# 9 Grid Strength and Fault Levels

A transmission line is primarily copper and can be modeled as a resistor. Transformers are coils of wire and hence have inductance. Therefore, a transmission system can be represented by an impedance Z_{T} = R + jX_{L} where R is the resistance and X_{L} is the inductive reactance. Ideally, utilities want to keep R and X_{L} small to minimize losses in transmission.

Fault level is defined to be maximum voltage times the short circuit current. For example, if a system has a 230 kV rating and a resistance of 10 Ohms, the short circuit current would be I = V/R = 230 kV/ 10 Ohms = 23 kA so the fault level is 23 kA*230 kV = 5.29 GVA. An acceptable voltage drop is roughly 5%. Hence, loads totaling up to 0.05*5.29 GVA = 264.5 MVA could be connected. (Lynn, 171)

**Works Cited**

Lynn, Paul A. *Electricity from Wave and Tide : An Introduction to Marine Energy*. Wiley, 2014.