IMPACT OF SYNCHRONOUS GENERATOR DIGITAL MULTIFUNCTION RELAYS

The latest technological leap in generator protection has been the release of digital multifunction relays by various manufacturers. With more sophisticated characteristics being available through software algorithms, generator protective function characteristics can be improved. Therefore, multifunction relays have many advantages, most of which stem from the technology on which they are based.

IMPROVEMENTS IN SIGNAL PROCESSING

Most multifunction relays use a full-cycle Discrete Fourier Transform (DFT) algorithm for acquisition of the fundamental component of the current and voltage phasors. Consequently, they will benefit from the inherent filtering properties provided by the algorithms, such as:

• Immunity from DC component and good suppression of exponentially decaying offset due to the large value of X/R time constants in generators;

• Immunity to harmonics;

• Nominal response time of one cycle for the protective functions requiring fast response.

Since sequence quantities are computed mathematically from the voltage and current phasors, they will also benefit from the above advantages.

However, it should be kept in mind that fundamental phasors of waveforms are not the only parameters used in digital multifunction relays. Other parameters like peak or RMS values of waveforms can be equally acquired through simple algorithms, depending upon the characteristics of a particular algorithm.

A number of techniques have been used to make the measurement of phasor magnitudes independent of frequency, and therefore achieve stable sensitivities over large frequency excursions. One technique is known as frequency tracking and consists of having a number of samples in one cycle that is constant, regardless of the value of the frequency or the generator’s speed. A software digital phase-locked loop allows implementation of such a scheme and will inherently provide a direct measurement of the frequency or the speed of the generator. A second technique keeps the sampling period fixed, but varies the time length of the data window to follow the period of the generator frequency. This results in a variable number of samples in the cycles. A third technique consists of measuring the root-mean square value of a current or voltage waveform. The variation of this quantity with frequency is very limited, and therefore, this technique allows measurement of the magnitude of a waveform over a broad frequency range.

A further improvement consists of measuring the generator frequency digitally. Precision, in most cases, will be one hundredth of a hertz or better and good immunity to harmonics and noise is achievable with modern algorithms.

IMPROVEMENTS IN PROTECTIVE FUNCTIONS

The following functions will benefit from some inherent advantages of the digital processing capability:

• A number of improvements can be attributed to stator differential protection. The first is the detection of CT saturation in case of external faults that would cause the protection relay to trip.

When CT ratios do not match perfectly, the difference can be either automatically or manually introduced into the algorithm in order to suppress the difference.

• It is no longer necessary to provide a Δ-Y conversion for the backup 21 elements in order to cover the phase fault on the high side of the voltage transformer. That conversion can be accomplished mathematically inside the relay.

• In the area of detection of voltage transformer blown fuses, the use of symmetrical components allows identification of the faulted phase. Therefore, complex logic schemes can be implemented where only the protection function impacted by the phase will be blocked. As an example, if a 51V is implemented on all three phases independently, it will be sufficient to block the function only on the phase on which a fuse has been detected as blown. Furthermore, contrary to the conventional voltage balance relay scheme, a single VT will suffice when using this modern algorithm.

• Because of the different functions recording their characteristics over a large frequency interval, it is no longer necessary to monitor the frequency in order to implement start-up or shut-down protection.

• The 100% stator-ground protection can be improved by using third-harmonic voltage measurements both at the phase and neutral.

• The characteristic of an offset mho impedance relay in the R-X plane can be made to be independent of frequency by using one of the following two techniques: the frequency-tracking algorithm previously mentioned, or the use of the positive sequence voltage and current because their ratio is frequency-independent.

• Functions which are inherently three-phase phenomena can be implemented by using the positive sequence voltage and current quantities. The loss-of-field or loss-of-synchronism are examples.

• In the reverse power protection, improved accuracy and sensitivity can be obtained with digital technology.

• Digital technology allows the possibility of tailoring inverse volt/hertz curves to the user’s needs.

Full programmability of these same curves is readily achievable. From that perspective, volt/hertz protection is improved by a closer match between the implemented curve and the generator or step-up transformer damage curve.

Multifunction generator protection packages have other functions that make use of the inherent capabilities of microprocessor devices. These include: oscillography and event recording, time synchronization, multiple settings, metering, communications, self-monitoring, and diagnostics.