User Spotlight Series: Week 9

Development of a Current Transformer Model with Saturation Characteristics

Abstract: Current transformer (CT) saturation may cause the mal-operation or operating time delay of protective relays. Therefore, using a CT simulation module with its saturation characteristics is essential for the research and development of the detection and compensation techniques for the CT saturation phenomena. A CT modeling method which can simulate the saturation characteristics using a given Vrms–Irms excitation curve which can be provided by a manufacturer through an open-circuit test is proposed in this paper. The RSCAD’s CBuilder is used to create the proposed CT model which can be utilized in the CT saturation studies. In the RSCAD, there are two ways to build the CT model with the saturation characteristics. First, the CT secondary circuit is modeled with the saturable reactor module provided by the RSCAD. However, because the saturable reactor only uses the saturation knee voltage and the air core inductance to compute the excitation curve, it is difficult to obtain the same Vrms–Irms curve of the CT from its simulation. Secondly, the CT module provided by the RSCAD can be used. The given Vrms–Irms excitation curve can be applied to the CT module. However, the problem is that the Vrms–Irms excitation curve obtained from its simulation doesn’t match with the applied Vrms–Irms excitation curve. In this paper, a new CT modeling method based on the already suggested iron core non-linearity modeling method[1] is proposed by using a given Vrms–Irms excitation curve under the CBuilder environment. The performance of the proposed model is verified by comparing the Vrms–Irms excitation curve obtained from the simulation of the proposed CT model to the Vrms-Irms excitation curve applied to the proposed model.

Automatic voltage regulators and power system stabilizers’ digital models verification experience using frequency responses obtained by RTDS

Abstract: The report presents the experimental frequency responses obtained from automatic voltage regulators (AVR). Different AVRs were tested via specialized test-band for frequency response estimation, which was developed in JSC «STC UPS» with the use Real-Time Digital Simulator (RTDS).

The manufacturer provides the mathematical model of the AVR and power system stabilizers (PSS). These models can be checked via test band. Test procedure is the comparison of the experimental device frequency response and the model frequency response. This comparison require the special software.

The developed software measures the generator electric power mode signals (U, I, P, Q, f) at the input of the AVR and the output signal of the AVR.

The points of the frequency response (amplitude and phase) of the AVR control system calculates by modulating the input parameters with a frequency in the range [0.1; 10] Hz.

The several international AVRs tests have shown that mathematical models often contain simplifications. That lead to inconsistencies in the frequency response of the experimental AVR and its model in the range [0.1; 10] Hz. That did not allow the mathematical model to reproduce the correct work of AVR.

Summing up the results, the initial mathematical models of AVR and PSS provided by the manufacturer cannot be used to create verified mathematical models. Since they describe the regulation structure in a simplified way. For this reason, it requires experimental verification and correction of mathematical models of AVR.

Mathematical models verification and correction is one of the areas of JSC «STC UPS».

Keywords: Automatic voltage regulator, AVR, power system stabilizers, PSS, mathematical model, power system, real time digital simulator, RTDS, frequency response.