TJ|H2b offers a time-proven service for the testing of Sulphur Hexafluoride (SF6) that significantly reduces the cost of circuit breaker and GIS maintenance. The Breaker Gas Analysis, BGA®, analytical test procedures are used to determine the composition of the gas including the decomposition products of SF6, moisture and dielectric breakdown voltage. Analysis of SF6 contaminants and breakdown products gives diagnostic information about the operating condition of the gas-filled equipment.
The use of SF6 analysis reduces the maintenance cost of breakers by identifying those breakers that have internal problems, safety issues, or special handling needs. The time required to remove the SF6 gas from a breaker, to perform an internal inspection and replace the gas requires 10 – 20 man-days. The variation in time depends upon the voltage class and model of the breaker. Perhaps more importantly, SF6 analysis reduces maintenance costs by identifying those breakers that do not require maintenance.
Gas analysis is useful throughout the life cycle of the equipment beginning with acceptance testing. This includes verification of the quality of the gas supply as well as the verification of the quality of the gas in equipment that is ready to be commissioned. This addresses purity and contaminant issues that can significantly reduce life.
Time and predictor driven testing during the life of the equipment can provide indication and identification of faults and failure modes. At the same time, the analysis provides indication of the wear out of key interrupter and contact components. This information can be used to schedule and direct in-tank maintenance activities as they are needed. Contact and interrupter parts can be made available before the maintenance has begun.
After performing a BGA® test on your SF₆ breaker, you will receive a condition rating. The recommended sampling interval ranges from ‘immediately’ to ‘24 months’, depending on the condition of your unit. Please refer to the scoring methodology or reach out to TJH2b for further explanation on recommended intervals.
BGA™ condition codes, in general, indicate the following+:
+Some variations of the recommended mitigations or retest intervals may occur at the discretion of the laboratory based on information provided. With breakers, we are dealing with a complex subject involving a number of variables. As such, general “rules of thumb” with regard to sampling intervals cannot be applied in every case. A number of other factors must be taken into consideration, such as:
As such, the appropriate action is up to the discretion of the Asset Manager who is best placed to take other influencing factors into account according to established internal procedures. Our codes and recommendations, while based on a vast amount of experience as well as guidelines published by IEEE and other standards bodies, are to be taken only as useful guides.
Breaker Gas Analysis (BGA®) includes a condition rating, diagnostics, and potential remediation actions where applicable. Upon receipt of the report from TJH2b, we recommend that results and diagnostics are reviewed promptly, following internal procedures. If further support is needed, TJH2b offers free, 24/7 consultation services to help you understand reports and provide guidance.
Obtaining a representative sample is essential for any analytical testing process to yield meaningful results. In SF₆ analysis, contamination of the sample with air and moisture or corruption of the sample by reactions with sampling equipment are critical issues. Special sampling equipment and procedures have been developed to address these issues. The containers are fabricated from stainless steel because of its chemical inertness to SF₆ decomposition products (Gas Sampling Collection Unit). These containers are dried and evacuated before being sent into the field. Special fittings are used for each type of equipment.
In the absence of an electrical discharge or extreme heat (>500°C), SF₆ is chemically inert. When a discharge occurs, fluorine atoms on the SF₆ may capture an electron and dissociate from the sulfur. When the discharge has ended, under ideal circumstances, each dissociated fluorine loses the captured electron and recombines with a dissociated SFx species to reform SF₆ (Figure 1). This is the ‘self-healing’ or regenerative property of SF₆. Regardless of circumstances, this is the predominate reaction occurring in SF₆ filled high voltage electrical equipment. A discharge does not result in each SF₆ in the discharge path being completely dissociated. Instead, a distribution of dissociation products is formed that looks something like the distribution shown in Figure 2. The amplitude of the distribution is determined by the duration of the discharge and the composition of the distribution is determined by the energy of the discharge. Discharge durations and energies can vary considerably (Figure 3).
For more details, please see: Using sf6 analysis for condition based maintenance of circuit breakers and gas insulated substations.
The circuit breaker in Case 1 is a 161 kV, 2000 amp, SF₆ single pressure, live tank unit with a true spring operating mechanism. The breaker is operated 2 to 3 times per day to connect a 168 MVAR capacitor bank. On September 20, 1997, a remote dispatcher operated the breaker. Pole 2 failed to clear, resulting in operation of the overcurrent and breaker failure relay. The operation of these relays isolated the bus section that feeds the circuit breaker. After the clearance, the dispatcher was able to remotely energize the bus section. This failure did not result in a rupture of the interrupter.
For details about the four case studies, please see: Using SF₆ analysis for condition based maintenance of circuit breakers and gas insulated substations.