Basic Principles

Low voltage and low current signals (i.e., at the secondary of a Voltage transformers and Current transformers) are brought into a low pass filter that removes frequency content above about 1/3 of the sampling frequency (a relay A/D converter needs to sample faster than 2x per cycle of the highest frequency that it is to monitor). The AC signal is then sampled by the relay's analog to digital converter at anywhere from about 4 to 64 (varies by relay) samples per power system cycle. In some relays, the entire sampled data is kept for oscillographic records, but in the relay, only the fundamental component is needed for most protection algorithms, unless a high speed algorithm is used that uses subcycle data to monitor for fast changing issues. The sampled data is then passed through a low pass filter that numerically removes the frequency content that is above the fundamental frequency of interest (i.e., nominal system frequency), and uses Fourier transform algorithms to extract the fundamental frequency magnitude and angle. Next the microprocessor passes the data into a set of protection algorithms, which are a set of logic equations in part designed by the protection engineer, and in part designed by the relay manufacturer, that monitor for abnormal conditions that indicate a fault. If a fault condition is detected, output contacts operate to trip the associated circuit breaker(s).

Protective element types

Protective Elements refer to the overall logic surrounding the electrical condition that is being monitored. For instance, a differential element refers to the logic required to monitor two (or more) currents, find their difference, and trip if the difference is beyond certain parameters. The term element and function are quite interchangeable in many instances.

For simplicity on one-line diagrams, the protection function is usually identified by an ANSI device number. In the era of electromechanical and solid state relays, any one relay could implement only one or two protective functions, so a complete protection system may have many relays on its panel. In a digital/numeric relay, many functions are implemented by the microprocessor programming. Any one numeric relay may implement one or all of these functions.

A listing of device numbers is found at ANSI Device Numbers. A summary of some common device numbers seen in digital relays is:

11 - Multifunction device.

21 - Impedance

24 - Volts/Hz

25 - Synchronizing

27 - Under Voltage

32 - Directional Power Element

46 - Negative sequence current

40 - Loss of Excitation

47 - Negative sequence voltage

50 - Instantaneous overcurrent (N for neutral, G for ground current)

51 - Inverse Time overcurrent (N for neutral, G from ground current)

59 - Over Voltage

62 - Timer

64 - Ground Fault (64F = Field Ground, 64G = Generator Ground)

67 - Directional Over Current (typically controls a 50/51 element)

79 - Reclosing Relay

81 - Under/Over Frequency

86 - Lockout Relay / Trip Circuit Supervision

87-Current Differential(87L=transmission line diff; 87T=transformer diff; 87G=generator diff)