Electromagnetic Transients Study – A Brazilian ISO Perspective

Take a first look at the Electromagnetic Transient Phenomena!

The accuracy of transient phenomena simulations requires a specific representation of the electric power network according to the frequency range. For example, let's imagine that you need to model a transient recovery voltage, phenomenon, TRV. The choice of mathematical models that represent power components needs to take into account the frequency-dependence of parameters.

Each range of frequencies corresponds to some particular transient phenomena. Table 1, obtained from [1], shows the frequency ranges per phenomenon. In 1990, CIGRE Study Committee published a technical brochure entitled "Guidelines for representation of network elements when calculating transients" [2].

This report gives guidance to deal with any kind of electrical transient phenomena in power systems. Also, this document describes the frequency ranges origin of electrical transients with associated frequency range.

What does the Brazilian ISO say?

Operador Nacional do Sistema elétrico, ONS, known as Brazilian Independent System Operator?(ISO), recommends that studies of electromagnetic transients are in accordance with some basic design guidelines. These guidelines describe how some components should be modeled in electromagnetic transients studies. For example, in isolation coordination and transient recovery voltage modeling, the parasitic capacitance of substation buses must be represented.

Brazilian ISO publishes an updated (free) version of the document "Submodule 2.3 - Premissas, critérios e metodologia para estudos elétricos". This describes all the guidelines and recommendations for modeling your electrical power system in electromagnetic transient studies. Figure 1 shows a diagram with the studies recommended by the ONS.

Figure 1 - Brazilian Independent System Operator, electromagnetic transients studies perspective.

The Good, Old Statistics

According to the ONS, in statistical studies, the modeling must consider the probabilistic characteristics of the circuit breakers. Statistical studies must contain at least (200) two hundred different simulations.

If you do a transmission line energization study, you need to simulate 200 times your electrical system. Circuit breakers are represented as a statistical switch, with the individual operating times of each of the three phases. Figure 2 shows a statistical switch block in ATPDraw.

Figure 2 - ATPDraw statistics circuit breaker.

Energization

In the probabilistic simulations of energizing transmission lines, the circuit breaker opening simulation must consider the application of a phase-to-ground short circuit at the remote end of the line.

The modeling of the power transformer to be energized must take into account the saturation characteristic and the positive, negative, and zero sequence impedance. ISO also recommends that three-phase transformers should be modeled with phase-to-phase couplings, with the phenomenon of saturation of the ferromagnetic core represented by a non-linear element.

Energizing the Shunt Capacitor Bank

If the system contains a bank of capacitors near to the bank to be energized, it is necessary to make the energization of the back-to-back configuration simulations. The objective is to quantify the level of their overvoltages.

Reclosing

In three-pole reclosing studies, the engineering must open the three-pole terminal closest to the fault, according to the time provided by the line protection. To ensure accuracy in the simulation results of single-pole reclosing studies, the following steps must be taken:

1. Apply a single-phase fault in one of the line terminals;
2. Apply the single-pole opening of the terminal closest to the fault, according to the time provided by the line protection
3. Apply the single-pole opening of the terminal opposite the fault in the trip transfer time provided by the line protection
4. In cases of successful reclosing, keep the short-circuit applied to the line after its total opening, eliminate it within a time of the order of three cycles after the opening of the last phase and consider a dead time up to 500 ms dead
5. Reconnect the line through one of the terminals after the dead time and follow the same system used for energizing the transmission line

Deterministic Eyes

Deterministic studies use the previously defined characteristics of the equipment in the operation of opening circuit breakers or reproducing the worst cases of a statistical study.

Load-Rejection

Deterministic studies use defined characteristics of the equipment, ie. modeling the worst scenario. The empirical analysis is necessary to evaluate the network configuration that gives the most conservative results.

In load-rejection studies, an electric power system must reproduce the maximum flow of active and reactive power in the transmission line. It is necessary to model the load-rejection without fault application, with a single-phase fault after the load-shedding.

Transient Recovery Voltage

In TRV studies, transmission lines must be represented by a model of distributed parameters without correction in the frequency domain. For transformers, reactors, models it is important to model the saturation effect and iron losses. Loads do not need to be represented on the electrical power system.

That's all folks!

Early in their career, engineers may feel lost. However, fortunately, Brazilian ISO gives a hand with guidelines, especially Submodule 2.3. That is essential, a specific guide to go out of the black cave.

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References

[1] Martinez-Velasco, Juan A., ed.?Power system transients: parameter determination. CRC press, 2017.

[2] THIEN, Myo Min. "Guidelines for Representation of Network Elements when Calculating Transients."

[3] ONS, Procedimentos de Rede. "Submódulo 2.3–Diretrizes e Critérios para Estudos Elétricos." (2021).