OLTC
On-load tap changers have been in the power industry for quite a long time now. On-load tap changer is a device that is used for voltage regulation and phase shifting of a transformer without power interruption. The tap changing is used mainly to change the turns ratio of the transformer and thus regulate the voltage of the transformer.
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The main feature of On-load tap changer is that the tap-changing process requires no power interruption of the transformer. The tapping is basically provided on the HV winding of the transformer. This is because as the high voltage side has less current the size of lead contacts will also be small.
The tap changer has a selector switch, and motor mechanism and it can be operated locally or remotely. Manual changing of the tap via a handle is also provided in case the motor malfunctions. The selector switch allows the operator to choose the required tap position as per the prevailing operating conditions. The motor mechanism or manually controlled shaft is responsible for physically shifting the tap to the selected position.
PRINCIPLE OF SWITCHING :
The On-load tap changer changes the ratio of a transformer by adding or subtracting turns from either the primary or the secondary winding. The transformer is therefore equipped with a regulating or tap winding which is connected to the On-load tap changer.
Simple changing of taps during an energized status is unacceptable due to momentary loss of system load during the switching operation. The “make before break contact concept”, is therefore the basic design for all OLTCs.
The transition impedance in the form of a resistor or reactor consists of one or more units that bridge adjacent taps for the purpose of transferring load from one tap to the other without interruption or appreciable change in the load current.
At the same time, they limit the circulating current (IC) for the period when both taps are used. Normally, reactor-type OLTCs use the bridging position as a service position and the reactor is therefore designed for continuous loading.
The voltage between the taps is the step voltage, which normally lies between 0.8 % and 2.5 % of the rated voltage of the transformer. The main components of an OLTC are contact systems for make and break currents as well as carrying currents, transition impedances, gearings, spring energy accumulators, and a drive mechanism.
RESISTOR TYPE OLTC
The OLTC design that is normally used for higher ratings and higher voltages comprises a diverter switch (arcing switch) and a tap selector. For lower ratings, OLTC designs in which the functions of the diverter switch (arcing switch) and the tap selector are combined in a selector switch (arcing tap switch) are used.
With an OLTC comprising a diverter switch (arcing switch) and a tap selector, the tap-change operation takes place in two steps. The next tap is first preselected by the tap selector at no load ( fig a-c). The diverter switch then transfers the load current from the tap in operation to the preselected tap (fig c-g). The OLTC is operated by means of a drive mechanism.
The tap selector is operated by a gearing directly from the drive mechanism. At the same time, a spring energy accumulator is tensioned, which operates the diverter switch – after release at a very short time interval independently of the motion of the drive mechanism.
The gearing ensures that this diverter switch operation always takes place after the tap preselection operation has finished. The switching time of a diverter switch is between 40 and 60 ms with today’s designs.
During diverter switch operation, transition resistors are inserted (fig d-f) which are loaded for 20–30 ms, which means the resistors can be designed for short-term loading. The amount of resistor material required is therefore relatively small. The total operation time of an OLTC is between 3 and 10 seconds, depending on the respective design.
REACTOR TYPE OLTC
The following types of switching are used for reactor oil-type OLTCs:
Selector switch (arcing tap switch)
Diverter switch (arcing switch) with tap selector
All reactor-type OLTCs are compartment types where the preventive autotransformer (reactor) is not part of the OLTC. The preventive autotransformer is designed by the transformer manufacturer and located in the transformer tank.
Now only selector switches (arcing tap switches) for voltage regulators are still in production whereas the reactor vacuum-type OLTCs are going to be the state-of-the-art in the field of power transformers.
