Method for Static and Dynamic Resistance Measurements of HV Circuit Breaker

Transcription

1 Method for Static and Dynamic Resistance Measurements of HV Circuit Breaker Zoran Stanisic Megger Sweden AB Stockholm, Sweden Abstract S/DRM testing methods usually use long, heavy cables to connect high current source from ground to the circuit breaker. Alternative is to move current source close to the circuit breaker by using heavy batteries, transformers, dc/dc etc. In new proposed method batteries are replaced with ultra capacitors and constant current charger, resulting in a few hundred grams of powerful current source, capable to generate few hundred amperes into the breaker contacts, or any other power contacts. Measuring voltage drop across the circuit breaker and the current through it continuously monitors discharging of ultra capacitors, giving a high accurate resistance value of the measured object. Keywords-component; contact resistance; circuit breaker; ultra capacitor I. INTRODUCTION Industries that consume electrical power always have energy losses due to different reasons. Part of these losses comes from unwanted heating of current conductive parts as a result of increase in resistance of the conductor itself or the resistances in contacts. The resistances of high current components are usually very low ( μω); nevertheless their increase above acceptable values may lead to serious problems, since operation of electrical equipment depends on the controlled flow of current within the design parameters of the given piece of equipment. As examples of elements where resistance must be kept under control are rail bonds, ground bonds, circuit breaker contacts, switches, transformer windings and tap changer contacts, bus bar connections, battery strap connections, motor windings, squirrel cage bars, bus bar with cable joints and bond connections to ground beds. 2-wire measurement is the simplest way to assess resistance. It is generally used when the contact resistance, series lead resistance or parallel leakage resistances has no effect on the quality of the measurement. Most multi-meters are based on 2-wire measurement, which provide sufficient accuracy in resistance range 10 Ω -10 MΩ. Three-wire technique is used for measurements of very high resistances (above 10 MΩ) and is typically applied when measuring insulation of high voltage equipment. The third wire acts as a guard preventing leakage currents across the surface to influence measurement. Four-wire testing is the most accurate method when measuring circuits below 10 Ω as this method eliminates errors due to lead and contact resistances. This test method is associated with low resistance ohmmeter and micro ohmmeter. Four-wire measurements use two current and two sensing leads as shown in Figure 1. This arrangement allows for cancellation of voltage drop across current cables thus ensuring accurate measurement of the resistance. Additional accuracy enhancement is achieved by using high current source, usually above 10 A. II. MEASUREMENT TECHNIQUES Depending of desired accuracy and absolute value of resistance there are three methods of measuring 2-wire, 3- wire and 4-wire measurements [1]. Figure 1. Example of 4-wire measurement

2 III. EXISTING APPROACHES FOR LOW RESISTANCE MEASUREMENT Different methods can be used to measure low contact resistance of power circuit breakers, isolating switches, and also of the bus bars and welded joints. Most of these include heavy weight current source and are based on four contacts connection to achieve high accuracy and eliminate power cables resistance. One of the simplest methods is based on rectified AC current source [2]. Figure 2. Diagram of microohmeter based on rectifed AC. Using this method, high pulsating current passing through the test object (15 μω) is phase shifted with the voltage across it, which indicate existence of not negligible inductance in the circuit as shown in Figure 4.. Here resistance measurement becomes impedance determination by dividing RMS values of voltage and current. This method can be acceptable if inductive component in the test circuit can be disregarded and if there is no need for high accuracy. It is possible to measure only resistive component of the test object by using current pulse injection, a method proposed in [3] and depicted in Figure 3.. Electrolytic capacitor bank is charged and then discharged through the test object. PC based data acquisition is used to record current through and voltage across the contacts. When the current derivative is zero, voltage and current value are divided, giving a resistance value. Current and voltage drop can take higher or lower value, depending on voltage level that capacitor bank was charged up and the contact resistance. It is obvious that noise level of the voltage drop across the contacts must be filtered in order to achieve high accuracy (see Figure 5. ). This makes determination of the resistance more complex and less accurate, especially if current transformer is a part of the test circuit. Core saturation may interfere with the voltage measurement, causing resistance calculation very difficult. Figure 4. Voltage and current waveforms using rectified AC current source. In order to avoid the difficulties in resistance measurement caused by inductive component of the circuit, a DC current source based micro ohmmeters are widely used. Battery or DC/DC converter is used as a high current source, able of generating more than 100 A through the test object. Battery solution can be heavy and is not attractive because of that as well as the possibility of battery degradation caused by accidental short circuiting. Figure 5. Voltage and current waveforms using discharge of large capacitor bank. Figure 3. Low resistance measurement with large capacitor bank. The concept with DC/DC converter depicted in Figure 6. is highly promising and giving a high current capability, low weight and good control possibilities. Current is slowly ramped

3 up, then kept constant during the resistance measurement and afterwards ramped down, offering a robust and safe operation. Still though, there is one problem: EMI. Switching noise is high and its intensity depends on generated current. It is evident from Figure 7. that switching noise has much higher intensity than voltage drop across the test object. With proper DC/DC topology and filtering the voltage drop, influence of the switching noise can be minimized but not eliminated completely. Figure 6. Microommeter based on switching current source A/s, depending on equivalent serial resistance, thus eliminating the influence of inductance of the circuit. Battery is not used to generate high current trough the contacts, instead of that it is used to supply DC/DC converter, which will then charge ultra capacitor with constant current. During the resistance measurement, DC/DC is turned off to eliminate high frequency switching ripple. Only linear analogue parts are then supplied with the battery, resulting in ripple free and high accurate, low resistance measurement. The proposed low resistance meter (Figure 8. ) is based on a large value capacitor with ultra-low internal resistance, a current switch, a charger for the capacitor plus control and measurement circuitry allowing for a high DC current (250 A to be generated [5]. Compromise between capacitance and equivalent serial resistance (ESR) must be taken to fulfill desired operational conditions and specification. Typical capacitance, ESR value for ultra capacitors used in this application are F and mω. Charging method with constant current provide full control over charging time and voltage across the capacitor. By charging ultra capacitor using different voltage values allows for generation of different currents through the testing object. Figure 7. Voltage and current waveforms using switching current source. Following descriptions of low resistance measurement solutions and methods, it becomes apparent that two parameters are of highest importance, namely low weight and high accuracy. Possibility to design a portable and accurate micro ohmmeter is possible when combining all the advantages of aforementioned methods. IV. LOW RESISTANCE METER WITH ULTRA CAPACITORS Present advances in capacitor technology allows for an ultra capacitor with capacitance in the range of few hundred Farads to be used as a current source, thus storing high amount of energy. When discharging the ultra capacitor, high capacitance value will then dominate the time constant of the test circuit, which means that the slope of discharged current is in range of Figure 8. Low resistance meter based on large capacitor. Measurement of the resistance takes place during a short (1 s) interval where a transient recorder is employed. The measurement and the operation of the switch are synchronized using an operation control (in the transient recorder). A mains powered supply or battery powers the ohmmeter. When powered, the control circuitry senses the voltage level of the capacitor. When the capacitor voltage is below a certain value the capacitor is charged (using a DC/DC converter giving a constant current output). Above a preset voltage level the charger stops charging and signals to the user that the unit is fully charged. The measurement is initiated by operating the transient recorder, which sends a trig signal to the current source. The current source activates its switch control during a preset time and the current flows through the resistance, which shall be measured.

4 The current is switched on/off using power transistors. The transient recorder measures both voltage and current and calculates the resistance or optionally uses an analog dividing circuit. During charging and discharging the capacitor and the surrounding circuitry may become hot. For this reason the control circuitry is provided which supervises the temperature and prevents from using the unit while too hot. V. STATIC AND DYNAMIC RESISTANCE MEASUREMENT Figure 9. Voltage and current waveform using an ultra capacitor During current generation and measurement the charging circuitry is disconnected. A typical unit may use a relay to disconnect the load from the circuitry during charging, to allow other measurements to be made on the unknown resistance. For example it could be a high voltage circuit breaker contact, which has opened during or after the resistance measurement and it may be desired to use another instrument for contact timing or for measuring a parallel grading capacitor. Combining an uncontrolled discharging of ultracapacitor with dc/dc converter (Figure 10), it is possible to regulate generated current through the resistor. Doing this, it is possible to simplify resistance recording by measuring voltage drop across the resistor only. For example, if output current is regulated to 100A, then unknown resistance equals to voltage drop divided by 100. In this case, time for generating constant current is limited and depends on measured resistance. Care must be taken of filtering high frequency components to avoid EMI and influence to measured results. Polyphase dc/dc converter is good candidate to use in this application, resulting in efficiency higher than 90% and very low current ripple. One application area where low resistance measurements are frequently employed is assessment of main contact resistance of high voltage circuit breakers. In addition there are two specific cases, namely, measurement of static and dynamic resistance. An ultracap based current source prototype was build with 350F ultracapacitor, charged to different voltage levels no higher then 2.7V, giving a possibility to generate currents up to 300Apeak. In case of static resistance the current and voltage supplied by ultra capacitor decay (see Figure 11. ); however providing that resistance is linear it is possible to simply average its value over measurement time. Using 12-bit A/D converter the accuracy of measurements on calibrated current shunts were in within accuracy limits (±0.5%) and the ripple seen in voltage as well as resistance graph is due to digitization of analog signal. Similarly this technique can be applied when testing circuit breaker contacts in operation. This test is called dynamic resistance measurement and it reveals the condition of arcing contacts or other moving electrical contact surfaces. Current is injected into the contacts few milliseconds before command for moving is given. When contacts begin to move, voltage drop and current though them are continuously measured, divided and displayed as dynamic resistance. By doing so, erosion of the contact surface as well as shortening of arcing contacts can be early detected, preventing damage in form of wrong timing information, high temperature developing or in worst case explosion. Figure 10. Low resistance meter based on combination of large capacitor and polyphase dc/dc power supply. Figure 11. Traces of measured current and voltage and calculated resistance during static resistance measurement. Voltage is multiplied by factor 5 for better illustration.

5 Figure 12. depicts an example of dynamic resistance measurement on high voltage circuit breaker where transition from static resistance value to open contact is shown. Rcb = (t / C(lnUmax lnuc)) R Where R represent known resistance of cables and static contacts. CONCLUSIONS Figure 12. Example of dynamic resistance measurement on main contacts ogf high voltage circuit breaker. Also here a high current source is used, previously heavy car batteries or similar, but the large capacitors have solved the weight and portability issues. If only estimation of the dynamic resistance is needed for calculating position of the circuit breaker contacts during operation, one simple way to do that is to calculate capacitance of ultracapacitor during charging with constant current and use this value to estimate total resistance of contacts, cables and circuit breaker contacts during discharging of ultracapacitor. Voltage across the capacitor and discharging time are continuously measured and resistance of the circuit breaker contacts is calculated as: In this paper several methods for low resistance measurement were presented and compared. Laboratory prototype with ultra capacitor solution was build and experimental result was shown, demonstrating benefits in form of high accuracy, low weight and extremely high portability. This solution has possibility to be used in high and low power applications. Charging ultra capacitor on different voltage levels, discharging current is indirectly predicted. There are possibilities for series/parallel connection of ultra capacitor based micro-ohm meters, increasing test current to very high levels. Accuracy of both static and dynamic resistance measurements depends for the most part on instrumentation used for voltage and current measurement. REFERENCES [1] A Guide To Low Resistance Testing. Megger Ltd. [2] Programma MOM200A datasheet. [3] M. Runde et al. Condition Assessment of Contacts in Gas-Insulated Substations, IEEE Trans. on Power Delivery, vol.19, no.2, April 2004 pp [4] Programma Mjölner 200 datasheet, [5] (WO/2009/131530) RESISTANCE MEASUREMENT IN HIGH POWER APPARATUS ENVIRONMENTS