What is Knee Point Voltage of Current Transformer?

This is the significance of saturation level of a CT core mainly used for protection purposes. The sinusoidal voltage of rated frequency applied to the secondary terminals of current transformer, with other winding being open circuited, which when increased by 10% cause the exiting current to increase 50%. The CT core is made of CRGO steel.

It has its won saturation level.

The EMF induced in the CT secondary windings is

E2 = 4.44φfT2

Where, f is the system frequency, φ is the maximum magnetic flux in Wb. T2 is the number of turns of the secondary winding. 

The flux in the core, is produced by excitation current Ie. We have a non-liner relationship between excitation current and magnetizing flux. After certain value of excitation current, flux will not further increase so rapidly with increase in excitation current. This non-liner relation curve is also called B – H curve. 

Again from the equation above, it is found that, secondary voltage of a current transformer is directly proportional to flux φ. Hence one typical curve can be drawn from this relation between secondary voltage and excitation current as shown below.

It is clear from the curve that, linear relation between V and Ie is maintained from point A and K. The point ′A′ is known as ′ankle point′ and point ′K′ is known as ′Knee Point′.

In differential and restricted earth fault (REF) protection scheme, accuracy class and ALF of the CT may not ensure the reliability of the operation. It is desired that, differential and REF relays should not be operated when fault occurs outside the protected transformer. When any fault occurs outside the differential protection zone, the faulty current flows through the CTs of both sides of electrical power transformer. The both LV and HV CTs have magnetizing characteristics. Beyond the knee point, for slight increase in secondary emf a large increasing in excitation current is required. So after this knee point excitation current of both current transformers will be extremely high, which may cause mismatch between secondary current of LV & HV current transformers. This phenomena may cause unexpected tripping of power transformer. So the magnetizing characteristics of both LV & HV sides CTs, should be same that means they have same knee point voltage Vk as well as same excitation current Ie at Vk/2. 

It can be again said that, if both knee point voltage of current transformer and magnetizing characteristic of CTs of both sides of power transformer differ, there must be a mismatch in high excitation currents of the CTs during fault which ultimately causes the unbalancing between secondary current of both groups of CTs and transformer trips.

So for choosing CT for differential protection of transformer, one should consider current transformer PS class rather its convectional protection class. PS stands for protection special which is defined by knee point voltage of current transformer Vk and excitation current Ie at Vk/2.

Accuracy Limit Factor or ALF of Current Transformer

This is the maximum value of primary current, beyond which core of the protection CT or simply protection core of of a CT starts saturated. The value of rated accuracy limit primary current is always many times more than the value of instrument limit primary current. 

Actually CT transforms the fault current of the electrical power system for operation of the protection relays connected to the secondary of that CT. If the core of the CT becomes saturated at lower value of primary current, as in the case of metering CT, the system fault will not reflect properly to the secondary, which may cause, the relays remain inoperative even the fault level of the system is large enough.

That is why the core of the protection CT is made such a way that saturation level of that core must be high enough. But still there is a limit as because, it is impossible to make one magnetic core with infinitely high saturation level and secondly most important reason is that although the protection care should have high saturation level but that must be limited up to certain level otherwise total transformation of primary current during huge fault may badly damage the protection relays.

So rated accuracy limit primary current, should not be so less, that it will not at all help the relays to be operated on the other hand this value must not be so high that it can damage the relays. So, accuracy limit factor or ALF should not have the value nearer to unit and at the same time it should not be as high as 100. The standard values of ALF are 5, 10, 15, 20 and 30.

Instrument Security Factor or ISF of Current Transformer

Instrument Security Factor or ISF of Current Transformer

 

Instrument security factor is the ratio of instrument limit primary current to the rated primary current. Instrument limit current of a metering current transformer is the maximum value of primary current beyond which current transformer core becomes saturated. 

 Security or Safety of the measuring unit is better, if ISF is low. If we go through the example below it would be clear to us.

 Suppose one current transformer has rating 100/1 A and ISF is 1.5 and another current transformer has same rating with ISF 2. 

That means, in first CT, the metering core would be saturated at 1.5 × 100 or 150 A, whereas is second CT, core will be saturated at 2 × 100 or 200 A.

That means whatever may be the primary current of both CTs, secondary current will not increase further after 150 and 200 A of primary current of the CTs respectively.

Hence maximum secondary current of the CTs would be 1.5 and 2.0 A.

Coordination of overcurrent relays

Correct overcurrent relay application requires knowledge of the fault current that can flow in each part of the network. The data required for a relay setting study are:

• Single-line diagram of the power system involved, showing the type and rating of the protection devices and their associated current transformers.
• The impedances in ohms, percent or per unit, of all power transformers, rotating machine and feeder circuits.
• The maximum and minimum values of short circuit currents that are expected to flow through each protection device.
• The maximum load current through protection devices.
• The starting current requirements of motors and the starting of induction motors and the transformer inrush, thermal withstand and damage characteristics.

The relay settings are first determined to give the shortest operating times at maximum fault levels and then checked to see if the operation will also be satisfactory at the minimum fault current expected. Also, the basic rules for correct relay co-ordination can generally be stated as follows: 

• Whenever possible, use relays with the same operating characteristic in series with each other.
• Make sure that the relay farthest from the source has current settings equal to or less than the relays behind it, that is, that the primary current required to operate the relay in front is always equal to or less than the primary current required to operate the relay behind it.

Among the various possible methods used to achieve correct relay co-ordination are those using either time or overcurrent, or logic coordination. The common aim of all three methods is to give correct coordination.

Time-Based Coordination (will update soon)

Current-Based Coordination (will update soon)

Logic Coordination (will update soon)

FARADAY’S LAW AND LENZ’S LAW OF ELECTROMAGNETIC INDUCTION

Faraday’s laws explain the relationship between electric circuit and magnetic field. This law is the basic working principle of the most of the electric motors, generators, transformers, inductors, etc. 

Faraday’s First Law

It states that whenever a conductor is placed in a varying magnetic field, an emf gets induced across the conductors (called an induced emf) and if the conductor is a closed circuit then induced current flows through it.

Faraday’s Second Law

According to this law, the magnitude of induced electromagnetic field (emf) is equal to the rate of change of flux linked with the coil. The flux linkage is the product of number of turns and the flux associated with the coil. The emf induced in a closed circuit is proportional to the time rate of change of magnetic flux linking the circuit. 

where e is the induced emf, f is the magnetic flux. The negative sign indicates that the direction of the induced emf is opposite to the cause that produces it.

Lenz’s Law

It states that when an emf is induced according to Faraday’s law, the polarity (direction) of that induced

emf is such that it opposes the cause of its production. 

Thus, by considering Lenz’s law,

The negative sign shows that the direction of the induced emf and the direction of change in magnetic field have opposite sign.

  

  

Testing & Commissioning engineer interview questions and answers (***Important***)

Dear Engineers spend some time to read this article fully which helps to crack most of the MNC interviews.

Ultimately the following points are important before undergo any interview.

*Solid foundation of knowledge about the position on the job.

*Knowledge about the employer, the background of the people interviewing you.

*It is important that you present yourself as polite.

*Practice makes your confidence level will naturally grow.

1. Tell me about yourself?

    (or)

    As Commissioning engineer position, please tell me about yourself?

One of the most frequently asked questions in all the type of Interviews

You have to be very careful on how you answer this question because your answer here sets the tone for the rest of the interview. This question is mostly asked as an icebreaker but if you did not prepare for it, it becomes a real problem.

Where do you start? 

What do they really want to know? 

Are you to begin from elementary school or college? 

The right approach to this is to discuss your key strengths and how they relate to the job. Talk about a few of your accomplishments. Talk about your current employer and then tell them how you see yourself fitting into a position at their company.

2. As Commissioning engineer position, what is the most difficult situation you have had to face and how did you tackle it?

Now a days all the interview involves this question, the order may vary someone will ask at beginning or at the end.

The reason why you are asked this question is to hear what you consider difficult and how you approached the situation. Select a difficult work situation, which wasn’t caused by you and can be explained in a few sentences. You can then show yourself in a positive light by explaining how you handled the situation.

3. What is your greatest strength as Commissioning engineer position?

This question is related to the previous question and important too.

This could be a very simple question if you are prepared for it. You just have to talk about the strengths that you know would be of value to the company.

DO:

• Make the most of this question. This question gives you the control to guide the interview to where you want it so take advantage.

• Emphasize the strengths you have that are crucial to the position

• Find out from the job description and from company research, the kind of strengths the company invests in.

DON’T:

• Do not be too modest or claim to be what you are not

• Do not try to mention things you cannot illustrate with a brief example

• Do not mention the strengths that aren’t relevant to the job at hand

4. As Commissioning engineer position, what are your weaknesses?

Turn this question into a strength question in disguise. For instance, say something like “I do not like not being challenged at work” or you could mention a weakness that has nothing to do with the job and that you can overcome with training. This way, you end up turning this potentially tricky question into a positive.

Sometimes, you may be asked about certain challenges you faced in your previous position. If you are asked this question, lean towards the problem that happened early in your carrier and that you were able to solve. Do not try to blame others, just identify the problem and the role you played in solving it.

5. As Commissioning engineer position, how do you respond to working under pressure?

The essence of this question is to test your composure, ability to solve problems and staying true to the task, even in unfavorable conditions. Give an example of a time where you were faced with a challenge and what you did to remedy the situation. In the process, highlight how you were calm and in control till everything was okay.

6. As Commissioning engineer position, why do you wish to leave your present job?

No matter what you say, do not mention negative things about your employer, neither should you mention anything about more money being the reason. The reason is simple; if you are leaving a company because of money to come to theirs, you will definitely leave them to move on to another if it promises a better paycheck. Your best bet is to ay it on responsibility and challenge and how your previous position want challenging you enough. Indicate that you yearn for more responsibility and how what you have to offer outweighs the responsibility and challenge posed by your former position.

7. As Commissioning engineer position, what sort of salary are you looking for?

Note that whenever you are going for an interview, this question may be asked. Before going, try to find out what the average salary for someone holding that position in that industry is paid. This would help prepare you for what is in front of you.

Do not forget that this is only an interview and you haven’t been offered the job, so do not go on negotiating. Just state something within the range you have researched and move on. Whatever you do, do not sell yourself short.

Current Transformer Burden

In CTs, the secondary has very small impedance referred to as burden, so the CT practically operates on short circuit conditions. The burden for CT is the volt-ampere (VA) loading which is imposed on the secondary at rated current. The burden can also be expressed as the ratio between secondary voltage and secondary current. 

A metering CT has lower VA capacity than a protection CT. A metering CT has to be accurate over its complete measuring range. Such a CT’s magnetising impedance at low current and hence low flux should be very high. 

The magnetising impedance is not constant for a CT’s operating range due to the non-linear characteristics of the B-H curve. It cannot give linear response during large fault currents. For protection CT, linear response is expected for up to 20 times the rated current. It is also expected to give precise performance in the normal operating currents up to high fault level currents.

Common Ingress Protection (IP) Ratings for Electric Motors

Enclosures of electrical equipment, per characteristics where they will be installed and their maintenance accessibility, should offer a certain degree of Protection also known as Ingress Protection (IP). Standard IEC 60034-5 defines the degrees of protection of electrical equipment by means of the characteristic letters IP, followed by two characteristic numerals. NEMA also defines IP ratings for enclosures.

Motor	Degree of Protection	First Characteristic Numeral		Second Characteristic Numeral
		Protected Against Accidental Contact	Protected Against Solid Objects	Protected Against Water
Open Motors	IP00	Non-Protected	Non-Protected	Non-Protected
	IP02	Non-Protected	Non-Protected	Protected against dripping water even when tilted 15° vertically
	IP11	Protection against accidental contact with the hand	Ingress of solid objects exceeding 50mm in diameter	Protection against dripping water falling vertically
	IP12	Protection against accidental contact with the hand	Ingress of solid objects exceeding 50mm in diameter	Protected against dripping water even when tilted 15° vertically
	IP13	Protection against accidental contact with the hand	Ingress of solid objects exceeding 50mm in diameter	Protected against dripping water even when tilted 60° vertically
	IP21	Protection against the touching with the finger	Ingress of solid objects exceeding 12mm in diameter	Protection against dripping water falling vertically
	IP22	Protection against the touching with the finger	Ingress of solid objects exceeding 12mm in diameter	Protected against dripping water even when tilted 15° vertically
	IP23	Protection against the touching with the finger	Ingress of solid objects exceeding 12mm in diameter	Protected against dripping water even when tilted 60° vertically
Closed Motors	IP44	Protection against the touching with tools	Ingress of solid objects exceeding 1mm in diameter	Protection against splashing water from any direction
	IP54	Protection against contacts	Protection against accumulation of harmful dust	Protection against splashing water from any direction
	IP55	Protection against touches	Protection against accumulation of harmful dust	Protection against any water jets from any direction

For special and more dangerous areas where electric motors are required to be applied, the following degrees of protection are commonly used:

IPW 55 (Weather protection)

IP56 (Protections against water jets)

IP65 (Totally protected against dust)

IP66 (Totally protected against dust and water jets).

What is a Motor Service Factor?

Motor Service Factor (SF) is the percentage multiplier that a motor can handle for short periods of time when operating within its normal voltage and frequency tolerance. In other words, it is a fudge factor that give extra horsepower when it's occasionally needed.

For instance, this 1/2 horsepower shown in the photo has a service factor of 1.25 so it can actually output 25% more power required for short periods of time. This comes in handy if the density of the liquid increases or a higher than normal flow rate is required.

Fractional horsepower motors usually have a higher service factor up to 1.5 since their power consumption does not lead to significantly higher winding temperatures. Motors of 10 hp and up usually have a service factor of 1.15.

The Canadian Electrical Code defines service factor as a multiplier that, when applied to:

• The rated horsepower of an AC motor,
• To the rated armature current of a DC motor, or
• To the rated output of a generator,

... indicates a permissible loading that may be carried continuously at rated voltage and frequency. Note how the CEC allows for continuous operation while we reserve it for only short periods of time to ensure reliability.

As electrical consultants, using the service factor gives us a margin of safety that allows our design to:

• Extend the life of the motor by lowering the temperature of the insulation winding.
• Compensate for low or unbalanced supply voltages.
• Accommodate the variability in horsepower. A 15% buffer is a nice margin to have especially for those occasional overload conditions.

Why Have a Service Factor

Operating a motor at its limit makes it more prone to overheating. A service factor allows the motor to operate below its theoretical maximum so it can run continuously with a cooler winding temperature at rated load. This leads to a longer life and better reliability.

Designing with the Service Factor

We never design systems to operate continuously at the maximum level (redundancy is a different topic). It is good practice to size a motor for continuously operation that is below the service factor percentage. An Electrical load monitoring test help to see how efficient a motor is running.

Operating in the service factor area may result in:

• Decreased efficiency (more energy usage).
• Decreased power factor (more reactive power usage).
• Overheating and damage to the wire insulation.
• Incorrect starter sizing which may lead to inadequate starting & pull-out torques.

A motor services factor is a margin of safety which increases the reliability of building systems.

Differential Relay | Transformer Differential | Line Differential | Differential relay concept

Differential relays are very sensitive to the faults occurred within the zone of protection but they are least sensitive to the faults that occur outside the protected zone. Most of the relays operate when any quantity exceeds beyond a predetermined value for example over current relay operates when current through it exceeds predetermined value. But the principle of differential relay is somewhat different. It operates depending upon the difference between two or more similar electrical quantities.

Definition of Differential Relay

The differential relay is one that operates when there is a difference between two or more similar electrical quantities exceeds a predetermined value. In the differential relay scheme circuit, there are two currents come from two parts of an electrical power circuit. These two currents meet at a junction point where a relay coil is connected. According to Kirchhoff Current Law, the resultant current flowing through the relay coil is nothing but the summation of two currents, coming from two different parts of the electrical power circuit. If the polarity and amplitude of both the currents are so adjusted that the phasor sum of these two currents, is zero at normal operating condition. Thereby there will be no current flowing through the relay coil at normal operating conditions. But due to any abnormality in the power circuit, if this balance is broken, that means the phasor sum of these two currents no longer remains zero and there will be non-zero current flowing through the relay coil thereby relay being operated.

In the current differential scheme, there are two sets of current transformer each connected to either side of the equipment protected by differential relay. The ratio of the current transformers are so chosen, the secondary currents of both current transformers matches each other in magnitude.

The polarities of current transformers are such that the secondary current of these CTs opposes each other. From the circuit is clear that only if any nonzero difference is created between this to secondary currents, then only this differential current will flow through the operating coil of the relay. If this difference is more than the peak up value of the relay, it will operate to open the circuit breakers to isolate the protected equipment from the system. The relaying element used in differential relay is attracted armature type instantaneously relay since differential scheme is only adapted for clearing the fault inside the protected equipment in other words differential relay should clear only the internal fault of the equipment hence the protected equipment should be isolated as soon as any fault occurred inside the equipment itself. They need not be any time delay for coordination with other relays in the system.

Types of Differential Relay

There are mainly two types of differential relay depending upon the principle of operation.

1. Current Balance Differential Relay
2. Voltage Balance Differential Relay

In current differential relay two current transformers are fitted on the either side of the equipment to be protected. The secondary circuits of CTs are connected in series in such a way that they carry secondary CT current in same direction.

The operating coil of the relaying element is connected across the CT’s secondary circuit. Under normal operating conditions, the protected equipment (either power transformer or alternator) carries normal current. In this situation, say the secondary current of CT1 is I1 and secondary current of CT2 is I2. It is also clear from the circuit that the current passing through the relay coil is nothing but I1-I2. As we said earlier, the current transformer’s ratio and polarity are so chosen, I1 = I2, hence there will be no current flowing through the relay coil. Now if any fault occurs in the external to the zone covered by the CTs, faulty current passes through primary of the both current transformers and thereby secondary currents of both current transformers remain same as in the case of normal operating conditions. Therefore at that situation the relay will not be operated. But if any ground fault occurred inside the protected equipment as shown, two secondary currents will be no longer equal. At that case the differential relay is being operated to isolate the faulty equipment (transformer or alternator) from the system.

Principally this type of relay systems suffers from some disadvantages

1. There may be a probability of mismatching in cable impedance from CT secondary to the remote relay panel.
2. These pilot cables’ capacitance causes incorrect operation of the relay when large through fault occurs external to the equipment.
3. Accurate matching of characteristics of current transformer cannot be achieved hence there may be spill current flowing through the relay in normal operating conditions.

Percentage Differential Relay

This is designed to response to the differential current in the term of its fractional relation to the current flowing through the protected section. In this type of relay, there are restraining coils in addition to the operating coil of the relay. The restraining coils produce torque opposite to the operating torque. Under normal and through fault conditions, restraining torque is greater than operating torque. Thereby relay remains inactive. When internal fault occurs, the operating force exceeds the bias force and hence the relay is operated. This bias force can be adjusted by varying the number of turns on the restraining coils. As shown in the figure below, if I1 is the secondary current of CT1 and I2 is the secondary current of CT2 then current through the operating coil is I1 – I2 and current through the restraining coil is (I1 + I2)/2. In normal and through fault condition, torque produced by restraining coils due to current (I1+ I2)/2 is greater than torque produced by operating coil due to current I1– I2 but in internal faulty condition these become opposite. And the bias setting is defined as the ratio of (I1– I2) to (I1+ I2)/2.

It is clear from the above explanation, greater the current flowing through the restraining coils, higher the value of the current required for operating coil to be operated. The relay is called percentage relay because the operating current required to trip can be expressed as a percentage of through current.

CT Ratio and Connection for Differential Relay

This simple thumb rule is that the current transformers on any star winding should be connected in delta and the current transformers on any delta winding should be connected in star. This is so done to eliminate zero sequence current in the relay circuit.

If the CTs are connected in star, the CT ratio will be In/1 or 5 A

CTs to be connected in delta, the CT ratio will be In/0.5775 or 5×0.5775 A

Voltage Balance Differential Relay

In this arrangement the current transformer are connected either side of the equipment in such a manner that EMF induced in the secondary of both current transformers will oppose each other. That means the secondary of the current transformers from both sides of the equipment are connected in series with opposite polarity. The differential relay coil is inserted somewhere in the loop created by series connection of secondary of current transformers as shown in the figure. In normal operating conditions and also in through fault conditions, the EMFs induced in both of the CT secondary are equal and opposite of each other and hence there would be no current flowing through the relay coil. But as soon as any internal fault occurs in the equipment under protection, these EMFs are no longer balanced hence current starts flowing through the relay coil thereby trips circuit breaker.