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How to Use a Micro-Ohmmeter as a Current Source for Dynamic Resistance Measurement (DRM) of a Circuit Breaker?

Introduction

The contact system of most HV circuit breakers consists of main and arcing contacts. One of the methods to assess the condition of arcing contacts is to inject high direct current (DC) through the circuit breaker contacts and measure the voltage drop during the breaker operation – a process called Dynamic Resistance Measurement (DRM). Lead batteries were used as a power source to produce pure DC current. Transportation and other technical reasons prevented wide use of lead and other rechargeable batteries for DRM testing. As an alternative, a micro-ohmmeter (RMO series) can be used as a direct current source.

RMO micro-ohmmeter is a portable, lightweight and easy to use. At the same time, it acts as a very powerful true DC ripple-free current source which is an efficient add-on component for DRM measurement execution. As a part of the same measurement set, DV Power circuit breaker analyzer (CAT series) records the voltage drop, current, contact motion during the breaker operation.

Connection and Measurement Procedure

Connection procedure for this type of measurement is as follows:

  1. Connect RMO’s current and sense cables to a circuit breaker.
  2. Connect current cables to corresponding current outputs on the RMO device
  3. Connect sense cables to the analog channel 1 on the CAT device
  4. Use Coil control cable to connect the CAT with the circuit breaker’s control circuit

NOTE: The voltage sense inputs on the RMO Instrument should be short-circuited. If not, it will display the “VOLTAGE SENSE” error message.

Figure 1 – RMO device connection diagram when it is used as a power source for DRM

The measurement procedure is described below:

  1. Set the test current and test duration (e.g. 300 A, 30 s) in the RMO’s CONTIN mode
  2. Press ENTER to change to the Ready state.
  3. Set the measurement parameters for OPEN test on the CAT device
  4. Press Start test button on the CAT to change to the Ready state
  5. Press START on the RMO device to start CONTIN test
  6. Press START on the CAT device to start OPEN test

NOTE: After the circuit breaker is opened, the RMO will display “OPEN CONNECTION” error. Press STOP button on the RMO to cancel it.

During an opening operation, sudden resistance change coincides with the separation of the main contacts and the separation of the arcing contacts. The resistance change would result in a sudden change of the voltage drop measured across the breaking element.

During each opening and closing operation, a fraction of the arcing contact material burns away. It is known that the temperature at the center of the arc is around 25,000 °C. This temperature is four times higher than the temperature on the surface of the sun. There is no material that can withstand such a high temperature. One of the consequences of material loss at the arcing contacts is that the time difference between separation of the main contacts and the separation of arcing contacts is shorter. This time difference is called the contact overlapping time, see Figure 2.

Figure 2 – Overlapping time

When the motion is measured, the difference between the moment of main and arcing contacts parting can be presented in distance units, for example, millimeter. (Figure 3)

NOTE: Either analog or a digital transducer can be connected to the CAT transducer input channels.

Figure 3 – Overlapping length shown in millimeters

Shunt Deployment for Current Recording During DRM

Except for a voltage drop and motion, the CAT device can also record a current during the breaker’s operation. For this type of measurement, a milliohm shunt (R ≤ 5 mΩ) should be connected in a series between the RMO output current cable and the circuit breaker. One additional current connection cable is needed to connect the shunt with the circuit breaker. Two voltage sense or analog channels cables (analog cables) can be used for recording the test current change (change of the voltage divided by a constant resistance of the shunt). These cables should be connected to the analog channel 2 (set at 2.5 V range).

The connection diagram is presented in Figure 4.

Figure 4 – Connection diagram for current recording using a testing shunt
Figure 5 – Current and voltage drop diagram

For a period after the main contacts open, a voltage drop and a current are not constant and a calculation does not represent a resistance, but rather an impedance. Industry practice is to name this calculation a “dynamic resistance” when the voltage drop and the current are not constant.

Overlapping length of main and arcing contacts is calculated manually. One cursor should be positioned at the starting point of the main contact part and the second at the ending point of the arcing contact separation.

NOTE: Very important advantage of CAT device is that DRM measurement can also be performed in “both sides grounded” (BSG) circuit breaker condition.

Dynamic Resistance Measurement in BSG Condition

Testing in conditions when the circuit breaker is grounded from both sides provides more safety to the device and the user since a current paths guide any induced current to the ground.

Figure 6 – Increased safety in “Both Sides Grounded” condition

The voltage drop and the current graphs recorded during the BSG test are presented below (Figure 7).

Figure 7 – Voltage drop and current – both sides of a circuit breaker grounded

Conclusions

In this application note, all the diagrams correspond only to the trip operation. The overlapping time and length are best calculated when the trip operation is performed. For example, the closing operation can be used to detect misalignment of arcing and main contacts. Such problems can be also detected during regular timing and motion testing while observing the velocity diagram during the first period of the arcing contact engagement.

The use of a micro-ohmmeter as a power source for the DRM test shows very good results in detecting the moment of main contact separation, as well as main and arcing contacts’ overlapping time and distance.

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November 27, 2019

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