Diagnostic testing of HV circuit breakers

Circuit breakers are not the most prominent items of equipment in a substation. They spend a lot of time doing nothing except waiting in anticipation. There comes a moment, however, when the circuit breaker must perform instantly and flawlessly. How can you test that yours will?

Unfortunately, all electrically operated devices are sooner or later likely to experience some kind of malfunction and, if a circuit breaker does not work as expected, problems can cascade with potentially catastrophic results.

By testing, however, technicians and substation managers can diminish their worries about circuit breaker performance. Circuit breakers provide protection for equipment that's an essential part of the infrastructure and expensive to replace; maintaining the breakers prevents outages, which reduces headaches — and saves money — for utilities and their customers. Additionally, there is a real public service component in ensuring the reliable supply of power, minimising business downtime and customer 'dark' time.

Substation breaker testing is an important task for all power utilities. The proper functioning of a breaker relies on many individual components that must be calibrated and tested at regular intervals. The factors used to determine maintenance intervals differ greatly between power utilities, but they often include time since last test, number of operations or severity of fault current operations. Environmental considerations such as humidity and temperature — whether the breaker is located in a desert or coastal region — also affect the maintenance schedule.

Types of circuit breakers

Circuit breakers can be classified in many different ways — by voltage, application, insulating medium, etc, as shown in Figure 1.

Depending on where the circuit breaker is positioned in a power network, different levels of reliability will be required from it. It is these requirements that usually determine the test schedule for the breaker and the amount of maintenance it will receive. In this two-part article we will look at the most common test methods for breakers as well as at some newer methods that are rapidly growing in popularity.

Conventional testing methods

The main functions of a circuit breaker are to open the circuit in response to faults and to connect/disconnect objects and parts of the electricity network. The majority of the switching operations of a circuit breaker are normal-load operations.

At first it may appear that there is not much to test in a circuit breaker, but a closer look reveals a complex mechanism that must perform flawlessly in a matter of milliseconds. Measuring those milliseconds — the main contact timing — is one of the key objectives of circuit breaker testing. In addition, measurement of contact movement is almost always included in the circuit breaker maintenance/service plan. Possible tests on circuit breakers are not limited to just these two, however, and we will discuss a number of diverse measuring techniques that help in reliably assessing circuit breaker status.

First trip test

An effective method to verify the condition of a circuit breaker is to examine its behaviour during the first open operation after it has been idle for long time. For a test of this type, the connections to the circuit breaker and the measurements are carried out while it is still in service. All test connections are made inside the control cabinet.

The major benefit of performing a first trip test is that it checks circuit breaker performance under 'real world' operating conditions. If the circuit breaker has not operated for long period of time, first trip testing will reveal whether its operation has become slower due to problems in the mechanism linkages or coil armatures caused by corrosion or dried grease. With the alternative test methods that have traditionally been used, testing is carried out after the circuit breaker has been taken out of service and has operated at least once.

During a first trip test on a gang-operated circuit breaker (a breaker with a common operating mechanism), one coil current is measured. On an independent pole-operated breaker, however, three coil currents are measured. Analysis of the coil current signatures provides information about the circuit breaker condition. The timing of the auxiliary contacts can also be measured.

The opening time of the circuit breaker can be measured by monitoring the secondary current in the protection CTs, but if this method is used, the arcing time will be included. If there is a parallel primary current path, the opening time can be determined more accurately since arcing is then minimised.

Examples of problems that can be revealed by first trip analysis:

Problem	Revealed by
Sticky trip latch components in the mechanism	Trip coil current graph comparison
Delay in trip or close initiations	Auxiliary contact timing measurement
Issues with voltage supply to circuit breaker	Coil voltage graph
Sluggishness in energy delivery by a spring/hydraulic/pneumatic operating mechanism	Speed measurement from motion graph
Loose connections in the control wiring	Trip/close coil current graph comparison

Main contact timing

Main contact timing is based on these IEC definitions: Opening time — the time interval from when the opening release (the trip coil, for example) is activated to the instant when the arcing contacts have separated at all poles; Closing time — the time interval from when the closing device (the closing coil, for example) is activated to the instant when the arcing contacts touch each other in all poles.

The aim of the main contact timing test is to make sure that the opening and closing times are as specified by the circuit breaker manufacturer. Times outside the manufacturer's specifications, especially when switching short-circuit currents, lead to an increased arcing time. This results in excessive contact wear (in the best-case scenario) and can also cause an equipment emergency, namely melting of the contacts. And, if the contacts melt, the breaker will need to be serviced or replaced.

As well as acceptable opening and closing times for the circuit breaker as a whole, correct synchronisation is imperative, both between phases and, in case of multiple breaks per phase, between contacts in the same phase.

Synchronism within a phase is essential where several contacts are connected in series. Here, the breaker becomes a voltage divider when it opens a circuit. If the time differences between the operations of the contacts are too great, excessive voltage will appear across one of them resulting in flashover, with the possibility of serious damage to the breaking chamber.

The time tolerance for simultaneity between phases is greater for a three-phase power transmission system running at 50 Hz since there is always 3.33 ms between zero crossovers. Nevertheless, even in such systems, the time tolerance is usually specified as less than 2 ms. It should also be noted that breakers that perform synchronised switching must meet more stringent requirements in both of the aforesaid situations.

IEC 62271-100 requires that circuit breaker synchronisation (phase versus phase) is better than 1/4 cycle for closing operations and better than 1/6 cycle for opening operations. Synchronisation between interrupters in the same phase is specified as better than 1/8 cycle.

Resistor contact timing

The resistor contacts can be of the pre- or post-insertion type. Timing of resistor contacts is performed simultaneously with the main contacts but it is only possible to detect the resistor contacts while the main contact is open. The resistance value is a good parameter for evaluation.

Auxiliary contact timing

There are no generalised limits for the time relationships between main and auxiliary contacts, but it is still important to understand and check auxiliary contact operation. The purpose of an auxiliary contact is to close and open a circuit. Such a contact might, for example, enable a closing coil when a breaker is about to perform a closing operation and then open the circuit immediately after the operation starts, to guard against coil burnout. Auxiliary contacts are also used for relay protection and signalling purposes.

Primary injection test

For primary injection testing, a high current is injected on the primary side of the current transformer. The entire chain — current transformer, conductors, connection points, relay protection and sometimes the circuit breakers as well — is covered by the test. During primary injection testing, the system under test must be taken out of service. This type of test is typically conducted as part of the commissioning process.

The only way to verify that a direct-acting, low-voltage circuit breaker operates properly is to inject a high current through it and observe/record its performance.

Main contact motion

A high-voltage breaker is designed to interrupt short-circuit currents in a controlled manner. This puts great demands on the mechanical performance of the operating mechanism and of all the components in the interrupter chamber. The breaker has to operate at a particular speed in order to build up adequate pressure for the cooling stream of air, oil or gas (depending on the type of breaker) to extinguish the arc that is generated after the contact separation until the next zero crossing.

It is important to interrupt the current to prevent a re-strike. This is achieved by ensuring that the contacts move sufficiently far apart before the moving contact enters the so-called damping zone. The distance throughout which the breaker's electric arc must be extinguished is usually called the arcing zone. From the motion curve, velocity and acceleration curves can be calculated which reveal even marginal changes that may have taken place in the breaker mechanics. The contact motion is captured by connecting a travel transducer to the moving part of the operating mechanism. The transducer provides an analog voltage related to the movement of the contact. Motion is usually presented as a time versus distance curve.

Travel

The travel trace indicates the instantaneous position of the circuit breaker contacts during an operation. The trace provides important information such as total travel, over-travel, rebound, under-travel, contact wipe or penetration of moving contact or operating-rod position at the time of close or open, and it also reveals many types of anomalies.

Speed and acceleration

Speed is calculated between two points on the motion curve. The upper point is defined as a distance in length, degrees or percentage of movement from either the closed or open position, or from the contact-closure or contact-separation point. The time that elapses between these two points ranges from 10 to 20 ms, which corresponds to 1-2 zero-crossovers. The lower point is determined based on the upper point. It can either be a distance below the upper point or a time before the upper point. The most important benefit derived from the instantaneous velocity and acceleration curves is the insight they provide into the forces involved during the operation of a circuit breaker. Average acceleration can also be calculated from the velocity trace.

Damping

Damping is an important parameter to monitor and test as the stored energy the operating mechanism uses to open and close a circuit breaker is considerable. The powerful mechanical stresses produced during open and close operations can easily damage the breaker and/or reduce its life. The damping of opening operations is usually measured as a second speed, but it can also be measured as the time that elapses between two points just above the breaker's open position.

Contact resistance measurement

Contact resistance is measured by injecting a known DC current through the main contact system when the circuit breaker is closed. By measuring the voltage drop the resistance can be calculated. The value of the main contact resistance reflects the condition of the conducting parts. This test is often called static resistance measurement (SRM).

The static resistance value provides a reference value for all types of electrical contacts and joints. IEC56 states that this resistance is to be measured using a current between 50 A and the breaker's nominal current. ANSI C 37.09 specifies a minimum test current of 100 A. Other international and national standards set forth similar guidelines in order to eliminate the risk of obtaining erroneously high measurements if the test current is too low. In some cases, heat generated by a high test current disperses any contact grease remnants or other impurities found on contact surfaces (resulting from numerous high-current breaking operations). When the circuit breaker contacts are in poor condition, the values obtained can differ dramatically from those measured at the factory when the breaker was new. ANSI mentions about 200% increase of resistance over the maximum value specified at the factory.

Dynamic resistance measurement (DRM)

This test is conducted by injecting DC current through the breaker main contacts and measuring both the voltage drop and current while the breaker is operated. These values are then used to plot the resistance as a function of time. If contact movement is recorded simultaneously, it is possible to determine the resistance at each contact position. This method is used mainly for contact diagnosis, but can also be used for main contact timing.

With DRM, the arcing contact length can be reliably estimated. The only alternative way of finding the length of the arcing contact is to dismantle the circuit breaker. In SF6 breakers the arcing contact is commonly made of tungsten. This contact is burned off and becomes shorter at each interruption of load current. Dynamic resistance measurements clearly reveal this shortening of the arcing contact. To obtain reliable DRM data, a high test current is required as well as test equipment with good voltage measurement resolution.

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