A Revolution in Current Transformer Testing

06/04/2020
CT Testing with CTAnalyzer

A Revolution in Current Transformer Testing

Different test devices and methods are used in the market to verify the performance of current trans- formers 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.

Equivalent  circuit diagram of a real current transformer

Sơ đồ mạch tương đương của một biến dòng điện thực
1.Equivalent  circuit diagram of a real current transformer

le            Excitation current

ls            Secondary current

lp            Primary current

Xm         Main inductivity of the core

Rm         Magnetic losses of the core

Np, Ns  Amount of turns of the ideal core

Rct        Ohmic resistance of secondary turns

EMF      Electro-Motive Force—secondary core voltage

Us   Secondary terminal voltage

Rb          Ohmic part of complex burden

Xb          Inductive part of complex burden

Applications areas

– 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 oper- ate 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 con- nected 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 influ- ence measurement results.

Ví dụ đấu nối một biến dòng có 6 nấc tỉ số
2.CONNECTION EXAMPLE FOR A 6-TAP CURRENT TRANSFORMER

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 current transformer is to apply a high current to the primary side and read the sig- nals 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 ana- lyzed. This method, however, is time-consuming and mate-rial-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).
  2. Another common testing scenario for current trans- formers is injecting a defined testing voltage on the secondary side and reading signals on the primary 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 approximate- ly 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 constan- cies and many others cannot be tested at all.

Because both methods have limitations, a new method of test-ing CTs was needed.

Hình 3: Thiết bị phân tích CT Analyzer với phụ kiện CT SB2 đính kèm
3. CT Analyzer with CT SB2 ATTACHED

New modeling concept

Omicron developed the CT Analyzer test device. The con- cept 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.

Hình 4: Đường cong từ trễ tại điểm bão hòa lớn nhất thể hiện vùng có thể của từ dư
4. HYSTERESIS CURVE AT MAXIMUM SATURATION POINT SHOWING THE POSSIBLE AREA FOR RESIDUAL MAGNETISM

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 assess- ment 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 metro- logical 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.)

Innovations

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 with- out the need for rewiring. An integrated connection check feature tests the secondary connection to the CT and indi- cates wiring mistakes before the measure-ment 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 dur- ing the operation. The SB2 can be attached to the back of the analyzer (see Figure 3) with all wiring connections made.

Hình 5: Nguyên lý khử từ của lõi sắt
5.DEMAGNETIZATION PRINCIPLE OF IRON CORES

As a new measurement option for the analyzer, RemAlyzer allows current trans- formers to be tested for residual magnetism after a system fault or local event where core saturation is suspected.

Residual magnetism might occur if a cur- rent  transformer  is  driven  into saturation. This can happen as a consequence of  high fault currents’ containing transient components or direct cur- rents applied to the current transformer during winding resis- tance 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 oper- ation, 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 demagnetiza- tion 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 reduc- ing the voltage gradually to zero (see Figures 5 and 6).

Hình 6: Thẻ thí nghiệm của CT Analyzer thể hiện kết quả đo của một thí nghiệm từ dư
6.TEST CARD OF CT ANALYZER SHOWING MEASUREMENT RESULTS OF A RESIDUAL MAGNETISM TEST

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 mag- netism, 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 con- dition to determine whether the core was affected by residual magnetism. The results are displayed as absolute values in volt- age per second, as well as in percent relative to the saturation flux (Ys: defined in the IEC 60044-1) on the residual magne- tism test card. In addition, the remanence factor Kr is shown on the test card.

The analyzer automatically demagnetizes the current trans- former when the test is complete.

Conclusion

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, manufac- turing process and installation is essential. Several quality tests are performed from development to installation. After installa- tion, CTs should be tested regularly to ensure correct function- ing 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.

OMICRON

www.omicronusa.com  

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