Distance protection of EHV long transmission lines considering compensation degree of shunt reactor

1 Introduction

Distance protection is the preferred backup protection for extra-high voltage(EHV)transmission lines,and it occupies an extremely important position in EHV transmission line protection.Distance protection has many advantages,such as weak influence from power system operation modes and structural change,longer protection domain for longdistance transmission line,and high durability against faults.Therefore,distance protection is the most important and widely used protection scheme in complex high-voltage power networks.

The continuous development of distance protection devices is accompanied by the development of power system and hardware technology.Distance protection devices have been available since the early 20 s of the last century.These devices have experienced several stages of development,resulting in electromechanical,rectifier,induction,transistor,integrated circuit,and microprocessorbased devices.With the rapid development of advanced technology in recent years,the cycle of relay protection devices is becoming increasingly short.Some integrated and microprocessor-based protection devices have gradually replaced rectifier and transistor protection devices,and become mainstream products.In grids with voltage levels of 220 kV and above,microprocessor-based distance protection devices occupy the leading position.

With the continuous development and construction of EHV transmission lines in China,it is of great significance to study relay protection related to EHV transmission lines.For relatively long EHV transmission lines,a major problem in existing distance protection is that the measured impedance of impedance relay may not be proportional to the fault distance.This study analyzed this problem in detail.Through theoretical analysis and simulation,the adaptability of distance protection and the effect of the shunt reactor on distance protection were verified [1-5].

2 Distance protection principle

The traditional distance protection principle is to calculate the impedance value of the fault line through the voltage and current of the protection side and compare the value with the setting line impedance to judge the fault situation.Depending on its characteristics,impedance relay can be classified into several types,such as linear,circular,and quadrilateral impedance relay [5-6].

(1)Linear impedance relay

Linear impedance relay can be mainly divided into three types:resistance relay,reactance relay,and phase limited relay.It is characterized by a straight line in the impedance complex plane.The action of resistance relay depends only on the measured impedance values,and the action of reactance relay depends only on the reactance component of measured impedance.Although the linear characteristic criterion is simple,it is non-directional and does not reflect the actual change of measured impedance accurately.Accordingly,the error could be very large if only resistance and reactance values are used as analysis parameters.Therefore,it is not ideal in practical application.

(2)Circle impedance relay

There are several types of circular impedance relay:full impedance circle,directional impedance circle,and offset impedance circle.It is the most widely used impedance relay in traditional relay protection.In fact,the action characteristic of impedance relay is expanded to a circle,making the manufacture and debugging of relay more convenient,and simplifying the wiring of relay.Among them,the full impedance circle characteristic is non-directional,whereas the directional impedance circle has a voltage dead zone.The characteristic of the offset impedance circle is the combination of the former two,with better characteristics and more applications.

(3)Quadrilateral impedance relay

The quadrilateral impedance relay synthesizes the linear characteristics of the resistance reactance type and takes into account the directivity of the impedance.It is a type of impedance relay that reflects fault measured impedance boundary more accurately,and has high transition resistance.However,it is seldom used in traditional relay protection because of its complex structure and difficult realization.Nevertheless,with the advent of microprocessorbased protection,it has been widely used in microprocessorbased distance protection.

3 EHV distance protection problem

The working principle of distance protection is to use the characteristics of simultaneous changes of shortcircuited voltage and current and the ratio of measured voltage to measured current to reflect the distance from the fault point to the relay location.The theoretical premise of distance protection is that the measured impedance should be proportional to the fault distance.There are two main problems for distance protection in EHV transmission lines.

When the length of the EHV transmission line is relatively long,the measured impedance of the line may not be proportional to its length,thus offsetting the premise of distance protection [7-9].This problem should be considered in conjunction with the allowable length of the corresponding EHV transmission lines.For reasons of limiting overvoltage,single-segment EHV transmission lines should not exceed 500 km and EHV transmission lines with switching stations in the middle should not exceed 2×350 km.Accordingly,the possible maximum line length is 500 km,and the maximum protection length of zone I of the distance protection may be about 400 km.

There are rich variety of fault electrical quantities in the EHV transmission lines’ transient process The slowly decaying non-periodic component and the high frequency component close to the fundamental frequency significantly affects the transient process of the impedance relay.However,the two fault components are directly related to the line length,back source impedance,fault type,and fault time.This problem is mainly attributed to the filtering algorithm.In terms of the four-stage distance protection,which is backup protection,because of the low action speed requirement,the reactor filter and full wave Fourier algorithm should have a good filtering effect after the preceding low-pass filters.Therefore,the measured impedance in this paper refers to the steady state measured impedance of the impedance relay [10-11].

4 Theoretical analysis of measured impedance for distance protection

4.1 Three-phase fault measured impedance

The structure of a transmission line is shown in Fig.1.The wave characteristic equations of EHV transmission lines are as follows(high voltage shunt reactor is not considered):

Fig.1 Structure of a transmission line

R1,L1,G1,C1 are the resistance,inductance,conductance,and capacitance of the unit length of the transmission line,respectively.

Where:are the voltage and current corresponding to the m side; are the voltage and current corresponding to the n side; is transmission constant of the transmission line; is wave impedance of the corresponding transmission line; l is the transmission distance.

When n is a short-circuit point(taking three-phase fault as an example),0=,at this point,the measured impedance of circuit breaker 1 is:

However,according to the assumption of conventional distance relay,the measured impedance is proportional to the length of the line.Then the measured impedance of circuit breaker 1 should be:

When the distance l between the short circuit point and the protection measuring point is long,the error of Zm to Zm1 is likely large.With increasing values of l,the calculated impedance value according to the distributed parameter is larger than the calculated impedance value in accordance with the linear relationship [12-13].

In fact,shunt reactors of certain capacity are installed on EHV transmission lines in order to limit overvoltage.In general,both ends of a non-segmented line should have high voltage shunt reactors.As shown in Fig.1,the voltage and current of the line at the distance of l should meet:

When n is a short circuit point,0=,Then the measured impedance is obtained by(4).

According to Equation(5),after the application of high voltage shunt reactors,the value of the measured impedance decreases slightly compared with Zm1.The magnitude of the measured impedance is related to the magnitude of the high voltage shunt reactors XL1 at the head side,which is further related to the compensation degree of the high voltage shunt reactors.For an EHV transmission line with high voltage shunt reactors installed on both terminals,the compensation degree of high voltage shunt reactors is high,generally about 80%-90%.Thus,the high voltage shunt reactor on a single side is considered to account for 40%-50%.In this study,the 1000 kV line of Southeast Shanxi-Nanyang was taken as an example.This line has a length of 500 km.The curves in Fig.2 show the relationship between Zm1,Zm,Zm2 and the length of lines.

Fig.2 Value change for measured impedance

As shown in Fig.2,with the application of the high voltage shunt reactor,the value of Zm2-Zm became less than that of Zm1-Zm.In other words,the high voltage shunt reactor rendered the measured impedance more linear.At the same time,the value of Zm2 is directly related to the compensation degree of XL1.When the compensation degree is large and the line is long,the value of Zm2 could be smaller than that of Zm,but the error is not obvious.

Fig.3 shows the relationship between the phase angle of Zm1,Zm,Zm2 and the length of lines.Irrespective of the condition considered,the phase angle change of measured impedance is not obvious,which is no need to be considered in the analysis.

Fig.3 Value change for measured impedance’s phase angle

From the analysis of the value and phase angle of Zm1,Zm,and Zm2,the connection of high voltage shunt reactors makes the measured impedance Zm2 more approximate to the impedance Zm obtained in the linear relationship.The premise of distance protection is still valid.

4.2 Three phase asymmetrical fault measured impedance

Based on the symmetrical component method,sequence components are generated when asymmetric faults occur.When the line is not long,the distributed parameter characteristics of the system cannot be considered.Thus,the measured voltage of the m side is(taking phase A as an example):

where,Zm =(R1 + jωL1)l.Similar to(3),other parameters are as follows:

Similarly,the measured voltage of the m side of the remaining two phases are

Conventional distance relays are based on(6)-(10),comprising 3 grounding relays and 3 phase relays.However,when the line is long,the distributed parameter characteristics are considered and the discussion of different faults is as follows:

4.2.1 Two-phase short circuit fault

Suppose that there is a short circuit fault between phase B and C,and the voltage of the fault point is equal,that is Then(11)can be obtained:

In this manner,the measured impedance of the phase to phase relay actually becomes Z1,the same impedance as the three-phase fault.Thus,the discussion on the measured impedance of phase to phase fault is the discussion of Zm1,Zm,and Zm2,and its conclusion is the same as that of the three-phase fault.The impedance measured by phase to phase relay can be considered to be proportional to the length of the line when the line is long.

4.2.2 Two phase grounding fault

Suppose that grounding fault occurs between phases B and C,then The measured impedance of phase to phase relay between B and C remains Z1.However,the measured impedance of grounding relays in phases B and C differs from Z1.In order to check the error of measured impedance without considering distributed parameter,for relays,K = Km.If the line does not have high voltage shunt reactors:

When the line is connected with a shunt reactor:

4.2.3 Single phase grounding fault

Suppose there is a metallic grounding fault in phase A,The measured impedance of the grounding relay can also be obtained.

When there is no high voltage shunt reactor:

When there is a high voltage shunt reactor:

The analysis of the grounding relay of single-phase grounding fault and two-phase grounding fault shows that the measured impedance of the grounding relay adds an additional measured impedance on the basis of the measured impedance of the three-phase fault.Taking metallic short circuit fault of phase A as an example,additional measured impedance ΔZmA_Ag is:

In this case,k0-k1 is k01-k11 when there is no high voltage shunt reactor,and it becomes k02-k12 in the presence of the reactor.K is Km1 and Km2,and Z is Zm1 and Zm2.In addition to the zero sequence voltage of the fault point and the zero sequence current of the measuring point,ΔZmA_Ag largely depends on the magnitude and phase angle of k0-k1 and K-Km.

5 Simulation analysis

5.1 Simulation analysis of three-phase symmetrical fault

The three-phase fault of different points between 315 km and 340 km from Southeast Shanxi was simulated,and the EMTDC simulation model was established for the Southeast Shanxi-Nanyang line(shown in Fig.4).The measured impedance of Southeast Shanxi side was calculated using MATLAB.

Fig.4 Transmission line model for Southeast Shanxi-Nanyang

The fault can be divided into two cases:with or without high voltage shunt reactor.The reactor filter and full wave Fourier algorithm were applied in MATLAB.The cut-off frequency of low-pass filter was set to 300 Hz in EMTDC.The simulation results and the previous theoretical calculation results are shown in Table 1.According to the results,when the line has a high voltage shunt reactor,the measured impedance is very close to the impedance value calculated in accordance with the linear relationship.The error lied within an acceptable range,and thus,the conclusion of theoretical analysis in section 4.1 was verified.

5.2 Simulation analysis of three-phase asymmetrical fault

A 500 km 1000 kV EHV transmission line was built using EMTDC/PSCAD.Several kinds of faults were set within the protection location between 370 km and 420 km to simulate the measured impedance.Firstly,the condition without shunt reactor in the protection side was simulated.The simulation results are shown in Table 2.When the protection side featured a shunt reactor,the compensation degree was set to 0.4 according to the line length of 500 km.The simulation results are shown in Table 3.From the result in Tables 2 and 3,when the phase-to-phase fault and the two-phase grounding fault occur,the measured impedance of the corresponding phase relay is equal to that of the threephase fault.Therefore,the result of the aforementioned theoretical analysis is verified.When the single-phase grounding fault and the two-phase grounding fault occur,the measured impedance of the corresponding grounding distance relay is less than that of the three-phase grounding fault.However,the simulation results show that the gap is very small,particularly for the line with shunt reactor.This is also consistent with the above theoretical analysis results.When the line is long,if the line is not equipped with shunt reactors,the measured impedance will be greater than the measured impedance Zm obtained considering the linear relationship.If the shunt reactor is equipped,this situation will be greatly improved,and the measured impedance will be very close to Zm.According to the results of theoretical analysis and simulation,the premise of distance protection in asymmetric faults is still valid.

Table 1 Impedance for Southeast Shanxi-Nanyang transmission line

Zm2/ohm(Simulation value)315 86.415 80.871 83.084 87 83 320 87.9 82.17 84.403 89 83.7 325 89.392 83.472 85.722 91.2 85.1 l Zm /km Zm1/ohm(Calculated value)Zm2/ohm(Calculated value)/ohm Zm1/ohm(Simulation value)

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l Zm2/ohm(Calculated value)/ohm Zm1/ohm(Calculated value)/km Zm1/ohm(Simulation value)Zm Zm2/ohm(Simulation value)330 90.888 84.776 87.04 92.6 86.3 335 92.391 86.082 88.359 94.5 87.8 340 93.9 87.39 89.678 96.1 89.3

Table 2 Impedance for different types of faults without shunt reactor

l Zm /km fault ABC Zma /ohm fault BC Zmbc /ohm fault Zmbc /ohm fault Zmb /ohm fault BCG Zmc /ohm fault AG Zma /ohm /ohm 370 106 106 106 104.5 103.5 102.5 97.591 380 109.3 109.25 109.3 107.7 106.3 105.8 100.23 390 112.7 112.7 112.6 111.1 109.5 109 102.87 400 116.2 116.3 116.2 114.3 112.7 112.1 105.5 410 119.8 119.7 119.7 117.8 116.2 115.2 108.14 420 123.3 123.3 123.3 121.3 119.5 118.6 110.78 430 126.8 126.8 126.9 125 123 122.1 113.42

Table 3 Impedance for different types of faults with shunt reactor

l Zm /km fault ABC Zma /ohm fault BC Zmbc /ohm fault Zmbc /ohm fault Zmb /ohm fault BCG Zmc /ohm fault AG Zma /ohm /ohm 370 96.9 96.9 96.9 96.7 96.2 96.3 97.591 380 99.8 99.7 99.8 99.6 99.1 99 100.23 390 102.6 102.6 102.6 102.3 101.8 101.7 102.87 400 105.45 105.5 105.5 105.1 104.5 104.5 105.5 410 108.3 108.3 108.3 108 107.3 107.2 108.14 420 111.3 111.3 111.3 110.8 110 109.8 110.78 430 114.2 114.2 114.2 113.8 112.8 112 113.42

6 Conclusion

This paper presents the principle and current situation of distance protection.The practical problems encountered in the application of distance protection in EHV long transmission lines were specified,and actual theoretical analysis was performed according to the protection principle.The problem of the measured impedance not being proportional to the fault distance in long EHV transmission lines was analyzed in detail.The theoretical results and simulation analysis verify that the EHV distance protection shows accurate response regardless of the existence of shunt reactors for different fault conditions through the change of compensation degree.Overall,the method was proved to be feasible and effective.