Current transformers are always used with the secondary windings circuit closed through ammeters, current coils of watt-meters or relay coils. Its secondary winding circuit should not be opened while its primary winding is energized. A violation of this precaution may lead to serious consequences. Here, I am discussing the reasons behind it.
the current flowing in the primary winding depends upon the current in the secondary winding whereas,
in a current transformer current flowing in the primary winding depends upon the current flowing through the line whose current is being measured.
This current is no way controlled by the conditions of the secondary winding circuit of the CT.
Under normal conditions, both primary and secondary windings produce MMF which opposes each other. The primary MMF is slightly more than the secondary MMF and consequently, the resultant MMF is small.
This resultant MMF is responsible for the production of flux in the core and as this MMF is small under normal operating conditions, a small voltage is induced in the secondary winding of the CT.
If the secondary winding is open-circuited with energized primary, the primary MMF remains the same while the opposing secondary winding MMF reduces to zero.
In this condition, the resultant MMF becomes very large. This large MMF produces a large flux in the core till it saturates.
This large flux links with secondary winding and induces a high voltage in the secondary winding. This could be dangerous to the transformer insulation and to the person who has opened the circuit.
Also, the eddy current and hysteresis losses would be very high under these conditions and due to this the CT may be overheated and damaged.
Even it does not occur, the core may become magnetized permanently and this gives considerable ratio and phase angle errors.
Mostly, CTs are provided with a switch or short-circuiting link at the secondary winding terminals. If such a link is available, it should always be short-circuited before any change is made in the secondary winding circuit with primary winding energized.
When a CT is used for measurement, its secondary winding can be short-circuited safely since it is practically short-circuited the impedance of the burden (i.e. an ammeter, CC of wattmeter etc.) is very small.
Buchholz relay is a gas actuated relay. It is generally used on all oil immersed transformers having a rating more than 500 KVA. It is installed between the conservator and main tank. Therefore, such relay can only be installed in the transformers equipped with conservator tanks.
The construction of a Buchholz relay is shown in Figure. It consists of two hinged floats in a metallic chamber. One of the floats actuates the mercury switch connected to the external alarm circuit and the other float actuates the mercury switch connected to the tripping circuit.
Whenever a fault occurs inside the transformer, the oil of the tank gets overheated and gases are generated. The generation of gases may be slow or violent according to nature of the fault.
When a predetermined amount of gases accumulate in the top of the chamber of the relay, the mercury type switch attached to the float is tilted, closes the alarm circuit and rings the bell.
When a severe fault occurs, a large volume of gas is generated, the lower float is tilted and the trip coil is energized. This opens the circuit breaker and supply to the transformer is switched off.
Buchholz relay is a very simple device used for transformer protection. Moreover, it detects the developing faults at a much earlier stage and enables us to protect transformer before serious damage occurs.
For example, the clock hour number 0 represents zero degrees phase displacement between primary and secondary EMFs. The clock hour number 6 is for 1800 phase displacement, 1 for – 300, 11 for +300.
On the nameplate of three-phase transformer vector group is printed as Yy0, Dd1 etc. Here Yy0 will mean it’s both the windings are star-connected and phase displacement between primary and secondary EMFs is zero degree. It is belonging to Group No.1 (refer to table).
Clock – Hour Number
Similarly, Dy1 will mean its primary winding is connected in delta, secondary winding in the star, phase displacement is – 300 and belonging to Group No. 3.
Information of vector group of a transformer is very significant when it has to operate in parallel. For a transformer working in isolation, the arrangement of its internal connections is of little importance.
Transformers will operate in parallel satisfactory if they have,
the same primary and secondary voltages
the same tap ratio
the same percentage impedance, and,
belonging to the same vector groups.
The two transformers may have their windings connected in star/star and yet it will not be possible to operate in parallel if one belongs to Group 1 and the other to group 2, unless the internal connections of the secondary winding of one of the transformers are changed.
Unlike DC system, there are no fixed positive and negative poles in AC system, and hence, transformers cannot have fixed positive and negative terminals.
The relative direction in which primary and secondary windings of a transformer are wound around the core determines the relative direction of the voltage across the windings.
If the windings of the high and low voltage coils are in opposite directions, the applied voltage and the induced voltage will have opposite directions and the transformer is said to have subtractive polarity.
If the windings of the two coils are in the same direction, the applied voltage and the induced voltage will have the same direction and the transformer is said to have additive polarity. It should be noted that the secondary voltage waveform is in phase with the primary waveform in additive polarity. This condition is known as “no phase shift.”
Similar polarity ends are indicated by a dot convention as shown in Figure. Usually the ends of LV winding are denoted by small letter of the alphabet and are suffixed 1 and 2 while HV windings ends are denoted by the correspondung capatial letter and are suffixed 1 and 2 . The ends suffixed 1 (H1, x1) have same polarity and so have the ends denoted by 2 (H2, x2).
Knowledge of polarity of transformer windings is essential when single-phase transformers are connected in parallel or three-phase configurations. An understanding of polarity is also required to connect potential and current transformers to power metering and protective relays.
Polarity Test on Single Phase Transformer
Assume, we want to determine the polarity of 220/12 V transformer. It may be determined by a simple voltage measurement, as follows:
Place a connection between the high-voltage and low-voltage terminals as shown in Figure.
Apply a low voltage, 220 volts, to the two high-voltage terminals.
Measure the voltage between H2 and X1 terminals.
If the voltage is lower than the voltage across the high-voltage terminals, the transformer has subtractive polarity. If it is higher, the transformer has additive polarity.
For measurement of large currents and high voltages in AC circuits, specially constructed accurate ratio transformers are used in conjunction with low range AC instruments. These specially constructed transformers are known as instrument transformers and are of two types:
Potential Transformers (PTs)
Current Transformers (CTs)
These instrument transformers are also used in power system in conjunction with protective relays. For safety purposes, the secondaries of these transformers are grounded.
Current Transformer Construction & Working
Current transformers are used in AC power circuits to feed the current coils of indicting and metering instruments (ammeters, watt-meters, energy-meters) and protective relays. These transformers make the ordinary low current instruments suitable for measurement of high current and isolate them from high voltage.
The current transformer basically consists of an iron core on which a primary and one or two secondary windings are wound. The primary winding has one or two turns of thick wire and is connected in series with the load. It carries the actual power system current. Primary current ratings vary from 10 A to 3000 A or more.
The secondary winding has a large number of turns of fine wire. It is connected across current coils of indicting and metering instruments and protective relays. The secondary current ratings are of the order of 5 A, 1 A, and 0.1 A. The latter is used for static relays. If for any reason the instrument connected to the secondary of CT is to be removed then the secondary of CT must be short-circuited by a fairly thick wire.
The ratio of primary current to the secondary current is known as transformation ratio of the CT. The transformation ratio of a CT is usually high.
The product of voltage and current on the secondary side when it is supplying its maximum rated value of current is known as the rated burden and is measured in volt-amperes (VA). The volt-ampere rating of CTs is low (5 – 150 VA) as compared to that of power transformers.
Also current in the secondary of CTs is governed by the current in the primary winding i.e. power circuit current. But in the case of power transformers, it is governed by load impedance.
Potential Transformers Construction & Working
Potential transformers are used in AC power circuits to feed the potential coils of indicting and metering instruments (voltmeters, watt-meters, energy-meters) and protective relays. These transformers make the ordinary low voltage instruments suitable for measurement of high voltage and isolate them from high voltage.
The PTs are highly accurate ratio step down transformers. Its primary winding has a large number of turns and is always connected across the supply system. Its secondary winding has few number of turns and is connected to the potential coil of indicting and metering instruments and protective relays. The primaries of PT are rated from 400 V to several thousand volts and secondaries always for 110 V.
The ratio of the rated primary voltage to the rated secondary voltage is known as turn or transformation ratio of PT.
The burden is the total external volt-ampere load on the secondary at rated secondary voltage.
The rated burden of a PT is the VA burden which must not exceed if the transformer is to operate with its rated accuracy.
The maximum burden is the greatest VA load at which the PT will operate continuously without overheating its winding beyond the permissible limits.
Let the voltage to be measured of a power system is 11 kV. It is impossible to measure such a high voltage directly by a voltmeter. Therefore, a PT having secondary to primary turn ratio 1:100 is used in conjunction with a voltmeter which steps down the voltage from 11 kV to 110 V as shown in the figure.
For measurement of power in a high voltage power system, both CT and PT are used. The CT is used to step down the system current and the PT is used to step down the system voltage up to the required value. The potential coil (PC) of the wattmeter is connected across the secondary of PT and the current coil (CC) of the wattmeter is connected across the secondary of CT as shown in the figure.