Tap Changing Transformer

       In case of power system networks, the voltage supplied by transformers can be varied by changing it s transformation ratio. This can be achieved by tappings which are provided on transformers. The tappings are the leads which are connected to various points on a transformer winding. These terminals are brought outside to permit access to the winding. Thus the number of turns present in the circuit with one tap are not same for another tap.

Key Point : The turns ratio is therefore different with different tappings and hence different voltages are obtained with different tappings.

       The tappings are placed either on high voltage or low voltages or sometimes on both high and low voltage windings. The Fig.1 shows transformer with tappings provided on high voltage winding.

Fig.  1  Transformer with tappings on high voltage side

      In many cases, the tappings are brought out at a terminal connection box located on the transformer. The line lead connection is made to the desired tap. When it is desired to change the voltage as per the requirement, the reconnection of line leads to other tap is to be done which is the most inexpensive method which is applicable where line voltage is stable and the change in the tappings are not required to be done periodically.

       The principle winding in case of a distribution transformer with voltage rating 11 KV/400 V is a tapping on high voltage winding which when connected to rated voltage of 11 KV gives rated voltage of 400 V on low voltage side. The rating of winding is related to this tapping. The number of turns to other tappings may be more or less than that on principal tapping. The positive tapping will have number of turns more than that on principal while in case of negative tapping the number of turns are less than the turns on principle winding.

       There are various reasons for providing voltage control in supply networks. Firstly the voltage at consumer's premises should be remain in the permissible limits as declared by the electric supply company. It is also required to control active and reactive power. With change in load, the voltage variations are to be adjusted. For all these reasons the tappings are rqeuired.

       Depending on constructional availability, the location of tappings are made. They can be provided either at phase end, at the neutral point or in the middle of the winding. If the tappings are provided at phase end then the number of bushing insulators are reduced which is advantageous where limited transformer cover space is available. The tappings when present at neutral point then insulation between various parts is small which is economical especially in high voltage transformers. The tappings are placed near the centre of the winding if large voltage variation is required which also reduces magnetic asymmetry. But this arrangement can not be used on low voltage winding placed next to the core.

1.1 Why Tappings are on High Voltage Side ?

      Normally the tappings are provided on high voltage (h.v.) winding due to following reasons,
1) A fine voltage regulation is possible with high voltage winding as it carries large number of turns.

2) The low voltage winding of the transformer carries large current. So if tappings are provided on low voltage side then then there are difficulties encountered in the interruption of high currents which makes its impracticable.

3) For the reasons of requirement of insulation, the low voltage (l.v.) winding is placed near the core while the l.v. winding is placed outside. Hence practically it is easier and simpler to provide tappings on high voltage winding.

4) In case of step down transformers, it is an added advantage to provide tappings on h.v. side. At light loads, the l.v. side side voltage increases. It is required to decrease this voltage by adjusting the tapping on h.v. side to a position where number of turns are large. With large number of turns, the flux and flux density decreases.

       This results in reduction of core loss which increases transformer efficiency at light loads.

5) If the tappings are provided on the l.v. side then the exact voltage regulation may not be provided. This can be explained by considering an example of a transformer with rating 3 phase, 11 KV/415 V, delta/star connected which is designed for 15 volts/turn. The l.v. side voltage is 415/√3 = 240 V. The number of turns on l.v. side are 240/15 = 16. Minimum number of turns that can be tapped is one. Hence minimum possible voltage regulation with tappings on l.v. side is 15 V or 6.25  %. It can be seen that if a voltage regulation of 5 % is desired with this then it is not possible with tappings on l.v. side.

       The magnetic asymmetry in a transformer is mainly caused due to the tappings. This is because the number of turns in the winding that contains tappings changes while the number of turns in the other winding remains same. Due to this, there is unbalance in the mmf which produces magnetic asymmetry in axial direction. This causes heavy mechanical forces and corresponding displacement of the windings in axial direction when fault occurs.

       In case of small transformers the tappings are provided on one end of the windings. For large transformers, the tappings are arranged at the centre of winding. This is shown in the Fig. 2.

Fig.  2

       If it desired to reduce the magnetic unbalance then a part of untapped winding is thinned which is opposite to part of that winding which contains tappings which is shown in the Fig. 2(d). There are another arrangements for reducing magnetic unbalance which are shown in the Figs. 2(e) and (f). If the untapped windings are split into number of parts and connecting these parts in parallel then it may achieve axial mmf balance which is shown in the Fig. 2(g).

       There are two ways by which the taps can be changed. When the tappings are changed after disconnecting it from the supply then it is called off circuit tap changing. It is used for occassional adjustment. In on load tap changing, the tappings are changed when the transformer is on load. With the help of on load tap changing gear, the daily and short time voltage adjustment can be done.

1.2 Off Circuit Tap Changing

       In this method of tap changing, the tappings are changed when the transformer is disconnected from the supply. As per the requirement the tappings are taken out on the respective winding and the connections are brought out near the top of transformer. Manually operated selector switches are provided for change in tappings. The commonly used switches are vertical tappings switches and faceplate switches.

       The off load tap changer is shown in the Fig. 3.

Fig. 3  Off load tap changer

       The above arrangement is normally used to get ±5 % change in the steps of  ±2.5 %. It consists of an insulating base on which six brass or copper terminals are mounted. The contactor is mounted on an arm of the shaft. The central or middle part of the winding contains the taps and the taps are connected to terminals of tap changer. The shaft can be rotated from one position to another so that the selector switch is connected to adjacent pairs of stationary terminals.

       Let us consider that the selector switch is at a position connecting taps 1 and 2. Hence total winding is in use. When contactor is moved one point to the left, it makes a connection between 1 and 6 thus cutting out part of the winding between taps 2 and 6. The next step connects taps 6 and 5 cutting out part of winding between taps 1 and 5. Thus the parts of the windings cut out gradually in steps with minimum number of turns remain in the winding with the position 5 and 6. Corresponding to each position of the selector switch different voltage regulation on positive as well as on negative side can be obtained.

1.3 Parallel winding Tap Changer

       This is another way of Off circuit tap changing in which transformers are specially wound for this purpose. It consists of two identical winding which operate in parallel and capable of supporting full load current of transformer. It is shown in the Fig. 4.

Fig  4

        Each tap changer is connected to circuit breaker one end of each winding is connected to supply line while other end is connected to power line through circuit breakers.

       In normal working condition, both the circuit breakers are closed. The power is supplied by both windings to the secondary circuit. Under the requirement of tap changing, one of the circuit breakers is opened while other is closed. The tap is then changed in that winding whose breaker is opened which is then closed and the one which was in closed position. Previously is not open to change its tap to the same position and it is reclosed then.

       There is a short interval within this opening during which the two windings are operated with different taps. This may lead to flow of large circulating currents within the two windings which can be avoided by allowing this tap change by only one step at a time. With excessive current circulates then circuit breakers will operate to protect the transformers.

       The need of identical winding is because during tap changing either of the winding is supplying total power to the load. Hence it should carry full load current.

1.4 On Load Tap Changing

       Under the load conditions, it is required to maintain the voltage on the secondary side of the transformer with the help of certain arrangement when transformer is connected to a system. If this arrangement works without making the load off from the transformer then it is called on load tap changing. Without interruption in the supply, the tap changing gear should change the turns ratio.

       Normally in case of on load tap changing the tappings are connected at the neutral end of high voltage winding. The tap changer is normally in the form of selector switch. There are various ways by which tap changer is operated viz. motor operated mechanism, remote control or with the help of handle for manual operation in case of emergency.

       The most vital factor in case of on load tap changing is continuity of circuit throughout the operation of tap changing. If the circuit is disconnected, the continuity of supply to load will be lost. As the selector switch should not break current, additional separate oil filled compartment is used to mount diverter switch which breaks the load current by interrupted arc which can form carbon. It should not be mixed with the oil in the main tank to decrease its dielectric strength.

       The main consideration is that when one tapping is opened, the contact must be established to other tapping. Hence make before break switch is used and in the transition period connection is made to adjacent taps which may result into short circuit of turns between adjacent tappings. This short circuit current can be limited by using resistors or reactors. The reactors which were used in old days sre replaced by resistors now.

      The on load tap change is shown in the Fig. 5.

Fig.  5

       The selector switch 1 and 2 are provided on taps 1 and 2 respectively. The diverter switch is connecting tap 1 to the neutral terminal of the transformer winding.

       If we want to change the tap from position 1 to 2 then following is the sequence of operation :

i) The resistance R₁ is short circuited as contacts a and b are closed. The load currents flows through contact a from tap. This is nothing but the running position at the tap 1.

ii) With the help of external operating mechanism, the diverter switch is moved to open the contact a. The load current now flows through resistance R₁ and contact b.

iii) The contact c closes to open the resistance R₁ when the moving contact of diverter switch continues its movement to the left. The resistance R₁ and R₂ are now connected across taps 1 and 2 so that the load current flows through these resistance to mid point of junction of b and c.

iv) With further movement of diverter switch to the left makes contact b to open. Now the load current flows from tap 2 through resistance R₂ and contact c.

v) At last the diverter switch moves to the extreme left position which closes the contact d. This short circuits resistance R₂. The load current flows from tap 2 through contact d which is the running position of tap 2.

       It can be sen that the change of tap from position 1 to 2 does not involve the movement of selector switches 1 and 2. But if is desired to have further tap change from tap 2 to tap3 then the selector switch S₂ is moved to tap 3 before the movement of diverter switch. Then then the same sequence as described above but in reverse order is to be followed and the diverter switch is moved.

       As the resistances are included in the circuit there will be some loss of energy which can be reduced by keeping these resistances in circuit for minimum time as possible. For economical consideration, as the resistors are designed for short time rating, they should by kept in the circuit for minimum time. This needs some form of energy storage in driving mechanism which ensures the completion of tap change once initiated under the failure of control supply. Modern on load tap changers use springs as energy storage elements which reduce the time of resistor in a circuit to minimum. This type of tap changer is compact in size while due to high speed breaking, contact wear reduces.

1.5 Single Winding Tap Changer

       This also one of the types of on load tap changer by connecting an inductor in series with tapped winding. The inductor is centertapped which facilities half step increments. It is shown i the Fig. 6.

Fig. 6  Single winding on load tap changer

       The secondary or low voltage side voltage remains constant with proper switching on and off of the circuit breakers which changes the voltage on high voltage side. Opening or closing of circuit breakers to obtain desired connection must be done in proper sequence.

       This type of tap changer is often used in case of tap changing motor which automatically regulates the output voltage. For operating the tap changing motor, the voltage sensitive relays are used. When the output voltage falls below certain level, the motor operates to change the taps to increase the voltage while if output voltage is high then the motors operates in a direction to decrease the voltage.

       Automatic on load tap changers are normally provided with time delay mechanism which provides some time to pass before operating the tap changing motor. That ensures that taps will not change whenever there is momentary voltage fluctuation which can occur when high current loads are added or removed from the line. Some means of manual operation is also provided in the form of a handle which permits tap changing in the event of failure of automatic circuit.

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