Tap changers are devices that can increase or decrease the output secondary voltage by changing the turn ratio of primary or secondary winding. A tap changer is usually installed on a high voltage section of a two-winding transformer, due to the low current in that area. The changers are also provided on high voltage windings of an electrical transformer if there is sufficient control of voltage. The change of voltage is affected when you change the number of turns of the transformer provided with taps.
There are two types of Tap Changers:
1. On-Load Tap Changer
Its primary feature is that during operation, the main circuit of the switch should not be opened. This means that no part of the switch should get the short circuit. Due to the expansion and interconnection of the power system, it becomes crucial to change the transformation taps numerous times every day for achieving the necessary voltage as per the load demand.
This demand of continuous supply doesn’t allow you to disconnect the transformer from the system for off-load tap changing. Hence, on-load tap changers are preferred in the majority of power transformers.
Two conditions must be fulfilled while tapping:
- The load circuit should be intact to avoid arcing and to prevent contact damage
- While adjusting the tap, no part of the windings should be short-circuited
In the above diagram, S is the diverter switch, and 1, 2 and 3 are the selector switches. The tap changing employs the center tapped reactor R as shown in the diagram. The transformer operates when the switches 1 and S are closed.
To change to tap 2, switch S must be opened and switch 2 must be closed. To complete the tap change, switch 1 is operated and switch S is closed. Remember that the diverter switch operates on-load and no current flows in the selector switches during tap changing. When you tap change, only half of the reactance that limits the current is connected in the circuit.
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2. Off-Load/No-load Tap Changer
You have to install an off-load changer on a transformer if the required change in voltage is infrequent. The taps can be changed after completely isolating a transformer from the circuit. This kind of changer is generally installed on a distribution transformer.
The tap changing can be carried out when the transformer is in Off-Load or No-Load condition. In a dry-type transformer, the cooling phenomenon takes place mainly with natural air. Unlike in on-load tap changing where the arc quenching is limited by oil when the transformer is on-load, the tapping with an off-load tap changer is only carried out when the transformer is in OFF-Switch condition.
It is often utilized in situations where turn-ratio doesn’t need to be changed much, and de-energizing is allowed in low power and low voltage transformers. In some, tap changing may be done with a rotary or slider switch. It can be mainly seen in solar power projects.
Off-load tap changers are also used in high voltage transformers. The system of such transformers includes a no-load tap changer on the primary winding. This changer helps accommodate variations within a narrow band around the nominal rating. In such systems, the tap changing will often be done only once, at the time of installation. However, it can also be changed during a scheduled outage to address any long-term change in the system’s voltage profile.
It is imperative that you choose the right kind of tap changer based on your requirements.
Jeson Pitt is part of the marketing department for D&F Liquidators and regularly writes to share his knowledge; educating readers about electrical products and problems. He has years of experience within the field and a keen insight into the industry. Jeson lives in Hayward, CA.