# Thevenin Equivalent Circuit

Consider again the Icelandic national power system shown in Figure 5.12. It is often useful to be able to calculate performance at one particular node, without having to perform a full load flow or fault analysis of the entire network. This can be achieved by representing the network by its Thevenin equivalent, as explained in the Appendix.

The values in the Thevenin equivalent circuit shown in Figure 5.15 must be calculated for the particular node in question. The magnitude of Zth can be found from the fault level at the node by

 Nominal system voltage (kV) Fault level (MVA) 132 5 000-25000 33 500-2500 11 10-250

Figure 5.15 Thevenin equivalent circuit

th th

Sk where V is the nominal line-to-line voltage. The angle of Zth could also have been found from the fault level, if only Isc and Sk had been calculated and stored as complex numbers. Instead of this, normal practice is to express the fault level as a scalar and to express the associated angle as an X/R ratio, which can be used in Zth = Rth+ jXth. The Thevenin source voltage can often be taken as the nominal voltage at the point of interest, being careful to use the phase-to - neutral or line - to - line value consistent with the calculation.

The fault level is an important design parameter, not only for predicting currents under fault conditions but also for predicting performance under normal operating conditions. It defines the strength of the network at a particular point. A weak grid is a network or part of a network where fault levels are low; i.e. the Thevenin or source impedance Zth is high. A high Zth implies that the node voltage would be sensitive to active and/or reactive power extraction or injection at the node.

The impact of a renewable energy generator on a network is very dependent on the fault level at the point of connection. Considering a proposed wind farm capacity (in MW) as a percentage of the fault level (in MVA) can provide a rough guide to acceptability. This topic is dealt in greater detail in Section 6.1.3. 