Tap changer

Mechanism to change turn ratios in transformers

A tap changer is a mechanism in transformers which allows for variable turn ratios to be selected in distinct steps. This is done by connecting to a number of access points known as taps along either the primary or secondary winding. Tap changers exist in two primary types, no load tap changers (NLTC), which must be de energized before the turn ratio is adjusted, and on load tap changers (OLTC), which may adjust their turn ratio during operation. The tap selection on any tap changer may be made via an automatic system, as is often the case for OLTC, or a manual tap changer, which is more common for NLTC. Automatic tap changers can be placed on a lower or higher voltage winding, but for high power generation and transmission applications, automatic tap changers are often placed on the higher voltage (lower current) transformer winding for easy access and to minimize the current load during operation.

No load tap changer (NLTC), also known as Off circuit tap changer (OCTC) or De energized tap changer (DETC), is a tap changer utilized in situations in which a transformer's turn ratio does not require frequent changing and it is permissible to de energize the transformer system. This type of transformer is frequently employed in low power, low voltage transformers in which the tap point often may take the form of a transformer connection terminal, requiring the input line to be disconnected by hand and connected to the new terminal. Alternatively, in some systems, the process of tap changing may be assisted by means of a rotary or slider switch. No load tap changers are also employed in high voltage distribution type transformers in which the system includes a no load tap changer on the primary winding to accommodate transmission system variations within a narrow band around the nominal rating. In such systems, the tap changer will often be set just once, at the time of installation, although it may be changed later to accommodate a long term change in the system voltage profile.

On load tap changer (OLTC), also known as On circuit tap changer (OCTC), is a tap changer in applications where a supply interruption during a tap change is unacceptable, the transformer is often fitted with a more expensive and complex on load tap changing mechanism. On load tap changers may be generally classified as either mechanical, electronically assisted, or fully electronic. These systems usually possess 33 taps (one at centre "Rated" tap and sixteen to increase and decrease the turn ratio) and allow for ±10% variation (each step providing 0.625% variation) from the nominal transformer rating which, in turn, allows for stepped voltage regulation of the output. Tap changers typically use numerous tap selector switches which may not be switched under load, broken into even and odd banks, and switch between the banks with a heavy duty diverter switch which can switch between them under load. The result operates like a dual clutch transmission, with the tap selector switches taking the place of the gearbox and the diverter switch taking the place of the clutch.

This is a relatively recent development which uses thyristors both to switch the transformer winding taps and to pass the load current in the steady state. The disadvantage is that all non conducting thyristors connected to the unselected taps still dissipate power due to their leakage currents and they have limited short circuit tolerance. This power consumption can add up to a few kilowatts which appears as heat and causes a reduction in overall efficiency of the transformer; however, it results in a more compact design that reduces the size and weight of the tap changer device. Solid state tap changers are typically employed only on smaller power transformers.

If only one tap changer is required, manually operated tap points are usually made on the high voltage (primary) or lower current winding of the transformer to minimize the current handling requirements of the contacts. However, a transformer may include a tap changer on each winding if there are advantages to do so. For example, in power distribution networks, a large step down transformer may have an off load tap changer on the primary winding and an on load automatic tap changer on the secondary winding or windings. The high voltage tap is set to match long term system profile on the high voltage network (typically supply voltage averages) and is rarely changed. The low voltage tap may be requested to change positions multiple times each day, without interrupting the power delivery, to follow loading conditions on the low voltage (secondary winding) network. To minimize the number of winding taps and thus reduce the physical size of a tap changing transformer, a 'reversing' tap changer winding may be used, which is a portion of the main winding able to be connected in its opposite direction (buck) and thus oppose the voltage.

Name Status IEC 60214 1:2014 Current IEC 60214 2:2004 Current IEEE Std C57.131 2012 Current ГОСТ 24126 80 (СТ СЭВ 634 77) Current IEC 214:1997 Replaced by a later version IEC 214:1989 Replaced by a later version IEC 214:1985 Replaced by a later version