by Benton Vandiver, Omicron
Different test devices and methods are used in the market to verify the performance of current transformers during development, production, installation and maintenance. An innovative solution exists to test current transformers at all life cycle stages by using the modeling concept.
Current transformers are used in electrical power systems for relaying and metering purposes. Depending on the application they are used for, the current transformers are designed differently.
The current transformers for metering and protection applications basically work the same way: translating high-power primary signals to secondary readable values. While current transformers used for protection applications operate to well above the nominal current, however, the current transformers for metering purposes must go into saturation directly above the nominal current level to protect the connected metering equipment.
Protection current transformers. Current transformers 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. Energy is supplied by many sources, including alternative energy sources such as solar and wind power. To guarantee accurate billing in this competitive electricity market, additional metering points are necessary. It is important to have the entire metering chain calibrated because the meter is only as accurate as the instrument transformers sourcing it. This makes the testing and calibration of current transformers up to the 0.1 accuracy class essential. However, on-site testing of CTs of the 0.1 accuracy class is particularly critical because disturbances from power lines can influence 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.
- The traditional way of testing a current transformer 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, 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 even feasible because very high currents are required (e.g., for on-site testing of current transformers designed for transient behavior (TP types) as they have very high knee-points values).
- Another common testing scenario for current transformers 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 be tested only 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 kV, which can be applied to the secondary side of current transformers. 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, a new method of testing CTs was needed.
New Modeling Concept
Omicron developed the CT Analyzer test device. The concept of modeling a current transformer allows for a detailed view of the transformer’s design and its physical behavior. The test device builds a model of the current transformer automatically by using nameplate data and data measured during the test. Based on this model, the test device can calculate additional parameters such as the Vb (secondary terminal voltage acc. IEEE) or the accuracy limiting factor (ALF) and the safety factor (FS acc. to IEC) and simulate the CT’s behavior, for example, under different burdens or with various primary currents.
Since its introduction in 2005, the analyzer has gained acceptance with more than 1,000 units operating in every corner of the globe, including the U.S. and Canada.
The analyzer is small, lightweight and conducts fully automated tests of current transformers within the shortest times possible.
It measures the current transformer’s copper and iron losses according to its equivalent circuit diagram (see 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 analyzer can model the current transformer and calculate the current ratio error, as well as the phase displacement for any primary current and secondary burden.
All operating points described in the relevant international standards for current transformers 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 the transient dimensioning factor (according to the IEC 60044-6 standard for transient fault current calculations) to be assessed.
With all of the relevant modeling data known, it can be used directly in power system simulation programs to give that CT or group of CTs actual response to modeled system conditions. This provides the power system engineer with improved fault simulations, making performance testing and trouble shooting of protection systems much more accurate.
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 analyzer offers a very high testing accuracy of 0.05 percent (0.02 percent typical) for current ratio and 3 minutes (1 minute typical) for phase displacement.
The accuracy of the analyzer is verified by several metrological institutes including the PTB in Germany, KEMA in the Netherlands and the Wuhan HV Research Institute in China. (Traceability is to national standards administered by EURAMET and ILAC members (e.g. ÖKD, DKD, NIST, NATA, NPL, PTB, BNM etc.)
For automated testing of multiratio CTs with up to six tap connections (X1 to X6), the analyzer was improved with the addition of the CT SB2 Switch-Box as an accessory. The CT SB2 is connected to all taps of a multiratio CT, as well as to the analyzer (see Figure 2).
Every ratio combination can be tested automatically without the need for rewiring. An integrated connection check feature tests the secondary connection to the CT and indicates wiring mistakes before the measurement cycle begins.
In addition, the analyzer checks the different ratios of the current transformer under test. The testing signal will then be adjusted to limit test voltages below 200 V, ensuring a high level of worker safety during the operation. The SB2 can be attached to the back of the analyzer (see Figure 3) with all wiring connections made.
3 CT Analyzer With CT SB2 Attached
As a new measurement option for the analyzer, RemAlyzer allows current transformers to be tested for residual magnetism after a system fault or local event where core saturation is suspected.
Residual magnetism might occur if a current transformer is driven into saturation. This can happen as a consequence of high fault currents’ containing transient components or direct currents applied to the current transformer during winding resistance tests or during a polarity check (wiring check). Depending on the level of remaining flux density, residual magnetism dramatically influences the response characteristic of a current transformer (see Figure 4).
Because remanence effects in protective current transformers are not predictable and barely recognizable during normal operation, these effects are even more critical. Unwanted operation of the differential protection may be caused. Protective relays also might show a failure to operate in case of real over-current as the current transformer’s signal is distorted because of the residual magnetism in the CT core.
Once the current transformer 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 which caused the remanence. In a second step, the current transformer is demagnetized by reducing the voltage gradually to zero (see Figures 5 and 6).
The analyzer performs the residual magnetism measurements prior to the usual CT testing cycle as it automatically removes residual magnetism after testing. To determine the residual magnetism, the analyzer drives the core into positive and negative saturation alternately until a stable symmetric hysteresis loop is reached. The 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 (Ψs: defined in the IEC 60044-1) on the residual magnetism test card. In addition, the remanence factor Kr is shown on the test card (see Figure 7).
The analyzer automatically demagnetizes the current transformer when the test is complete.
After installation, current transformers are typically used for 30 years. To guarantee a reliable and safe operation over the life of the CTs, a high level of quality during design phase, manufacturing process and installation is essential. Several quality tests are performed from development to installation. After installation, CTs should be tested regularly to ensure correct functioning over the entire life.
Benton Vandiver III is technical director at Omicron electronics and has more than 32 years’ experience in power systems protection. He is a registered professional engineer in Texas and a member of IEEE/PES/SPRC, IEEE-SA and USNC/CIGRE. Benton holds a U.S. patent for “Communication-based Testing of IEDs” and has authored or co-authored more than 80 technical papers and articles in the U.S. and internationally.