An Overview of Dynamic Contact Resistance Measurement of HV Circuit Breakers
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With the deregulation of the electrical power industry, utilities and service companies are operating in a changing business environment. High voltage circuit breakers are extremely important for the function of modern electric power supply systems. The need to predict the proper function of circuit breaker grew over the years as the transmission networks expanded. The maintenance of circuit breakers deserves special consideration because of their importance for routine switching and for protection of other equipments. Electric transmission system breakups and equipment destruction can occur if a circuit breaker fails to operate because of a lack of preventive maintenance. Dynamic Contact Resistance Measurement (DCRM) is known as an effective technique for assessing the condition of power circuit breakers contacts and operating mechanism. This paper gives a general review about DCRM. It discusses the practical case studies on use of DCRM for condition assessment of high voltage circuit breakers.
KeywordsCircuit breaker testing Dynamic contact resistance measurement Metallic fluorides Ultra capacitors
A circuit breaker is an important equipment to power electric networks. Its importance is due to the role it is playing. According to CIGRE report , more than 90 % of circuit breaker failures are due to mechanical causes. In view of this, circuit breakers are primarily mechanical devices that are performing an electric function. Hence it is imperative to assure its proper operation. This is only possible by applying suitable maintenance. Various utilities, people and organizations have different viewpoints on and approaches to maintenance strategies. Testing and maintenance methodologies have changed over the years and in all likelihood will continue to evolve as new technologies become available. Condition-based maintenance is being used more and more. It provides excellent opportunities to improve reliability and cut costs, but it requires effective diagnostic methods.
In POWERGRID, there have been failures of breakers due to damages in components in the interrupting chambers much before 10 years, that is, before carrying out the first major overhaul. Defects were noticed in the components of few circuit breakers before completion of 10 years, whereas few circuit breakers which were 17 to 20 years in service were healthy. Power Grid Corporation India Ltd. (PGCIL) then started Dynamic Contact Resistance Measurement (DCRM) for diagnostic testing to decide for overhauling .
Static Contact Resistance Measurement
The modern High Voltage SF6 circuit breakers have two parallel contact sets. The main contacts are low resistance contacts, which are silver plated, whereas arcing contacts are of tungsten-copper which helps in initiating arc quenching and current interruption. Measurement of the static contact resistance with the breaker in closed position gives the resistance of main contacts only because the arcing contacts are bypassed. Healthiness of main contacts only gets checked during this test. The minimum dc test current should be used according to manufactures specification; however, the IEC and ANSI recommended levels are: 50 A IEC 60694 and 100 A ANSI.
Static contact resistance is measured by injecting a dc current through the breaker and measuring the milli-voltage drop. A four wire measurement method is used. The breaker must be in the closed position. If low resistance readings are obtained when testing the breaker contact resistance using a low current, then it is recommended to re-test the contacts at a higher current. A higher current will have the ability to overcome connection issues and oxidation on terminals, where a lower current may produce higher readings under these conditions .
Dynamic Contact Resistance Measurements
The limitation of static contact resistance measurement is that it does not give information about the arcing contact condition. Erosion of arcing contact, contact misalignment, damage to driving mechanism cannot be detected from static contact resistance measurement. Dynamic contact resistance measurement for circuit breakers was introduced in 1992.
Review on Dynamic Contact Resistance Measurement
Measurement at Low Speed
It was observed by Landry et al.  that DCRM curves at rated speed were not reproducible from one test to another. It was difficult to identify the main contact part. It was anticipated that this phenomenon is caused by partial contact part and due to high contact speed and acceleration. They found that performance of DCRM at low contact speed of 0.2 and 0.15 m/s were almost identical. At low contact speed, the DCRM curve was smoother and the main contact part can be easily identified. However, for some breaker mechanisms, the method is intrusive since some adjustments of the operating mechanism are required for low-speed breaker operation. There is a potential risk of damaging the operating mechanism when restoring it back in service. Also the DCRM at low speed may not give the correct information of the contacts because in service the circuit breaker is operated with rated speed only.
DCRM in the Presence of Metallic Fluorides
Metallic fluorides are usually present in the form of nonconductive dust powder deposited on the breaker contacts. The effects of metallic fluorides on contact resistance have been dealt . High resistance was observed on a capacitor bank circuit breaker which had undergone quite large number of operations. Short-circuit breaking tests have revealed that the presence of metallic fluorides does not decrease the breaker’s breaking capacity. High contact resistance will appear when there is no scraping motion between contacts . Static contact resistance measurements of the main contacts using conventional equipment injecting 10-A dc current was carried out as a first check. An extremely high value of the main contact resistance of the order of 4,500–6,000 μΏ was measured, which could be interpreted as defective contacts. A measurement method was developed by Michel Landry et al. to determine the reason for high resistance with the aim of avoiding the dismantling of the breaker’s interrupting chambers. The measuring system consists of three current sources connected in parallel for delivering the measuring current of 2800 A dc. Six 4/0 copper cables were used to carry the high dc current from source to the breaker terminals. A data acquisition system along with a measuring shunt of 51.32 Ώ was used for recording the relevant signals. With the breaker contacts maintained closed, contacts heating at currents from 100A to 2800A for different intervals from 1–15 min were performed in order to vaporize the deposited metallic fluorides at the actual points of contact. DCRM for different currents were taken for each phase. Michel Landry et al. observed that 2800A current for at least 15 min was required to reduce the arcing contact resistance to an acceptable level .
Low Resistance Meter with Ultra Capacitors
The conventional method of DCRM measurement requires long, heavy cables to connect high current source to the breaker. Alternative is to move current source close to the circuit breaker by using heavy batteries, transformers dc/dc etc. A paper  was published on the use of ultra capacitors as a current source. The batteries can be replaced with ultra capacitors and constant current charger which is light weight powerful current source capable to generate few hundred amperes. They developed a low resistance meter which was based on a large value capacitor with ultra-low internal resistance, a current switch, a charger for the capacitor plus control and a measurement circuitry allowing for a high dc current of about 250A to be generated. This low resistance meter can be used for static and dynamic resistance measurements. The accuracy of measurement depends on the instrumentation used for voltage and current measurement.
DCRM Signature Analysis
This is how by enlarging the signature in the trip free portion and knowing the design details of circuit breaker, length of arcing contact can be found out from the signature.
Electromagnetic Forces on Contacts
The constriction at the point of contact is responsible for the contact resistance and consequently for the heat being generated at the contacts and also is the source of electromagnetic force acting upon the contact structure.
Condition assessment of circuit breaker through DCRM technique is being used by POWERGRID and various utility companies in India. Following case studies depicts the use of DCRM signature in detecting the defects at early stage in circuit breakers.
Case No. 1
The external connections were confirmed normal by re-verification. The test leads connecting main contact to analyzer were checked. To eliminate electrical signaling fault, the test was repeated with different analyzer. But same problem was observed. It was confirmed that the problem is inside the interrupter assembly.
Static contact resistance of the pole was verified and found to be very high 120–160 μΏ.
The DCRM test of B-phase front side interrupter was conducted to verify the contact bouncing seen in no-load closing characteristic.
DCRM test showed lot of fluctuations in the current and resistance curve in the steady state part of the contact travel curve seen in Fig. 6.
The rear side of same pole was also showing abnormal DCRM signature, even though no abnormal contact bounce was seen in no-load closing operation.
After investing subassemblies in steps, the final cause was detected in moving contact assembly. The joint of puffer cylinder and cylinder support was found loose fitted. Tightening Torque was not applied during assembly.
Case No. 2
Abnormal contact bouncing in closing for longer duration in R and Y poles for more than 5 ms was observed.
The wiping was less in opening operation for R and Y pole compared to B pole.
Case No. 3
Case No. 4
In this paper, an overview of dynamic contact resistance measurement is presented. DCRM allows the evaluation of the wear and tear of the arcing and main contacts without opening the CB. It helps to save on the cost of an extended down time that is caused by complete disassembly of the CB to visually assess the state of the arcing contact. Electrical lifetime can be calculated by analyzing DCRM. Four case studies were studied. DCRM at rated speed & injection of 100A dc current was carried out. Measurements were recorded with a resolution of 100 μs with a sampling frequency of 10 kHz. It is observed that DCRM is useful to the manufacturers as the manufacturing error can be detected before dispatch. Wipe adjustment, improper torque for tightening, assembly of wrong arcing contacts, etc. can be detected at the works only avoiding further complications. Utility companies like PGCIL and state transmission companies can avoid major breakdown through condition assessment of high voltage circuit breakers. However, analysis of DCRM signature needs knowledge of anatomy of CB that is CB design, operating mechanism etc. to conclude the case. DCRM is found to be a powerful diagnostic tool for circuit breaker health management.
This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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