CVA500 is DV Power’s device specially made for testing current transformers (CT), inductive voltage transformers (VT) and capacitive voltage transformers (CVT). It can perform following tests:

• Winding resistance measurement
• Demagnetization
• Saturation test
• Turns ratio measurement and polarity check
• Burden test
• Insulation test 

Main focus of this application note is related to Saturation test and knee point detection.

Introduction

Knee point of a CT is represented as a voltage at CT secondary terminals above which CT becomes saturated. In case CT is saturated, it is unable to accurately measure current on its secondary side and therefore is unable to perform its main purpose. Consequently, CT may operate correctly at low primary current levels but fail to do so under faulty conditions.

Field experience indicates that the knee point detection test using the CVA500 takes longer than other tests available on this device. This application note presents method to reduce test duration. Two different CTs are analyzed and discussed in the following sections.

CT Saturation test (knee point detection)

Parameter settings

Firstly, user should ensure proper connection to testing object (Figure 1.).

Figure 1. Connecting CVA500 to a multi-tap CT

Since only Saturation Test is performed in this case, user can choose not to connect primary cables, burden cables, and insulation test cables. This makes connection diagram significantly simpler.

After selecting New Test button, a window will appear as shown in Figure 2. CT should be selected as test object. For this purpose, the Application and Class fields can be left with default selection, as their setup will not affect measurement. If user wants to test a multi-tap transformer, that option can also be selected.

Figure 2. New test menu

After pressing test type button, user should select only Saturation test.

Figure 3. Test type selection menu

When user selects Test options button and toggle from basic to advanced settings, user can set Saturation test parameters.

Figure 4. Test options menu

In the case of multi-tap CT, it is necessary to set maximum output test voltages for each tap combination. CVA500 automatically calculates these values according to entered nameplate ratio.

When determining maximum output test voltages for multi-tap CT, users should be aware of the secondary terminals maximum input voltage limitation. CVA500 secondary terminals are rated to 2100 V AC. Any voltage higher than this value present at these terminals may damage the CVA500 instrument, cause incorrect measurement, or both. Therefore, maximum output voltages have to be set so that inducted voltage at any tap combination does not exceed 2100 V AC. Output limits menu for multi-tap transformer is shown in Figure 5.

Figure 5. Output limits for CT with 5 taps

During Saturation test, CVA500 increases test voltage starting from minimum value (1 V, default value) up to the maximum value (2000 V, default value) or to 10% above the detected knee point value, whichever is reached first. User can adjust the start and end voltage, especially if the CT knee point range is already known by the user. Narrowing the voltage range may significantly speed up the testing time, which will be showcased in two case studies.

Maximum current is the upper limit of output current during saturation test. If this limit is reached before the end voltage is reached, test will be stopped, and the application will indicate an overcurrent error.

The maximum current that can be selected is 10 A, and this value is set by default.

Figure 6. Saturation test parameter setup

Test results

Case study 1 – Multi-tap CT

Testing object and its nameplate are shown in Figure 7.

Figure 7. Multi-tap transformer

Since test object is a multi-tap CT configured with four taps, this information should be entered in CVA500 SBC application. When a CT has four taps, a total of six knee points can be tested. In this case, three of them are selected (Figure 8).

Figure 8. Selection of tested taps

Once device is properly connected and all parameters are configured, CVA500 initiates the test. Upon completion, it displays three graphs along with measured knee current and knee voltage values. When the user enters the rated knee voltage, the device automatically checks the results and indicates pass or fail.

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Figure 9. Saturation graphs

Table 1. presents test results for Case study 1. Multi-tap transformer was tested using multiple start test voltages setups ranging from 1% (default value) to 80% of rated knee voltage. User needs to ensure that test values correspond to tap with lowest rated knee voltage. In this case, rated knee voltages are, respectively, 100, 200, and 400 V (X1-X2, X1-X3, and X1-X4).

It is clearly observed that increasing the start test voltage value can significantly reduce duration of saturation test while maintaining results accuracy.

Table 1. Test results for Case study 1

User needs to ensure that start test voltage does not have higher value than knee point of CT, which can cause significant measurement error. This is the reason why default value is set to 1 V.

Case study 2 – Single-tap CT

Figure 10. Single-tap transformer

In this case study, a single-tap CT with a rated knee point of 490 V was tested. Start test voltages were applied within the same range (percentage wise) as in previous case. In the best-case scenario, test duration was almost four times shorter.

Table 2. Test results for Case study 2

August 27, 2026