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ELECTRICAL SUBSTATIONS CHANNEL

DYNAMIC RESISTANCE MEASUREMENT

Condition testing of circuit breaker’s arcing contacts

BY BUDO MILOVIC, DV-Power

The preventive maintenance on a high-voltage (HV) breaker consists of several routine tests. The timing and motion tests are methods used to assess the breaker’s mechanical condition. When the tim ing

and motion results indicate an abnormality, the dynamic resistance meas-urement (DRM) test can be an effective way to further diagnose the internal condition of the breaker contacts.

As an example, a high-voltage puffer-type sulfur hexaflouride (SF6) gas circuit breaker is based on the switching of two parallel contact sets. First, the low-resistance silver-plated contacts, or the main contacts, are designed specifically to carry the load current without any excessive temperature rise. The second, tungsten-copper arcing contacts operate at the breaker opening following the main contact part. The electrical arc starts after the sep aration of the arcing contacts. The tungsten-copper material is de-signed to carry the arc until it is cleared at the next zero-crossing.

The new dynamic-contact resistance measurement is a tool to diagnose the condition of these arcing contacts. The measuring method has been val-idated by numerous field tests. It is based on the breaker contact resistance measurement during an opening operation at rated contact speed.

The dynamic resistance measurement test requires a circuit breaker anal-yzer with a high-resolution measurement. The points of a position change from the main to arcing contacts are observed from the test results presented as corresponding resistance plots.

In addition, the DRM test requires a high-resolution circuit breaker anal yzer to observe the point of a position change from the main to arcing contacts. The resistance curve, as a function of a contact travel, is used to reveal potential problems related to the arcing contact conditions. The actual length of the arcing contacts is calculated by measuring the motion.

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DRM TEST TECHNIQUES The dynamic resistance measurement test requires a micro ohmmeter as a power source to generate a high (several hundred amperes) injecting current through the breaker contacts. The voltage drop across the main contact is monitored during breaker operations. We will use a tested 145-kilovolt (kV) SF6 circuit breaker as an example.

A number of different methods are available for this type of testing. The simplest one is by measuring a voltage drop across breaker terminals using a regular 12 DC (direct current) automotive bat-tery. Deployment of lead acid or other rechargeable batteries as a source is not practical, due to inaccuracy, potential accidental battery short-circuiting and poor contact connections.

An alternative method uses a DC current injection and operates the circuit breaker at low speeds. There fore, the contact system is not tested under realistic con di-tions. While the only advantage of the aforementioned meth od is an ability to differentiate between different arcing events, this alternative does not provide realistic testing conditions and is intrusive to some circuit breaker mech anisms requiring adjustments to the operating mechanism necessary for simulations. There is also a potential risk of damaging the operating mechanism when restoring the breaker to service. DRM tests are recommended to be at the rated opening contact speed while simultaneously injecting test currents.

Transition from the static resistance value to open contact is accurately identifiable, as resistance values ob-tained are from 400 microohms (μΩ) to 1400 μΩ after the main contacts open. Those values are within the range expected for arcing contacts.

When comparing results from different current values in DRM tests (see Figures 2 and 3 on pages 14 and 15), it becomes clear that interpreting the DRM curve at 100 amperes may lead to a wrong diagnostic conclusion, es pecially for the main contact separation that occurs at ap proximately 19 milliseconds. The measured length of the arcing contact was approximately 20.6 mm (57.6 degrees), from 19.6 to 24.9 milliseconds. As the injected current is increased from 100 to 200 A, the graph is far smoother and the main contact separation can be identified easily.

Another DRM testing for 24 kV-oil-type circuit break-ers confirmed benefits to use high-test currents. Figure 4 on page 15 shows dynamic resistance meas urements from testing by 500 amperes currents to detect differences between operational (phase A) and damaged (phase C) arcing contacts. RECOMMENDATIONSDynamic resistance measurement test methods are described as an additional tool for diagnostics of cir-cuit breaker contacts. The benefits of injecting as high cur rents as possible are identified to ensure a significant vol tage drop reading, thus allowing exact detection of the arcing contact performance.

Upon reviewing the characteristics of the dynamic resistance curve, all the measuring parameters, and test results extracted from the resistance curve, it is recommended to apply higher injected currents of at least 200 amperes when performing DRM tests at the

rated contact speed for tested circuit breakers. High-test currents guar antee reliable test results from the resistance curve for detecting contact anom-alies, wear, and/or misalignments. DV-Power has a standardized cir cuit break er analyzer for 500-amperes dynamic resistance measurement test-ing and 500-amperes micro ohmmeter targeted for high-voltage circuit break er testing. . ET

VISIT: www.dv-power.com

High-test currents guarantee reliable results from the resistance curve for detecting contact anomalies, wear, and misalignments.

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Figure 1

Figure 2

A car battery used as a power source for

high current injection; Voltage drop is measured across breaking elements

during trip-free operations

The DRM results obtained at 200 amperes

current provide a much smoother curve. The main contacts separa-

tion is clearly identified at 19.6 milliseconds (ms). It means that the arcing contact overlapping time is approximately 5.2 ms, which is the

expected time, and overlapping length is approximately 20.6

millimeters (mm).

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Figure 3

DRM test with 200 amperes of current, a smoother graph with

the point of main contact parting easily

identified

Figure 4

ADRM test with 500 amperes

current has identified problem on

C phase

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Figure 5

Typical test set up with

DV-Power breaker circuit analyzer