Patent ID: 8775104

Claim:
A method for protecting an electrical power transmission network comprising the steps of: (a) measuring, at a particular instant in time, a voltage and a current at a first location in the network other than at a generator in the network, the first location having a load connected thereto, wherein measuring the voltage comprises measuring all phases of the voltage and measuring the current comprises measuring all phases of the current; (b) determining, from the measurements taken at the particular instant in time, a positive sequence measured voltage and a negative sequence measured voltage based on the measured voltage; (c) determining, from the measurements taken at the particular instant in time, a positive sequence measured current and a negative sequence measured current based on the measured current; (d) determining a negative sequence Thévenin impedance based on the negative sequence measured voltage and the negative sequence measured current; (e) defining a positive sequence Thévenin impedance as equal to the negative sequence Thévenin impedance; (f) determining a Thévenin voltage based on the positive sequence measured voltage, the positive sequence measured current and the positive sequence Thévenin impedance; (g) determining a load impedance based on the positive sequence measured voltage and the positive sequence measured current; and (h) determining the stability of the network using the load impedance and one or more of the Thévenin voltage and the positive sequence Thévenin impedance.