Fundamentals of Distance Protection

Introduction

Impedance relays and automatics are devices whose function is based on the magnitude and angle of impedance. The main group of impedance relays is distance protection devices. Other types of impedance relays are e.g. loss of synchronism protection, loss of excitation protection, or impedance automatics like fault locator.

Impedance relays measure and evaluate the magnitude and angle of impedance and therefore these relays are adjusted to the power line parameters. Also because of their versatility and wide range of applications, these relays have a very complex algorithm that can deal with the most difficult transient phenomena.

The distance protection theme is very extensive. This article aims to give the reader a simple overview and introduction to distance protection.

Distance protection

The principle of distance protection is based on the determination of the fault impedance from the measured short-circuit voltage and current at the relay location as illustrated in figure 1.

The measured fault impedance is compared with the known value of power line impedance (which is approximately constant). If the measured fault impedance is smaller than the power line impedance, an internal fault is detected and a trip command is sent to the appropriate circuit breaker. For this basic protection decision, no further information is required. The measurement in distance protection is principally realized that for each fault type the line impedance of the fault loop is determined. It means the protection is measuring three fault loops for phase-to-phase faults (L1-L2, L2-L3, L3-L1) and three phase-to-ground fault loops (L1-N, L-2N, L3-N). The combination of all six loops covers all 11 types of short-circuit faults that can occur in a solidly grounded network. Modern distance protection first determines the type of fault based on voltage difference in the pre-fault and the fault voltage and then calculates the fault loops only for the identified short-circuit. This contributes to shorter operation times of distance protection. Such algorithm is shown in figure 2.

The basic formulas for impedance loop can be derived from the following diagram (see Figure 3) where:

• E is the equivalent emf,
• Z_S is the source impedance,
• Z_SC is the short-circuit impedance,
• U_SC is the short-circuit voltage at the relay location,
• I_SC is the short-circuit current at the relay location,
• R_F is the fault resistance.

*Z_fwd is the forward path; Z_ret is the return path

Let us consider the following 20 kV power system in figure 4 (taken from [1] and modified). In normal operation, the distance protection device ‘D’ at point A sees an impedance equal to the sum of the impedance of power lines Z_V1, Z_V2, and the load impedance, Z_Load. Since Z_Load >> Z_V1 and Z_V2, the impedance measured by the distance protection will be very high.

For a two-phase short-circuit, L2 – L3, occurrence on power line V1, an equivalent impedance diagram such as shown in figure 3 can be constructed. In this case, referring to figure 3, the forward path is L2 while the return path is line L3.

Next, let us assume that distance protection in point A measured following values:

From the measured values, the impedance can be calculated using the fault loop for two-phase short-circuit L2-L3.

Since the impedance of the fault loop is smaller than the impedance of power line V1, an internal fault is detected and a trip command is sent to the appropriate circuit breaker.

The distance to the fault can be estimated as follows,

You may want to check-out discussions and step-by-step examples of setting Transformer Differential Protection.

Zones of distance protection

Distance protection uses grading type of tripping characteristic. In figure 5, a typical 3-zone set-up of distance protection is shown. The zone and time grading set-up provide protection of the power line through zone 1 and the adjacent line through zones 2 and 3.

The impedance reach of each zone is set with a certain overreach and underreach with respect to the protected line. This is done because of the measurement errors of distance protection which can be caused by one of the following factors:

• calculation algorithm,
• accuracy of power line parameters,
• accuracy of voltage and current transformers,
• accuracy of protection (A/D converter),
• impact of parallel lines,
• type of fault,
• actual topology of the power system.

From figure 5, the ideal impedance reach of distance protection is marked with a dashed line. Distance protection “D” isolates all faults in this area with no intentional time delay. However, because of measurement errors, the impedance reach of the first zone is set in the range of 80% to 90% of the ideal impedance reach in order to avoid misoperation for faults close to or behind the opposite substation “B”.

For faults closer to substation “B”, protection is accomplished by adding a zone 2 distance protection (shown inside the green line). Zone 2 serves as protection beyond the zone 1 reach and provides backup protection for the part of the outgoing lines from substation “B”.  A time delay of 400 ms to 500 ms is usually applied in order to coordinate with the zone 1 distance protection “D2”.

To provide back-up protection for complete outgoing power line from substation “B”, a third zone of distance protection “D” (blue line) is configured. Impedance reach of zone 3 is set to cover the entire length of the power line. A time delay of 1 s to 1.1 s is usually applied in order to coordinate with the distance protection “D2” in substation “B”.

Again, for faults along the segment A-B and closer to substation “B”, the zone 2 distance protection of “D” will isolate the fault in 400 ms to 500 ms. However, from the perspective of the distance protection “D1”, the fault falls on its zone 1 distance protection and isolation is done with no intentional time delay.

Biased Differential Protection for Transformers

Learn More Biased Differential Protection for Transformers

Vector Group of Transformer Dyn1|Dyn11|Ynd1|Ynd11

Learn More Vector Group of Transformer Dyn1|Dyn11|Ynd1|Ynd11

Differential Current Pickup Setting Calculation

Learn More Differential Current Pickup Setting Calculation

Distance Protection Operational Characteristics

Generally, the following impedance characteristics are available but not all are used in practice:

• Circular Impedance Characteristic,
• Offset Mho,
• Polarized Mho,
• Reactance Characteristic,
• Resistance Characteristic,
• Directional Straight Line,
• Polygonal (quadrilateral).

These characteristics are shown in the following figure.

The tripping characteristic in figures 6a and 6c are typical for older electromechanical distance protection relays. As a basic type of impedance characteristic in the early years, a circle characteristic is shown in figure 6a (so-called the fast impedance) was used. The main disadvantage of this characteristic is no directionality and no compensation of arc occurrence.

The Offset Mho characteristic in figures 6b and 6c represent improved circle characteristics, where the center of circle is offset to the 1st quadrant. This solution removes the problem with directionality and misoperation on arcing faults. The disadvantage is the low voltage measurement element of distance protection for close faults (from point of distance protection). This results in inaccuracy in the measurements. This disadvantage, however, is eliminated using the so-called Polarized Mho characteristic, where polarized voltage is used for measurement purposes. Distance protection with polarized mho characteristic has 100% directionality, low sensitivity to arc resistance, or power swing in the network.

The tripping characteristic in figure 6d belongs to older static electronic protection relays.

Finally, the tripping characteristic in figures 6e and 6f are used in modern digital distance protection. It is a polygonal (quadrilateral) type that was created using a directional straight line, reactance, and resistance type of characteristic. The straight lines in most cases pass through the center of the coordinate system with the axis +R forming an angle, α = 115° to 125°, and α₂ = -15° to -25°. The Quadrilateral characteristic is bounded with straight lines parallel to the real and imaginary axis. For selectivity and security, the tripping characteristic is ordered to 5 zones each with an appropriate time delay. Zones 1 to 4 are set to protect towards the direction of the power line while zone 5 is set to protect towards the direction of the busbar (see Figure 7).

Factors Influencing Distance Protection

The principle of distance protection is fairly simple, however, its application can be quite complex. Protection engineers are dealing with a lot of factors that can affect the operation of distance protection. Some of them are briefly discussed below.

Fault Resistance

The proper function of distance protection during single-phase to ground faults is very important because 70-90% of all faults on power lines are these single-phase to ground short-circuits. Fault resistance is composed of three components: arc resistance, tower construction resistance, and tower footing resistance. In single in-feed faults, the fault resistance will increase the measured value of impedance. In case of in-feed faults from both sides, the situation is more complicated because of a higher voltage drop on the power line, which can cause measurement error also in the reactance part of the impedance and results in incorrect fault location by both opposite distance protections.

Arc Short-Circuit Faults

Because the arc is highly resistive, the voltage and current are in phase. The arc, therefore, appears as an additional resistance in the measured fault loop. Arc also contains higher-order harmonics and therefore the voltage deformation is also present. Arc affects the measuring accuracy only on extremely short lines. The influence of arc can be neglected due to the applied digital filtering techniques.

Parallel Lines

The measured impedance (fault loop) for parallel lines is lower. In the case of long lines, it is important to consider also the mutual coupling of zero-sequence between parallel lines.

Intermediate Infeed

The intermediate infeed will affect the measurement of the impedance (fault loop). Increased measured impedance means the protection is evaluating the fault virtually at a longer distance which will operate the time-delayed zone of protection. This effect arises because of an additional voltage drop on short-circuit impedance generated by current from intermediate infeed which causes an increase in voltage at the location of the distance protection relay. The greater is intermediate infeed current the greater is the error.

Non-symmetry of Power Lines

The power line impedance is determined by the geometry of towers, material, and cross-section of the conductors. The arrangement of the conductors at the towers causes a natural non-symmetry. To avoid large negative-sequence and earth-currents, long lines (> approx. 30- 40 km) are usually transposed. Also from the point of view of the zero-sequence impedance, the ground and soil characteristic has an additional influence.

Long Transmission Lines

The power transfer through long transmission lines will cause a phase shift of the system voltage. In the case of a short-circuit, the feeding EMFs, therefore, have different angles. The distance protection measures a reactance that is too small at the power sending end, and tends to overreach, while at the receiving end, the measured impedance is too large, i.e. a tendency to underreach exists. Therefore, the phase-shift between both ends of long transmission lines must be reflected in the configuration of distance protection to ensure correct operation.

Series Compensation

The power transfer capacity of long transmission lines can be increased using series compensation (capacitor banks). The negative reactance of the capacitors compensates for the longitudinal reactance of the line and reduces the transmission angle. The capacitor bank can be placed e.g. at the mid-point of the power line. The short-circuit current and voltage at the relay depends on the location of the series capacitor in relation to the relay location. The following fundamental phenomena can arise; reduced fault reactance, voltage, and current inversion. Each of them can significantly affect distance protection operation and must be considered in the set-up process of distance protection.

Conclusion

Distance protection is a very extensive aspect of power system protection. This article aims to give the reader a simple overview of distance protection fundamentals. This is essential as an introduction to the topic. A lot of interesting aspects were not involved and will be discussed in detail in the subsequent articles.

References

[1] Ziegler, G.: Numerical Distance Protection. Principles and Applications. Publicis Erlangen, Zweigniederlassung der PWW GmbH. Germany, 2011.

[2] Chladny, V.: Digital Protections in Electrical Networks. Technical University of Kosice, Department of Electric Power Engineering. Kosice, Slovakia, 2007.

[3] Maslo, K. et al.: Control and Stability of Power system. ČEPS and Energy Managers’ Association. Prague, Czech Republic, 2013.

[4] Procházka, M.: Distance Protection. West Bohemia University in Pilsen. Czech Republic, 2004.

Visit our community. Check us out on our LinkedIn, Facebook, and Instagram Pages.