Safety and portability in circuit breaker testing

September 11th, 2015, Published in Articles: Energize

 

A high voltage circuit breaker is the most important component of a HV substation as it has the crucial task of removing the fault, no matter what the fault current is. Even a nominal current could be difficult to open as 2000 A on a 380 kV line would equate to approximately 500 MW per phase. However, the current to be opened can often be a lot higher at 20, 40 or even 80 times the nominal current. This article discusses the reality of high and medium voltage circuit breaker testing with both sides connected to ground.

When the question of what should be tested in a substation is raised, nobody argues about the certainty of whether the circuit breaker of the plant is operating correctly. On the other hand, performing tests is an expensive task, demanding highly skilled engineers, strictly observed procedures, programming and a relevant test execution time. Additionally, laws and standards dictate how the test should be performed in order to avoid any risk to people at work.

All this conflicts with the need for reducing time and the level of skilled people involved. The logistics of performing the tests can be a significant factor in the process. The test equipment itself can impact the process through its portability or lack thereof, and ease of use in the field. Traditionally these devices have been heavy and impractical to transport. If and when they are delivered to a site for use, they can often need to be used in conjunction with a laptop or PC to view the results, further impacting on ease of use and pre-requisite skills.

In 2008 the national French distribution company RTE commissioned ISA for the supply of HV and MV circuit breaker analysers. Two circuit breaker analysers (CBA 1000 and CBA 2000) were used at various sites and the information supplied has been included in this article.

Both sides connected to ground

The most important tests performed on circuit breakers are:

  • Main contacts timing: measures the opening and closing times of contacts. It must be the same for each of the three phases.
  • Contact resistance: measures the contact resistance variations as the contacts close.

A test set operates by injecting the test current across the poles and measuring the voltage drop. If the pole is closed the voltage is zero; if it’s open the voltage is the same as the circuit supply. When the test set is connected, the operator’s safety is ensured by connecting both sides of the test object to earth. During the test, the safety grounding is removed on one side to avoid the test current from flowing into the two ground connections in order to perform the timing test. This problem does not exist when performing a contact resistance test because the contact resistance is much less than the earth connection resistance.

Although this is not a recommended procedure it has become standard practice through necessity. When the contact is closed there is no risk, but if during the test the contact is opened the risk becomes present; high voltage induction with no protection in case of a fault. Grounding both sides using a device such as the BSG 1000 (a circuit breaker tester with both sides grounded) answers the need to operate safely when performing timing and synchronisation tests.

BSG connected to a CBA during a real test

The technology to measure the operating time of a circuit breaker when it is grounded on both sides is based on resistance measurement. The BSG solution is based on high current, short duration contact resistance measurement:

  • Current injection at least 20 A
  • Voltage measurement
  • Resistance computing: the value of the resistance shows whether the contact is open or closed.

The resistance of an open contact is much higher than a closed contact. The ground resistance can be programmed in two different ways. Manually by the operator or automatically by means of test and data management (TDMS) software.

How the BSG system works

Each BSG system is made up of the following elements:

  • Three BSG remote heads (Fig. 1): the heads are connected close to the main breaker contact that includes the current generator and the two level resistance comparator which communicates the result in digital form to the main unit. Each head has two cables, one for the connection of the test current to the CT under test and the other (shielded) for the open/close detection of the circuit breaker contacts.
  • One BSG main unit which includes the microprocessor supervising the heads and the DACs for CBA inputs. It is connected to the heads and to the analyser by a cable kit.
Fig. 1: BSG 1000 remote head, type 1.

Fig. 1: BSG 1000 remote head, type 1.

The connection diagram in Fig. 2 shows how the BSG remote head is connected to the circuit breaker pole. High current connections are shown with bold lines and the voltage measurement cables are shown as thin lines.

Fig. 2: Connection between circuit breaker and BSG remote head.

Fig. 2: Connection between circuit breaker and BSG remote head.

The BSG’s remote heads connect to the BSG tester which in turn is connected to the CBA module to provide a complete functional system.

Connection between analyser and tester

Fig. 3: BSG 1000 connected to CBA 1000 during a test in a substation.

Fig. 3: BSG 1000 connected to CBA 1000 during a test in a substation.

The connection is made of two cables (Fig. 3):

  • Driving cable: allows the analyser to drive the BSG tester as well as gather, process and report test results.
  • Circuit breaker contact cable: when connected to the analyser detects active poles. As the data is processed numerically all contacts must be connected, even if some of them are not going to be used during the test (e.g. during a maintenance test of a single break per phase).

Once the cables have been connected the testing procedure is identical to the analyser. However, the timing measurement is simpler as the BSG device’s interface is easier to use. The test setting and results can be controlled and viewed locally on the wide graphical display at the front of the analyser. Aside from the coil control which opens and closes the circuit breaker coils, the BSG functions independently.

Both the CBA modules and the BSG unit have been designed and manufactured to be fully operational and easy to use in severe environment and substation conditions. As such they are suitable for use in extreme and severe conditions such as extra high voltage (EHV) substations.

Connecting the remote heads

The three BSG remote heads are connected to each circuit breaker pole via a multi-pole connector and four measurement clamps:

  • Injection clamps clip to the poles to be tested. These create a closed ring and generate the current needed to perform the test.
  • Measurement clamps measure the voltage drop of the circuit breaker contact.

With resistance testing it is mandatory that the contact point be as close to the circuit breaker as possible in order to avoid stray voltages. It is very important to take care of the connection direction during the test. The measurement clamps must be positioned inside the current ring created by the injection clamps.

The circuit breaker connection

The cable connection is the most important factor when performing a test with a micro-ohmmeter. The measured voltage values are small (typically in the mV range) thus a high accuracy is needed. The final result can be influenced by elements such as contact oxidation and unstable contacts.

It is strongly recommended that the test engineer be aware of the importance of a good connection as a poor connection can lead to incorrect results. The BSG connection is terminated by crocodile clamps which provide a strong and tight lock on the contacts.

As this type of clamp cannot be mounted directly from the ground, the company provides other clamps for this function:

  • SAFO clamp: has a telescopic bar that can be directly mounted and connected to the circuit breaker contacts. The locking system assures a good accuracy and stability of the contacts during the testing process (Fig. 4).
  • Arc clamp: has hooks that can be easily connected to the circuit breaker poles from the ground. The most important characteristic of this type of clamp is that it fits perfectly with any circuit breaker (Fig. 5).
Fig. 4: SAFO clamp.

Fig. 4: SAFO clamp.

Fig. 5: Arc clamp with connection cable  (without telescopic crank).

Fig. 5: Arc clamp with connection cable
(without telescopic crank).

On-site results

The results obtained by the BSG tester were the same as the analyser when operated on its own; showing that the results were not compromised by using the BSG system for the safety and protection of the operator.

Figs. 6 and 7 are an example of an opening cycle of an Alstom FL 245 circuit breaker during an ordinary maintenance test performed by a BSG device connected to an analyser.

Fig. 6: BSG 1000 test result: opening cycle.

Fig. 6: BSG 1000 test result: opening cycle.

Fig. 7: TDMS test result, ready  to be saved and printed.

Fig. 7: TDMS test result, ready
to be saved and printed.

Conclusion

When using the BSG option, circuit breaker timing tests can be safely performed with both sides grounded. Unlike conventional circuit breaker test options, the grounding connection is not removed during testing which allows for:

  • Time-saving during circuit breaker testing and analysis
  • Preserves all timing and motion test integrity
  • The testing of graphite nozzle equipped circuit breakers

In addition to its safety feature, the analyser offers all the functionality of a circuit breaker test instrument with the added benefit of being compact, portable and standalone in operation. The base CBA unit has a mass of only 10 kg, making the system practical and cost effective for transportation to all types of locations. When paired together, the CBA and BSG modules provide a safe, fully independent, portable and completely functional circuit breaker testing system.

Acknowledgement

This article was published in Transmission and Distribution, October/November 2014, and is republished here with permission.

Contact Sean Goodwin, HV Test, Tel 011 782-1010, sales@hvtest.co.za

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