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.