T&D World Magazine
Testing Current Transformers with 'the Modeling Concept'

Testing Current Transformers with '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.

Applications Areas
The current transformers for metering and protection applications work basically the same way - transforming high power primary signals to lower secondary values. However, while current transformers used for protection applications operate to well above the load current, the current transformers for metering purposes must go into saturation directly above the load current level to protect the connected meter.

Protection current transformers
Current transformers play an important role in the protection of electrical power systems. They provide the protection relay with a replication of the primary current so that 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 can be expected).
Metering current transformers

Today, energy is supplied by many different sources including alternative energy sources like solar and wind power. To guarantee accurate billing in this competitive electricity market, additional metering points are necessary. It is therefore important to have the entire metering circuit calibrated, as the meter is only as accurate as the instrument transformers sourcing it. This makes the testing and calibration of current transformers up to the 0.15 accuracy class essential. However, on-site testing of CTs of the 0.15 accuracy class is particularly critical as 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 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, various situations can be simulated and the signals on the secondary side can be measured and analyzed. However, this method is time-consuming and requires a lot of equipment. Sometimes it is not even feasible as very high currents are required, e.g. for on-site testing of bushing current transformer inside a power transformer or a shunt reactor.
  2. Another common testing scenario for current transformers is injecting a defined test voltage on the secondary side and reading the reverse transformed value on the primary side. Unfortunately using this scenario some parameters, like accuracy and knee point (excitation curve), can only be tested with limitations. This is due to 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 like the transient dimensioning factor, the accuracy limit factor, the safety factor, composite errors, time constancies and many others cannot be tested at all.

As both methods have limitations, OMICRON has developed a method of testing CTs.


New Modeling Concept

The concept of modeling a current transformer allows for a detailed view of the transformer's design and its physical behavior. The CT analyzer device from Omicron builds up a model of the current transformer by using initial data, measured automatically during the test. Based on this model, the test device is able to calculate parameters like 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.

The CT Analyzer is small, lightweight and conducts fully automated tests of current transformers within the shortest times possible.

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.

Therefore, all operating points described in the relevant 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 even the transient dimensioning factor (according to the IEC 60044-6 standard for transient fault current calculations) to be assessed.

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 very high testing accuracy of 0.05% (0.02% typical) for current ratio and 3 minutes (1 min typical) for phase displacement.

The accuracy of the CT Analyzer is verified by several metrological institutes like 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.)

New Innovations - CT SB2

For its latest release,the CT Analyzer was improved by 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.
Thus every ratio combination can be tested automatically with the CT Analyzer 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.
Additionally, the CT Analyzer checks the different ratios of the current transformer tested. The testing signal will then be adjusted to make testing voltages above 200 V impossible. This ensures a high level of worker safety during operation.


New Innovations – RemAlyzer
As a new measurement function for the CT Analyzer, the RemAlyzer allows current transformers to be tested for residual magnetism.

Residual magnetism may 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 functionality of a current transformer.
Since 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 may show a failure to operate in case of real over-current as the current transformer's signal is distorted due to the residual magnetism in the CT core.
Once the current transformer is magnetized a demagnetization process is necessary in order to remove residual magnetism. This can be achieved e.g. 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.

The CT Analyzer performs the residual magnetism measurements prior to the usual CT testing cycle as it automatically removes residual magnetism after testing. In order 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 (Ψs: defined in the IEC 60044-1) on the residual magnetism test card. Additionally, the remanence factor Kr is shown on the test card.
The CT Analyzer automatically demagnetizes the current transformer when the test is complete.

After installation, current transformers are typically used for 30 years. In order to guarantee a reliable and safe operation over the life time of the CTs, a high level of quality during design phase, manufacturing process and installation is essential. Therefore, several quality tests are performed from development to installation. After installation CTs should be tested on a regular basis to ensure correct functioning over the entire life time.

The lightweight and mobile CT Analyzer now offers the possibility to conduct all of these tests in a fast and cost-effective manner. Its wide functionality range and high accuracy make it a suitable solution for testing single and multi-tap current transformers for protection and metering purposes.

Biography

Peter Fong received a BS in electrical engineering from the University of British Columbia in 1988. He joined OMICRON in 2000, where he presently holds the position of Application Engineer. Prior to joining OMICRON, he worked for 12 years at BC Hydro and two years at a relay manufacturer in the US. Peter Fong is a Professional Engineer (APEGBC) and a member of IEEE.

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