Inter-area oscillation detection by modern digital relays

The inter-area oscillation between large interconnected power systems is a common problem worldwide, where the generators of the interconnected systems oscillate against each other causing power flow and frequency oscillations on the tie lines, in a low frequency range of 0.1-0.7 Hz. This type of oscillations not only limits the amount of power transfer, but also threatens the system security, because it may lead to system instability and as a result in cascading outages in the system. This paper describes an inter-area oscillation detector, which can be implemented in a regular digital protective relay. This inter-area oscillation detector can monitor power transfer over several tie lines of the interconnected systems or use measurement from one tie line. When the oscillation at the specified frequency band with appreciable magnitude exceeding a threshold is detected, the detector will issue an alarm and may initiate an action for damping the inter-area oscillations. If a higher magnitude threshold is exceeded, the detector will issue a trip command to separate the interconnected systems in order to maintain stability of each individual system. The detector operation can be based on either power or frequency measurements at the tie lines, and can be deployed in a single relay or in multiple relays at the tie lines. The measurements between digital relays can be sent to the master relay using IEC 61850 GOOSE analogs, or synchrophasors and are synchronized at the master relay. Filtering technique has been used to eliminate DC offset and high frequency noise from the input signals. The oscillation frequency is estimated first using a time domain algorithm, and then an adaptive window length Fourier technique is used to estimate the oscillation magnitude with high degree of accuracy. Alarm or trip decision is made accordingly at the end based on these measurements and estimations. Various testing results are also discussed.