection and Control. 

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Description : What is the standard CT polarity?

Last Answer : Primary current enters at P1 and secondary current leaves at S2.  

A CT has 2 – secondary windings. If we use only one secondary winding can we keep the unused secondary winding short circuited?

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Last Answer : No. If it is short-circuited then the ratio will not get correctly. The turns of primary winding will be shared between 2 secondary windings. So the unused secondary winding should kept open. 

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Description : What are the errors of the following CT’ s 5p. 10p. 15p. At rated current?  

Last Answer : 5p - 1 % Ratio error ± 60 min phase error 10p - 3 % Ratio error ± 60 min phase error 15p - 5 % Ratio error ± 60 min phase error 

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Description : What are the specifications of CT? 

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Description : Why CT should not be open circuited?

Last Answer : Very high voltage will be induced in secondary due to less back emf resulting in the failure of the insulation. 

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Description : How can you de-saturate the CT? 

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Description : What is the problem anticipated due to CT saturation?  

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When will you say that the CT is saturated?

Description : When will you say that the CT is saturated?

Last Answer : When checking the CT with the secondary injection method a 10% increases in the voltage causes a 50% increase in the current the CT is said to be saturated. 

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Description : Show the CT characteristics.

Last Answer : CT characteristics :  

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Description : How to reduce the CT error? 

Last Answer : Error reduces if load increases.  

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Description : Explain with simple diagram the core balance CT.

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Last Answer : Polarity test: Connect the circuit as shown in figure with a battery, switch and AVOmeter. Now momentarily close the switch S and see the deflection in the AVOmeter. If it is in the direction as shown in the ... A2. Then see whether it confirms to reading of nameplate. Ratio = A1/A2. 

Where core balance CT is used?

Description : Where core balance CT is used?

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

Draw a neat sketch and explain the use of CT and PT for measurement of power in a single-phase circuit?

Description : Draw a neat sketch and explain the use of CT and PT for measurement of power in a single-phase circuit?

Last Answer : If wattmeter of proper range is not available or if voltage and current ranges are high we can usr CT and PT of suitable ratio. Connect the CT and PT as shown in figure. The reading of wattmeter can be multiplied by the ratio to get the actual power.   

The boundary of the protective zone is determined by the (A) Location of CT (B) sensitivity of relay used (C) Location of PT (D) None of these

Description : The boundary of the protective zone is determined by the (A) Location of CT (B) sensitivity of relay used (C) Location of PT (D) None of these

Last Answer : Ans: B The boundary of the protective zone is determined by the sensitivity of relay used. If the relay is more sensitive, the protective zone will be increased.

The knee voltage (cut in voltage) of Si diode is a. 0.2 V b. 0.7 V c. 0.8 V d. 1.0 V

Description : The knee voltage (cut in voltage) of Si diode is a. 0.2 V b. 0.7 V c. 0.8 V d. 1.0 V

Last Answer : b. 0.7 V

‘Germanium diode knee voltage is lower than silicon diode knee voltage.' Justify.

Description :  ‘Germanium diode knee voltage is lower than silicon diode knee voltage.' Justify.

Last Answer : Justification: The band gap between conduction and valence band for Germanium (0.66eV) is less as compared to Silicon (1.11eV).Hence less energy is required to start conduction in Germanium diode. 

A diabetic develops a severe perineal infection with skin necrosis, subcutaneous crepitance, and drainage of a thin, watery, grayish and foul-smelling fluid. Management should consist of: a. Gram stain of the fluid, which will likely demonstrate multiple bacteria including predominantly gram-positive rods b. A CT scan is indicated in a stable patient to define the extent of the disease c. Broad spectrum antibiotics followed with prompt extensive debridement is indicated d. A safe guideline is to resect infected necrotic tissue so that a several centimeter margin of grossly normal, healthy tissue can be achieved e. A colostomy is of little benefit in this situation

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Define the term voltage regulation of Alternator.

Description : Define the term voltage regulation of Alternator.

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Define voltage controlled bus?

Description : Define voltage controlled bus?

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Description : Define voltage controlled bus(generator bus/PV bus).

Last Answer : A bus is called voltage controlled bus if the magnitude of voltage |V| and real power (P) are specified for it. In a voltage controlled bus, the magnitude of the voltage is not allowed to change. Voltage controlled bus is also called as Generator bus and PV bus.

Define the followings. Insulation level, Rated short circuit breaking current, Making current, Short time rating, Impulse withstand voltage, Power frequency withstand voltage .

Description : Define the followings.   Insulation level, Rated short circuit breaking current, Making current, Short time rating, Impulse withstand voltage, Power frequency withstand voltage .

Last Answer : Insulation level - it is the combination of rated voltage, the corresponding impulse withstand voltage, which together characterize the insulation of the equipment as regards its ability to withstand ... wave of power frequency (50 Hz) which the insulation of equipment should withstand. 

Define the percentage voltage regulation and efficiency of a power transformer.

Description : Define the percentage voltage regulation and efficiency of a power transformer.  

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Description : The most inefficient method of voltage reduction from the stand point of power loss, is a/an _____________. A. capacitor in series with the load B. inductor in series with the load C. capacitor and inductor in series with the load D. resistor in series with the load

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