Circuit breaker types and technologies have a rich history. Whether we are referring to arc-extinction methods such as oil, air-blast, air magnetic, SF6, newer non-SF6 technologies, and vacuum, or to energy storage methods such as mechanical springs, pneumatics, and hydraulics, the use of and preference for these technologies has changed vastly over time.
It is essential to understand diagnostic testing of high-voltage (HV) circuit breakers. When diagnostic tests are performed on HV circuit breakers, valuable information can be extracted. From a technical maintenance perspective, these diagnostic tests provide critical information about the condition of HV circuit breakers.
Standard field tests widely applied in HV circuit breaker diagnostics include:
- Timing and travel
- Insulation power factor
- Static and dynamic contact resistance
- Minimum operating coil pick-up
CIRCUIT BREAKER TECHNOLOGY
Circuit breaker technology varies depending on the application, and the preferred technology is dependent on the geographical region in which it is applied. For example, dead-tank SF6-filled circuit breakers are popular in North America in HV applications, while other regions of the world prefer live-tank circuit breaker technology.
Overall, circuit breakers, regardless of type and technology, are designed with three functions in mind:
- Direct current flow between desired sections of an electrical power system
- Interrupt current flow under abnormal power system events and conditions, such as faults
- Carry load current under normal power system conditions with minimal losses
These three functions must be performed under normal and abnormal (fault) conditions and must adhere to strict performance specifications.
Circuit breakers vary by subsystem:
- Insulation system
- Arc-quenching method
- Mechanism
- Contact technology
- Control circuit schemes
- Current transformers (CTs)
These subsystems must be analyzed separately and as a complete electromechanical system. Table 1 lists several properties related to circuit breakers.
Diagnostic testing can be performed in an on-line/in-service state or an off-line/de-energized state. The manufacturer’s recommendations, duty, number of operations, and experience should be considered when justifying the test and maintenance requirements.
Table 2 and Table 3 list recommended and commonly practiced on-line/in-service and off-line/de-energized tests, respectively.
CIRCUIT BREAKER TYPES
Let’s focus on bulk oil circuit breakers (OCB) and dead-tank SF6 circuit breakers. These dead-tank design types were selected because they are the most popular for high-voltage applications in North America. OCBs are considered old technology and have been steadily replaced by the newer dead-tank SF6 circuit breakers. This change-over has been occurring for roughly 30 years.
Bulk Oil Circuit Breakers(OCB)
- Bulk OCBs use a large volume of oil to extinguish an arc.
- Arcing occurs in oil, which creates gases (hydrocarbons).
- By using a vented interrupter chamber, gas bubbles create pressure that forces the arc to expand further into the vents until it can extinguish itself at a zero crossing current.
- Interrupter chambers often attract moisture.
- Bulk OCBs often use condenser-type bushings, resin, or oil equipped with tap electrodes. These bushings can be isolated and tested; current transformers (CTs) are often mounted on the lower ground sleeve.
Dead-Tank SF6 Circuit Breakers
Dead-tank SF6 circuit breakers can use three methods for arc extinction. Each method utilizes SF6 gas pressure to blow out or extinguish the arc.
- SF6 puffer circuit breaker: Mechanical compression of the arcing chamber generates SF6 gas pressure.
- SF6 self-extinguishing circuit breaker: Heat generated in the arcing chamber generates SF6 gas pressure.
- SF6 double (dual) pressure circuit breaker: Uses a pressurized SF6 gas chamber that is released in the arcing chamber during operation.
Dead-tank SF6 circuit breakers are often equipped with SF6-gas-filled bushings that cannot be isolated for field testing. The bushings often have CTs mounted on the upper external ground sleeve near the mounting flange.
TIMING AND TRAVEL MEASUREMENT
Circuit breaker timing and travel measurements entail three steps:
- Perform a dynamic timing and travel measurement.
- Calculate performance characteristics.
- Compare results to the manufacturer’s recommendations, trending of historical data, and/or user-defined limits.
Table 4 provides the fundamental tests and calculations involved in circuit breaker timing measurements and diagnostics.
Most breakers utilize a 125 VDC control circuit. However, 48 VDC, 250 VDC, 120 VAC, and 240 VAC control circuits are not uncommon. Table 5 lists typical control signal timing values used in performing timing and travel tests.
Measured Signal
Five primary signals are of interest when performing circuit breaker timing and travel measurements:
- Displacement
- Contact state (open-resistor-close)
- Command coil current
- Auxiliary contact state (OPEN Wet-OPEN Dry-CLOSE)
- Battery voltage
It is worth noting that the main contacts can take on three states: OPEN, CLOSE, and RESISTOR because some breaker applications require pre-insertion resistors (PIRs). When the breaker performs a CLOSE operation, a resistor will be placed across the open contacts for a few to several milliseconds to limit potential overvoltage associated with long transmission line applications. It is important to capture the operation, specifically, the timing of this resistor switch.
Depending on the use and availability of the auxiliary contacts, such as 52a and 52b, these contacts may be wet (voltage present) or dry. The measurement must be configured for such conditions.
All of the above signals are not always included. However, omitting signals limits the effectiveness of the analysis.
Performance Characteristics
Table 6 lists all of the 11 pertinent circuit breaker characteristics, including 5 related to timing, 5 related to displacement, and 1 for velocity.
Timing and travel results are directly compared to the manufacturer’s performance specifications and previous results[1]. All of the performance characteristics listed in Table 6 will have pass/fail criteria. Table 7 illustrates typical performance characteristics. It should be noted that not all manufacturers document all performance characteristic limits; it may be worthwhile to establish a characteristic baseline for any missing limits with commissioning tests.
INSULATION POWER FACTOR
Power factor and capacitance testing provide a means to verify the integrity of the insulation of circuit breaker components. Problems that impair the insulation integrity can be detected by measuring the power factor and capacitance.
- Deterioration of entrance bushing insulation
- Deterioration of interrupter assemblies, insulated operating rods, and support insulators due to arcing byproducts
- Presence of particles, impurities, and contamination of SF6 insulating medium
- Moisture ingress
- Damages resulting from partial discharge and tracking
Test Procedure
Power factor and capacitance test procedures depend on the design and type of apparatus. The following test procedures are those required to test SF6 dead-tank circuit breakers. However, these test procedure concepts apply to a number of breaker types. The applied test voltage should not exceed the line-to-ground rating of the test specimen or otherwise stated by the manufacturer. The test specimen should be solidly grounded for safety and proper measurement.
Table 8 shows the nine recommended and three optional tests performed on dead-tank SF6 breakers.
Analysis of Results
Tests 1–6 primarily measure the insulation integrity of the energized bushing including any insulated support structure, operating rod, and SF6 gas.
Tests 7–9 assess the condition of the contact assembly and SF6 gas within the interrupter chamber.
Tests 10–12 are optional tests that are performed on circuit breakers with more than 1 contact chamber per phase. This test mode helps stress the additional support structures that will not be seen while the circuit breaker is in the open position.
For low-capacitance specimens like dead-tank SF6 breakers, it is generally recommended to assess losses in watts instead of power factor. This is especially true for open-breaker UST measurements. The capacitance is so low that most dead-tank SF6 breakers must be analyzed in watts. Power factor should not be used to determine the integrity of insulation if the measured current is less than 0.3 mA.
At low measured currents, PF calculations are susceptible to large swings, which could be misleading[2]. In those cases, the test results should be evaluated based on current and loss readings. Please note: Not all dead-tank SF6 breakers are assessed according to measured watt losses — only those units with very low capacitance.
Elevated power factor or loss readings can indicate degradation of bushings, insulated support structure, operating rod, contact assembly, and/or SF6 gas.
On circuit breakers with grading capacitors, Tests 7–9 are dominated by the grading capacitors. High power factor or loss readings may indicate deteriorated capacitors. An unexpected increase in capacitance may indicate short-circuited capacitance layers.
Figure 1 illustrates typical results obtained from dead-tank SF6 breakers. It can be seen that tests [1 3 5], tests [2 4 6], and tests [7 8 9] can be compared.
CONTACT RESISTANCE
Contact resistance can be a complicated subject. Contact assemblies can consist of main and arcing contact components. To see the main and arcing contact components, the contact resistance is analyzed statically and dynamically.
Static Contact Resistance
The microohm measurement or static contact resistance measurement determines the continuity integrity of the main contact components. Abnormal readings may indicate improper alignment, pressure, or damaged contact surfaces, such as plating or coating. This is the standard test performed to measure the actual resistance value of contact continuity and associated series components, such as bushing connections and tulips. Static measurement produces a single temperature-dependent value in Ohms (W), more specifically (µΩ).
Static contact measurement is to be performed on each phase using a DC source. Typical measurements are close to or less than 100 µΩ (Figure 2). However, the manufacturer’s literature should be used to determine the actual expected value.
Experience has shown that measurements performed on dead-tank SF6 breakers range from 75 µΩ to 150 µΩ, with 100 µΩ being a very common result. It is recommended to inject at least 100 A DC for this test[3]. It should be noted that if the breaker is equipped with CTs, it may take several seconds to saturate the opposing effects. Precautions should be taken to ensure that the injected high primary current does not affect protection circuits.
Due to the very low resistances in the µΩ range, it is recommended that a high DC source be used in conjunction with a Kelvin connection. The Kelvin 4-wire method is the most effective method used to measure very low resistance values. The Kelvin 4-wire method will exclude the resistance from the measurement circuit leads and any contact resistance at the connection points of these leads. The concept of the Kelvin 4-wire method is to apply the voltage and current leads separately. This is shown in Figure 3.
Dynamic Resistance (DRM)
Dynamic resistance measurement is a diagnostic tool to assess the condition of the arcing contacts in SF6 nozzle-style interrupters. By measuring the current, voltage, and displacement associated with the contact assembly, it is possible to determine the wear level and integrity of the arcing contact. This measurement, like the static contact resistance measurement, requires high current injection to be successful. Common practice is to use at least 100 A DC.
Caution must be taken when analyzing the results. As implied by the name (DRM), resistance is being isolated and measured. Due to the speed of the contact interaction (roughly 15–20 ms), it is actually impedance, which includes real and reactive components, that drives the response. Source leads, CTs, and stray and fixed capacitances contribute to the additional measured reactance.
Figure 4 illustrates a typical dynamic resistance measurement. Motion and the resistance (impedance) response are plotted together. The length of what is left of the arcing contact is determined by comparing it to the distance traveled.
Minimum Pick-Up
Minimum pick-up measurement is performed to determine the minimum command coil (trip or close) voltage required to operate the trip coil and actuate the circuit breaker’s operation. This is the minimum energy needed for the command coil to release the latch. The latch can be a mechanical release mechanism or a value used to control a pneumatic or hydraulic system.
This test is done for each control coil of a circuit breaker. Different considerations must be given to ganged versus independent pole operation (IPO) circuit breakers. The test needs to be done for all command coils independently. The IPO breaker may require several more tests to include all command coils.
The test procedure steps:
- Determine the command coil parameters and ratings, AC or DC, and operating voltage.
- Determine a start and stop voltage for the command coil under test. Example: 125 VDC command coil, start [10 VDC], stop [125 VDC].
- Determine pulse time, which should be limited so the command coil does not overheat; 300 ms is the default starting point.
- Determine dead time, which is the time the command coil pauses between pulses. The dead time should be long enough to assist in cooling the command coil. A reasonable starting point is 2 seconds.
- Determine the voltage step increment, which is the amount the voltage is increased between command coil pulses. A reasonable starting point is 5 V DC.
Voltage is increased linearly until the command coil operates. This behavior is illustrated in Figure 5. The blue response is current (A); the purple response is voltage (V).
The smaller the steps, the more accurate the test result. In general, experience has shown that most healthy command coils will operate at less than 50% of the rated voltage. However, this is not an official limit.
CONCLUSION
Timing and travel measurements determine and validate the performance characteristics of circuit breakers. Control coils, mechanical linkages, energy storage devices, contacts, and dashpots are all monitored for proper operation.
Power factor and capacitance testing provide means to verify the integrity of the insulation of circuit breaker components. Depending on the type of breaker and the insulation components that are present, the proper test procedure and analysis strategy must be implemented.
Contact resistance measurements can be performed in either static mode or dynamic mode. Traditionally, static measurements have been performed allowing only the main contacts to be assessed. With the introduction of DRM, the integrity of the arcing contacts on SF6 nozzle-style contacts can be determined.
The minimum pick-up measurement is performed to determine the minimum command coil (trip or close) voltage required to operate the circuit breaker. This will ensure that the circuit break will operate at a specified reduced voltage.
REFERENCES
ANSI/NETA MTS–2019, Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems.
IEEE Std. C57.152–2013, IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors.
P. Gill: Electrical Power Equipment Maintenance and Testing, Second Edition, CRC Press, 2009.
Charles Sweetser received a BS in electrical engineering in 1992 and an MS in electrical engineering in 1996 from the University of Maine. He joined OMICRON electronics Corp USA in 2009 and presently holds the position of PRIM Manager for North America. Before joining OMICRON, he worked for 13 years in the electrical apparatus diagnostic and consulting business. He has published several technical papers for IEEE and other industry forums. As a member of the IEEE Power & Energy Society (PES) for 20 years, Sweetser actively participates in the IEEE Transformers Committee, where he holds the position of Chair of the FRA Working Group PC57.149. He is also a member of several other working groups and subcommittees. Additional interests include condition assessment of power apparatus and partial discharge. Sweetser was the 2023 recipient of NETA’s Alliance Recognition Award.