Off-Nominal Frequency Measurements in Electric Power Systems

Some power system protection applications require accurate estimates of voltage magnitude and frequency over a wide measurement range. Most digital techniques for measuring frequency have acceptable accuracy over a small range in the neighborhood of the nominal value. This paper presents a method for extending the measurement range of the Least Error Squares (LES) technique. The paper also examines the effectiveness of the technique and presents some results from simulated studies and laboratory experiments. It is essential to maintain the frequency of a power system close to its nominal value. Usually, frequency deviations in the range of two to three percent only are allowed for short durations of time. In this range, the Least Error Squares algorithm (LES) provides accurate estimates and is, therefore, suitable for implementing in most frequency relays. However, the measurement errors increase as the frequency deviation from the nominal value increases. During start-up and shut-down of a generator, its frequency varies over a wide range. If the generator is connected to a transformer, both the generator and the transformer can experience overexcitation. Excessive volts-perhertz (V/Hz) indicates that overexcitation has occurred. One method of estimating this parameter is to measure the voltage magnitude and frequency and then calculate volts-per-hertz. Voltsper-hertz relays that provide accurate measurements over 10 to 70 Hz range would be desirable for use on steam turbine driven generators. The frequency of the voltage of a thermal generator is low during start-up and shut-down procedures.