Current Transformer Performance study Using Software Tools.

Transcription

1 Current Transformer Performance study Using Software Tools. A. Mechraoui, A. Draou, A. Akkouche, and S. AL Ahmadi Department of Electronics Technology Madinah College of Technology, Madinah Council of Technical Education and Vocational Training, Saudi Arabia, Abstract Power system protective relaying systems are striving to meet the demands of systems which are ever increasing in complexity.. High-speed digital relays must make decisions while the power system is still in transition. Understanding the operation and performance of microprocessor based relays requires new engineering tools. Current transformer (CT) and relay modeling are practical tools to evaluate protection equipment performance. Models for instrument transformers are especially important for studying ct saturation, ferroresonance phenomena, harmonics, and their effects on the performance of protective relaying. This paper presents some software tools (ATP-EMTP,MODEL-TACS, TOP and Mathcad) that are used to investigate and to understand the transient events to better understand and visualize the response of digital relay models during transient conditions Keywords : Software, ATP/EMTP, MATHCAD, Modeling, Simulation, Transients, Protective Relaying. 1 Introduction. The dynamic performance of high-speed protective relays depends to a large extent on the signals produced by the instrument transformers, and these signals depend on the overall transient response of the instrument transformers and the type of transients generated by the power system. Computer simulation of power systems and protective relays eases the burden of relay testing and relay performance evaluations. The subject of computer modeling and simulation for the study of the power system transients on the current transformers are summarized in [1,2,3,4,5,6,7]. The relay design assessment is needed when a new relay design is introduced. In industry, a vendor or utility may want to evaluate the relay design characteristics to make sure that its performance is as expected [5]. In education, students learn by emulating an algorithm of a given relay (modeling) and testing it (simulation). Designing, setting, testing and evaluating protective relays call for software tools capable of modeling the protective relays of various designs as well as the surrounding power system. It is relatively difficult to explain the relationship of transformer modeling to the performance of the large interconnected system. The primary role of software is help bridge this gap. This paper presents software tools for modeling, testing and evaluating protective relays. 2 Software Tools. ATP/EMTP The ATP version of EMTP [8] is the basic software tool for electric system transient modeling. Different computer operating systems use different versions of the program. Version ATPMING works very well with MS Windows 98, 2000 and XP. MODEL-TACS [8] Figure 1 strategy for implementing relay in ATP/EMTP Figure 1 illustrates a general strategy for implementing relay simulations in EMTP. Since this strategy operates in closed-loop, a solution for each time-step in EMTP requires a

2 solution in TACS. The function blocks identified in Fig. 1 by {1} and {9} are programmed in the network portion of EMTP. Function blocks {2} through {8} are programmed in TACS. Block {2} can be implemented in either the network portion of EMTP or in TACS. In some relay simulation, blocks {2}, {3}, and {8} may be omitted in this strategy depending upon several factors including the desired simulation detail or quality level, the primary focus to the simulation, and the total system. Block {4} may be omitted if the DSP algorithm assumes a sampling interval equal to that of the EMTP simulation time-step. If this is not the case, EMTP measurements must be sampled as required by blocks {5} through {7}. ATPDraw [9] With the introduction of graphical user interfaces for EMTP (ATPDRAW) knowledge of the subtleties of EMTP is less important. ATPDraw is a graphical, mouse-driven preprocessor to ATP on the MS Windows platform and uses a standard Windows layout. Users build a picture of an electric circuit by selecting components from menus and using dialog boxes to enter component values and ATP parameters. ATPDraw then creates the ATP input file and runs ATP. Basic ATP model development is much easier in this environment, particularly for new users. TOP[3] TOP, written and supported by Electrotek Concepts, Inc., is a graphical postprocessor for transient data. TOP will graph ATP output files (*.pl4) and allow users to save the data in different formats, including COMTRADE and comma separated variable (CSV) text files. This program is the bridge between ATP and Mathcad. Mathcad 2002 Professional Mathcad worksheets process the transient data generated by ATP. The Mathcad desktop interface uses mathematical equations similar to those seen in textbooks. Concepts are easy to see and understand, although the same results can be achieved in other programs such as MATLAB. 3 CT Modelling using EMTP/ATP Models for instrument transformers are especially important for studying ct saturation, ferroresonance phenomena, harmonics, and sub harmonics and their effect on the performance of protective relaying. The major nonlinear effects in iron cores are saturation, eddy currents, and hysteresis. The predominant effect in cts is saturation. A number of iron core models described in the literature and their summary is described below: EMTP and ATP Inductor Models The EMTP and alternative transients program (ATP) support two classes of nonlinear elements, a true nonlinear model (Type-93), and two pseudo-nonlinear models (Type-96 and Type-98). In the true nonlinear model the nonlinearity of the element is explicitly defined as a nonlinear function, i.e., the flux as a function of the current. The EMTP and ATP then solve the combination of nonlinear equations and an appropriate system equivalent at each time step using a Newton Raphson iterative procedure. In the pseudo-nonlinear model, the nonlinearity is defined as a number of piece-wise linear segments. Such linear segments are represented by the program with a resistor in parallel with an appropriate current source. In the particular case of a nonlinear inductor, the flux is monitored at each time step in order to determine which linear segment should be used to compute the inductance at that time step. Note that this methodology does not model the true nonlinearity since the program relies on previous time step results to decide on what segment to operate next. The EMTP and ATP change segments only after they have operated illegally outside the range of the current segment. It is therefore important to use a small time step during the simulation so that the operating point moves up and down the nonlinearity in small increments. EMTP and ATP[9,10] support the following two additional magnetic saturation routines:

3 Subroutines CONVERT (in EMTP) and SATURA(in ATP) are designed to convert rms saturation curve data into peak data with the hysteresis loop being ignored. Subroutines HYSDAT (in EMTP) and HYSTERESIS (in ATP) are designed to provide the hysteresis loop data required by the Type-96 pseudo-nonlinear reactor model. This model uses data points from the ct secondary excitation curve, as inputs to subroutine CONVERT to obtain data points of peak flux, versus peak current. The result is a piece-wise linear model because a small finite number of data points are used, usually 10 or less. These EMTP/ATP models allow the user to represent the effects of residual flux left in the ct following primary current interruption. They can also be easily integrated directly into EMTP/ATP power system studies. Finally because the model parameters are derived from ct test data, a degree of validity is implied. The main disadvantage is the need to preprocess the data. The model used in this works is shown in Figure 3. Figure 4: CT primary and secondary current during saturation Figure 5 shows the voltage developed across the CT secondary during the simulation. Figure 5: CT burden voltage during an asymmetrical fault Figure 3 ATP saturable transformer model. 4 Simulation results obtained Simulation results are obtained to show the effects of saturation of the CT on a digital overcurrent and distance protection relays. Effect on Overcurrent relays The digital relay block diagram in Figure 6, demonstrates that you can model saturated CTs and relay elements, to better understand relay performance during transient events. elements if you know the relay parameters. Consider Effect of saturation on the secondary current. The currents applied to the primary sides of transformers 1 and 2, are shown in Figure 4. Saturation reduces the magnitude of CT secondary current from its ideal value as shown in Figure 4.

4 Figure 6: Overcurrent relay response to saturated CT secondary current Figure 6: Digital Overcurrent Relay Block Diagram The following example demonstrates a digital overcurrent relay response to saturated CT secondary current. Assume a sample rate of 16 samples per cycle. The analog low-pass filter is set at a cutoff frequency of 540 Hz to limit signal aliasing. After sampling by the A/D converter, the relay converts the current samples to complex vectors. The relay then compares the absolute values of the complex currents to the 50 element setting to determine if the element should operate. Figure 6 shows the relay response to the saturated current from Figure 4. Saturation initially reduces the relay magnitude response by one half, a reduction that may affect relay performance in different ways. For example, a high-set instantaneous 50 element could pick up for one cycle and then drop out for one to two cycles. A time-delayed overcurrent element could respond up to three cycles late. Effect on distance relays A typical digital relay model is derived from public information and may include: Anti-alias low-pass filter (cutoff slightly above one-half sampling frequency) Sampling function Full-or Half cycle cosine filter Sample-to-vector converter Sequence current and voltage calculation Phase-distance calculation Ground-distance calculation Negative-sequence impedance calculation. Zero-sequence impedance calculation Figure 7 Digital Distance Relay Block Diagram Mathcad formulae and calculations describe the different blocks or functions of the digital relay models used in this example. Figure 8 shows the effects of CT saturation during a phase-to-phase-to-ground fault at 15 percent of the line length, applied at 7 cycles into the simulation. Force saturation by increasing CT burden to 4 Ohms. Open the line breaker after 5 cycles.

5 Figure 8: Saturated B- and C-phase CT secondary current, phase-to-phase-to-ground fault After analog filtering, sampling, and digital filtering, the CT secondary current appears as shown in Figure 9. Figure 10: Phase-to-Phase Impedance Calculation During CT Saturation The phase angle calculated by the relay remains close to the actual phase angle as the CT recovers from saturation, as shown in Figure 11. Figure 9: Filtered Secondary Current, Phase-to- Phase-to-Ground Fault Saturation causes the relay to under-reach, as shown in Figure 10. Without saturation, the relay calculates the ideal B-phase to C-phase impedance (MBC) = Ohm at cycles after fault inception. With saturation, the relay calculates MBC = 2.09 Ohms at cycles after fault inception. At 5 cycles after fault inception, with the B-C-phase CTs still slightly saturated, the relay calculates 1 Ohm. Figure 11: Filtered Secondary Current, Phaseto-Phase-to-Ground Fault Figure 12 shows the effects of CT saturation on Z2 and Z0 calculations. These directional elements are very secure. Notice that Z0 has a brief positive excursion. Security counters in the directional logic ensure that the calculation has stabilized before allowing a directional determination.

6 7 References Figure 12: CT saturation effects on Z2 and Z0 Calculations 6. Conclusions. The most significant features that the instrument transformer (CT) models must include, have been described in the paper. It is imperative that relay systems be tested under transient conditions in order to assure a high of dependability and security in their design and applications. The use of electromagnetic transient programs to adequately model the different power system components and to generate transient data to assess the performance and correct application of modem static and microprocessor high-speed protective relays has become essential. This paper has investigated a number of relay models and the nonlinear behavior of the magnetic core of instrument transformers. It was shown that software tools are effective, inexpensive tools for power system transient analysis and relay simulation. [1] F. de León and A. Semlyen, Complete transformer model for electromagnetic transients, IEEE Trans. on Power Delivery, vol. 9, no. 1, pp , Jan [2] Joseph B. Mooney, Charlie F. Henville, and Frank P. Plumptre, Computer Based Relay Models Simplify Relay- Application Studies, Proceedings of the 20th Annual Western Protc. Relay Conference, Spokane, WA, October 19 21, [3] J. R. Lucas and P. G. Mc Laren, Improved simulation models for current and voltage transformers in relay studies, IEEE Trans. Power Delivery, vol. 7, pp , [4] Joseph B. Mooney, Charlie F. Henville, and Frank P. Plumptre, Computer Based Relay Models Simplify Relay- Application Studies, Proceedings of the 20th Annual Western Protective Relay Conference, Spokane, WA, October 19 21, [5] J. H. Chan, A. Vladimirescu, X. Gao, P. Liebmann, and J. Valainis, Nonlinear transformer model for circuit simulation, IEEE Trans. Comput. Aided Design, vol. 10, pp , [6] J. G. Frame, N. Mohan, and T. Liu, Hysteresis modeling in an electromagnetic transients program, IEEE Trans. Power Apparatus Syst., vol. 101, pp , [7] S. Prigozy, PSPICE computer modeling of hysteresis effects, IEEE Trans. Educ., vol. 36, pp. 2 5, [8] Alternative Transients Program (ATP) Rule Book, Copyright by Canadian / American EMTP User Group. [9] László Prikler and Hans Kr. Høidalen, ATPDraw for Windows 3.1x/95/NT version 1.0 User s Manual, SINTEF Energy Research, Trondheim, Norway, October 15, [10] A. Morched, L. Martí, and J. Ottevangers, A high frequency transformer model for the EMTP, IEEE Trans. Power Delivery, vol. 8, pp , [11] A. Mechraoui, M, Al Zahrani, A Choucha and A. Nouar, Current Transformer performance and suitability using Software Tools, Int.Conf.EEE'2004, vol. 2, pp , April 2004.