Transformer outages have a considerable economic impact on the operation of a power supply network. Therefore, it is the aim to ensure an accurate assessment of the transformer condition. Partial discharges, leakage currents, hot spots, and arcing are phenomena that contribute to the degradation of oil insulating performances by generating dangerous gas flow inside the transformer tank. Internal gas accumulation and oil flow are monitored and kept under control by the Buchholz relay. Buchholz relay has many advantages and disadvantages for the transformers. Let’s list them one by one.
Advantages of Buchholz Relay
- The Buchholz relay has a long history. It was first developed in 1921 by Max Buchholz, Oberrat at Preußische Elektrizitäts-A.G. (Prussian electricity company) in Kassel.
- It is an important protection and monitoring device for insulating liquid-filled transformers with the conservator and choke coils.
- It is the simplest form of transformer protection. A Buchholz relay is a fundamental device for transformers, it keeps internal gas accumulation and oil flow monitored and under control.
- It detects incipient faults much earlier than the other forms of protection.
- 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.
- The Buchholz relay is suitable for open-air as well as indoor installations.
- 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.
- Normally a protective relay does not indicate the appearance of the fault. It operates when a fault occurs. But Buchholz relay gives an indication of the fault at a very early stage, by anticipating the fault and operating the alarm circuit. Thus, the transformer can be taken out of service before any type of serious damage occurs and accidents can be prevented.
- It has a high degree of ingress protection.
- It can operate at very high and low temperatures effectively.
- The Buchholz relay does not need periodic maintenance; however, it is advisable to check regularly the electric contact and the free movement of the float.
- The relay is designed to withstand corrosion.
Disadvantages of Buchholz Relay
- The Buchholz relay can only be implemented in transformers and other electrical equipment having absorbed oil.
- The relay is only detecting faults in cases when the level of oil is below.
- The relay is not guarding connecting cables, therefore dedicated protection is required for cables.
- The response time of the Buchholz relay is a bit too high.
- The relay is slow to operate having a minimum operating time of 0.1 seconds and an average time of 0.2 seconds.
- The setting of the mercury switches cannot be kept too sensitive otherwise the relay can operate due to bubbles, vibration, earthquakes mechanical shocks, etc.
- Using the relay in transformers for rating less than 500 kVA is not economical.