 |
|
|
|
This document is available for downloading
as an Adobe PDF document. Current Transformer (CT) Testing Using the Powermate System Analyzer 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 Powermate offers three methods for testing the integrity of an installed Current Transformer:
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 Powermate are given in Figures 1 to 4:
Figure 1. Plot of Ratio Change (Burden + Ratio Test)
Figure 2. Burden + Ratio Test Summary Table
Figure 3. Plot of Burden Change (Burden 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 BURDEN input of the Powermate using the heavy burden cable (with redundant conductors for safety). The fundamental (ie, 60 Hz only) CT secondary current is measured to determine a reference "no applied burden" current output. 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, then 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 Powermate 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 Powermate. The
fundamental (ie, 60 Hz only) currents 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 to be low. 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 (BURDEN) 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 Powermate measures both currents, 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, a warning is displayed to the operator. 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 Powermate 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 Powermate BURDEN 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. 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 Figures 1 and 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 Considerations The Powermate has connections to allow a variety of current probes for CT Ratio testing. In most cases the "Burden" or "Phase 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. The available current Input Connectors are:
CAUTION: When a probe is required to perform a CT
Ratio test (Main Menu, Option 5), the following steps should be performed:
Other Documents Available for CT Testing
|
|
