In addition to the standard function of the Buchholz relay, i.e. a mechanical switching mechanism responds to the release of gas, oil surge or complete loss of oil, the sensor technology offers a number of useful functions allowing extended and reliable monitoring of the transformer.
The Buchholz relay can be equipped with the following sensors:
• Gas volume sensor - NM series (No. 1 in the figure)
• Temperature sensor (No. 2 in the figure)
• Moisture-temperature sensor (No. 3 in the figure)
These allow permanent monitoring of gas generation, water in oil and temperature so that transformer defects can be detected and corrective measures taken at an early time and finally the transformer life extended.
Code 60 - Gas volume sensor (NM series)
The standard Buchholz relay detects unsolved gases in the insulating liquid and generates a signal indicating the presence of gases when the specified threshold is exceeded, i.e. up to a certain gas volume no signal is generated. It does not allow conclusions with regard to the time of generation of gases.
The time of the generation of unsolved gases in the insulating liquid is a significant criterion for the evaluation of the defect. The composition and volume of the fault gases depend on the type and energy content of the defect leading to the generation of gases. Spontaneous and high-energy faults cause significant gas volumes within a short period of time whilst minor and low-energy faults produce low gas volumes.
Continuous and analog measurements in the Buchholz relay by the gas volume sensor supply information about the development of unsolved gases which allow early evaluation of the defect.
The additional function is realised by a gas volume sensor with capacitive action. The supply voltage of this component is 24 V DC provided by the user. The output signal is a current signal of 4 to 20 A DC. Processing of the information supplied by this standardised signal is the user’s responsibility.
Code 61 - Buchholz relay with temperature sensor
The temperature sensor monitors the insulating liquid temperature in the Buchholz relay continuously. The common Pt1000 resistance sensor type allows straightforward integration in the monitoring system of the transformer. Via a shielded connector the temperature sensor is connected (two-wire technology) to a signal transducer to be provided by user.
Code 62 – Buchholz relay with moisture-temperature sensor
The measuring device is a combined sensor measuring water content and temperature of transformer oil. Measurement is performed independent of type and age of oil and its additives. The water content can be expressed as water activity, relative saturation or as an absolute value. The temperature range is scalable.
The risk of free water generation can be derived directly from both measured values.
The supply voltage of this component is 24 V DC and made available by user. Measured data output is via either an analog signal (0(4) ... 20 mA) or a digital signal (RS-485 with Modbus RTU). The device can also be parameterised via a digital interface.