Diagnostic testing of HV circuit breakers — Part 2
Circuit breakers are complicated, mechanically sophisticated devices that require periodic adjustment. Sometimes a technician can see the need for these adjustments with a visual inspection and the problem can be solved without testing. However, with most circuit breaker issues, testing will be required.
This article, the second in a two-part series on circuit breaker testing, details more test techniques and, in particular, looks at the relatively new approach of resonant frequency testing. The first part of the article covered circuit breaker types and some of the most commonly performed tests.
Coil test
If the current in the operating coil of a circuit breaker is monitored during a trip operation, a curve similar to that shown in Figure 1 will be obtained.
When the trip coil is first energised [1], current flows through its windings. The magnetic lines of force in the coil magnetise the iron core of the armature, in effect inducing a force in the armature.
The current flowing through the trip coil increases to the point where the force exerted on the armature is sufficient to overcome the gravitational and frictional forces that tend to keep the armature at rest. When this point is reached, the armature is pulled [2] through the trip coil core. The magnitude of the initial current [1–2] is proportional to the energy required to move the armature from its initial rest position. The movement of the iron core through the trip coil generates an electromagnetic force in the coil that in turn has an effect on the current flowing through it.
The rate of rise of current depends on the change in the inductance of the coil. The armature operates the trip latch [3–4], which in turn releases the trip mechanism [4–5]. The anomaly at [4] is the point where the armature momentarily stops as contact is made with the prop. More energy is required for the armature to resume motion and overcome the additional loading of the prop. The anomaly may be caused by degradation of the prop bearings, lubrication, changes in temperature, excessive opening spring force or mechanism adjustment. The armature completes its travel [4–5] and hits a stop [5].
Of particular interest is the section of the curve between [4] and [5]. As the armature moves from the point where the trip mechanism is unlatched [4] to the stop [5], the inductance of the coil changes. The curve is an indication of the speed of the armature. The steeper the curve, the faster the armature is moving. After the armature has completed its travel and has hit the stop [5], there is a change in current signature. The magnitude of the current [7] is dependent on the DC resistance of the coil. The ‘a’ contact opens at [8] to de-energise the trip coil and the current decays to zero.
The interpretation of the circuit breaker operating coil signature often provides useful information about the condition of the latching systems.
Minimum voltage test
This test is often neglected even though it is specified and recommended in international standards. The test objective is to make sure that the breaker can operate at the lowest voltage level provided by the station battery during a power outage. The test is performed by applying the lowest specified operating voltage and verifying that the breaker operates within specified parameters. The standard test voltages are 85% and 70% of nominal voltage for close and open operations respectively.
Minimum voltage required to operate the breaker
This test, which should not be confused with the minimum voltage test just described, determines the minimum voltage at which the breaker is able to operate. It is a measure of how much force is needed to move the coil armature. This test is not concerned with contact timing parameters — only whether the breaker operates or not. The test starts by sending a control pulse at a low voltage to the breaker. If the breaker doesn’t operate, the voltage is increased by, say, 5 V and the test is repeated. This procedure is continued, with gradually increasing voltage, until the breaker eventually operates. The voltage at which this occurs is recorded and, if the test is repeated next time the breaker is maintained, a comparison between the old and new figures will indicate whether significant changes have occurred.
Vibration testing
Vibration testing is based on the premise that all mechanical motion in equipment produces vibrations and that, by measuring them and comparing the result with the results of previous tests, the condition of the equipment in question can be evaluated.
The easiest parameter to measure is the total vibration level. If it exceeds a specified value, the equipment is deemed to be in the fault or at-risk zone. For all types of vibration testing, a reference level must have been previously measured on equipment known to be fault free.
All measurements on the equipment tested are then related to this reference signature in order to determine whether the measured vibration level is ‘normal’ or whether it indicates the presence of faults.
Vibration analysis is a non-invasive test technique that uses an acceleration sensor with no moving parts. The breaker can stay in service during the test; an open-close operation is all that is required for the measurement. First-trip operation can be different from the second and third because of corrosion and other metal-to-metal contact issues. Vibration is an excellent method to capture the data about the first operation after the breaker has spent a long time in the same position.
The analysis of vibration data involves comparing the latest results with the reference. Vibration measurement can detect faults that are barely noticeable using other conventional methods. However, if data such as contact timing, travel curve, coil current and voltage are available in addition to the vibration data, even more precise condition assessment is possible.
Vacuum bottle test
Vacuum bottles in vacuum circuit breakers are tested with high-voltage AC or DC to confirm the integrity of the vacuum. The electrical behaviour of the vacuum in the bottle is identical for AC and DC. The main difference in using DC and AC is that AC measurements are influenced by capacitance. The resistive current component is typically between 100 and 1000 times smaller than the capacitive current component; the resistive component is therefore difficult to distinguish when testing with AC. As a result, AC requires much heavier equipment for testing compared to DC test instruments.
Synchronised switching
In order to test the function of a controlled switching device, one or more currents from current transformers and reference voltages from voltage transformers are recorded, along with controller output signals, while issuing an open or close command. Details of the test set-up depend on the test instrumentation, as well as the available voltage and current sources.
SF6 leakage
SF6 leakage is one of the most common problems with circuit breakers. The leakage can occur in any part of the breaker where two components are joined, such as valve fittings, bushings and flanges. In rare cases, SF6 can also leak straight through the aluminium as a result of poor casting. Leaks can be found by using gas leak detectors (sniffers) or thermal imaging.
Humidity test
As humidity can cause corrosion and flashovers inside a breaker, it is important to verify that the moisture content inside an SF6 breaker is minimal. This is done by venting a small amount of SF6 gas from the breaker through a moisture analyser, which will determine the moisture content of the gas.
Air pressure test
Air pressure testing is carried out on air-blast breakers. Pressure level, pressure drop rate and airflow are measured during various operations. The blocking pressure that will block the operation of the breaker in the event of very low pressure may also be measured.
A new approach: resonant frequency testing
Preparation for testing a circuit breaker involves the safe isolation of previously energised high-voltage equipment. Ground connections are then applied to the isolated equipment, normally leaving breaker grounded on both sides.
Present practice for performing timing tests requires, however, that the ground connections on one side of the breaker are removed during the test to allow correct operation of the test equipment. The potential safety issues with this practice require the adoption of special safety procedures. In most cases, an ‘authorised person’ will be involved with the test as well as a central office that issues the special work permits required. This means that the test takes longer, tying up equipment and the test engineer unnecessarily. In addition — and most importantly — the network is out of service for longer.
Engineers also need more training so that they can deal with the necessarily complex safety procedures. To address these issues, a new technology was introduced in 2006 that allows main contact timing tests to be performed on a circuit breaker with both sides grounded. Dangerous voltages can, therefore, be kept at distance — a safe area around the circuit breaker can be created and clearly marked with security fencing. Accidents with electric arc and electrocution can be avoided.
The main contact timing results produced by this new technology are fully compatible with the conventional main contact timing measurement. For field personnel, the new way of working is much faster but is otherwise familiar. This new timing technique is based on the capacitance formed between the parts of the breaker contact, which are separated by an insulating medium — usually oil, air, vacuum or SF6. Any circuit breaker contact can, therefore, be seen as a capacitor. This capacitor is a part of the resonant circuit formed by breaker itself and other surrounding components such as busbars, connections and ground connections, as shown in Figure 2.
The resonant frequency of the circuit depends on the value of the capacitance between circuit breaker contacts. The circuit response will vary with the movement of the contacts, as shown in Figure 3.
Timing of main contacts can be performed using this technique, which is also called the DualGround method. This method allows circuit breakers to be tested more safely and more efficiently than with conventional timing techniques. Safety dictates that both sides of a breaker should be grounded during field tests, but conventional timing methods require ground to be disconnected on one side of the breaker to allow the instrument to sense the change in contact status. This procedure makes the test cables and the instrument part of the induced current path while the test is being performed.
The DualGround method allows for reliable measurements with both sides of the circuit breaker grounded, thus making the test safer, faster and easier. This technique also makes it possible to test circuit breakers in configurations such as GIS applications, generator breakers and transformer applications, where conventional timing methods require the removal of jumpers and busbar connections, which is difficult and inconvenient.
Comparison with other methods
DualGround timing is an excellent solution when ground loop resistance is low, since it has no lower limit for ground loop resistance. The ground loop can even have lower resistance than the main contact/arcing contact path without affecting the results. This is particularly crucial when testing GIS breakers and generator breakers, as well as for AIS breakers having good grounding appliances. The changes in resonant frequency of the whole circuit (breaker and ground loop) can easily be used for close/open status detection.
Often dynamic resistance measurement is proposed as a tool for timing circuit breakers with both sides grounded. In this case, the determination of circuit breaker state is made by evaluating the resistance graph against an adjustable threshold. If the resistance is below the given threshold, the circuit breaker is considered closed; if the resistance is above the threshold, the circuit breaker is considered open.
Problems can arise, however, when choosing the threshold, since it must be below the ground loop resistance (which is initially unknown) and above the resulting resistance of the arcing contact (which also is unknown) and the ground loop in parallel. The reason is that, according to the IEC standard, it is the closing/opening of the arcing contact that is considered as the operation time of the circuit breaker, not the main contact, and the difference between main and arcing contact operation time can, depending of contact speed, be as much as 10 ms. For example, a 2 x 10 m copper grounding cable with 95 mm2 cross-section area has a resistance of about 3.6 mΩ (not counting the resistance of connector devices). An arcing contact is usually also in the mΩ range, from a couple of mΩ up to about 10 mΩ, depending on the type of circuit breaker and on the condition of the arcing contact. All this taken together makes it a near-impossible task to adjust the threshold, as the exact value to use is unknown. It may require several attempts to achieve a reasonable result and may be even more difficult if the resistance graph is not recorded during measurement.
Furthermore, a method that relies on evaluation against thresholds is more sensitive to induced AC currents through the test object. When a circuit breaker is grounded on both sides, a loop is formed with a large area exposed to magnetic fields from surrounding live conductors. The alternating magnetic field will induce a current in the circuit breaker/grounding loop. This current can reach a few tens of amps, which corresponds to a significant proportion of a typical 100 A test current. If the evaluation threshold is at the limit, these induced currents will definitely affect the timing results. The resonant frequency technique is, on the other hand, completely insensitive to 50/60 Hz interference.
Conclusions
When maintaining a circuit breaker, technicians should start with timing and motion measurements. In fact, if that technician only has time for a single measurement, that measurement should be timing.
Electrical power network growth and asset development requires that all available technologies be implemented to ensure reliable electricity supply. New technology for circuit breaker testing offers a more cost-efficient test procedure and, since it allows both sides of the breaker to be grounded during testing, it ensures safety for key employees in accordance to national laws, standards and social partner demands.
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