EMB - Buchholz Relays for Transformer Protection

The Buchholz relay was developed in 1921 by Max Buchholz, Oberrat (senior councillor) at Preußische Elektrizitäts-A.G. (Prussian electricity company) in Kassel. Since that time it has been an important protection and monitoring device for insulating liquid filled transformers with conservator and choke coils. It also allows separate monitoring of oil-filled bushings or cable terminal boxes. It is mounted in the cooling cycle of the device to be protected and responds to faults such as gas generation, loss of as well as high flow rates of the insulating liquid.

For transformers with hermetical closure by means of a hydro-type compensator (rubber sack) in the conservator, the Buchholz relay can be used also as monitoring device (air cell failure relay) of the hydro-type compensator.

The Buchholz relay is suitable for open-air as well as indoor installations.

The type diversity of the Buchholz relay is tailored to norms and standards as well as to special customer demands. The type of relay to be used depends on the nominal rating and construction features of the device to be protected. Our range of products permits optimum adaptation to actual requirements.

Elektromotoren und Gerätebau GmbH (EMB GmbH) provides more than 70 years experience in producing Buchholz relays and other protective devices for liquid-cooled and liquid-insulated devices. It ranks today among the most distinguished manufacturers of this type of equipment.

EMB Buchholz relays are in compliance with EN 50216-2 as well as IEC 60076-22-1 and are known for their easy operation, high reliability and extremely long life. The functionality of the EMB Buchholz relays is given by using all conventional insulating liquids. The functionality in combination with new insulating fluids will be tested in close cooperation with the manufacturers.

EMB has been certified according to: DIN EN ISO 9001:2015, AEO F, Known Consignor (Airfreight Security) and EAC. Further certifications have been awarded by well-known independent test laboratories such as TÜV Rheinland and TZO.

Casing

The casing is made of weather-resistant cast aluminium alloy provided with a paint coating. It is supplied either with flanged (Fig. 1/ Number 1) or threaded connection (Fig. 2/ No.2). The different casing designs available are shown in Single-Float Buchholz Relays and Double-Float Buchholz Relays. Others are available on request.

To check the switching system for proper function, the casing is provided with sightglasses (Fig. 2/ No. 3). The sightglasses provided with scales permit reading of collected gas volume. The relays can be provided with hinged lids (Fig. 1/ No.4) to protect the sightglasses.

Cover

The cover is made of weather-resistant cast aluminium alloy provided with a paint coating. The upper section of the cover accommodates the terminal box (Fig. 3/ No. 1). In front of the terminal box are arranged the bleeding valve (Fig. 3/ No. 2) and the test key covered by a cap nut (Fig. 3/ No. 3) as well as a plate (Fig. 3/ No. 4) with instructions for actuating the test key. The terminal box accommodates the earth terminal (Fig. 3/ No. 5) and bushings (Fig. 3/ No. 6) for the terminals provided in the base of the cover. The number of these bushings determines the design of the switching systems in terms of type and quantity of the magnet contact tubes.

The terminal box is sealed by an cap (Fig. 3/ No. 7) so that it is safe to touch and protected against pollution. If the cap is opened instructions for installation (Fig. 3/ No. 8) are shown e.g. the graphic symbol and the connection diagram. The cable can be inserted through the cable gland (Fig. 3/ No. 9).

The Buchholz relay is used for the protection and monitoring of insulated liquid-filled transformers with expansion tank and earth leakage coils. It is installed in the cooling circuit of the unit to be protected and reacts to faults such as gas formation, loss as well as excessive flow of the insulating liquid with a warning or disconnection signal. This prevents devastating damage to the transformer or even its destruction.

The Buchholz relay is installed in the pipe between the tank of the device to be protected (transformer, reactor) and the conservator. During normal operation it is filled completely with insulating liquid.

Due to buoyancy the float of the single-float relay and both floats of the double-float relay are at their top position.

The upper and lower switching systems form a functional unit in the single-float Buchholz relay so that in the event of a fault the transformer is immediately disconnected from the power system.

In the following the function of a Buchholz relay is explained using the example of a double-float Buchholz relay. If a fault occurs inside the transformer, the Buchholz relay responds as follows: