Klaus Spitzenberg - Megger Support, Germany

Condition analysis of gas-insulated medium voltage circuit breaker systems is now practical and cost effective.

Working with energy distribution utilities Syna and Westnetz, Megger has developed techniques that make it possible to determine the condition of enclosed medium-voltage circuit breakers safely and cost-effectively. These techniques, which also allow first-trip testing to be performed, are based on connecting a Megger TM1800 or TM1700 circuit breaker analyser to the Voltage Detection System (VDS) via an adaptor. This opens up a whole range of new possibilities for system operators, as it means that it is no longer necessary to spend time isolating the circuit breakers for tests.

A definite need for testing

As gas-insulated medium-voltage circuit breaker systems are housed in enclosures, it is often impossible to use established condition analysis methods – the process would simply be too laborious, time consuming and uneconomical. However, there is a definite need to test these systems, as even components that are nominally maintenance-free need testing.Indeed, German law, and the law of most other countries, dictates that all work equipment must be tested regularly, using tests based on an assessment of the hazards that might arise from the equipment. And gas-insulated medium voltage circuit breakers most certainly fall into the category of work equipment. The new testing techniques make it easier for switchgear users to meet their legal obligations, as well as providing them with invaluable information to aid effective maintenance planning.

Isolation no longer needed

With the new techniques, it is no longer necessary to isolate the circuit breakers when evaluating their condition. The test instrument is connected to the capacitive Voltage Detection System via an adapter. It’s also connected to a trigger box and, via current clamps, to the medium-voltage circuit breaker system. A TM1700 circuit breaker analyser or a TM1800 analyser , shown in Figures 1 and 2, may be used. All key parameters can be measured and recorded without the time-consuming isolation and grounding procedures that are needed with other testing methods. In addition, no connections are needed to secondary circuits.

Fig. 1: TM1700 circuit breaker analyser

Fig. 2: TM1800 modular circuit breaker analyser

Present-day test techniques

The most common testing methods for medium voltage circuit breakers include timing and contact resistance measurements, but both of these tests traditionally require electrical access to the main contact connections. In air-insulated systems, this is not usually a problem. Furthermore, air-insulated circuit breakers can often be withdrawn from the assemblies they form part of, making it possible to disconnect them safely from all components that carry high voltages. In such cases, the full range of connection options is available.Even if the circuit breaker is not a withdrawable type, modern testing technologies ensure that the device undergoing testing can be grounded on both sides not only during the connection phase, but, crucially, also during the testing procedure. These technologies include timing using the DCM method, which is one of the DualGroundTM technologies exclusively available on Megger instruments. 

Fig. 3: DualGroundTM allows grounding of both sides of the circuit breaker, even during measurement 

The challenge of gas-insulated medium-voltage systems

The straightforward connection options available for air-insulated circuit breakers are unfortunately not available for circuit breakers in enclosed, gas-insulated switching systems. All of the primary components that come into contact with high voltages are enclosed in a hermetically sealed gas compartment. Electrical access to these components is theoretically possible via cable terminal compartments, reserve fields or similarroutes. At best however, such access is usually available only during the setup and commissioning phases, and even then it is subject to many restrictions.These options do not provide a practical approach for routine and periodic tests, as far more of the system than just the device under test would need to be isolated. In many cases the entire system would needed to be isolated. Furthermore, arranging access for measurements via the cable connections is extremely time consuming and often brings more risks than benefits. What the operators of medium-voltage systems really need, are practical testing options and concepts that are compatible with the testing technology they currently use. Not testing or performing only limited tests are not satisfactory solutions!

New possibilities

The approach most often adopted for timing measurements in enclosed gas-insulated medium voltage systems is to use the capacitive measuring point for voltage meters or an integrated capacitive voltage meter in accordance with IEC 61243-5 VDE 0682-415. This method has been developed and extensively tested by Syna and Westnetz, working with Megger. The capacitive measuring points, also called the Voltage Detection System or simply the VDS, are a safe interface to the high-voltage components. They are the only connection to the active part of the system, if no voltage transformer is present.Several versions of the VDS, including HR, MR and LRM types, are in use, but the electrical properties of all types have been standardized. Adapters are available for all of the systems to bring them up to the modern LRM standard. In developing the new analysis technology for gas-insulated medium voltage circuit breakers it was, therefore, only necessary to develop a measurement adapter (Fig. 4) to suit the LRM VDS.

Fig. 4: Connection of the VDS box to the LRM voltage meter 

To measure the switching times of the circuit breaker, test operations are performed on the relevant field with no load, but at the normal operating voltage. Capacitive voltage meters are also used in air-insulated systems, which makes it easy to validate the new technique by comparing conventional timing measurements with measurements obtained via the VDS. The variations found are in line with the normal tolerances for conventional measurements carried out several times on the same circuit breaker. 

Fig. 5: Comparison of conventional switching times and switching times measured using VDS

Fig. 5 shows an example of such a comparison for one make and one break operation.As the graph shows, parameters such as the tripping current and the operation of the auxiliary contacts can be recorded in the usual way when measuring switching times via the VDS. Synchronous time-travel measurements are also possible.An important benefit of timing measurements made via the VDS is the possibility of performing first-trip measurements. “First trip” refers to the first break operation that the circuit breaker performs after being closed for many years. This break operation can now be recorded, allowing problems such as those caused by lubricant aging (resinification) to be identified.This was not possible with conventional timing measurement techniques, as with these it was necessary to isolate the switch before the measurement could be performed. This meant that in the past, first trip measurements could only be made by recording the secondary currents of the transformer by using current clamps. The test setup with VDS measurement adapters is significantly faster, easier and safer.

Safe Switching

When performing measurements using the VDS, the system remains energised. This has the advantage that isolation in line with the five safety rules is not needed. Nevertheless, all locking procedures and protective functions must still be observed.As conventional timing measurements were only performed on isolated circuit breakers, the switching commands were issued by the testing device. It is much better, however, for the circuit breaker analyser to behave as a passive recording device that does not issue switching commands. Instead, these commands come from the on-site control system or the control room – the analyser simply receives the trigger signals from the system.To help perform the test safely and to display the results clearly, a trigger box has been developed.  This can be seen adjacent to the test set in Fig. 6. All switching commands from the system for disconnectors, grounding switches, multi-position switches, circuit breakers and spring-charged motors are routed via the trigger box and are sent to the trigger input on the circuit breaker analyser as a grouped command. The tripping and motor currents are recorded by the analyser using current clamps. This means that field-based analysis of all switching devices can be performed quickly, conveniently and safely.

Fig. 6: Trigger box with Megger TM1700 low-voltage analyser

Conclusion

Timing measurements for circuit breakers in enclosed medium-voltage systems can now be made quickly, safely and with minimal effort using the VDS. The cost-effective measurement adapter - the trigger box - is the only device required in addition to the existing TM1800 and TM1700 circuit breaker analysers. First-trip  measurements can also now be performed automatically. VDS measurement, combined with triggering from the system under test, enables comprehensive assessment of the switching devices in MV-GIS systems, and is already proving its worth in practice.