Adaptive restarting method for LCC-HVDC based on principle of fault location by current injection

0 Introduction

The conventional high-voltage direct current (HVDC) transmission system has a long-distance transmission line and is generally located in harsh environments.This makes it vulnerable to fault on the line.According to statistics, among all the types of faults that occur in the HVDC transmission system, the probability of line faults is over 50%.Furthermore, the probability of temporary faults is 90% of this [1].When a fault in the DC line induces the protection action, the control system activates the DC line fault recovery sequence (DFRS) to reduce the system outage period.It uses the emergency phase shift operation of the line commutated converter (LCC) to rectify the temporary fault and attempt to restart [2].

The restarting method of the existing project adopts a fixed time delay and a fixed number of times to wait for the arc to extinguish.However, this method is non-selective.If the strategy acts on a permanent fault, the key equipment of the system would sustain the secondary impact.If it acts on a temporary fault with complex characteristics, the restart strategy may fail owing to an insufficient delay time.In addition, the conventional HVDC transmission system has a large capacity, repeatedly restarting it may adversely affect the stable operation of the system [3].To summarize, by studying the method of identifying the fault property after DC line protection and by realizing the adaptive restart of the LCC-HVDC system, the success rate of restart operation can be improved effectively, the secondary impact on the equipment can be reduced, and the system outage period can be shortened.

Scholars worldwide have conducted a few studies on this issue [4-6].Liang Chenguang studied the voltage dominant frequency characteristics under different fault properties in [4].Subsequently, he demonstrated the method for identifying fault properties that is based on the phasedomain voltage.[5] injected a voltage signal into a fault line according to the coordination control of a converter and DC circuit breaker (DCCB).They then identified the fault property using the traveling wave theory.[6] proposed a capacitor-clamping DCCB with adaptive reclosing capability.Among the above methods, [4] is based on traveling wave information, whereby its reliability is affected.[5] and [6] are based on the flexible-HVDC system installed with DCCB.Hence, it is difficult to be applied in the LCC-HVDC system without DCCB.Apart from this, methods proposed by [5] and [7-12] provide a new concept for solving the problem.In these methods, the power electronic equipment in the existing system is used to inject the detection signal in the process of the fault.However, existing studies are based mostly on flexible-HVDC systems with a voltage source converter (VSC) or DCCB, and few scholars have studied this method based on the LCC.Therefore, a method for injecting detection signals by using LCC is proposed in this article.Furthermore, a fault property method based on the non-traveling wave principle is proposed on the basis of the distributed parameter model of the line.After the HVDC line protection action, the detection signals injected into the fault pole by the LCC is used to identify the fault property.If it is determined to be a temporary fault, the restarting strategy is adopted.If it is determined to be a permanent fault, the system is blocked and, the result of fault location is sent.

Chapter 1 proposes the method for injecting detection signals and the response characteristics of the transmission line to the detection signal.The method for identifying fault properties is proposed in Chapter 2.Chapter 3 provides the flow chart of the adaptive restart method.Chapter 4 proposes the constraint conditions of the parameters of the injected detection signal in combination with the equipment performance and system limits.Chapter 5 presents the simulation results.Finally, the conclusions are presented in Chapter 6.

1 Injection method and response

1.1 Signal injection method based on LCC

The LCC based on a semi-controlled device has the following characteristics compared with the VSC based on a fully-controlled power electronic device: 1) The converter valves conduct unidirectionally.Therefore, the DC current flowing through the converter cannot alter its direction.2) The converter valves are incapable of turning off by themselves.These can be turned off only when the current through the converter valves is zero.Therefore, the turning-off process depends on the external circuit.3) The blocking of LCC is similar to directly cutting off the fault circuit.Thus, it has the capability to clear the temporary fault [13-16].

Limited by the above characteristics, when the LCC is used to inject the detection signals, the polarity of the current signals cannot be altered, and the frequency cannot be excessively high.Because the rectifier operates in the constant current/DC power mode in the steady state, the detection signals are selected as the current signals.

The scheme of signal injection and the control block diagram are shown in Fig.1.

Here, Iorder is the reference value of the DC current, idc is the measured value of the DC current, ierror is the deviation of the DC current, βrec is the trigger leading angle of the rectifier, αrec is the trigger angle of the rectifier, and idet is the detection signal command.

The control function [17-18] of constant current control is

where P and TI are the proportional coefficient and integration time constant, respectively, of the PI regulator.

Fig.1 and (1) show that by adjusting the DC-current reference value, the DC fault current can be limited and detection signals can be injected.

As shown in Fig.2, in the detection signal injection stage, a DC component is added to the characteristic frequency signal to ensure that the current direction does not vary.This is considering the unidirectional conduction characteristic of the thyristor.

where kinj is the coefficient of the detection signal command.It determines the amplitude of the injected signal.The coefficient is obtained by setting the PI regulator to eliminate the influence of the earth capacitance of the line, the fault resistance, and other factors.In addition, the feedback element helps improve the response speed when injected.ωdet and φdet are the angular frequency and initial phase, respectively, of the detection signal.||I˙detAC is the amplitude of the detection signal phasor.

The following needs to be explained: when the rectifier is switched to the detection signal injection control mode, the inverter side can adopt only the bypass control mode to control the voltage to zero.On the one hand, this control mode provides a clear boundary condition for identifying fault property.On the other hand, it prevents overvoltage on the line when the rectifier injects a current signal into the healthy line.

1.2 Response characteristics of transmission line

Consider the single-pole system as an example to facilitate analysis (see Fig.3).The distribution [19-20] of the detection signal and its response are shown in (5):

where are the voltage and current phasors at the characteristic frequencies measured at the head of the line and before the fault branch, respectively.and are the reflection coefficient and wave impedance, respectively, of the line.The specific calculation equation is

where r, l, c, and g are the resistance, inductance, capacitance, and conductance, respectively, per unit length of the line.

The fault branch with transition resistance shunts the DC current, and the voltage and current have the following relationship:

where and are the currents in the fault branch and in the line after fault branch, respectively.

Then, and are used to calculate the voltage and current at the end of the line according to (5):

Because the inverter is put into the bypass pair, and have the following relationship at the end of the line:

where d is the total length of the line.is the combined equivalent impedance of the boundary units at the inverter side (smoothing reactor, DC filter equivalent impedance, and valve on-state resistance).

By substituting (5), (7), and (8) into (9),

where are known parameters.and are measured after the detection signal is injected.The fault distance x and transient resistance Rf are unknown parameters.Thus, the fault information can be obtained by solving (10).

2 Principle of fault property identification

2.1 Equation of fault location

1) Positive pole grounding fault

In the bi-pole system, the positive and negative lines are coupled.Therefore, it is necessary to use the Karrenbauer transformation [21] to rewrite (5):

where m is the model subscript.m = 1 is a one-mode component, and m = 0 is a zero-mode component.Correspondingly, are calculated by the corresponding one-mode and zero-mode line parameters, respectively.

When a positive pole rounding fault occurs, the following relationship is valid for the fault branch:

where and are the one-mode and zero-mode components, respectively, of .

At the end of the line, the faulty pole is put into the bypass pair.However, the healthy pole operates normally.Therefore, the relationship is as follows:

By substituting (11), (12), and (13) into (14), we can obtain the equation of positive pole grounding fault:

2) Negative pole grounding fault

When a negative pole rounding fault occurs, the following relationship is valid for the fault branch:

The relationship between the voltage and current at the end of the line is

By substituting (11), (16), and (17) into (14), we can obtain the equation of negative pole grounding fault:

3) Positive to negative short circuit

When a positive to negative short circuit fault occurs, the following relationship is valid for the fault branch:

The relationship between the voltage and current at the end of the line is

By substituting (11), (19), and (20) into (14), we can obtain the equation of positive to negative short circuit fault:

(15), (18), and (21) are nonlinear complex equations.The equations for their real and imaginary parts are written.Then, the numerical solution method is used to determine x and Rf.

2.2 Identification of fault property

For a permanent fault, the above solutions would yield the actual fault distance and transition resistance.However, for a temporary fault, the fault circuit does not attain a form that includes the fault branch established above.Therefore, the above equations would have no solution within the limited solution range.

Considering the measurement error and calculation error, the limit range of the equation is

Therefore, the corresponding equation of different fault types is solved within the limited solution range.If there is no solution, it is identified as a temporary fault.If there is an effective solution, it is identified as a permanent fault, and the solution is the corresponding fault location.

3 Adaptive restarting method

3.1 Activation of the method

The method proposed in this paper is a part of fault clearing and system recovery.Therefore, the transmission line protection criterion [22-26] should be used as the startup criterion of this method.

After the startup criterion is activated, the control strategy is altered to fault current limiting control in order to limit the short-circuit current to zero.It should be noted that the additional current limiting strategy is applied in the main station of the system (generally, the converter station).At this time, the reference values of constant current control on the rectifier and inverter are 0 p.u.and -0.1 p.u., respectively.This is caused by the current margin control.However, based on the unidirectional conduction characteristics of the thyristor, the DC current would be limited to zero and would not become negative.

3.2 Flow chart

The flowchart of adaptive restart is shown in Fig.4.

When a fault occurs, the method is activated by the line protection criterion.Then, the control mode is switched from constant current control to fault current limiting control.Then, a delay of t ms is imposed to extinguish the arc of the temporary fault.

Subsequently, the rectifier’s controller of the faulty pole is switched to signal injection control, and the inverter bypass pair is controlled to realize detection signal injection.Then, the fault property is identified according to the solution of the corresponding equation.If there is no solution, it is determined to be a temporary fault.Otherwise, the delay time is increased, and a reassessment is performed.This is continued until the number of assessments attains the specified upper limit nset.Then, it is designated as a permanent fault.

The restart operation is executed only when it is identified as a temporary fault.When the fault is identified as a permanent one, the faulty pole is blocked, and the result of fault location is sent.

4 Constraint conditions of injected signal

4.1 Frequency

The selection of the detection signal frequency should comprehensively consider the factors such as the dominant characteristic frequency of the LCC and the sampling frequency of the measuring device.

1) Dominant characteristic frequency of LCC

The LCC-HVDC system in the project adopts dual 12-pulse wave converters.These complete 12 commutations in a cycle, i.e., the dominant characteristic frequency is 600 Hz [27].The injection effect of detection signals with identical amplitude and different frequencies under this limitation are shown in Fig.5.

As show in Fig.5, limited by the LCC, the quality of the detection signal deteriorates gradually as the frequency increases.The recommended detection signal frequency is less than 50 Hz.

2) Sampling frequency of measuring device

To accurately obtain the detection signals and their response signals, the frequency of the detection signals should be at most the sampling frequency of the measuring device.

To summarize, the recommended frequency of the detection signal is 20 Hz.

4.2 Amplitude

The limitation on the amplitude of detection signals is mainly considering the influence on the system and the accuracy of the measuring equipment [28].

When the fault occurs, the impact of the signal injection process on the system should be minimum.Therefore, the detection signal amplitude should be minimum.

However, the amplitude should not be excessively marginal.This is to ensure that the measuring equipment can detect it accurately.

To summarize, the recommended amplitude of the detection signal is 0.15Iorder.

4.3 Durations

The selection of injection durations mainly considers the length of line, delay of measuring equipment, and phasor extraction requirements [29-30].

1) Propagation time of detection signals

The rule of the traveling wave velocity is

According to (24), considering a transmission line of 1500 km as an example, the propagation time of detection signals in the line is approximately 7.469 ms.

2) Delay of measuring equipment

Considering the transmission delay of the measuring equipment and the delay characteristics of the digital filter, the injection time should be at least 50 ms.

3) Phasor extraction requirements

According to the sampling theory, a length that is two times that of the characteristic frequency is required to accurately extract the detection signal and its response signal.Furthermore, the detection signal is generally delayed for a cycle after injection to ensure stable injection and reliable detection.

To summarize, considering the characteristic frequency of 20 Hz as an example, the injection time is selected as 400 ms.

5 Simulation results

5.1 System parameters

The bi-polar LCC-HVDC transmission model based on PSCAD/EMTDC is shown in Fig.6.

As shown in Fig.6, each pole of the system adopts dual 12-pulse wave converters in a series structure.In normal operation, the rectifier adopts the constant DC power control mode, and the inverter adopts the constant voltage control mode.The specific performance parameters are shown in Table 1.

5.2 Single-pole grounding fault

1) Positive metallic grounding fault

As shown in Fig.7, the fault occurs at t = 100 ms, and the line protection acts after 20 ms.In addition, the converter is switched to current limiting control, to limit the fault current to zero.Then, a delay of 200 ms is imposed to ensure that the fault arc is extinguished.Subsequently, the rectifier is switched to the injection control mode, and the inverter bypass pair is controlled to complete the signal injection.

Fig.7 is the fault waveform of a permanent fault.The effective fault distance is solved according to the solution of (14).

In addition, the simulation results of the positive grounding and faults on different locations are shown in Table 2.

2) Negative metallic grounding fault

As shown in Fig.8, the fault occurs at t = 100 ms and is cleared at the fault current limiting stage.Therefore, there is no fault branch on the line during the stage of detectionsignal injection.Because the inverter adopts the operation of the bypass pair, no overvoltage would occur on the line during the current injection process.

The simulation results of the negative grounding and faults on different locations are shown in Table 3.

5.3 Inter-pole short circuit fault

As shown in Fig.9, when a short-circuit fault occurs between the positive and negative poles, the positive and negative converters simultaneously adopt fault current limiting control and detection signal injection control to complete fault identification.

The simulation results of the inter-pole short circuit and faults at different locations are shown in Table 4.

The simulation results show that the method can accurately identify temporary and permanent faults, and send the locations of permanent faults.The relative error is within 1%.

5.4 Effect of transition resistance

The solution results of the fault location method under different transition resistances are shown in Table 5.

Theoretically, the method considers the shutting effect of the line after the fault branch, and the equations include the transition resistance.Therefore, it is not affected by the transition resistance.However, the transition resistance would also affect the measurement accuracy of the signal and the extraction of the phasor.The simulation results show that the method can accurately identify the fault property within 500 Ω.

6 Conclusion

A method for injecting detection signals after the occurrence of a fault is proposed.It utilizes the high controllability of the LCC.The method can inject current detection signals to the faulty line to locate the fault and identify the fault property.The theoretical analysis and simulation results show the following:

1) Considering the unidirectional conduction characteristic of the thyristor, a DC component is added to the characteristic frequency signal to ensure that the current direction does not vary, during the detection signal injection stage.

2) On the basis of the conventional fault location method, the distributed parameter model is used to construct the equivalent circuit of the entire system including the line after the fault branch.Then, the fault solution equation is obtained.It overcomes the influence of the transition resistance and increases the accuracy.

3) The limitation for the frequency, amplitude, and durations of the detection signal are proposed by combining the operation principle of LCC, performance of measuring equipment, influence on the system, and phasor extraction requirements.

The simulation verifies the effectiveness of the method.It can accurately locate a fault within a resistance of 500 Ω, and the maximum relative error is less than 1%.Furthermore, it can identify the fault property and then, decide whether to restart.This realizes the adaptive restarting of the LCC-HVDC system.

Acknowledgements

This project is supported by Science and Technology Project of State Grid Corporation of China (52094020006U), National Natural Science Foundation of China (NSFC) (52061635105), and China Postdoctoral Science Foundation (2021M692525).

Declaration of Competing Interest

We declare that we have no conflict of interest.