Different test devices and methods are used in the market to verify the performance of current transformers (CTs) during development, production, installation and maintenance. This article describes an innovative solution to test CTs at all lifecycle stages by using a new testing method called the modeling concept.
CTs are used in electrical power systems for relaying and metering purposes. Depending on the application they are used for, the CTs are designed differently.
The CTs for metering and protection applications basically work the same way–translating high-power primary signals to secondary readable values. While CTs used for protection applications operate to well above the nominal current, however, the CTs for metering purposes must go into saturation directly above the nominal current level to protect the connected metering equipment.
Protection Current Transformers
CTs play an important role in the protection of electrical power systems. They provide the protection relay with a ratio of the primary current so it can operate according to its settings. The transformation of the current values from primary to secondary must be accurate during normal and especially during fault conditions on the primary side–when currents up to 30 times the nominal current are not an exception.
Metering Current Transformers
Today, energy is supplied by many different sources including alternative energy such as solar and wind power. To guarantee accurate billing in this competitive electricity market, additional metering points are necessary. It is important, therefore, to have the entire metering chain calibrated, as the meter is only as accurate as the instrument transformers sourcing it. This makes the testing and calibration of CTs up to the 0.1 accuracy class essential. On-site testing of CTs of the 0.1 accuracy class, however, is particularly critical because disturbances from power lines can influence the measurement results.
Testing of Current Transformers
Conventional testing methods apply a signal on one side and read the output signal on the other side.
Several ways 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 over-currents, various situations can be simulated and the signals on the secondary side can be measured and analyzed. This method, however, is time-consuming and material-intensive. Sometimes it is not feasible because very high currents are required–for example, on-site testing of CTs designed for transient behavior.
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, using this scenario some parameters such as accuracy and knee point–excitation curve–can only be tested with limitations. This is because of the scenario's restrictions in accuracy caused by the very low signals in use and the maximum voltage of approximately 2 kilovolts that 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 constancies and many others cannot be tested at all.
Because both methods have limitations, OMICRON has developed an innovative method of testing CTs.
|Figure 1: Equivalent circuit diagram of a real current transformer|
New Modeling Concept
OMICRON developed the CT Analyzer test device, which uses a new testing concept. The concept of modeling a CT allows for a detailed view of the transformer's design and its physical behavior. 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 secondary terminal voltage (Vb), according to the Institute of Electrical and Electronics Engineers (IEEE), or the accuracy limiting factor (ALF) and the security factor (FS) according to the International Electrotechnical Commission (IEC) and simulate the CT's behavior, for example, under different burdens or with various primary currents.
Since it's introduction in 2005, the OMICRON CT Analyzer has rapidly gained wide acceptance with well over 1,000 units operating in every corner of the globe, including many utilities and service providers in the U.S. and Canada.
The CT Analyzer is small, lightweight and conducts fully automated tests of CTs very quickly.
It measures the transformer's copper and iron losses according to its equivalent circuit diagram. 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 CT Analyzer is capable of modeling the current transformer and calculating the current ratio error as well as the phase displacement for any primary current and secondary burden.
All operating points described in the relevant standards for current transformers, therefore, can be determined. The model also allows important parameters such as the residual magnetism, the saturated and unsaturated inductance, the symmetrical short-current factor–over-current factor–and even the transient dimensioning factor, according to the IEC 60044-6 standard for transient fault current calculations, to be assessed.
|Figure 2: Connection example for a six-tap current transformer|
Within seconds a test report, including an automatic assessment according to IEEE C57.13 or C57.13.6, Standard for High Accuracy Instrument Transformers, is generated. The CT Analyzer offers a high testing accuracy of 0.05 percent, 0.02 percent typical, for current ratio and three minutes, one minute typical, for phase displacement.
The accuracy of the CT Analyzer is verified by several metrological institutes such as the Physikalisch Technische Bundesanstalt (PTB) in Germany, KEMA in the Netherlands and the Wuhan HV Research Institute in China. Traceability is to national standards administered by European Association of National Metrology Institutes (EURAMET) and International Laboratory Accreditation Conference (ILAC) members. See Figure 1.
New Innovations–CT SB2
For its latest release, the CT Analyzer was improved with new hardware accessories and software functions.
For automated testing of multi-ratio CTs with up to six tap connections–X1 to X6–the CT SB2 Switch-Box is now available as an accessory to the CT Analyzer. The CT SB2 is connected to all taps of a multi-ratio CT as well as to the CT Analyzer. See Figure 2.
|Figure 3: Hysteresis curve at the maximum saturation point showing the possible area for residual magnetism|
Every ratio combination, therefore, can be tested automatically with the CT Analyzer without rewiring. An integrated connection check function tests the secondary connection to the CT and indicates wiring mistakes before the measurement cycle begins.
In addition, the CT Analyzer checks the different ratios of the current transformer tested. The testing signal is then adjusted to make testing voltages above 200 volts impossible, ensuring a high level of worker safety during operation.
As a new measurement function for the CT Analyzer, the Rem-Alyzer allows CTs to be tested for residual magnetism.
Residual magnetism might occur if a CT is driven into saturation. This can happen as a consequence of high fault currents containing transient components or direct currents applied to the CT during winding resistance tests or during a polarity check–wiring check. Depending on the level of remaining flux density, residual magnetism greatly influences the functionality of a CT. See Figure 3.
|Figure 4 and Figure 5: Demagnetization principle of iron cores|
Since remanence effects in protective CTs are not predictable and barely recognizable during normal operation, these effects are even more critical. Unwanted operation of the differential protection might be caused. Protective relays also might show a failure to operate in the event of real over-current as the CTs' signal is distorted because of the residual magnetism in the CT core.
Once the CT is magnetized, a demagnetization process is necessary to remove residual magnetism. This can be achieved, for example, by applying an ac current with similar strength as the current that caused the remanence. In a second step, the CT is demagnetized by gradually reducing the voltage to zero. See Figures 4 and 5.
The CT Analyzer performs the residual magnetism measurements prior to the usual CT testing cycle because it automatically removes residual magnetism after testing. To determine the residual magnetism, the CT Analyzer drives the core into positive and negative saturation alternately until a stable symmetric hysteresis loop is reached. The CT Analyzer then calculates the initial remanence condition to determine whether the core was affected by residual magnetism. The results are displayed as absolute values in voltage per second as well as in percent relative to the saturation flux on the residual magnetism test card. In addition, the remanence factor Kr is shown on the test card. See Figure 6.
The CT Analyzer automatically demagnetizes the CT when the test is complete.
|Figure 6: Test card of the CT Analyzer showing the measurement results of a residual magnetism test|
After installation, current transformers are usually used for 30 years. To guarantee a reliable and safe operation over the CT's life, a high level of quality during design phase, manufacturing process and installation is essential. Several quality tests, therefore, are performed from development through installation. After installation, CTs should be tested on a regular basis to ensure correct functioning over the entire life.
The lightweight and mobile CT Analyzer now offers the possibility to conduct all these tests in a fast and cost-effective manner. Its wide functionality range and high accuracy make it a very good solution for testing single and multi-tap CTs for protection and metering purposes.