Current Transformer (CT) Testing Using the PowerMaster®

Modern current transformers (CT) produce a secondary output current that is in-phase with and proportional to the primary input current (ie, divided by the CT ratio) when supplying current to a circuit with burden that does not exceed the burden rating. In a normal meter, the burden (resistance) is very low, but long secondary leads or degraded wiring contact integrity can increase the installed resistance. Furthermore, as in any equipment exposed to the environment, CTs can internally degrade or even fail.

The PowerMaster®  offers three methods for testing the integrity of an installed Current Transformer:

  1. Burden Testing

  2. Ratio Testing

  3. Burden + Ratio Testing

Burden Testing checks only the effect of additional burden (resistive load) on the CT secondary output current.

Ratio Testing checks only the CT ratio (and angle) as measured from the actual CT primary and secondary currents.

Burden+Ratio Testing simultaneously checks the CT secondary current and ratio as additional burden is applied. If practical, Burden+Ratio Testing is recommended since it provides ratio data that can be directly compared to the nameplate value and the percentage ratio change plot gives unambiguous indications of CT burdened response. Each type of test is described in detail below and examples of the test results generated by the PowerMaster®  are given in Figures 1 to 4:

Figure 1. Burden + Ratio Test Results

Figure 2. Burden + Ratio Test Summary Table

   
   

Figure 3. Plot of Burden Only Test

Figure 4. Plot of CT Class Accuracy

(Burden + Ratio Test)

 

Burden Testing

In Burden Testing, the secondary current from the CT passes directly through the CURRENT input of the PowerMaster®  using the Current Direct probes. The CT secondary current is measured to determine a reference "no applied burden" current output. The user may select to view the current with or without harmonics (fundamental only).  A resistive burden of 0.1 ohm is automatically applied to the current path and the CT secondary current is again measured. Additional resistance values from 0.1 to 4.0 ohms are sequentially applied up to the limit specified by the operator. The entire process takes less than 2 seconds and the test results are displayed as a percentage change in CT secondary current as a function of added burden.

The Burden Test results are interpreted by looking for a progressive decrease in the CT secondary current (ie, negative percentage change) as the applied burden increases. If such a progressive decrease is observed, the test indicates that the CT output is decreasing with applied load. If the drop is significant at less than the rated burden of the CT, the PowerMaster® will alert the user a problem exists in the CT or its wiring. If a progressive decrease is not observed (ie, change stays near zero), the test indicates stable CT output even with additional burden. 

One problem that occurs in field Burden Testing (but not in a lab test) is that the test is dependent on the actual field currents which depend on customer demand. If the customer demand changes during the test interval, the CT primary current changes which directly changes the CT secondary current. This change in current will appear to be caused by the change in burden since only the secondary current is being measured. The PowerMaster®  reduces the likelihood of this problem due to its short test interval (2 seconds) and incorporation of numerous tests to identify customer demand changes. However, if the customer demand changes, there will be a sudden, erratic, or positive jump in the percentage current change (ie, not flat or a smooth decrease). If this occurs, simply repeat the test until a Burden Test is complete under more stable customer demand conditions. (An example of a customer demand change that cannot be reliably detected by Burden Testing is shown in Figure 3. As shown by Figure 1, Burden+Ratio testing can eliminate this problem.)

Ratio Testing

In Ratio Testing, both the primary and secondary currents from a CT are simultaneously measured by the PowerMaster® . The  currents (with or without the presence of harmonics) are used to calculate the ratio and phase angle between the primary and secondary of the CT. This value can be directly compared to the nameplate ratio to verify the proper field performance of the CT. Additionally, the phase (angle) shift can be verified. Since the ratio results can be directly compared, there is no complicated data interpretation required.


In most situations, Ratio Testing can be performed using the same direct (CURRENT) secondary CT current input as is used in Burden Testing (even though any current input may be used). At the same time, the CT primary current is measured using any compatible current probe (or multiple probes). The PowerMaster®  measures both currents for validity, calculates the Ratio and phase angle, and displays the results. If a reversed polarity or a cross-phase connection is indicated by the measured data, an error is displayed to the operator. If the nameplate ratio is measured outside of the user selectable limits, an error will also be displayed.  Because only a comparison between 2 currents is being performed, Ratio Testing can be performed using current probes for both the CT primary and secondary current measurements.

In some cases, Ratio Testing can be more complicated because measurement of the primary CT current can be physically difficult or requires an understanding of the CT wiring. For example, the CT may be connected to a high-voltage line or may be located in a tightly-packed pad-mount CT enclosure. Optional probes are available to make measurements in these situations but must be available for the operator. Furthermore, for meter installations with "summed" currents (ie, Form 5 metering on a 4-wire Y service), the first CT primary current consists of phase A current in the forward direction and phase B current in the reverse direction. The person performing the Ratio Test must be familiar with the installation to connect the test current probe around ALL of the primary current conductors in the proper direction. Alternately, 2 probes can be used to independently measure currents with the PowerMaster®  performing internal current summations.

Burden + Ratio Testing

In Burden+Ratio Testing, the burden and ratio analyses are performed at the same time. Since a Burden Test is being performed, the CT secondary current must be directly connected to the PowerMaster®  CURRENT input. As for Ratio Testing, the CT primary current must be measured using appropriate probe(s). When started, the testing is performed automatically with sequential burdens applied. As in the Burden Test, the total test time is less than 2 seconds per phase.

The results of the Burden+Ratio Test include all of the data described for the separate tests plus an additional display of percentage change in ratio as a function of applied burden and a Class Accuracy (see Figure 4). The display of percentage change in ratio removes the problems associated with customer demand changes that may occur in Burden Testing since the ratio is calculated from the secondary current relative to the primary current. Therefore, the percent change in ratio plot may be directly interpreted by looking for a progressive increase (ie, secondary current drop produces a higher ratio) without external variations causing ambiguous results.

CT Testing: Ratio Testing Probe Information

The PowerMaster®  has connections to allow a variety of current probes for CT Ratio testing. In most cases the CURRENT inputs are used to measure CT secondary currents, but ratio testing usually requires a larger or higher voltage rated probe to measure CT primary currents. Two probe connectors are provided: "PROBE SET 1" and "PROBE SET 2".  Each probe set can consist of up to three probes.  The three probes do not have to be of the same type and can be used to measure any phase (A, B, C or N).  When a probe is connected, the system recognizes the probe and can look up the correct calibration.  Probes can be moved from one system to another without loss of accuracy.

For CT installations, Powermetrix recommends the following current probes:

 

Flexible Current Probes

Available in different widths (36", 24" or 48").  These probes can be used as a single probe or as a set of three for true 3-phase measurements.  They have a 600V, 1000A rating (up to 3000A special order).  These popular probes are know for their versatile design and are mainly used for testing pad mount CT installations.  They are ideal for CT cabinets with large bus bars and multiple primary current conductors.  Also, two probes can be used to correctly calculate CT ratios for non-Blondel service types.

 

Amp LiteWire

Available in different widths (2" and 4").  These probes can be used as a single probe or as a set of three for true 3-phase measurements.  They have a 150kV, 2000A rating.  These probes are mainly used for primary metering installations, but they can also be used for secondary metering as well.  This probe is designed to be mounted to a hot stick, so it is useful for testing overhead CT installations without the need of a bucket truck. 

 

 

SR752

These probes can be used as a single probe or as a set of three for true 3-phase measurements.  They have a 600V, 1200A rating.  These probes are mainly used for self-contained metering installations, but they can also be used for CT testing as well.  These probes are very accurate, but are limited to the max conductor size due to the jaw opening (2.25" or 57mm) of the clamp-on probe. 

 

Some precautions when using current probes are:

  1. Verify the probe is rated for the voltage and current being tested.

  2. Never connect a current probe around a conductor before terminating it to its cable and to the PowerMaster® .

  3. For LiteWire probes, verify the optic cable is clean to prevent surface arcing.

  4. Position the probe head perpendicular to the current carrying conductor as nearly in the center of the probe opening as practical.

  5. For clamp-on probes, clean contact surfaces and then snap and release the jaws to verify a metal-to-metal contact of the jaws (dirt and grease may contaminate the surfaces.)

  6. For flexible probes, the plastic closure connector should be fully closed and be positioned as far as possible from the conductor (also observe #7 below).

  7. Position the probe as far as practical from other current carrying conductors. (All current probes will respond to external currents.)

  8. For LiteWire probes, press the black button on the head assembly to force the 2000A range when currents are above 100 amps. (Note that the decimal is not displayed on the receiver unit when the 2000A range is active.)

  9. For LiteWire probes, position the open end such that it is not pointing toward other current carrying conductors.

  10. IN GENERAL, if a CT Ratio test has an erroneous ratio result or a phase angle greater than ±1°, check the installation of the current probe before suspecting a CT problem.

 

Other Documents Available for CT Testing

Instrument Transformer Theory and Testing

Selecting the Appropriate CT for the Metering Installation

 
 

 

  Powermetrix, A Division of TEC
10737 Lexington Drive
Knoxville, TN 37932-3294
  Phone:(865) 966-5856
Fax: (865) 675-1241
E-Mail: main@powermetrix.com
 
 

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