Principles and characteristics of distance protection

Principles and characteristics of distance protection

Introduction to distance protection

Distance relays are one of the most significant transmission line protection components.

These relays can be configured as a percentage of line impedances in some cases. Zone 1 may be adjusted to 80 percent line impedance to avoid reaching the far end, zone 2 to 120 percent line impedance to reliably exceed the line, and zone 3 may be deactivated or configured to cover a neighboring line.

Mho, Quadrilateral, Offset Mho, and more features of distance relays exist. Extra caution may be necessary to keep safe under big loads if quadrilateral features or Mho traits are present.

The mutual connection of parallel wires can cause underreach and overshoot of distance relays. As a result, the relay configuration must account for this impact; some relays include algorithms to compensate, but the parallel line current must be used, which complicates the installation.

Distance protection may not be achieved at other voltage levels in some countries because fault clearing times in transmission sub-levels are slower than fault clearing periods at the transmission level.

The difficulty of combining quick clearing with selective plant tripping is critical for power system protection.

High-speed protection systems for transmission and primary distribution circuits, which may be employed with auto-reclosing circuit breakers, are continually improving and are frequently used to satisfy these needs.

Distance protection is a non-unitary protection system with significant economic and technological advantages in its most basic form.

The fundamental advantage of distance protection, as opposed to phase and neutral overcurrent prevention, is that its fault coverage of the circuit being protected is essentially unaffected by fluctuations in source impedance.

Distance protection is a simple technique that may be applied rapidly to faults along a protected circuit. In addition, it can perform both main and remote backup duties in a single scheme. When combined with a signaling channel, it can easily be adapted to create a unitary protection scheme.

It's perfect for quick reclosing applications, such as protecting key transmission lines, in this form.

Principles of Distance Relay

Because the impedance of a transmission line is proportional to its length, it is preferable to employ a relay capable of monitoring the impedance of a line up to a preset point for distance measurement (the point of reached).

A distance relay, for example, is designed to work only for faults that occur between the relay's position and the designated access point, enabling faults to occur in various parts of the line to be distinguished.

The voltage at the relay point is divided by the measured current as the basic concept of distance protection. The computed apparent impedance is compared to the impedance of the impact spot. If the measured impedance is less than the reach point's impedance, a failure on the line between the relay and the reach point is presumed.

A relay's hit point is the place on the line impedance locus where the relay's boundary characteristic intersects.

It may be shown on an R/X diagram since it is dependent on the voltage and current ratios as well as the phase angle between them. The locations of the power system impedances detected by the relay during faults, as well as fluctuations in power and load, may all be depicted on the same diagram, allowing the relay's performance in the presence of faults and system disturbances to be evaluated.

Relay performance

The performance of a distance relay is measured in terms of range accuracy and operation time. The actual ohmic reach of the relay under real-world conditions is compared to the relay's preset value in ohms to determine to reach accuracy.

The degree of voltage delivered to the relay under fault situations determines the accuracy of the reach.

The impedance measuring techniques used in specific relay systems have an effect as well. The fault current, the position of the fault relative to the relay setting, and the point in the voltage wave where the fault occurs can all affect the time it takes for the relay to operate.

Transient measurement mistakes, such as those induced by capacitor voltage transformers or saturating CTs, might also delay relay operation for failures around the dip point, depending on the measuring techniques employed in a specific relay design. Maximum and minimum operation times are typically required for electromechanical and static distance relays.

The discrepancy between them is minor for modern digital or digital distance relays over a wide variety of system operating circumstances and fault situations.

The Distance Relay's Characteristics

Some digital relays assess if the operation is necessary by measuring the absolute fault impedance and comparing it to the impedance restrictions established on the R/X diagram.

Traditional distance relays, as well as digital relays that mimic traditional relay impedance components, do not monitor absolute impedance. To establish whether the fault is in-zone or out-of-zone, they compare the observed fault voltage to a replica voltage produced from the fault current and the zone impedance parameter. Distance relay impedance comparators or algorithms that mimic classic comparators are classed based on their polar features, number of signal inputs, and signal comparison mechanism.

When plotted on an R/X chart, common kinds compare the relative amplitude or phase of two input numbers to generate operational characteristics that appear as straight lines or circles. The development of impedance functional forms and their sophistication has been dictated by available technology and acceptable cost at each stage of the history of distance relay design.

A quick review of impedance comparators is necessary since many classic relays are still in use, and some digital relays imitate old relay systems.

A contemporary distance protection relay is one example.

Simple illustration of SIPROTEC 7SA522 protective relay (provides complete distance protection and incorporates all the functions generally required for the protection of a power line)

AINSI	description	AINSI	description
21 / 21N	Distance protection	50HS	Fault Protection
FL	Fault locator	50BF	Breaker failure protection
50N / 51N; 67N	Directional earth fault protection	59/27	Over/under voltage protection
50/51/67	Back-up overcurrent protection	81O / U	Over/under frequency protection
50 STUB	STUB bus overcurrent threshold	25	Synchronized Verification
68 / 68T	Power swing detection/triggering	79	Self-reclosing
85/21	Tele protection for distance protection	74TC	Trip Circuit Supervision
27WI	Low power protection	86	Lockout (CLOSE command - interlocking)
85 / 67N	Tele protection for earth fault protection