Step switch for power transformers

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Three single-phase step switches mounted on the transformer cover; the rule derivation is already connected

Step switches for power transformers are used to set the transformation ratio. The winding of the transformer on its high or low voltage side usually comprises a main winding and a control or step winding with several taps that are led to the tap changer. The power control with parallel connection is also implemented via the step switch.

Step switch divided into load tap changer ( English OLTC - On Load Tap Changer ) and diverter ( English NLTC - No Load Tap Changer , DETC - De-Energized Tap Changer or OCTC - Off Circuit Tap Changer ).

On-load tap-changer

Step switch from a power transformer
Load switch in the lower area with eight preselection switches (1..8) on the primary side of a high-voltage transformer. The illustration shows the uninterrupted switching process from level 2 to level 3 and vice versa.

On-load tap-changers are used for uninterrupted switching under load and are divided into load selectors and load switches . Depending on the operating currents to be handled and the installation location in the transformer circuit, tap changers are installed with one or three phases. This means that a tap changer column switches either one or three phases. Three single-phase step switches require more space in the transformer tank than a three-phase step switch. The use of three-phase step switches usually requires the installation location in the star point of a star connection . Single-phase switches are usually required for higher currents, higher switching capacity or use in a delta connection.

Load switches consist of a selector , a diverter switch and resistors or coils that limit the short-circuit current between the stages when switching. The selector forms a unit with the active part of the transformer, it is rigidly connected to the taps of the step winding. The selector contacts are switched load or current-free. The unit consisting of diverter switch and resistors or coils (diverter switch insert) is located in its own oil compartment, which in turn is integrated into the transformer tank, due to the oil contamination from the switching arc . The diverter switch insert is filled with the same oil (usually a mineral oil-based or synthetic oil or an ester ) as the boiler, but is generally not in contact with the transformer oil . The step switch with current limiting choke represents a modification of this principle. This type of step switch contains all components except for the current-limiting elements in a kind of oil-filled backpack that is welded or screwed onto the boiler. A current limiting choke, also known as a preventive autotransformer , is required to limit the current when switching . The switch is connected to the taps on the step winding and the current limiting choke via a connection plate. This tap changer technology is mainly used in America.

To enlarge the control range or the number of stages , the selector can comprise a preselector , which connects the main winding with the stage winding , and a fine selector , which contacts the desired tap. The preselector can for example be designed as a turner , coarse stage or multiple coarse stage .

The on-load tap-changer is relatively error-prone due to the relatively frequent switching operations and the high proportion of mechanically and electrically highly stressed components, which requires thorough operation during maintenance work. A weak point of the system described lies in the oil chamber of the voter. This can only be maintained with great effort, which makes it necessary for the transformer to come to a standstill.

The trend in diverter switches is towards vacuum switches . According to the manufacturer, these increase the maintenance intervals from 7 to 25 years, or a higher number of levels is possible based on a time interval.

Tap changer monitoring systems offer constant status diagnosis through total current generation and torque monitoring.

Diverter

In principle, tap changers perform the same tasks as on-load tap-changers, but can only be adjusted without load or voltage. Diverter are usually made with a few steps and are often only operated by hand. They are largely maintenance-free.

Manufacturer

literature

  • Wilfried Knies, Klaus Schierack: Electrical systems engineering. 5th edition, Carl Hanser Verlag, Munich and Vienna, 2006, ISBN 3-446-40574-7
  • Patent specification - German Reich No. 474613 Class 21d² Group 53: Device for switching two taps of a step transformer during operation by circuit breakers moving in opposite directions with pre-contacts

Web links

Individual evidence

  1. ↑ Oil type OLTCs MR product brochure , approx. 1.52 MB. Retrieved May 6, 2017.
  2. ↑ On-Load Tap-Changers For Power Transformers MR product brochure , approx. 1.5 MB. Retrieved May 6, 2017.