The purpose of a tap changer is to regulate the output voltage of a transformer. It does this by altering the number of turns in one winding and thereby changing the turns ratio of the transformer. There are two types of transformer tap changers: an on-load tap changer (OLTC) and a deenergised tap changer (DETC). Note that not all transformers have tap changers.
An OLTC varies the transformer ratio while the transformer is energized and carrying load. The switching principle uses the “make before break” contact concept. An adjacent tap is bridged before breaking contact with the load carrying tap for the purpose of transferring load from one tap to the other without interrupting or appreciably changing the load current. While in a bridging position (i.e., contact is made with two taps), some form of impedance (resistive or reactive) is present to limit circulating current. A high speed resistive type OLTC uses a resistor pair to absorb energy and does not use the bridging position as a service position. A reactive type OLTC uses a reactor that is designed for continuous loading, e.g., a preventative autotransformer, and therefore uses the bridging position as a service position.
There are two primary OLTC designs. A diverter design, used for higher voltages and power, has both a tap selector and a separate diverter switch (also called arcing switch). The switching arc may occur in oil or may be contained in a vacuum bottle. A non-diverter design, used for lower voltage ratings, simply uses a so-called selector switch (also called arcing tap switch) that combines the functions of a diverter switch and tap selector.
A DETC is a tap changer that cannot be moved while the transformer is energized. It often has 5 positions (A,B,C,D,E, or 1,2,3,4,5). If a DETC is not exercised on a regular basis, there is increased risk that the DETC will not make properly when next moved.
Tap changers have historically been one of the top causes of transformer failures (Cigre_WG 12-05 “An international survey on failures in large power transformers in service“, Electra No. 88, 1983, and ANSI/IEEE, 1985). Faults in OLTC’s can be classified as dielectric failures (oil quality or clearance related), thermal failures (due to coking or crimp problems), or mechanical failures (contact wear and misalignment, limit switches, sheared pins on the linkage that operates the reversing switch, lubrication problems, etc). The following electrical field tests provide information about the integrity of a transformer tap changer.
Electrical field tests:
- Exciting current; exciting current tests have the ability to detect a myriad of transformer tap changer problems (DETC and OLTC), including: misalignment, coking and wear of contacts, loose moveable contacts, improper wiring from the tap winding to the OLTC, reversed connections to the preventative autotransformer (PA) of an OLTC, open- or short-circuited turns or high resistance connections in the OLTC PA, series autotransformer or series transformer, and more.
- DC winding resistance; a DC winding resistance test is used to detect any problem which impacts the integrity of the current carrying path between terminals of a winding, including the tap changer. It is particularly adept in identifying partial open-circuited conditions.
- Dynamic winding resistance; a dynamic winding resistance test is the measure of the DC current and resistance (as a function of time) as the OLTC changes tap position. It is particularly effective at identifying problems with the diverter switch, diverter switch contacts and transition resistors of a resistive style OLTC. Generally, the test assesses the integrity of any component which makes, carries or breaks current during an OLTC operation.
- Sweep frequency response analysis (SFRA); the mechanical integrity of the tap windings and their leads are assessed in the mid- to upper-frequency ranges of an FRA test on a transformer
- DGA; normal gassing patterns (produced as insulating materials deteriorate) vary for each family of OLTCs. DGA on a sample of oil from the OLTC is an effective tool for identifying problems such as localised overheating or excessive arcing, which will result in a change in the OLTC’s typical gassing behaviour (for example, the ratio of the hydrocarbon gasses change).
- Dielectric Strength; checks that the dielectric breakdown voltage of the oil in the OLTC is above a minimum threshold. This is influenced by the relative saturation of water in oil and presence of conductive particles (number and size).
- Moisture; a test for excessive water in the tap changer, which reduces the dielectric breakdown strength of the oil and can accelerate aging of the contacts
- Infrared; checks for a temperature difference between the transformer’s main tank and tap compartment; it is atypical for the tap compartment to be as hot or hotter than the main tank,