Background on EMT

Electromagnetic transient (EMT) simulation differs from other types of electrical power system simulation in that power system signals are modeled as their fundamental time varying quantities, such as voltage and current, instead of the more abstract analytic representation– the phasor. Figure 2 illustrates the high level difference between these two representations, with both the time-domain representation (“Time Domain Plot”) and phasor representation (“Phasor Plot”) shown.

The basic phasor is a representation for signals of the form \(M\cos(\omega t+\theta)\) where all parameters are constant except for the time parameter, \(t\). This is a good assumption for many types of analysis, such as understanding power flow throughout a network, voltage drop, short circuit current, and transmission line contingency analysis, to name just a few. As a result, many types of analysis software popular among transmission planners and facility engineers alike rely on this representation.

However there are many cases where this assumption is not sufficient. The study of switching transients is one of the must fundamental examples. When studying switching transients, relationship of time-varying voltages and currents in the inductive and capacitive elements of a system come into play:

\begin{equation} V(t) = L \frac{dI(t)}{dt} \end{equation}

\begin{equation} I(t) = C \frac{dV(t)}{dt} \end{equation}

For instance, the relationship between current through an inductor \(I(t)\) and voltage across an inductor \(V(t)\) indicates that a rapid change in current will result in a large voltage across the inductor.

In power systems (usually in the context of transimssion systems), one way we encounter this phenomena is during the opening of a current-carrying circuit breaker. The current interruption can cause a large transient voltage across the breaker’s terminals. This voltage is called the transient recovery voltage (TRV), and its severity is dependent on a variety of factors including the characteristics of the fault and local power system. It is important to study TRV to ensure it is properly controlled and the breakers are properly rated for their expected operating conditions, and this type of study is typically performed using an EMT program. Further, transients such as these occur microsecond scale time intervals, so compared to phasor domain simulations, an EMT simulation might be studied over a fraction of a 50/60 Hz cycle or even less!

Breaker TRV is just one example where the enhanced detail of EMT may be necessary. Other classical examples include insulation coordination, which requires modeling of lightning induced surges, or power quality analysis, where voltage/current harmonics beyond the fundamental 50/60 Hz need to be taken into account. Studies involving inverter based resources (IBRs) also often require the use of EMT models because of their fast-acting control systems and semi-conductor based switching components.

At its core, Kestrel EMT uses a trapezoidal integration based engine to model circuits.