ELECTRICAL ENGINEERING INTERVIEW QUESTION & ANSWER
ELECTRICAL ENGINEERING INTERVIEW QUESTION & ANSWER
How do we select the Voltage for Megger in the Electrical System?
The minimum IR Value is usually recommended as (kV+1) MΩ.
Where kV is the Equipment Voltage rating.
E.g: In 400 kV rated equipment a periodical 5000 V DC Insulation Resistance (Megger) test is recommended.
Explain about charging and discharging time of capacitor?
Ans: The time constant (T) of a capacitor is Calculated by taking the Circuit Resistance and multiplying it by the circuit Capacitor. For a 1k Ω resistor and 1000 Pfarad Capacitor. The Time Constant should be 1 Second .This is the amount of time it takes for the Capacitor Voltage to increase 63.2% from its present Value to its final value.
The discharging circuit provides the same kind of changing capacitor Voltage this time the voltage slowly falls down .Experiment once again with different Combination of resistors and Capacitor polarity is Corrector, make sure as always the Capacitor polarity is correct.
What is the Difference between Series and Shunt Reactor? What are the Advantages?
Ans: Shunt Reactor: It is used for EHV transmission lines to Compensate shunt Capacitance of transmission line during low load Period.
Shunt Reactors are connected to the line bus without any circuit breaker for Switching.
Series Reactor: It is used to limit Current Surges associated with fluctuating loads.
Why Star point is required in CT and VT?
Ans 1: Star point of CT as well as VT is used to neglect the Zero Sequence Components in the System.
Ans 2:
CT star Point also used to avoid the Unbalancing Current circulating in the System.
Why do we need single point earthing in CT?
Ans: Earthing is Safety for Equipment as well as Persons those who are working nearer to Electrical Equipment.
In normal Condition there is no fault current flow to Earth. During Fault Condition the Fault Current should flows through Earth only.
Single point Earthing is needed in Current transformer in order to neglect the Effect of Splitting of Current in other Earthing.
If there is single Earthing in CT, During Fault Condition the Current to flow in only one closed Path. If double earthing means the fault current Split into two Directions. During this Condition the Relay should not operate properly due to insufficient amount of Fault Current to trip the CB.
What will happen when the CT opens Circuit?
Ans: If secondary is open circuited the Voltage across the secondary rises to a high value .The Peak Value may reach some kilovolts open Circulating of Secondary’s results in Zero Secondary Current hence reduced back emf.The working flux Increases and core gets Saturated.
The Secondary emf Increases due to Increased flux .The primary gets over heated and the core also gets over heated .Voltage are induced in the Secondary by Electro-magnetic Induction.
The Peak value of the Secondary on open Circuit may be several times the rms value .Since the core is saturated and wave form of voltage is distorted. This may cause danger to personnel working on Secondary Side.
What is Saturation Point of CT?
Ans: This is the point .During this Condition the CT behaves like a Open Ciruit.The primary current is fully utilized for Magnetizing only.
What do you mean by Knee point Voltage?
Ans: By Applying Voltage gradually from Secondary of CT Keeping primary winding open circuited.
Knee point is defined as where 10% Increase in Flux Density cause 50% Increase to and Saturated Exciting Ampere turns.
The knee point or effective point of Saturation is defined by ANSI/IEEE Standard as the Intersection of the Curve with a 45 deg tangent line.
However, the IEC defines the Knee as the Intersection the Straight lines from the non saturated parts of the exciting curve.
What is CT Burden?
Ans: 1 The External load Connected to the Secondary of a CT is called the Burden. The rated Secondary burden of a CT is the maximum resistance that can be connected to its Secondary winding without exceeding the Permissible error specified for the given class of accuracy.
Ans:2 The Number of Electrical measuring Instruments that can be Safely Connected to a single CT is determined by the total resistance of the instruments and the connecting leads . This Value should not exceed the burden of the CT. All Manufactures can Supply the burden of their Individual Device.
Define ALF?
Ans: ACCURACY LIMIT FACTOR
A current Transformer is designed to maintain its ratio within Specified limits up to a certain Value of primary Current, Expressed as a multiple of its rated primary Current. This multiple is known as the Current transformers rated Accuracy Limit Factor.
Eg: 5P10, 15 VA
5P--- Accuracy Class
10---ALF
15---Accuracy Power
Why we are using Y phase Earthing in 11 KV VT Circuit?
The three phase, three wire circuit is in common use today for VT circuits in metal clad switchgear. It is common practice to ground one of the phases, usually Y. This arrangement is used when it is desirable to eliminate coordination problems between primary and secondary VT protective devices for phase-to-neutral (ground) short circuits or phase-to- ground insulation failures. Because each mentioned fault in secondary side appears as two phase faults in primary side.
In metering CT, What is mean by FS?
Ans. SAFETY FACTOR
In order to protect the Metering Device Connected to the CT from high Current on the primary side. This Instrument Transformers must have Early Saturation Characteristics.
To ensure that the Appliance is not destroyed should a fault occur in the Primary the Current Transformer must be able to Saturate before 10 In in the Secondary.
Eg: 400/5 A, 15 VA, Cl 0.5, and FS 10
400A: Primary Current.
5A: Secondary Current.
15 VA: Accuracy power.
Cl 0.5: Accuracy Class.
FS: Safety Factor.
Explain about the magnetization Curve in CT?
Ans: The excitation Curve may be Sub-divided into four regions..
A. From origin to Ankle Point.
B. from Ankle point to knee Point.
c. Knee Region.
d. Saturation region.
Knee point is defined as where 10% Increase in Flux Density cause 50% Increase to Exciting Ampere turns.
Protective CT generally operates over working range of Flux density extending from the Ankle – point to Knee region of above. While the Measuring CT has the flux density in the region of ankle point only.
Prior to Saturation the flux density in core is Proportional to ampere turns. On reaching Saturation, magnetizing Inductance becomes low and the total primary current is utilized in exciting the core alone and therefore Secondary output of CT disappears.
The Saturation Continues till the Primary transient current is reduced below Saturation level .On energy in Saturation Zone; the CT behaves as open Circuited.
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Give formula for Knee Point Voltage?
Ans: 1:
VK = (VA*ALF / IN)
Where: VK-Knee Point Voltage
VA-Burden
ALF-Accuracy Limit Factor
IN-Secondary Current of CT
Eg: 5P10; 15VA; 500/1A
VK = ((15*10)/1)
150 v
SUMMATION CT:
Summation C.T.′s are designed to summarize several synchronous A.C. currents of equal phase relation with any angle of phase difference, i.e. summarize the secondary currents of a number of main C.T.’s. The secondary circuits of the main C.T.’s are to be connected to the corresponding marked primary terminals of the summation C.T., i.e each main C.T. feeds with its secondary current a specific portion of the primary winding of the summation C.T. The number of turns of the particular sections of the primary winding must fit in with ratios of the main C.T’s. If all main C.T.’s have identical ratios, it is irrelevant for their secondary circuits to which section of the primary winding of the Summation C.T. they are connected.
INTERPOSING CT
Transformer differential relays compare the phase and magnitude of the current entering one winding of the transformer with that leaving via the other winding(s). Any difference in phase or magnitude between the measured quantities will cause current to flow through the operate winding of the relay. If this current exceeds the relay setting, tripping of the transformer circuit breakers will be initiated. To enable a comparison to be made, the differential scheme should be arranged so that the relay will see
rated current when the full load current flows in the protected circuit. In order to achieve this, the line current transformers must be matched to the normal full load current of the
transformer. Where this is not the case it is necessary to use an auxiliary interposing current transformer to provide amplitude correction. The connection of the line CTs should compensate for any phase shift arising across the transformer. Alternatively the necessary phase correction may also be provided by the use of an interposing CT.
Continuous Thermal Rating Factor - (TRF) :
Normally designated for current transformers and is the factor by which the rated primary current is multiplied to obtain the maximum allowable primary current without exceeding temperature rise standards and accuracy requirements. Example - if a 400:5 C.T. has a RTF of 2.0, the C.T. will continuously accept 400 X 2 or 800 primary amperes with the 5 X 2 or 10 amperes from the secondary. The thermal burden rating of a voltage transformer shall be specified in terms of the maximum burden in volt-amperes that the transformer can carry at rated secondary voltage without exceeding a given temperature rise.
Impedance
Impedance (symbol Z) is a measure of the overall opposition of a circuit to current, in other words: how much the circuit impedes the flow of current. It is like resistance, but it also takes into account the effects of capacitance and inductance. Impedance is measured in ohms, symbol .
Impedance is more complex than resistance because the effects of capacitance and inductance vary with the frequency of the current passing through the circuit and this means impedance varies with frequency! The effect of resistance is constant regardless of frequency.
The term 'impedance' is often used (quite correctly) for simple circuits which have no capacitance or inductance - for example to refer to their 'input impedance' or 'output impedance'. This can seem confusing if you are learning electronics, but for these simple circuits you can assume that it is just another word for resistance.
Four electrical quantities determine the impedance (Z) of a circuit: resistance (R), capacitance (C), inductance (L) and frequency (f). Impedance can be split into two parts: Resistance R (the part which is constant regardless of frequency) Reactance X (the part which varies with frequency due to capacitance and inductance) The capacitance and inductance cause a phase shift* between the current and voltage which means that the resistance and reactance cannot be simply added up to give impedance. Instead they must be added as vectors with reactance at right angles to resistance as shown in the diagram.
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