After installation, current transformers (CTs) are typically used for 30 years. In order to guarantee a reliable and safe operation over the lifetime of the CTs, a high level of quality during the design phase, manufacturing process and installation is important. Therefore, several quality tests are performed from development to installation. After the installation CTs should be tested on a regular basis to ensure correct functioning over their entire lifecycle.
Several methods of conventional testing are possible:
1. The traditional way of testing a CT is to apply a high current to the primary side and read the signals on the secondary side. By using different burdens or injecting overcurrents, various situations can be simulated and the signals on the secondary side can be measured and analysed. However, this method is time-consuming and material-intensive. Sometimes, it is not even feasible as very high currents are required, for example, for on-site testing of CTs designed for transient behavior (TP types) as they have very high knee-point values.
2. Another common testing scenario for CTs is injecting a defined testing voltage on the secondary side and reading signals on the primary side. Unfortunately, when using this method some parameters, such as accuracy and knee-point (excitation curve), can only be tested with limitations. This is due to the restrictions in accuracy caused by the very low signals in use and the maximum voltage of approximately 2 kV which can be applied to the secondary side of CTs. Other important parameters, such as the transient dimensioning factor, the accuracy limit factor, the safety factor, composite errors, time constancy, and many others, cannot be tested at all.
As both methods have limitations, another established approach to test CTs is by using a modeling concept.
The concept of modeling a CT allows for a detailed view of the transformer’s design and its physical behaviour. The test device builds up a model of the CT by using initial data, measured during the test. Based on this model, the test device is able to calculate parameters such as the accuracy limiting factor (ALF) and the safety factor (FS) and simulate the CT’s behaviour, for example, under different burdens, or with various primary currents.
It measures the transformer’s copper and iron losses according to its equivalent circuit diagram (Figure 1). While copper losses are described as the winding resistance RCT, iron losses are described as the eddy losses or eddy resistance Reddy, and hysteresis losses as hysteresis resistance RH. With this detailed information about the core’s total losses, the test device is capable of modeling the CT and calculating the current ratio error as well as the phase displacement for any primary current and secondary burden.
Therefore, all operating points described in the relevant standards for CTs can be determined. The model also allows important parameters such as the residual magnetism, the saturated and unsaturated inductance, the symmetrical short-current factor (overcurrent factor) and even the transient dimensioning factor (according to the IEC 60044-6 standard for transient fault current calculations) to be assessed.
The modeling approach is perfectly suited for tests from production to maintenance inspections.
With the CT Analyser, OMICRON developed the first test device using the above-mentioned modeling concept. The CT Analyser (Figure 2) is small, lightweight and conducts fully automated tests of CTs in less than one minute. For added operational safety, it uses only low-test signals of up to 120 V.
Using the modeling concept for testing CTs, the CT Analyser can offer an outstanding accuracy of 0.05 per cent in ratio and 1 min in phase angle deviation. This makes it the most suitable portable test device for testing metering CTs up to accuracy class 0.1.
The accuracy of the CT Analyser is verified by several metrological institutes such as the PTB in Germany, KEMA in the Netherlands and the Wuhan HV Research Institute in China.
The CT Analyser can also test CTs for residual magnetism and automatically demagnetises the test object when the test is complete. Additionally, it can be used as a multimeter with AC/DC current and voltage sources for manual tests, such as L, Z, R, ratio, polarity and burden. For VTs, the CT Analyser can perform ratio measurements of inductive voltage transformers.
Testing of Multi-ratio CTs
For automated testing of multi-ratio CTs with up to six tap connections (X1 to X6), the CT SB2 Switch-Box is available as an accessory to the CT Analyser (Figure 4). The CT SB2 is connected to all taps of a multi-ratio CT as well as to the CT Analyser (Figure 3).
Thus, every ratio combination can be tested automatically with the CT Analyser without the need for rewiring. An integrated connection check function tests the secondary connection to the CT and indicates wiring mistakes before the measurement cycle begins.
Software Supported Testing and Assessment
The CT Analyser software (Figure 5) supports users through every single step of the testing process. During test preparation, all necessary test and asset-related entries can be undertaken in the structured software form. Before test execution, wiring diagrams help check the correct wiring of the measuring setup. Immediately after the tests, CT Analyser gives an overview of the test results and an automated assessment of the CT condition.
Thereby, the CT assessment is not only conducted in compliance with international standards (IEC, IEE) but CT Analyser also offers the flexibility to define and use local national standards (for example, Canadian or British Standards) as well as self-defined corporate standards or assessment rules for all important CT parameters.
In order to make CT testing even more practical, CT Analyser offers various transport accessories.
The multi-functional transport case (Figure 6) is a heavy-duty option with wheels and serves as a “sturdy outer housing”. All control elements of the CT Analyser are on the front, allowing the device to be left in the case while testing. The lid is designed to be raised for use as a bench for a laptop while the CT Analyser stays in the case. Attachable end plates can be used for mouse control or technical documents and offer further space for accessories.
The lightweight and portable CT Analyser offers the possibility to conduct all of the above-mentioned tests in an accurate, fast and cost-effective manner. Its wide functionality range and high accuracy make it the ideal solution for testing single and multi-tap CTs for protection and metering purposes.
Courtesy: OMICRON Energy Solutions
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