Power electronic converters help to support voltage while the network voltage is near its nominal value. However, during a fault, power electronic converters can supply no more than their current ratings, and the current they supply during a fault will be much smaller than a synchronous machine of similar rated capacity (in absence of any explicit grid code requirements, each model of converter delivers a short-circuit current of its own). Thus, a power electronic converter will provide very little contribution to Short Circuit Capacity in the faulted system.
Therefore, we can no longer use a common terminology for two fundamentally different characteristics:
1. Current that would flow into a fault at the given bus (the short-circuit capacity).
2. Change of voltage in response to P/Q load changes with no fault, at near-nominal voltage.
It is proposed that the following terminology should be used for the nonfault, near-nominal voltage characteristics:
1. System Impedance (in Ohms or per unit).
2. System Strength (in MVA).
System impedance ZSYS is the effective impedance of the network from power sources to the load bus. This is traditionally expressed as Short Circuit Capacity (SCC) in units of MVA, as an ideal voltage source (VS ) multiplied by the current that would flow from the network into a fault at the bus being assessed.
In general, an equivalent impedance ZSYS and the associated System Strength [1] in MVA is a useful measure of the ability to maintain a stable voltage at the load bus. Changes in active and reactive power of the load will influence the load bus voltage less when ZSYS is small and the System Strength is high, compared with a larger influence on the voltage when ZSYS is high and the System Strength is low. However, using an equivalent impedance is a very simplified representation of a complex and non-linear power system, and in reality the Thevenin equivalent parameters in Figure 1 vary with the condition of the power system. This nonlinearity is becoming more significant as synchronous generation is displaced by power electronic converters.