Friday, 22 December 2017

HSV / NSV Rating of Electric Equipment

You might surprise to hear about HSV & NSV Rating of Electric Equipment. If you ever get a chance to visit a EHV Switchyard, you will notice that HSV / NSV rating is mentioned on every equipment connected to the Switchyard. I was also wondering to see this rating on every connected equipment in EHV Switchyard. 

HSV stands for Highest System Voltage and NSV for Nominal System Voltage. As you can guess, nominal system voltage rating is the expected continuous voltage of system in which the equipment is connected. For example, let us assume that a Circuit Breaker is connected in 400 kV Switchyard, the NSV will be 400 kV.

HSV is the highest continuous system voltage. Considering 400 kV Switchyard, it may happen under light load condition or because of some other reason that system voltage is maintain say 410 kV or 420 kV. Therefore it is expected that the connected equipment like CT, PT, and Breaker etc. should withstand this voltage without any damage. Therefore it is very important to study the system for highest continuous voltage which system / grid can achieve to assign or design HSV of equipment.

Highest continuous voltage of system should not be confused with switching surge or voltage surge due to lightening. HSV is based on highest continuous voltage of system. 

Thus HSV rating of equipment is an important factor for designing insulation requirement. Basically insulation requirement of equipment depends on switching voltage surge, lightening over voltage, highest power frequency withstand voltage and HSV. Insulation of equipment is so designed to withstand lightening over voltage for a time of the order of micro second, surge overvoltage for a time of the order of mili second and highest power frequency over voltage for 1 minute. But equipment insulation is designed based on HSV for continuous operation.

Wednesday, 20 December 2017

Instrument Safety Factor of Current Transformer

Instrument Safety Factor

Instrument Safety Factor (ISF) is defined as the ratio of CT saturation current to its rated current. Suppose the CT ratio is 2000/1 and the CT gets saturated if there is flow of 10 kA current through its primary, then Instrument Safety Factor is given as

Instrument Safety Factor, ISF = CT saturating Current / Rated current

                                                = 2000x5 / 2000 = 5

ISF is defined only for metering current transformer (CT). Metering CT is nothing but a CT used for metering purpose. 

Need of Instrument Safety Factor

Generally a CT have more than one core say 4 cores. Different cores are designed for different purpose like Core-1, Core-2 and Core-4 are meant for protection purpose whereas Core-3 is meant for metering as shown in figure below.

Instrument Safety Factor

Thus ISF will be defined for metering core i.e. core-3 of CT. Since meters are only designed for low value of current, therefore it is very important to protect them from high value of current. As meters are connected directly with the terminals of metering core of CT, it may happen so that during fault condition the secondary current of CT may be high which in turn will flow through the connected meter. This may lead to the damage of meter coil. Therefore some measure must be taken to protect meters from such event. This is the reason we define Instrument Safety Factor, ISF for metering CT. Now the question arises, how does defining ISF protects connected meters from over current?

Well, suppose there occurs some fault in the system. Assume that the fault current is 6 times the rated primary current of CT i.e. 6x2000 A. In this case, if the ISF value of CT is 5 then it is most likely to saturate and hence the secondary current of CT metering core will become zero. Thus there will not any flow of current through the connected meters during such fault. In this way, meters are protected from over current during fault. You may think of overload condition like everything is normal but the load current is say 4000 A. In this case CT secondary current will be 2 A which will flow through the meter but it should be noted that meters are designed for certain overloading. Based on meter overloading, Instrument Safety Factor of CT is chosen. Thus meters always remain protected.

Why secondary current of CT becomes zero during saturation?

Since during CT saturation, the magnetic flux in the core will become almost constant, this means that there will be no change in the flux and hence no induced emf. As there is no induced emf, hence there will not be any transformer action. This means that there will not be any CT secondary current.

If you see the name plate of metering core of a Current Transformer (CT), you will notice ISF value mentioned.

Compensating Device - Purpose

A compensating device is generally used in metering CT as shown in figure above. The basic purpose of this compensating device is to achieve the Instrument Safety Factor. This device is nothing but high resistance.

If you carefully observe the figure, you will notice that a secondary terminal S’ is connected to 3S1 through a compensating device. If we want to connect this CT terminal 3S1 and 3S4 then first of all S’ terminal is shorted with 3S4 and then 3S1 & 3S4 are connected to meters.

Thus compensating device forms a parallel path to the connected meter through high resistance. In case of fault, if CT do not saturate above its ISF (it is most likely to saturate above ISF), then excess current will be shunted through the compensating device. But under normal condition, the flow of current through the compensating device will be negligible.

Saturday, 16 December 2017

Why PT/VT Secondary Terminal should not be Shorted?

Before discussing "Why PT/VT Secondary Terminal should not be Shorted?", it is good to have a brief idea of Potential Transformers / Voltage Transformers. PTs/VTs are Instrument Transformer used for the purpose of protection and measurement. The construction of PT/VT is same as that of power transformer except for insulation level, cooling, sealing etc. PTs are designed for of specific voltage rating like 400 kV / 110 V. This means that when a PT primary is connected to 400 kV line, the secondary voltage will be 110 V. This secondary voltage is then connected to various measuring instruments like voltmeter, energy meter etc. and protection relays like distance relaydirection earth fault relay etc.

Thus we can say, PT steps down the primary line voltage to some lower voltage suitable for relays and meters. This means that PT design should be such that to have low voltage regulation to maintain its secondary voltage constant.

Why PT/VT Secondary Terminal should not be Shorted?

Let us now come to the point, why PT/VT Secondary Terminals should not be shorted? Unlike Current Transformer (CT), PTs are connected in line to ground as shown in figure below. Figure below depicts the connection of three PTs connected in three phases. Note that a neutral point is made by shorting a terminal of three PTs and then grounding the neutral point. 


You may like to read Difference between Current Transformer & Potential Transformer

Due to low voltage regulation, the secondary terminal voltage will remain constant and hence if we keep the PT terminals open, nothing is going to happen as the secondary voltage is low (110 / 1.732 = 63.5V). Mind that the same is not true for CT. CT secondary terminals should never be kept open. In normal condition, PT secondary is connected to some impedance offered by relay / measuring instrument. Therefore the current through the secondary circuit is low.

But when we short the secondary of PT, a high current will flow thorough the secondary circuit. This is because of low voltage regulation. PT will try to maintain its secondary voltage and for doing this it will try to flow high current through shorted terminals. This high current will lead to overheating and consequent damage to the PT.
To avoid damage due to short circuit of PT terminals, fuses are installed in PT junction box. In case of short circuit of secondary terminals, these fuses will blow out and thus will open the circuit. It shall be noted that fuse should be installed as near to the PT as possible to avoid heating of connecting cables.

Thursday, 9 November 2017

Core Balance Current Transformer

Core Balance Current Transformer or CBCT is a ring type current transformer through center of which a three core cable or three single core cables of three phase system passes. This type of current transformer is normally used for earth fault protection for low and medium voltage system. A typical Core balance Current Transformer is shown in figure below.


Secondary of CBCT is connected to Earth Fault Relay. During normal operating condition as the vector sum of three phase current i.e. (Īa + Īb + Īc =0) is zero therefore no residual current in the primary will be present. Here residual current means zero sequence current. Therefore there will not be any flux developed in the CBCT core and hence no current in the secondary circuit of CBCT.

Working Principle of CBCT:

Let Īa, Īb and Īc be the three line currents and Φa, Φb and Φc be corresponding components of magnetic flux in the core. Assuming that the CT is operating in the linear region (Read B-H Curve to get idea of linearity), magnetic flux because of individual phase current will be directly proportional to the phase current and hence we can write as below,

Φa = kIa

Φb = kIb

Φc = kIc

where k is constant of proportionality. Mind here that same constant of proportionality is used as all the three phase current are producing magnetic flux in the same core i.e. magnetic material.

Thus the resultant magnetic flux in the CBCT core,

Φr = k(Īa + Īb + Īc) …………………..(1)

But we know from theory of symmetrical components,

Īa + Īb + Īc = 3Ī0 = Īn

Where, Io is zero sequence current and In is neutral current. Hence we can write as

Φr = kĪn  …………………………(2)

Now let us consider two cases:

Case1: During normal condition

Īa + Īb + Īc = 0

Hence from equation (1),

Net resultant flux in the CBCT Core, Φr = 0 which means no secondary current and therefore the Earth Fault Relay won’t operate.

Case2: During earth fault, three phase current passing through the center of Core Balance Current Transformer will not be balanced rather a zero sequence current will flow. For example for single line ground fault,

If = 3Ia0 = In

Thus from equation (2),

Net magnetic flux in the CBCT core, Φr will have some finite value which in turn will induce current in the secondary circuit due to which earth fault relay will operate. Because of this reason, a Core Balance Current Transformer or CBCT is also called Zero Sequence Current Transformer.

Advantage of Core Balance Current Transformer:

The advantage of using CBCT for earth fault protectionis that only one CT core is used instead of three core as in conventional system where the secondary winding of three cores are connected residually. Thus the magnetizing current required for the production of a particular secondary current is reduced by one third which is a great advantage as the sensitivity of protection is increased.

Also, the number of secondary turn does not need to be related to the cable rated current because no secondary current flows under normal operating condition as the currents are balanced. This allows the number of secondary turns to be chosen to optimize the effective primary pick-up current.

Core Balance Current Transformer is normally mounted over a cable at a point close to the cable gland of the Switchgear. In case cables are already laid in a Switchgear, physically split core, which is also known as Slip-over type CT, are used.

Equal Area Criterion and Out of Step Condition

Equal Area Criterion is the method of studying the Transient Stability of system of two machines or single machine connected to infinite bus. The study of Transient Stability tells us whether or not synchronism is maintained i.e. whether or not load angle δ settles down to steady state stable value after the clearance of fault or disturbance. In this post we will focus on
  • Equal Area Criterion
  • Transient Stability Limit and Margin
  • How Transient Stability Margin depend on Breaker Opening time?
  • Meaning of Out-of-Step condition in power system
  • Meaning of Loss of Synchronism
  • Critical Clearing Angle

Concept of Equal Area Criterion:

As we know that under steady state operation of a generator, there exists a balance between the power generation and demand. In other words we can say that mechanical input to generator is balanced with the generated power due to which Generator rotates at a constant synchronous speed. Basically, the output electric power from the generator produces an electric torque that balances the mechanical torque applied to the generator rotor shaft. The generator rotor therefore runs at a constant speed with this balance of electric and mechanical torques. Mind that electromagnetic torque and mechanical torque opposes each other and because of this balance between the mechanical & electromagnetic torque, mechanical energy is converted to electrical energy.

Let us consider a two source system as shown in figure below. The equation of power transfer between the two sources assuming lossless system is given as

P = VsERSinδ / X

Where Vs = Sending end Voltage

VR = Receiving end voltage

X = Transmission Line Reactance + Source Reactance


Thus from the above equation, it is clear that power transfer between the two sources is inversely proportional to the Reactance. If the net reactance increases, power transfer will decrease and vice versa.

During fault condition like Single Line to Ground Fault, Double Line to Ground Fault, Three Phase Fault etc. the effective transmission reactance between the two sources increases depending upon the type of fault. Due to this increase in the effective transmission line Reactance the power transfer between the two sources will reduce.  Due to this reduction of power transmission, the electric torque that counters the mechanical torque is also decreased. Thus

Mechanical Torque > Electromagnetic Torque

If the mechanical power is not reduced during the period of the fault, the generator rotor will accelerate with a net input torque (equal to Mechanical Torque - Electromagnetic Torque).

Assume that the two source power system shown in above figure is initially operating at a balance point of δ0 and hence transferring electric power P0. After a fault, the power output is reduced to PF, the generator rotor therefore starts to accelerate, and δ starts to increase. Let’s say the fault is cleared when the angle difference between the sources reaches δC. At this point, fault reactance will not be considered as the system is has attained normal configuration and hence the power transfer will follow the Pre Fault Power Angle Curve as shown in figure below.

But at point δC power output PC is larger than the mechanical power input P0 which will result in deceleration of rotor as

Electromagnetic Torque > Mechanical Torque

However, because of the inertia of the rotor system, the angle does not start to go back to δ0 immediately. Rather, the angle continues to increase till δF such that energy lost during deceleration in Area 2 is equal to the energy gained during acceleration in Area 1. This is called Equal Area Criterion.

Let us consider two cease now for the sake of better understanding of out-of-step condition and application of Equal Area Criterion.

Case-1: If δF < δL or Area 1 < Area 2

Under this condition, the rotor of Generator will oscillate. But because of presence of damping, the amplitude of oscillation will continuously reduce and eventually δ will settle to balanced angle δ0.

Thus we can say that, in this situation the system is Transiently Stable. How much margin we have for transient stability?

If you carefully observe the above Power Angle Curve then you will definitely say that the margin is (δL- δF). Great!

But next question arises, who decides the value of δF?

You must say it is δC i.e. the point of fault clearing.  That is why δC is also known as Critical Clearing Angle.

How Transient Stability Margin depends on Breaker Opening Time?

Since the value of δC depends on the point of clearing fault. Therefore, Transient Stability margin i.e. (δL- δF) depends upon the fault clearing time. The less the fault clearing time, less will be the value of δC which means less value of δF. Hence Transient Stability margin will be more. Thus we can say that Transient Stability Margin is dependent on Breaker Opening time to clear the fault. Got it? I guess you get. Please write in comment box.

Case-2: If δF < δL or Area 1 > Area 2


If Area 2 is smaller than Area 1 then at the time the angle reaches δL, then further increase in angle δ will result in an electric power output that is smaller than the mechanical power input. Therefore, the rotor will accelerate again and δ will increase beyond recovery. This is a transiently unstable scenario, as shown in figure above. When an unstable condition exists in the power system, one equivalent generator rotates at a speed that is different from the other equivalent generator of the system. We refer to such an event as a loss of synchronism or an out-of-step condition of the power system.