An electrical power system operates under a steady-state condition when there exists a balance between generated and consumed active power for the system. Power system disturbances may cause oscillations in machine rotor angles that can result in conditions like a power swing, when internal voltages of system generators slip relative to each other. Power system faults, line switching, generator disconnection, or the loss or sudden application of large amounts of load are examples of system disturbances that may cause a power swing event to occur in a power system. Depending on the severity of the disturbance and power system control actions, the system may return to a stable state or experience a large separation of load angle and eventually lose synchronism. Large power swings, stable or unstable, may cause unwanted relay operations at different locations in the system, which can aggravate the system disturbance and can result in major power outages or blackouts.
Further, asynchronous operation of interconnected generators in the power system as an effect of unstable power swing may initiate uncontrolled tripping of circuit breakers resulting in equipment damage and posing a safety concern for utility operators. Therefore, the asynchronous system areas may need to be separated from each other quickly and dynamically in order to avoid extensive equipment damage and shutdown of major portions of the system. In order to contain these risks, it is required as per international standards to have an optimal generator protection device, such as a generator relay, in place to isolate generators from the rest of the system within a half-slip cycle. The need to meet the international standards challenges protection engineers to ensure selective and reliable relay operation.
In a conventional relaying approach, a variation in system impedance determined at generator terminals is analyzed for detecting power swing. Various impedance-based protection approaches including power swing block (PSB) and out-of step trip (OST) are currently being used. However, these protection approaches may need an extensive power system stability study to arrive at an optimal setting for selective and reliable relay operation. Protection engineers typically use preliminary settings that are not adapted to accommodate variation in system configurations or operational dynamics, for example, changes in transmission and distribution layout during implementation phase or dynamically during operational phase. Extensive study and non-dynamic preliminary settings may result in the protection device being unable to selectively, reliably and dependably detect power swings and isolate generators during such events.
Other known relaying approaches estimate swing center voltage (SCV) for detecting power swings. Such approaches use approximate estimation that does not take into consideration real time power system dynamics. In some relaying approaches, a high-speed communication network such as fiber optic or global positioning system (GPS) communication is used to obtain data at a source end from one or more generators at receiving end(s), which is at a remote location from the source end, for SCV estimation. However, such approaches have economic challenges due to the cost associated with implementing and maintaining a high-speed communication network. Some approaches for SCV directly measure the rotor angle between the generator's internal voltage and terminal voltage for detecting power swing. In the absence of direct measurements, it is difficult to determine the power swing condition.
In one known SCV approach, the relationship between the SCV and a swing angle (θ) of a two-source equivalent system may be determined as per the below equation:
  SVC  =            ±      E        ×          cos      ⁡              (                  θ          2                )            where, E is an internal voltage of a source-end generator
In such approaches, the power swing may be detected by calculating a rate of change of the SCV. The time derivative of the SCV is given by below equation:
            d      ⁡              (        SCV        )              dt    =            E      2        ×          sin      ⁡              (                  θ          2                )              ⁢                  d        ⁢                                  ⁢        θ            dt      
In this equation, for sin (θ/2) to be close to one, θ should be around 180 degrees (for example, between 90 and 180 degrees). Therefore, the above equation can be used for detecting power swing when the value of θ is around 180 degrees. However, for values of θ between 0 and 90 degrees, the above equation will result in sin (θ/2) to be close to zero. In other words, this approach is not suitable for a smaller range of values of θ (for example, between 0 and 90 degrees).