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Frequently Asked Questions (FAQ)

Circuit Breaker Analyzers & Timers

Is it possible to measure timing on GIS circuit breaker with CAT device?

Yes, it is possible. Maintenance ground switches of GIS substations are almost always provided with an insulating mount or an insulating bushing for the ground connection, which is also used as an access point for the CB main contacts terminals.

A detachable shunt has to be removed from one side of a GIS circuit breaker to provide separation of the CB main contact terminals access points from the grounded GIS enclosure, to remove the parallel circuit (with the main circuit) through grounding and GIS enclosure.

More information can be found in the application note GIS Circuit Breaker Timing Measurement.

Can CAT devices be used to record motor current waveform?

If needed to record a spring-charging motor current waveform of a circuit breaker operating mechanism, it is necessary to use one of the CAT devices that include the analog channels. In addition, this measurement requires using the current clamp accessory.

More information can be found in the application note Recording Spring-Charging Motor Current Waveform.

How to get results of motion in mm if it is measured with rotary transducer?

Motion measurements from the rotary transducer (expressed in degrees) can be converted to a linear contact motion (expressed in mm) using one of two conversion methods available in DV-Win software, Linear or Non-linear transfer function.

More information can be found in the application notes Use of the Transfer Function for Rotary to Linear Motion Conversion and Using Non-linear Transfer Function for Rotary to Linear Motion Conversion.

Can Dynamic Resistance Measurement (DRM) be performed without using a transducer?

Yes, DRM test can be performed without using a motion transducer. The contact voltage drop (resistance) waveform is the most important information obtained in the DRM test. It can be obtained using a CAT device with built-in micro Ohmmeter, current cables, voltage sense cables, and coil control cables.  Using a transducer for motion measurements during the DRM test is not mandatory but provides a measurement of additional parameters like arcing contact wipe (overlapping distance).

I can't find results for bouncing time. Can it be measured with CAT devices?

Yes, it can be measured, but it is required to use DV-Win software for controlling the CAT device during timing measurement. These parameters are shown in the window of the DV-Win software for results view.

How to get results with pass/ fail comparison of measured parameters against limit values that I have in the manufacturer’s manual?

To compare test results to limit values stated in a breaker factory specification, the user should use the Test plan option in the DV Win set of applications.

More information can be found in applications notes Test Plan Creation and Comparing CB Test Results with Factory Specified Limits.

Is it possible to create common reports for more results obtained with CAT devices?

Yes, it is possible. Common reports can be created from results available in the database (Result database within Test plan option) or by loading more single test results files.

More information can be found in chapters 8.1.1 and 11.4 of the Manual for DV-Win software.

How can I choose the result I want to display on CAT Handheld instruments?

In the Analysis menu, a user can switch the results by pressing the LEFT/RIGHT arrow keys.

How can I record coil current by using CAT Handheld instrument?

The coil current can be recorded by increasing the duration of the test (e.g. from 350 ms to 20 s). Before that, a user should choose an appropriate measuring resolution. These settings are available in the Test settings menu.

Battery Test Instruments

Does capacity testing speed up aging and damage the battery?

The answer is no. It is nothing but a myth that capacity/discharge testing decreases battery life and/or irreversibly damages the battery. Batteries are designed to be deep discharged between 100 and 1000 times, so 1 test per year does not significantly affect battery life.

Is the capacity test possible without disconnecting the load from the battery?

Absolutely yes. Sometimes a battery must not be disconnected from the load it supplies, for example when used as a backup supply in hospitals or some industrial applications. To properly measure battery capacity in these cases, the load current needs to be measured and taken into account during the discharge process.

BLU series of discharge units enables accurate capacity measurement of batteries under load by using an external current probe to measure the total battery current or the load current, depending on the selected test mode.

How to properly select current and duration for the capacity test?

The purpose of the capacity test is to check if the battery string is still “capable” to supply the required Ahs and/or if it still complies with the performance data provided by the manufacturer. Battery manufacturer will usually provide nominal capacity values for 1-hour (C1), 5-hour (C5), 8-hour (C8), 10-hour (C10) discharge test and possibly some other C-values.

To follow the aging trend, capacity test current and duration should be equal as for the previous capacity test. It should be paid attention that reduced test duration implies battery can supply less Ah, due to increased energy loss on internal resistance and faster-reaching cut-off voltage.

For example, if a manufacturer declares C10 capacity to be 300 Ah, the battery should be able to provide 30 A for 10h before it reaches the cut-off voltage. However, trying to extract 300 Ah from the battery within 3h test (which implies current of 100 A) would not be successful, as the battery would be completely discharged much before 300 Ah energy is discharged.

How important is it to regularly measure cell voltage during the capacity test?

Proper condition assessment of cells is not possible by monitoring only the total battery string voltage. The inability of the battery string to supply the nominal Ah is usually a consequence of a few bad cells in the string, which should be detected and replaced. Regular cell voltage measurement (at least every 60 min during the discharge test) will pinpoint the weak cells in the string – the ones with a significantly lower voltage than the other cells in the string.

Transformer Winding Resistance Meters & Tap Changer Analyzers

What is the difference between RMO-T and TWA instruments?

RMO-T series of devices are single-phase instruments with three measurement channels, capable of simultaneously measuring one primary, one secondary and one tertiary winding connected in series. TWA instruments are true three-phase winding ohmmeters, capable of one-time connection to six transformer windings, and of performing a three-phase test.

What is the recommended test current for winding resistance test?

Transformer test current can reach a stable state faster when the current magnitude is higher. The inductance value of a transformer depends on the current injected into the windings. When the transformer is saturated the inductance is minimized. Power transformers are normally designed to reach saturation when the current is 1,2 times the peak value of the no-load current. The no-load current is normally in the range of 0,2 to 5 percent of the nominal winding current.

When measuring DC resistance the test current should be at least 1,2 times the no-load current of the transformer. This is to ensure that the transformer core is saturated to obtain more accurate results. Note also that the test current is NOT to exceed approx. 10-15 percent of the nominal winding current.

Where do you connect the current clamps and what are they used for?

The current clamps are connected to one of the supply connections for the on-load tap changer motor, and they are used for measuring the AC current which the tap changer motor uses during its operation. The graph of the current is displayed together with the DVtest graph if that test was selected in the software. This enables detection of various motor and mechanical tap changer problems since obstructions in the operation of the mechanism will result in a higher amplitude of the motor current.

What happens if an open circuit is detected during the tap changer operation?

If an open circuit condition lasts for longer than 200 ms, the DISCONTINUITY error message will be displayed on the instrument screen. Shorter open circuits will also be easily detected on the DVtest graph, and it is recommended to perform this test for a detailed tap changer analysis.

Which starting demagnetization current should be selected?

If the demagnetization is performed after a DC current test, it is recommended to use the same current which was used for the DC test. If the DC test wasn’t performed, then the starting demagnetization current should be the highest available current which is lower than 10% of the winding rated current.

How long does it take to perform a winding resistance test on a power transformer?

It depends on the MVA rating of the transformer, selected test current, number of tested windings and number of tested tap positions. In general, the result stabilization time will be longer in case of large L/R ratio, which is especially significant when testing the low-voltage winding of power transformers with several hundred MVAs of rated power. Higher test current will enable faster stabilization of results. If the HV and LV winding on the same transformer leg are connected in series, with the magnetic flux produced by the DC current flowing in the same direction, another significant improvement of stabilization time can be achieved. Finally, the total test time on a three-phase transformer can be reduced by using a three-phase instrument, which reduces the total number of connections and disconnections of test leads to the transformer terminals and improves operator safety.

Transformer Turns Ratio Testers

What is the advantage of higher test voltages?

Higher test voltage provides better turns ratio accuracy, especially on larger transformers. Using higher test voltage will move the transformer operating point closer to the linear part of the B-H saturation curve. If the operating point is below the linear part, the induced voltage at the transformer LV side is lower and measured turns ratio will, therefore, be higher, leading to higher turns ratio deviation.

What is the application of simultaneous three-phase turns ratio test?

Simultaneous three-phase turns ratio test is performed using true three-phase test voltage, and it is most suitable for testing transformers with special configurations, such as phase shifting, arc furnace, etc. where it is not possible or convenient to perform turns ratio test using single-phase test voltage. However, the simultaneous three-phase test can also be done on transformers with standard configurations, such as delta/star, star/delta, star/star, etc. especially if excitation currents obtained when true three-phase test voltage is applied are of interest. However, for these transformers, sequential three-phase turns ratio test results should be taken as referent results in case there are significant differences between sequential and simultaneous three-phase turns ratio results.

Can I compare excitation currents obtained with different test voltages?

No, it is not possible to directly compare excitation currents this way. Excitation current depends on the test voltage – the higher voltage you use, the higher the excitation current will be. However, the pattern of excitation currents, which is usually H-L-H (high-low-high), will remain the same no matter what test voltage is used.

Can I compare excitation currents obtained in sequential and simultaneous three-phase test?

No, these currents can’t be directly compared, since they are obtained under different test conditions. Even the pattern of excitation currents can be different for some vector groups, especially for transformers with delta connected HV side.