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Non-communication based protection scheme using transient harmonics for multi-terminal HVDC networks
Electrical Power and Energy Systems 127 (2021) 106636
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International Journal of Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Non-communication based protection scheme using transient harmonics for multi-terminal HVDC networks Muhammad Haroon Nadeem a, b, Xiaodong Zheng a, b, *, Nengling Tai a, b, Mehr Gul a, b, Moduo Yu a, b, c, Yangyang He a, b a b c
Key Laboratory of Control of Power Transmission and Conversion, Shanghai Jiao Tong University, Ministry of Education, 200240 Shanghai, China Department of Electrical Engineering, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, 200240 Shanghai, China School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: Fault detection Fault classification HVDC protection Protection unit Multi-terminal HVDC
DC faults render hindrance in the flourishment of multi-terminal flexible high voltage DC (MTDC). The fault current rises rapidly, making a sharp peak, however, under a specific fault instance, isolation of the whole network is an impractical solution. For effective reliability of the whole network, rapid and accurate fault detection and selection of faulty pole is mandatory. A fault detection scheme is proposed in this paper to draw transient current harmonics from initial fault current by utilizing Discrete Fourier transform (DFT). Current harmonics employed the fault detection promptly at the converter stations without communication among the MTDC networks. Additionally, the proposed scheme can discriminate internal from external fault by tracking the number of pulse count in a specific interval of the harmonics of the voltage. In this way, the reliability of the protection scheme enhances and escape from the mal-operation of the circuit breaker. For four terminal MTDC transmission network, the simulations are carried out in PSCAD to authenticate the implementation of the proposed scheme and protection units (PU) are utilized for the isolation of faulty part with the combination of hybrid circuit breaker (HCB). A keen analysis of test results authenticated the ability of the proposed scheme to rapidly detect the faults with high precision, along with efficient classification of the nature of faults. Further more, consequential results provide a strong evidence for its key role in handling a multitude of MTDC faults.
1. Introduction
self-blocking strategy to avoid any damage, in case the protection fails to initiate timely or limit the fault current [10]. Hence, the protection of MTDC network is significant to guarantee security along with the reli ability of the entire system [11–14]. Consequently, if the protection operates prior to the blockade of the converters, the power losses in the entire network can be avoided, thereby emphasizing the strict condition of operating speed. To meet the optimal speed requirement, protection schemes based on single-end measurement appears as an appropriate choice for rapid protection, in contrast to the pilot protection which is reliable on the physical communication system and causes unavoidable time delay [15]. The prominence of several protection schemes for MTDC have been investigated and recognized in [15–23]. The most common method of fault detection is the current differential scheme [15]. However, this method is not suitable for enormous transmission networks as a time lag originates hindrances in repossessing the fault detection signal. More over, the signal becomes discrepant distributed owing to the distributed
With the advancements in renewable energy integration, longdistance power transmission for reliable system operation, voltagesource converter based high-voltage DC (VSC-HVDC) is rendered to have a very significant part in the forthcoming power grid [1–3]. In this instance of large-scale transmission system, the transmission line crosses rough territory and faces extreme atmospheric conditions [4,5], raising the probability of fault occurrence. Hence, reliable and efficient pro tection scheme is significant for the multi-terminal HVDC transmission networks [6]. For the protection of MTDC, speed is prioritized amongst the essential requisites [7,8]. Owing to the low inertia coefficient of multiterminal HVDC network, the fault occurring in the transmission line can cause rapid escalation of the current, which could affect the entire network within milliseconds [9]. The insulated-gate bipolar transistor (IGBT) based converters, that are sensitive to overcurrent, can initiate a
* Corresponding author. E-mail addresses: [email protected] (M.H. Nadeem), [email protected] (X. Zheng). https://doi.org/10.1016/j.ijepes.2020.106636 Received 22 May 2020; Received in revised form 9 September 2020; Accepted 3 November 2020 Available online 11 December 2020 0142-0615/© 2020 Elsevier Ltd. All rights reserved.
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line capacitance [16]. For effective fault detection in the MTDC network, a dignified approach is used by multiple measurements at numerous locations along the line [15]. However, rapid and reliable communica tion across the abundant measuring points can limit its realistic imple mentation. Traveling wave protection is a well-recognized single-end measurement protection, which, by means of recognizing the traveling wave front, can perform fault detection within milliseconds, and is implicated expansively in LCC-HVDC for a long time [17,18]. Though, its sensitivity to fault resistance can cause mal-operation in case of highresistance fault [19]. In addition, this drawback will become prominent while applying in MTDC, as the inverter of MTDC could provide current to the faulted line constantly, thereby increasing the influence of fault resistance. Some issues are rectified by utilizing the current data signal with the approach of non-communication principle [20]. However, the inclusion of a shunt capacitor may raise the cost with affection of the system parameter. A non-communication based protection scheme can be employed by using the transient components for fault identification [21]. A protection scheme for two terminal VSC-HVDC based on tran sient harmonic is presented in [22,23]. Fault detection done with the single frequency element would undoubtedly get influenced by filter capacitors and signal noise. Therefore, expansive investigation is required for adapting harmonic components to detect a fault and develop a novel protection for MTDC networks. A fault detection scheme is proposed in this paper to separate current harmonics from the fault current by utilizing DFT for the VSC-MTDC network. Fault detection is quickly retained by current harmonics at each terminal of converter stations of the MTDC networks without physical communication among the terminals. The modulated scheme is also adept of discriminating external fault from internal by counting the number of pulse count in a specific time interval of 6 ms of the voltage harmonics. In this way, the reliability of the protection scheme enhances and escape from the mal-operation of the circuit breaker. Furthermore, selection of faulty pole identification criterion is erected, based on voltage drop of positive and negative poles of the same line. By imple menting the proposed protection scheme, DC line faults have been identified and discriminated with greater reliability. The validation of the proposed protection scheme has been done through PSCAD by simulating a four terminal MTDC transmission network. A detailed exploration of test results affirms the ability of the proposed scheme to accurately detect faults, as well as promptly classify internal and external faults. In succession to classifying, the scheme adeptly isolates the faulty section from the healthy system by utilizing the HCB. Furthermore, its implications also conferred auspicious performance for various MTDC faults. The remaining paper is structured as: Section 2 presents the calcu lation of fault current in multi-terminal HVDC network. The proposed scheme for fault detection and classification is illuminated in Section 3. To analyse the performance of proposed scheme, a simulation model is explained in Section 4. In Section 5, the viability and robustness of the proposed protection is validated in a four-terminal VSC-HVDC system. The discovered results indicate the protection to identify and discrimi nate faults with single-end measurement, accurately. Finally, the con clusions are drawn in Section 6.
considered as a current-controlled source due to the small drop of DClink voltage, and for the analysis of fault current contribution, the AC side contribution can be neglected [12]. Hence, in the following anal ysis, the VSC can merely be considered as the parallel coupled capacitors. A three terminal MTDC network having a solid LG fault in the center of line 13 is expressed in Fig. 1. At the DC side, inductors are normally used to lessen the DC current ripples, which are having the same influ ence as the smoothing reactor in the LCC-HVDC. For controlling the rapid rate of change of DC fault current, large inductor with hundreds of mH is applied in MTDC system so that DC CD can interrupt the fault current in several milliseconds. To expedite the fault current analysis, DC transmission lines are aligned as a series RL circuits as illustrated in Fig. 1. Lab and Rab are the inductance and resistance of the Line ‘ab’ correspondingly. Similarly, the inductance and resistance to the fault location from side ‘a’ is expressed with La0 and Ra0 respectively. LLab is the DC auxiliary inductor at the ‘a’ side of branch ab. Meanwhile, Cn represents the DC-link capacitance of VSCn. 2.2. Short-circuit DC current calculation In the following analysis, the steady-state element can be ignored in comparison to the fault current element, since only the faulty part of DCline current is analysed. The calculation of DC current component can be done by using only a single fault DC voltage source Vo with a passive network according to the superposition principle. For simplification of the analysis, the progression of DC line current from healthy to faulty line is considered positive. For evaluating the fault current component, Laplace transformation of the stated three-terminal DC network is exhibited in Fig. 2. The capacitor voltage of terminal 2 is obtained as follows: V2 (s) =
I12 (s) + I23 (s) sC2
(1)
The voltage of the DC-link capacitor at terminal 2 is represented with V2. The DC current in the lines 12 and 23 are expressed with I12 and I23 respectively. In order to further simplify the analysis, the capacitor C2 is divided into two independent branches to supply each branch (line 12 and line 23) independently. The analogous impedance Zc12, Zc23 should be fulfilled as follows, ZC12 (s) =
[I12 (s) + I23 (s)]/sC2 I12 (s)
(2)
ZC23 (s) =
[I12 (s) + I23 (s)]/sC2 I23 (s)
(3)
The proportional coefficient of the DC current of the line 12 and 23 can be defined as follows:
2. Calculation of fault current in multi-terminal HVDC network 2.1. MTDC network topology for calculation of fault current In the instance of a DC fault in a MTDC network, the faulted DC line should precisely be localized in first few milliseconds for the operation of DC circuit breaker (CB). Therefore, power system protection design has the main concern within the initial period of the DC current after the occurrence of fault. In case where a line-to-ground (LG) fault emerges on the DC side of the system, the parallel linked capacitors of converter station initiate to discharge rapidly with a sharp peak. Meanwhile, during the initial small interval after fault, the VSC can still be
Fig. 1. Layout of a typical RLC three terminal DC network. 2
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the fault interval by considering the DFT. Subsequently, for DC fault analysis in the high-frequency zone, the entire impedance of the DC link Capacitor at terminal 1 (Zf10) could be considered as parallel connected DC-link capacitor, that is inscribed as, ( ) 1 ′ Zf 10 (s) = (9) sC1 Similarly, DC fault line current in the high-frequency part of Laplace Transform is formulated as, ′
I10 =
I23 (s) I12 (s)
3. Proposed protection scheme
(4)
A novel protection scheme based on transient DC fault current har monics for MTDC network is proposed in this section, which is appli cable for the identification of fault in the system. Calculating the number of pulse count in a specific time period of voltage harmonics is utilized to distinguish external from internal fault without communication be tween the terminals. In the proposed scheme, rapid identification of the faulted line is done through single-end signal.
where k is associated with fault location on line 12, and the measure ment of the transmission line 13 and 23. The supplementary inductance of the inductor is defined as correlated proportional coefficient. The capacitor C2, which is divided into two independent branches desig nated as C12 and C23 to supply line 12 and line 23 respectively and can be exhibited as follows, C12 =
C2 (k + 1)
(5)
C23 =
C3 (1/k + 1)
(6)
3.1. Fault identification Remarkable changes in current and voltage are observed with fault occurrence in the system. Characteristic DC fault current harmonics are utilized to identify these variations. The sample of current signal is considered x(t) at an arbitrary time ‘t’, having the fundamental frequency measured in radians per second.
Consequently, the multi-terminal DC network could be revealed into a couple of sovereign radial networks expressed in Fig. 3 to calculate the component of fault current in the faulted line I10, which could be derived as, I10 =
s(L10 (
Zf 10 (s) =
V0 /s + LL13 ) + R10 + Zf 10 (s) s(L12 + LL12 + LL13 ) + R12 + 1/sC12 s(L12 + LL12 + LL13 ) + R12 + 1/sC1 + 1/sC12
x(t) = Xc cosωo t + Xs sinωo t
(7) )
( ×
1 sC1
(10)
Equation (10) demonstrates the equivalent model of VSC based multi-terminal DC network for the calculation of fault current during initial small interval following a fault occurrence, which is based on DFT and utilized the current harmonics for the fault identification.
Fig. 2. Laplace transformation for evaluating the fault current component of a three terminal DC network.
k =
V0 /s s(L10 + LL13 ) + R10 + 1/sC1
(11)
where Xc and Xs are real numbers, x(t) is the instantaneous value of the current signal. The rectangular configuration of DFT is utilized for splitting a periodic signal into harmonics and fundamental of the signal for calculation of approximate expression of initial short period after fault as explained in Section 2. The fundamental frequency elements for K samples per cycle demonstrated and the hth harmonics obtained as [24]. The equations are derived to determine the newest data window by utilizing the previous data window as derived in Ref. [13] and comprehensive form is expressed in (12) and (13). The effectiveness of the equations is for any interval of the window.
) (8)
where Zf10 is the total impedance of DC-link capacitor of terminal 1, which has parallel connection with branch line 12. The mathematical expression derived from the Inverse Laplace Transform of (8) is difficult to express I10. Hence, approximate expres sion of I10 can be implemented to evaluate the initial fault current during
Xc’(new) = Xc’(old) + [xnew − xold ]cos(hθ)
(12)
Xs’(new) = Xc’(old) + [xnew − xold ]sin(hθ)
(13)
xnew is the newest sample and xold is the previous sample, accurately one complete cycle prior. The setting value of the transient current harmonics may be recog nized by utilizing the values of different parameters IH.set = IH.int × kline × kF.r × kint
(14)
where IH.set is the setting value of transient harmonic current protection scheme and IH.int is the initial value of transient harmonics current. Moreover, kline and kF.r are correction coefficient reflecting the length of transmission line and the fault resistance correspondingly. kint is expressing the correction coefficient deliberating the change of IH.int . For calculating IH.set , the values of the correction coefficient in the simulations are given as: kint = 0.5 kline = 1 kF.r = 0.2IH.int = 0.1 p.u. Thus, the setting value can be calculated. The characteristic harmonics
Fig. 3. Two independent radial network derived from the meshed MTDC network. 3
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current can be computed at the protection units close to the terminals and the frequency of sampling is taken 6000 Hz in this paper. 3.2. Fault classification Power conversion harmonics are generated in the system with inte gral multiple frequencies of the fundamental frequency during the power conversion process at converter stations [22]. The output voltage of a 12-pulse converter mainly produced 12th harmonics in the system. Internal and external faults can be discriminated by these harmonics by counting the number of pulse count in a specific time interval [13]. For external faults, harmonics are absorbed by the DC capacitors before arriving at the fault detection relay (FDR) as aforementioned. Contrarily, on occurrence of internal faults, the FDR easily recognizes the harmonics with the number of pulse count in one cycle. Considering the 12 pulse harmonics, pulse count in one cycle should be 12 for in ternal fault. The proposed scheme used the window length of 6 ms, as for 12th harmonic system, three peaks appeared in specific time frame as can be seen in Fig. 4. The height of the window depends on the average value of the ripples of the harmonics. NI(N.set) = NI(N.int)
(15)
NI(N.set) is indicating the number of pulses in sampling window and NI(N.int) the number of initial harmonics pulse in 6 ms correspondingly. For the 12th harmonics, the number of pulse count is 3 for the proposed topology. The protection algorithm is presented in Fig. 5. { IH.M ⩾ IH.set (16) NV(N.M) ⩽ NV(N.set) {
IH.M ⩾ IH.set NV(N.M) ⩾ NV(N.set)
(17)
According to the Eq. (16), an external fault occurs, and if Eq. (17) is satisfied, an internal fault arises in the system, subsequently tripping the HCB without communication among the terminals. 4. Simulation model A four-terminal bipolar VSC-HVDC test system is simulated in PSCAD, as depicted in Fig. 6 for analyzing the performance of proposed scheme under numerous fault conditions. MTDC system, protection unit and cable models are comprehensively explicated as follows.
Fig. 5. Algorithm for the fault detection and classification scheme.
utilized in [13]. Power generation with the off-shore energy is attached with Terminal 1 and 4, however an AC side grid is connected with Terminal 2 and 3. Evaluation of the effectiveness of proposed scheme has been done with the different (internal and external) faults. F1 and F2 faults are indicating internal and external faults respectively. Addi tionally, VSC converters are connected with DC link capacitors and designed on ±300 kV bipolar half-bridge topology. At respective ter minals, capacitors are grounded independent of each other. An appro priate model of VSC converter is separately represented in Fig. 6.
4.1. System model Four terminals MTDC network is modeled in PSCAD, which is illus trated in Fig. 6 and the system parameters are listed in Table 1, as
4.2. Protection unit A novel protection unit having HCB with the combination of FDR is designed for the required protection operations. The basic HCB is not suitable for most of the protection schemes as most of the protection schemes required some delay to verify the fault. However, the fault current rising-rate is too high in the beginning of the fault, which may surpass the maximum breaking capacity of the HCB. By enlarging the value of current limiting reactance (CLR), the aforementioned problem can be solved, but it will influence the dynamic aspects of the DC transmission network. The practicability of upcoming DC networks re lies mainly on their abilities to survive during DC faults.
Fig. 4. Response of harmonic current indicating the sampling window. 4
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Fig. 6. Structure of MTDC network and VSC-based HVDC converter.
absorption path, which protects IGBT from over voltage. Furthermore, the configuration of the SB also has three paths: path 1 is the low loss branch for the normal operation, having the LCS and UFMD. Path 2 contains the capacitor bank, and path 3 comprises of the energy ab sorption path. The SBs do not have FDR and can operate with the fault current threshold level. The control logic of protection unit for MTDC network is divided into three stages; normal working stage, fault condition/fault energy reduction stage and fault clearance stage, which are categorized as 0 ≤ t ≤ t1, t1 ≤ t ≤ t2, and t > t2 respectively and listed in Fig. 8. During normal working conditions, whole current passes via nominal
Table 1 Parameters of implementation model. System parameters
Values
Rated power of converter AC voltage DC voltage X/R in AC network Reactance of Transformer leakage Entire resistance of converter diodes Phase reactor of converter
1200 MW 660 kV ±300 kV 10 0.2 p.u. 0.006 p.u. 0.06 p.u.
The proposed protection unit consists of a main breaker (MB) and secondary breakers (SB) with the combination of reactance, as depicted in Fig. 7. The MB comprises of three paths: nominal path is a low loss path containing ultrafast mechanical disconnector (UFMD), load com mutation switch (LCS) and FDR. The second path is the commutation path which has the MB, and the fault current is diverted from nominal path to commutation path as the fault has been verified, and commu tation branch can isolate the fault rapidly. The third path is the energy
Fig. 8. Protection unit operating stages.
Fig. 7. Protection unit model. 5
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path of the main breaker blocking all the IGBT’s in the commutation path. Furthermore, for the secondary breakers, the current flows in first path having low power losses as it only contains the UFMD and LCS. Hence, the current in the protection unit flows through the low impedance path as expressed with green dash line in Fig. 7. Meanwhile the FDR continuously monitored the fault in the system through the instrument transformer. As a fault is detected in the system, the fault condition occurs and the second stage initiates. In the meantime, two operations have been executed at the same time. Firstly, the nominal path of the main breaker verifies the fault verification criteria and did not trip until the fault has been verified. Moreover, the secondary breaker’s UFMD isolates conducting path in case of increasing the line current after the occurrence of fault and the current limiting mode arises by the addition of reactors in series of each branch to restrain the sus pected rate-of-rising of the fault current as illustrated by the dotted red line in Fig. 7. After t2, the commutation branch switches are turned off and the residual fault current is commutated to the absorber path on the confirmation of internal fault. In that moment, the fault current is enforced to zero and complete isolation from the system occurs. The schematic diagram of the protection unit is expressed in Fig. 9 to illus trate the operation sequence. 4.3. Cable model
Fig. 10. Cable layout.
The VSC-HVDC submarine cable cross-section is simulated from an actual 230 kV cross-linked polyethylene electrical insulation (XLPE) [12]. The thickness of the copper conductor is demonstrated in accor dance with the 300 kV transmission line model. Attributes of the ma terial are deliberated conferring to the standards specified in [25]. Cross-section of each layer of the cable is catalogued in Fig. 10.
critically observed to investigate the execution of proposed scheme by simulating multiple fault scenarios. 5.1. Performance of the proposed scheme for internal faults With the occurrence of fault, some notable transformations can be perceived in current and voltage. The harmonics are sieved from current and voltage, and these vicissitudes can certainly be recognized via DFT by the implementation of the proposed scheme. The validation is grounded on the protection of transmission network. Pole-to-Ground fault (F1) is applied at the center of the line between terminal 3 and 4 at 0.50 s as shown in Fig. 6. The terminal voltage starts declining after a minuscule delay contingent on extent with the fault location as shown in Fig. 11. Across the terminals of MTDC network, the 12th harmonics of current and voltage can be extracted. Characteristic response of current harmonics for each terminal is expressed in Fig. 12. Accordingly, a comparison of the measured harmonic of current signal IH.M with that of the set value IH.set is carried out, and the output exhibits the harmonic current of terminal 3 and 4 at the top most level, that is about 0.11 p.u during the first peak. In contrast, the measured current harmonic of terminal 1 and 2 are lower than the set value. In conclusion from the outcomes, a fault is proximal to terminal 3 and 4. For evaluating external or internal fault, the periodic pulse count of the harmonics of the voltage
5. Results and analysis The performance evaluation for internal and external faults is
Fig. 11. Response of the terminal voltage of MTDC network in case of a P-G fault F1.
Fig. 9. Schematic diagram of protection unit. 6
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Fig. 12. Characteristic harmonic current response for fault detection in case of an internal fault (F1); (a) Response at terminal 1 and 2, (b) At terminal 3 and 4.
in 6 ms is evaluated as expressed in Fig. 13, subsequently, the antici pated value of pulse count NN.M is equal to the set value. The outcomes explicit that, there is an internal fault and the faulty portion should isolate with the aid of protection unit. The protection unit also restrains the fault current in the permissible limits during the categorization of internal and external fault. The fault current response of the SB1 is illustrated in Fig. 14 (a). Both SB’s have the same waveform as they are connected in parallel to the MB. Subsequently, Fig. 14 (b) shows the response of the MB. According to the Eq. (16), an external fault occurs, while in case if Eq. (17) is satisfied, an internal fault arises in the system, subsequently tripping the HCB without communication among the terminals. 5.2. Performance of the proposed scheme for external faults An external fault F2 is incepted in the MTDC network near the ter minal 1 to process the response of proposed scheme as shown in Fig. 6. Across the terminals of MTDC network, the protection units continu ously monitor the 12th harmonics which is extracted from the current and voltage. During the fault F2, the typical response of the current harmonics of each terminal is shown in Fig. 15. Comparison of measured harmonic current IH.M with the set value IH.set, exhibits the harmonic current of terminal 2 at maximum level, that is almost 0.315 p.u during the first peak. The results indicate a fault in the system in close proximity to the terminal 2. To differentiate internal fault from external, the number of periodic pulse count of the voltage harmonic in 6 ms is evaluated as expressed in Fig. 16, thus, the measured value of pulse count NN.M is not able to reach the set value due to DC capacitors, which are used as a filter. Hence, the external fault isolated by the AC circuit breaker on the AC side and the proposed protection scheme does not trip in the instance of external fault.
Fig. 14. Fault current response of the protection unit during an internal fault (F1); (a) Secondary breaker, (b) Main breaker.
Fig. 13. Characteristic harmonic voltage response for fault classification in case of an internal fault (F1); (a) Response at terminal 1 and 2, (b) At terminal 3 and 4. 7
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Fig. 15. Characteristic harmonic current response for fault detection in case of an external fault (F2); (a) Result of terminal 1 and 2, (b) Result of terminal 3 and 4.
Fig. 16. Characteristic harmonic voltage response for fault classification in case of an external fault (F2); (a) Result of terminal 1 and 2, (b) Result of terminal 3 and 4.
5.3. Impact of fault resistance and signal noise
(2) The sensitivity of the protection scheme decreases in response to an increase in fault resistance at fault point. (3) With noise condition at 30 dB in the system, the sensitivity of the proposed protection scheme increases in contrast to no noise condition.
Various values of fault resistances are considered for the evaluation of the sensitivity of the proposed protection scheme. Fault resistances are varied from 1 Ω to 200 Ω. Furthermore, when the frequency of sampling is high, signal noise is inevitable in a power transmission line. Therefore, in this analysis, the noise with 30 dB SNR is enclosed with the dc current. Proposed protection scheme is verified for various fault re sistances and noise conditions, which are presented in Table 2. Ac cording to the outcomes, the conclusions are inferred as:
5.4. Validation of proposed scheme under different fault scenarios Some other fault scenarios are established to authenticate the pro tection scheme, and the outcomes are recorded in Table 3, in which different fault locations and types are considered as illustrated in the Fig. 17. For a comprehensive investigation, A ~ F as pre-set fault loca tions are considered. The highest values of the harmonic current are approximated from each terminal (IHM1 to IHM4) and comparison of the measured values with the set values (IH.set) is done. In addition, the number of periodic pulse count of the voltage harmonic in 6 ms is evaluated and contrasted with the set value. It is evident from the inference of the results, that the proposed protection scheme can pre cisely recognize all faults. The obtained outcomes are tabularized in Table 4, that indicates the tripping operation during internal faults. In case of external faults, the periodic pulse count of the voltage harmonic lags behind its set value, in conclusion, the protection scheme does not
(1) The proposed protection scheme corresponds precisely for various fault conditions comprising of high resistance ground faults and noise conditions.
Table 2 Simulation results of pole-to-ground fault under different fault resistances and signal noise. Fault Resistance /Ω
No noise
0 10 50 100 200
0.0062 0.0148 0.0054 0.0142 0.0047 0.0133 0.0032 0.0124 0.0023 0.0112 30 dB noise 0.0065 0.0151 0.0059 0.0146 0.0053 0.0138 0.0035 0.0130 0.0026 0.0115
0 10 50 100 200
IHM1 (p.u.)
IHM2 (p.u.)
IHM3 (p.u.)
IHM4 (p.u.)
Periodic Pulse count
Action
0.1175 0.1159 0.1143 0.1119 0.1098
0.1121 0.1108 0.1092 0.1074 0.1051
3 3 3 3 3
Trip Trip Trip Trip Trip
0.1182 0.1165 0.1150 0.1124 0.1101
0.1125 0.1112 0.1098 0.1081 0.1055
3 3 3 3 3
Trip Trip Trip Trip Trip
Table 3 Fault scenarios for performance validation of proposed scheme.
8
Fault Scenario
Description
I II III IV V VI
Fault at the middle of DC line 1 Fault on AC side near the terminal 1 Fault at AC busbar near the terminal 4 Fault at the middle of the DC line 3 Fault at DC busbar 3 Fault on AC side near the terminal 3
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Fig. 17. Numerous scenarios for validation of proposed scheme. Table 4 Performance of proposed scheme for numerous fault locations. Fault Name
IHM1 (p.u.)
IHM2 (p.u.)
IHM3 (p.u.)
IHM4 (p.u.)
Periodic Pulse count
Identification Result
Operation
I II III IV V VI
0.129 0.317 0.0119 0.0153 0.029 0.0187
0.117 0.082 0.0347 0.0786 0.0431 0.0326
0.0296 0.061 0.0291 0.1179 0.178 0.0419
0.0122 0.023 0.497 0.155 0.0327 0.0519
3 1 1 3 3 1
Internal fault External fault External fault Internal fault Internal fault External fault
PU11, PU12 No trip No trip PU31, PU32 PU41, PU42 No trip
trigger tripping of the system.
and voltage harmonics) which increases reliability of the system as well as prevents mal-operation of the breaker. The proposed scheme does not require any physical communication among the terminals, which makes it faster and applicable for longer distance transmission lines. Moreover, a novel protection unit is introduced, having HCB with the combination of FDR designed for the required protection operations. The basic HCB is not suitable for most of the protection schemes as they require some delay to verify the fault. On the other hand, the proposed PU is effective for the rapid isolation of fault.
5.5. Comparative analysis The proposed protection scheme is compared with some state-of theart techniques. A commonly used protection scheme based on transient harmonic for VSC-HVDC is applied and demonstrated in [22,23]. Detection of fault carried out with single frequency element would most likely get affected by signal noise and filter capacitors. Furthermore, it is applicable only for two terminal HVDC network. Traveling wave pro tection is a well-recognized single-end measurement protection, which, by means of recognizing the traveling wave front, can perform fault detection within milliseconds, and is implicated expansively in LCCHVDC for many years [17,18]. Though, its sensitivity to fault resis tance can cause mal-operation in case of high-resistance fault [19]. In addition, this drawback will become prominent while applying in MTDC, as the inverter of MTDC could supply current to the faulted line constantly, thereby increasing the influence of fault resistance. By applying the non-communication principle, a few discrepancies are corrected while utilizing the current data signal [20]. However, by including the shunt capacitor, the system parameter may get affected in addition to the raising of its cost. The proposed scheme is based on two factor identification (Current
6. Conclusion A non-communication based transient harmonic protection scheme for MTDC network is proposed in this paper. Rapid fault detection can be secured to sieve transient current harmonics from initial fault current by using DFT, without communication among the terminals. Categorical classification of internal and external faults is carried out by counting the number of pulse count in a specific interval of the harmonics of the voltage and contrasted to the pre-set value. In case of internal fault, the proposed protection scheme revealed the current harmonic and its analogous pulse count to distinguish the fault and trip the faulty part of the MTDC network. However, during the external faults, the harmonics pulse count of a specific interval lags in reaching the measuring point 9
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vide DC filter capacitors, therefore, the harmonics pulse count is unable to reach the set value, thus, DC protection does not operate. An advanced protection unit is also employed with the combination of HCB for required protection operation. The proposed protection scheme is validated in a four-terminal VSC-HVDC modeled in PSCAD/EMTDC platform and the consequential results evidenced its promising perfor mance for numerous faults in case of multi-terminal network. The results exhibit that the protection can operate precisely during internal fault at various locations in the network. Furthermore, it is validated that the protection is robust to the HCB operation with single-end measurement.
[6] Akhmatov V, Callavik M, Franck CM, Rye SE, Ahndorf T, Bucher MK, et al. Technical guidelines and prestandardization work for first HVDC grids. IEEE Trans Power Delivery 2013;29(1):327–35. https://doi.org/10.1109/ TPWRD.2013.2273978. [7] Leterme W, Wang M, Chaffey G, Van Hertem D. HVDC grid protection algorithm performance assessment. In: Proc. CIGRE Symposium Aalborg, Denmark; 2019, pp. 1–12. CIGRE. [8] Le Blond S, Bertho Jr R, Coury DV, Vieira JC. Design of protection schemes for multi-terminal HVDC systems. Renew Sustain Energy Rev 2016;1(56):965–74. https://doi.org/10.1016/j.rser.2015.12.025. [9] Bahirat HJ, Høidalen HK, Mork BA. Thevenin equivalent of voltage-source converters for dc fault studies. IEEE Trans Power Delivery 2015;31(2):503–12. https://doi.org/10.1109/TPWRD.2015.2465812. [10] Li C, Zhao C, Xu J, Ji Y, Zhang F, An T. A pole-to-pole short-circuit fault current calculation method for DC grids. IEEE Trans Power Syst 2017;32(6):4943–53. https://doi.org/10.1109/TPWRS.2017.2682110. [11] Nadeem MH, Zheng X, Tai N, Gul M, Tahir S. Analysis of propagation delay for multi-terminal high voltage direct current networks interconnecting the large-scale off-shore renewable energy. Energies. 2018;11(8):2115. https://doi.org/10.3390/ en11082115. [12] Nadeem MH, Tai N, Zheng X, Huang W, Gul M. Multi-terminal hvdc fault current analysis during line to ground fault. In: 2017 IEEE Innovative Smart Grid Technologies-Asia (ISGT-Asia). IEEE; 2017, pp. 1–5. doi: 10.1109/ISGTAsia.2017.8378367. [13] Nadeem MH, Zheng X, Tai N, Gul M. Identification and isolation of faults in multiterminal high voltage DC networks with hybrid circuit breakers. Energies 2018;11 (5):1086. https://doi.org/10.3390/en11051086. [14] Nadeem MH, Zheng X, Tai N, Gul M, Yu M. Detection and Classification of Faults in MTDC Networks. In: 2018 IEEE PES Asia-Pacific Power and Energy Engineering Conference (APPEEC). IEEE; 2018, p. 311–16. doi: 10.1109/ APPEEC.2018.8566331. [15] Sneath J, Rajapakse AD. Fault detection and interruption in an earthed HVDC grid using ROCOV and hybrid DC breakers. IEEE Trans Power Delivery 2014;31(3): 973–81. https://doi.org/10.1109/TPWRD.2014.2364547. [16] Baran ME, Mahajan NR. Overcurrent protection on voltage-source-converter-based multiterminal DC distribution systems. IEEE Trans Power Delivery 2006;22(1): 406–12. https://doi.org/10.1109/TPWRD.2006.877086. [17] Kong F, Hao Z, Zhang B. A novel traveling-wave-based main protection scheme for ±800 kV UHVDC bipolar transmission lines. IEEE Trans Power Delivery 2016;31 (5):2159–68. https://doi.org/10.1109/TPWRD.2016.2571438. [18] Wu J, Li H, Wang G, Liang Y. An improved traveling-wave protection scheme for LCC-HVDC transmission lines. IEEE Trans Power Delivery 2016;32(1):106–16. https://doi.org/10.1109/TPWRD.2016.2549565. [19] Farhadi M, Mohammed OA. Protection of multi-terminal and distributed DC systems: design challenges and techniques. Electr Power Syst Res 2017;1(143): 715–27. https://doi.org/10.1016/j.epsr.2016.10.038. [20] Abu-Elanien AE, Abdel-Khalik AS, Massoud AM, Ahmed S. A non-communication based protection algorithm for multi-terminal HVDC grids. Electr Power Syst Res 2017;1(144):41–51. https://doi.org/10.1016/j.epsr.2016.11.010. [21] Zheng X, Tai N, Wu Z, Thorp J. Harmonic current protection scheme for voltage source converter-based high-voltage direct current transmission system. IET Gener Transm Distrib 2014;8(9):1509–15. https://doi.org/10.1049/iet-gtd.2013.0377. [22] Zheng X, Tai N, Thorp JS, Yang Y. A transient harmonic current protection scheme for HVDC transmission line. IEEE Trans Power Delivery 2012;27(4):2278–85. https://doi.org/10.1109/TPWRD.2012.2201509. [23] Liu J, Tai N, Fan C, Huang W. Protection scheme for high-voltage direct-current transmission lines based on transient AC current. IET Gener Transm Distrib 2015;9 (16):2633–43. https://doi.org/10.1049/iet-gtd.2015.0792. [24] Miano G, Maffucci A. Transmission lines and lumped circuits: fundamentals and applications. Elsevier; 2001. [25] Worzyk T. Submarine power cables: design, installation, repair, environmental aspects. Springer Science & Business Media; 2009.
CRediT authorship contribution statement Muhammad Haroon Nadeem: Conceptualization, Methodology, Writing - original draft. Xiaodong Zheng: Data curation, Software, Validation. Nengling Tai: Funding acquisition, Supervision. Mehr Gul: Visualization, Investigation. Moduo Yu: Writing - review & editing, Writing - review & editing. Yangyang He: Formal analysis, Software. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was partially supported by National Natural Science Foundation of China (51877135), Shanghai Rising-Star Program (18QA1402100) and National Key Research and Development Program of China (2016YFB0900600). References [1] Flourentzou N, Agelidis VG, Demetriades GD. VSC-based HVDC power transmission systems: an overview. IEEE Trans Power Electron 2009;24(3): 592–602. https://doi.org/10.1109/TPEL.2008.2008441. [2] Van Hertem D, Ghandhari M. Multi-terminal VSC HVDC for the European supergrid: obstacles. Renew Sustain Energy Rev 2010;14(9):3156–63. https://doi. org/10.1016/j.rser.2010.07.068. [3] Feng W, Tjernberg LB, Mannikoff A, Bergman A. A new approach for benefit evaluation of multiterminal VSC–HVDC using a proposed mixed AC/DC optimal power flow. IEEE Trans Power Delivery 2013;29(1):432–43. https://doi.org/ 10.1109/TPWRD.2013.2267056. [4] Sadovskaia K, Bogdanov D, Honkapuro S, Breyer C. Power transmission and distribution losses–A model based on available empirical data and future trends for all countries globally. Int J Electr Power Energy Syst 2019;1(107):98–109. https:// doi.org/10.1016/j.ijepes.2018.11.012. [5] Lin YK, Chang PC, Fiondella L. A study of correlated failures on the network reliability of power transmission systems. Int J Electr Power Energy Syst 2012;43 (1):954–60. https://doi.org/10.1016/j.ijepes.2012.06.060.
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Non-communication based protection scheme using transient harmonics for multi-terminal HVDC networks Muhammad Haroon Nadeem a, b, Xiaodong Zheng a, b, *, Nengling Tai a, b, Mehr Gul a, b, Moduo Yu a, b, c, Yangyang He a, b a b c
Key Laboratory of Control of Power Transmission and Conversion, Shanghai Jiao Tong University, Ministry of Education, 200240 Shanghai, China Department of Electrical Engineering, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, 200240 Shanghai, China School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: Fault detection Fault classification HVDC protection Protection unit Multi-terminal HVDC
DC faults render hindrance in the flourishment of multi-terminal flexible high voltage DC (MTDC). The fault current rises rapidly, making a sharp peak, however, under a specific fault instance, isolation of the whole network is an impractical solution. For effective reliability of the whole network, rapid and accurate fault detection and selection of faulty pole is mandatory. A fault detection scheme is proposed in this paper to draw transient current harmonics from initial fault current by utilizing Discrete Fourier transform (DFT). Current harmonics employed the fault detection promptly at the converter stations without communication among the MTDC networks. Additionally, the proposed scheme can discriminate internal from external fault by tracking the number of pulse count in a specific interval of the harmonics of the voltage. In this way, the reliability of the protection scheme enhances and escape from the mal-operation of the circuit breaker. For four terminal MTDC transmission network, the simulations are carried out in PSCAD to authenticate the implementation of the proposed scheme and protection units (PU) are utilized for the isolation of faulty part with the combination of hybrid circuit breaker (HCB). A keen analysis of test results authenticated the ability of the proposed scheme to rapidly detect the faults with high precision, along with efficient classification of the nature of faults. Further more, consequential results provide a strong evidence for its key role in handling a multitude of MTDC faults.
1. Introduction
self-blocking strategy to avoid any damage, in case the protection fails to initiate timely or limit the fault current [10]. Hence, the protection of MTDC network is significant to guarantee security along with the reli ability of the entire system [11–14]. Consequently, if the protection operates prior to the blockade of the converters, the power losses in the entire network can be avoided, thereby emphasizing the strict condition of operating speed. To meet the optimal speed requirement, protection schemes based on single-end measurement appears as an appropriate choice for rapid protection, in contrast to the pilot protection which is reliable on the physical communication system and causes unavoidable time delay [15]. The prominence of several protection schemes for MTDC have been investigated and recognized in [15–23]. The most common method of fault detection is the current differential scheme [15]. However, this method is not suitable for enormous transmission networks as a time lag originates hindrances in repossessing the fault detection signal. More over, the signal becomes discrepant distributed owing to the distributed
With the advancements in renewable energy integration, longdistance power transmission for reliable system operation, voltagesource converter based high-voltage DC (VSC-HVDC) is rendered to have a very significant part in the forthcoming power grid [1–3]. In this instance of large-scale transmission system, the transmission line crosses rough territory and faces extreme atmospheric conditions [4,5], raising the probability of fault occurrence. Hence, reliable and efficient pro tection scheme is significant for the multi-terminal HVDC transmission networks [6]. For the protection of MTDC, speed is prioritized amongst the essential requisites [7,8]. Owing to the low inertia coefficient of multiterminal HVDC network, the fault occurring in the transmission line can cause rapid escalation of the current, which could affect the entire network within milliseconds [9]. The insulated-gate bipolar transistor (IGBT) based converters, that are sensitive to overcurrent, can initiate a
* Corresponding author. E-mail addresses: [email protected] (M.H. Nadeem), [email protected] (X. Zheng). https://doi.org/10.1016/j.ijepes.2020.106636 Received 22 May 2020; Received in revised form 9 September 2020; Accepted 3 November 2020 Available online 11 December 2020 0142-0615/© 2020 Elsevier Ltd. All rights reserved.
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line capacitance [16]. For effective fault detection in the MTDC network, a dignified approach is used by multiple measurements at numerous locations along the line [15]. However, rapid and reliable communica tion across the abundant measuring points can limit its realistic imple mentation. Traveling wave protection is a well-recognized single-end measurement protection, which, by means of recognizing the traveling wave front, can perform fault detection within milliseconds, and is implicated expansively in LCC-HVDC for a long time [17,18]. Though, its sensitivity to fault resistance can cause mal-operation in case of highresistance fault [19]. In addition, this drawback will become prominent while applying in MTDC, as the inverter of MTDC could provide current to the faulted line constantly, thereby increasing the influence of fault resistance. Some issues are rectified by utilizing the current data signal with the approach of non-communication principle [20]. However, the inclusion of a shunt capacitor may raise the cost with affection of the system parameter. A non-communication based protection scheme can be employed by using the transient components for fault identification [21]. A protection scheme for two terminal VSC-HVDC based on tran sient harmonic is presented in [22,23]. Fault detection done with the single frequency element would undoubtedly get influenced by filter capacitors and signal noise. Therefore, expansive investigation is required for adapting harmonic components to detect a fault and develop a novel protection for MTDC networks. A fault detection scheme is proposed in this paper to separate current harmonics from the fault current by utilizing DFT for the VSC-MTDC network. Fault detection is quickly retained by current harmonics at each terminal of converter stations of the MTDC networks without physical communication among the terminals. The modulated scheme is also adept of discriminating external fault from internal by counting the number of pulse count in a specific time interval of 6 ms of the voltage harmonics. In this way, the reliability of the protection scheme enhances and escape from the mal-operation of the circuit breaker. Furthermore, selection of faulty pole identification criterion is erected, based on voltage drop of positive and negative poles of the same line. By imple menting the proposed protection scheme, DC line faults have been identified and discriminated with greater reliability. The validation of the proposed protection scheme has been done through PSCAD by simulating a four terminal MTDC transmission network. A detailed exploration of test results affirms the ability of the proposed scheme to accurately detect faults, as well as promptly classify internal and external faults. In succession to classifying, the scheme adeptly isolates the faulty section from the healthy system by utilizing the HCB. Furthermore, its implications also conferred auspicious performance for various MTDC faults. The remaining paper is structured as: Section 2 presents the calcu lation of fault current in multi-terminal HVDC network. The proposed scheme for fault detection and classification is illuminated in Section 3. To analyse the performance of proposed scheme, a simulation model is explained in Section 4. In Section 5, the viability and robustness of the proposed protection is validated in a four-terminal VSC-HVDC system. The discovered results indicate the protection to identify and discrimi nate faults with single-end measurement, accurately. Finally, the con clusions are drawn in Section 6.
considered as a current-controlled source due to the small drop of DClink voltage, and for the analysis of fault current contribution, the AC side contribution can be neglected [12]. Hence, in the following anal ysis, the VSC can merely be considered as the parallel coupled capacitors. A three terminal MTDC network having a solid LG fault in the center of line 13 is expressed in Fig. 1. At the DC side, inductors are normally used to lessen the DC current ripples, which are having the same influ ence as the smoothing reactor in the LCC-HVDC. For controlling the rapid rate of change of DC fault current, large inductor with hundreds of mH is applied in MTDC system so that DC CD can interrupt the fault current in several milliseconds. To expedite the fault current analysis, DC transmission lines are aligned as a series RL circuits as illustrated in Fig. 1. Lab and Rab are the inductance and resistance of the Line ‘ab’ correspondingly. Similarly, the inductance and resistance to the fault location from side ‘a’ is expressed with La0 and Ra0 respectively. LLab is the DC auxiliary inductor at the ‘a’ side of branch ab. Meanwhile, Cn represents the DC-link capacitance of VSCn. 2.2. Short-circuit DC current calculation In the following analysis, the steady-state element can be ignored in comparison to the fault current element, since only the faulty part of DCline current is analysed. The calculation of DC current component can be done by using only a single fault DC voltage source Vo with a passive network according to the superposition principle. For simplification of the analysis, the progression of DC line current from healthy to faulty line is considered positive. For evaluating the fault current component, Laplace transformation of the stated three-terminal DC network is exhibited in Fig. 2. The capacitor voltage of terminal 2 is obtained as follows: V2 (s) =
I12 (s) + I23 (s) sC2
(1)
The voltage of the DC-link capacitor at terminal 2 is represented with V2. The DC current in the lines 12 and 23 are expressed with I12 and I23 respectively. In order to further simplify the analysis, the capacitor C2 is divided into two independent branches to supply each branch (line 12 and line 23) independently. The analogous impedance Zc12, Zc23 should be fulfilled as follows, ZC12 (s) =
[I12 (s) + I23 (s)]/sC2 I12 (s)
(2)
ZC23 (s) =
[I12 (s) + I23 (s)]/sC2 I23 (s)
(3)
The proportional coefficient of the DC current of the line 12 and 23 can be defined as follows:
2. Calculation of fault current in multi-terminal HVDC network 2.1. MTDC network topology for calculation of fault current In the instance of a DC fault in a MTDC network, the faulted DC line should precisely be localized in first few milliseconds for the operation of DC circuit breaker (CB). Therefore, power system protection design has the main concern within the initial period of the DC current after the occurrence of fault. In case where a line-to-ground (LG) fault emerges on the DC side of the system, the parallel linked capacitors of converter station initiate to discharge rapidly with a sharp peak. Meanwhile, during the initial small interval after fault, the VSC can still be
Fig. 1. Layout of a typical RLC three terminal DC network. 2
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the fault interval by considering the DFT. Subsequently, for DC fault analysis in the high-frequency zone, the entire impedance of the DC link Capacitor at terminal 1 (Zf10) could be considered as parallel connected DC-link capacitor, that is inscribed as, ( ) 1 ′ Zf 10 (s) = (9) sC1 Similarly, DC fault line current in the high-frequency part of Laplace Transform is formulated as, ′
I10 =
I23 (s) I12 (s)
3. Proposed protection scheme
(4)
A novel protection scheme based on transient DC fault current har monics for MTDC network is proposed in this section, which is appli cable for the identification of fault in the system. Calculating the number of pulse count in a specific time period of voltage harmonics is utilized to distinguish external from internal fault without communication be tween the terminals. In the proposed scheme, rapid identification of the faulted line is done through single-end signal.
where k is associated with fault location on line 12, and the measure ment of the transmission line 13 and 23. The supplementary inductance of the inductor is defined as correlated proportional coefficient. The capacitor C2, which is divided into two independent branches desig nated as C12 and C23 to supply line 12 and line 23 respectively and can be exhibited as follows, C12 =
C2 (k + 1)
(5)
C23 =
C3 (1/k + 1)
(6)
3.1. Fault identification Remarkable changes in current and voltage are observed with fault occurrence in the system. Characteristic DC fault current harmonics are utilized to identify these variations. The sample of current signal is considered x(t) at an arbitrary time ‘t’, having the fundamental frequency measured in radians per second.
Consequently, the multi-terminal DC network could be revealed into a couple of sovereign radial networks expressed in Fig. 3 to calculate the component of fault current in the faulted line I10, which could be derived as, I10 =
s(L10 (
Zf 10 (s) =
V0 /s + LL13 ) + R10 + Zf 10 (s) s(L12 + LL12 + LL13 ) + R12 + 1/sC12 s(L12 + LL12 + LL13 ) + R12 + 1/sC1 + 1/sC12
x(t) = Xc cosωo t + Xs sinωo t
(7) )
( ×
1 sC1
(10)
Equation (10) demonstrates the equivalent model of VSC based multi-terminal DC network for the calculation of fault current during initial small interval following a fault occurrence, which is based on DFT and utilized the current harmonics for the fault identification.
Fig. 2. Laplace transformation for evaluating the fault current component of a three terminal DC network.
k =
V0 /s s(L10 + LL13 ) + R10 + 1/sC1
(11)
where Xc and Xs are real numbers, x(t) is the instantaneous value of the current signal. The rectangular configuration of DFT is utilized for splitting a periodic signal into harmonics and fundamental of the signal for calculation of approximate expression of initial short period after fault as explained in Section 2. The fundamental frequency elements for K samples per cycle demonstrated and the hth harmonics obtained as [24]. The equations are derived to determine the newest data window by utilizing the previous data window as derived in Ref. [13] and comprehensive form is expressed in (12) and (13). The effectiveness of the equations is for any interval of the window.
) (8)
where Zf10 is the total impedance of DC-link capacitor of terminal 1, which has parallel connection with branch line 12. The mathematical expression derived from the Inverse Laplace Transform of (8) is difficult to express I10. Hence, approximate expres sion of I10 can be implemented to evaluate the initial fault current during
Xc’(new) = Xc’(old) + [xnew − xold ]cos(hθ)
(12)
Xs’(new) = Xc’(old) + [xnew − xold ]sin(hθ)
(13)
xnew is the newest sample and xold is the previous sample, accurately one complete cycle prior. The setting value of the transient current harmonics may be recog nized by utilizing the values of different parameters IH.set = IH.int × kline × kF.r × kint
(14)
where IH.set is the setting value of transient harmonic current protection scheme and IH.int is the initial value of transient harmonics current. Moreover, kline and kF.r are correction coefficient reflecting the length of transmission line and the fault resistance correspondingly. kint is expressing the correction coefficient deliberating the change of IH.int . For calculating IH.set , the values of the correction coefficient in the simulations are given as: kint = 0.5 kline = 1 kF.r = 0.2IH.int = 0.1 p.u. Thus, the setting value can be calculated. The characteristic harmonics
Fig. 3. Two independent radial network derived from the meshed MTDC network. 3
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current can be computed at the protection units close to the terminals and the frequency of sampling is taken 6000 Hz in this paper. 3.2. Fault classification Power conversion harmonics are generated in the system with inte gral multiple frequencies of the fundamental frequency during the power conversion process at converter stations [22]. The output voltage of a 12-pulse converter mainly produced 12th harmonics in the system. Internal and external faults can be discriminated by these harmonics by counting the number of pulse count in a specific time interval [13]. For external faults, harmonics are absorbed by the DC capacitors before arriving at the fault detection relay (FDR) as aforementioned. Contrarily, on occurrence of internal faults, the FDR easily recognizes the harmonics with the number of pulse count in one cycle. Considering the 12 pulse harmonics, pulse count in one cycle should be 12 for in ternal fault. The proposed scheme used the window length of 6 ms, as for 12th harmonic system, three peaks appeared in specific time frame as can be seen in Fig. 4. The height of the window depends on the average value of the ripples of the harmonics. NI(N.set) = NI(N.int)
(15)
NI(N.set) is indicating the number of pulses in sampling window and NI(N.int) the number of initial harmonics pulse in 6 ms correspondingly. For the 12th harmonics, the number of pulse count is 3 for the proposed topology. The protection algorithm is presented in Fig. 5. { IH.M ⩾ IH.set (16) NV(N.M) ⩽ NV(N.set) {
IH.M ⩾ IH.set NV(N.M) ⩾ NV(N.set)
(17)
According to the Eq. (16), an external fault occurs, and if Eq. (17) is satisfied, an internal fault arises in the system, subsequently tripping the HCB without communication among the terminals. 4. Simulation model A four-terminal bipolar VSC-HVDC test system is simulated in PSCAD, as depicted in Fig. 6 for analyzing the performance of proposed scheme under numerous fault conditions. MTDC system, protection unit and cable models are comprehensively explicated as follows.
Fig. 5. Algorithm for the fault detection and classification scheme.
utilized in [13]. Power generation with the off-shore energy is attached with Terminal 1 and 4, however an AC side grid is connected with Terminal 2 and 3. Evaluation of the effectiveness of proposed scheme has been done with the different (internal and external) faults. F1 and F2 faults are indicating internal and external faults respectively. Addi tionally, VSC converters are connected with DC link capacitors and designed on ±300 kV bipolar half-bridge topology. At respective ter minals, capacitors are grounded independent of each other. An appro priate model of VSC converter is separately represented in Fig. 6.
4.1. System model Four terminals MTDC network is modeled in PSCAD, which is illus trated in Fig. 6 and the system parameters are listed in Table 1, as
4.2. Protection unit A novel protection unit having HCB with the combination of FDR is designed for the required protection operations. The basic HCB is not suitable for most of the protection schemes as most of the protection schemes required some delay to verify the fault. However, the fault current rising-rate is too high in the beginning of the fault, which may surpass the maximum breaking capacity of the HCB. By enlarging the value of current limiting reactance (CLR), the aforementioned problem can be solved, but it will influence the dynamic aspects of the DC transmission network. The practicability of upcoming DC networks re lies mainly on their abilities to survive during DC faults.
Fig. 4. Response of harmonic current indicating the sampling window. 4
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Fig. 6. Structure of MTDC network and VSC-based HVDC converter.
absorption path, which protects IGBT from over voltage. Furthermore, the configuration of the SB also has three paths: path 1 is the low loss branch for the normal operation, having the LCS and UFMD. Path 2 contains the capacitor bank, and path 3 comprises of the energy ab sorption path. The SBs do not have FDR and can operate with the fault current threshold level. The control logic of protection unit for MTDC network is divided into three stages; normal working stage, fault condition/fault energy reduction stage and fault clearance stage, which are categorized as 0 ≤ t ≤ t1, t1 ≤ t ≤ t2, and t > t2 respectively and listed in Fig. 8. During normal working conditions, whole current passes via nominal
Table 1 Parameters of implementation model. System parameters
Values
Rated power of converter AC voltage DC voltage X/R in AC network Reactance of Transformer leakage Entire resistance of converter diodes Phase reactor of converter
1200 MW 660 kV ±300 kV 10 0.2 p.u. 0.006 p.u. 0.06 p.u.
The proposed protection unit consists of a main breaker (MB) and secondary breakers (SB) with the combination of reactance, as depicted in Fig. 7. The MB comprises of three paths: nominal path is a low loss path containing ultrafast mechanical disconnector (UFMD), load com mutation switch (LCS) and FDR. The second path is the commutation path which has the MB, and the fault current is diverted from nominal path to commutation path as the fault has been verified, and commu tation branch can isolate the fault rapidly. The third path is the energy
Fig. 8. Protection unit operating stages.
Fig. 7. Protection unit model. 5
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path of the main breaker blocking all the IGBT’s in the commutation path. Furthermore, for the secondary breakers, the current flows in first path having low power losses as it only contains the UFMD and LCS. Hence, the current in the protection unit flows through the low impedance path as expressed with green dash line in Fig. 7. Meanwhile the FDR continuously monitored the fault in the system through the instrument transformer. As a fault is detected in the system, the fault condition occurs and the second stage initiates. In the meantime, two operations have been executed at the same time. Firstly, the nominal path of the main breaker verifies the fault verification criteria and did not trip until the fault has been verified. Moreover, the secondary breaker’s UFMD isolates conducting path in case of increasing the line current after the occurrence of fault and the current limiting mode arises by the addition of reactors in series of each branch to restrain the sus pected rate-of-rising of the fault current as illustrated by the dotted red line in Fig. 7. After t2, the commutation branch switches are turned off and the residual fault current is commutated to the absorber path on the confirmation of internal fault. In that moment, the fault current is enforced to zero and complete isolation from the system occurs. The schematic diagram of the protection unit is expressed in Fig. 9 to illus trate the operation sequence. 4.3. Cable model
Fig. 10. Cable layout.
The VSC-HVDC submarine cable cross-section is simulated from an actual 230 kV cross-linked polyethylene electrical insulation (XLPE) [12]. The thickness of the copper conductor is demonstrated in accor dance with the 300 kV transmission line model. Attributes of the ma terial are deliberated conferring to the standards specified in [25]. Cross-section of each layer of the cable is catalogued in Fig. 10.
critically observed to investigate the execution of proposed scheme by simulating multiple fault scenarios. 5.1. Performance of the proposed scheme for internal faults With the occurrence of fault, some notable transformations can be perceived in current and voltage. The harmonics are sieved from current and voltage, and these vicissitudes can certainly be recognized via DFT by the implementation of the proposed scheme. The validation is grounded on the protection of transmission network. Pole-to-Ground fault (F1) is applied at the center of the line between terminal 3 and 4 at 0.50 s as shown in Fig. 6. The terminal voltage starts declining after a minuscule delay contingent on extent with the fault location as shown in Fig. 11. Across the terminals of MTDC network, the 12th harmonics of current and voltage can be extracted. Characteristic response of current harmonics for each terminal is expressed in Fig. 12. Accordingly, a comparison of the measured harmonic of current signal IH.M with that of the set value IH.set is carried out, and the output exhibits the harmonic current of terminal 3 and 4 at the top most level, that is about 0.11 p.u during the first peak. In contrast, the measured current harmonic of terminal 1 and 2 are lower than the set value. In conclusion from the outcomes, a fault is proximal to terminal 3 and 4. For evaluating external or internal fault, the periodic pulse count of the harmonics of the voltage
5. Results and analysis The performance evaluation for internal and external faults is
Fig. 11. Response of the terminal voltage of MTDC network in case of a P-G fault F1.
Fig. 9. Schematic diagram of protection unit. 6
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International Journal of Electrical Power and Energy Systems 127 (2021) 106636
Fig. 12. Characteristic harmonic current response for fault detection in case of an internal fault (F1); (a) Response at terminal 1 and 2, (b) At terminal 3 and 4.
in 6 ms is evaluated as expressed in Fig. 13, subsequently, the antici pated value of pulse count NN.M is equal to the set value. The outcomes explicit that, there is an internal fault and the faulty portion should isolate with the aid of protection unit. The protection unit also restrains the fault current in the permissible limits during the categorization of internal and external fault. The fault current response of the SB1 is illustrated in Fig. 14 (a). Both SB’s have the same waveform as they are connected in parallel to the MB. Subsequently, Fig. 14 (b) shows the response of the MB. According to the Eq. (16), an external fault occurs, while in case if Eq. (17) is satisfied, an internal fault arises in the system, subsequently tripping the HCB without communication among the terminals. 5.2. Performance of the proposed scheme for external faults An external fault F2 is incepted in the MTDC network near the ter minal 1 to process the response of proposed scheme as shown in Fig. 6. Across the terminals of MTDC network, the protection units continu ously monitor the 12th harmonics which is extracted from the current and voltage. During the fault F2, the typical response of the current harmonics of each terminal is shown in Fig. 15. Comparison of measured harmonic current IH.M with the set value IH.set, exhibits the harmonic current of terminal 2 at maximum level, that is almost 0.315 p.u during the first peak. The results indicate a fault in the system in close proximity to the terminal 2. To differentiate internal fault from external, the number of periodic pulse count of the voltage harmonic in 6 ms is evaluated as expressed in Fig. 16, thus, the measured value of pulse count NN.M is not able to reach the set value due to DC capacitors, which are used as a filter. Hence, the external fault isolated by the AC circuit breaker on the AC side and the proposed protection scheme does not trip in the instance of external fault.
Fig. 14. Fault current response of the protection unit during an internal fault (F1); (a) Secondary breaker, (b) Main breaker.
Fig. 13. Characteristic harmonic voltage response for fault classification in case of an internal fault (F1); (a) Response at terminal 1 and 2, (b) At terminal 3 and 4. 7
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International Journal of Electrical Power and Energy Systems 127 (2021) 106636
Fig. 15. Characteristic harmonic current response for fault detection in case of an external fault (F2); (a) Result of terminal 1 and 2, (b) Result of terminal 3 and 4.
Fig. 16. Characteristic harmonic voltage response for fault classification in case of an external fault (F2); (a) Result of terminal 1 and 2, (b) Result of terminal 3 and 4.
5.3. Impact of fault resistance and signal noise
(2) The sensitivity of the protection scheme decreases in response to an increase in fault resistance at fault point. (3) With noise condition at 30 dB in the system, the sensitivity of the proposed protection scheme increases in contrast to no noise condition.
Various values of fault resistances are considered for the evaluation of the sensitivity of the proposed protection scheme. Fault resistances are varied from 1 Ω to 200 Ω. Furthermore, when the frequency of sampling is high, signal noise is inevitable in a power transmission line. Therefore, in this analysis, the noise with 30 dB SNR is enclosed with the dc current. Proposed protection scheme is verified for various fault re sistances and noise conditions, which are presented in Table 2. Ac cording to the outcomes, the conclusions are inferred as:
5.4. Validation of proposed scheme under different fault scenarios Some other fault scenarios are established to authenticate the pro tection scheme, and the outcomes are recorded in Table 3, in which different fault locations and types are considered as illustrated in the Fig. 17. For a comprehensive investigation, A ~ F as pre-set fault loca tions are considered. The highest values of the harmonic current are approximated from each terminal (IHM1 to IHM4) and comparison of the measured values with the set values (IH.set) is done. In addition, the number of periodic pulse count of the voltage harmonic in 6 ms is evaluated and contrasted with the set value. It is evident from the inference of the results, that the proposed protection scheme can pre cisely recognize all faults. The obtained outcomes are tabularized in Table 4, that indicates the tripping operation during internal faults. In case of external faults, the periodic pulse count of the voltage harmonic lags behind its set value, in conclusion, the protection scheme does not
(1) The proposed protection scheme corresponds precisely for various fault conditions comprising of high resistance ground faults and noise conditions.
Table 2 Simulation results of pole-to-ground fault under different fault resistances and signal noise. Fault Resistance /Ω
No noise
0 10 50 100 200
0.0062 0.0148 0.0054 0.0142 0.0047 0.0133 0.0032 0.0124 0.0023 0.0112 30 dB noise 0.0065 0.0151 0.0059 0.0146 0.0053 0.0138 0.0035 0.0130 0.0026 0.0115
0 10 50 100 200
IHM1 (p.u.)
IHM2 (p.u.)
IHM3 (p.u.)
IHM4 (p.u.)
Periodic Pulse count
Action
0.1175 0.1159 0.1143 0.1119 0.1098
0.1121 0.1108 0.1092 0.1074 0.1051
3 3 3 3 3
Trip Trip Trip Trip Trip
0.1182 0.1165 0.1150 0.1124 0.1101
0.1125 0.1112 0.1098 0.1081 0.1055
3 3 3 3 3
Trip Trip Trip Trip Trip
Table 3 Fault scenarios for performance validation of proposed scheme.
8
Fault Scenario
Description
I II III IV V VI
Fault at the middle of DC line 1 Fault on AC side near the terminal 1 Fault at AC busbar near the terminal 4 Fault at the middle of the DC line 3 Fault at DC busbar 3 Fault on AC side near the terminal 3
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International Journal of Electrical Power and Energy Systems 127 (2021) 106636
Fig. 17. Numerous scenarios for validation of proposed scheme. Table 4 Performance of proposed scheme for numerous fault locations. Fault Name
IHM1 (p.u.)
IHM2 (p.u.)
IHM3 (p.u.)
IHM4 (p.u.)
Periodic Pulse count
Identification Result
Operation
I II III IV V VI
0.129 0.317 0.0119 0.0153 0.029 0.0187
0.117 0.082 0.0347 0.0786 0.0431 0.0326
0.0296 0.061 0.0291 0.1179 0.178 0.0419
0.0122 0.023 0.497 0.155 0.0327 0.0519
3 1 1 3 3 1
Internal fault External fault External fault Internal fault Internal fault External fault
PU11, PU12 No trip No trip PU31, PU32 PU41, PU42 No trip
trigger tripping of the system.
and voltage harmonics) which increases reliability of the system as well as prevents mal-operation of the breaker. The proposed scheme does not require any physical communication among the terminals, which makes it faster and applicable for longer distance transmission lines. Moreover, a novel protection unit is introduced, having HCB with the combination of FDR designed for the required protection operations. The basic HCB is not suitable for most of the protection schemes as they require some delay to verify the fault. On the other hand, the proposed PU is effective for the rapid isolation of fault.
5.5. Comparative analysis The proposed protection scheme is compared with some state-of theart techniques. A commonly used protection scheme based on transient harmonic for VSC-HVDC is applied and demonstrated in [22,23]. Detection of fault carried out with single frequency element would most likely get affected by signal noise and filter capacitors. Furthermore, it is applicable only for two terminal HVDC network. Traveling wave pro tection is a well-recognized single-end measurement protection, which, by means of recognizing the traveling wave front, can perform fault detection within milliseconds, and is implicated expansively in LCCHVDC for many years [17,18]. Though, its sensitivity to fault resis tance can cause mal-operation in case of high-resistance fault [19]. In addition, this drawback will become prominent while applying in MTDC, as the inverter of MTDC could supply current to the faulted line constantly, thereby increasing the influence of fault resistance. By applying the non-communication principle, a few discrepancies are corrected while utilizing the current data signal [20]. However, by including the shunt capacitor, the system parameter may get affected in addition to the raising of its cost. The proposed scheme is based on two factor identification (Current
6. Conclusion A non-communication based transient harmonic protection scheme for MTDC network is proposed in this paper. Rapid fault detection can be secured to sieve transient current harmonics from initial fault current by using DFT, without communication among the terminals. Categorical classification of internal and external faults is carried out by counting the number of pulse count in a specific interval of the harmonics of the voltage and contrasted to the pre-set value. In case of internal fault, the proposed protection scheme revealed the current harmonic and its analogous pulse count to distinguish the fault and trip the faulty part of the MTDC network. However, during the external faults, the harmonics pulse count of a specific interval lags in reaching the measuring point 9
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International Journal of Electrical Power and Energy Systems 127 (2021) 106636
vide DC filter capacitors, therefore, the harmonics pulse count is unable to reach the set value, thus, DC protection does not operate. An advanced protection unit is also employed with the combination of HCB for required protection operation. The proposed protection scheme is validated in a four-terminal VSC-HVDC modeled in PSCAD/EMTDC platform and the consequential results evidenced its promising perfor mance for numerous faults in case of multi-terminal network. The results exhibit that the protection can operate precisely during internal fault at various locations in the network. Furthermore, it is validated that the protection is robust to the HCB operation with single-end measurement.
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CRediT authorship contribution statement Muhammad Haroon Nadeem: Conceptualization, Methodology, Writing - original draft. Xiaodong Zheng: Data curation, Software, Validation. Nengling Tai: Funding acquisition, Supervision. Mehr Gul: Visualization, Investigation. Moduo Yu: Writing - review & editing, Writing - review & editing. Yangyang He: Formal analysis, Software. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was partially supported by National Natural Science Foundation of China (51877135), Shanghai Rising-Star Program (18QA1402100) and National Key Research and Development Program of China (2016YFB0900600). References [1] Flourentzou N, Agelidis VG, Demetriades GD. VSC-based HVDC power transmission systems: an overview. IEEE Trans Power Electron 2009;24(3): 592–602. https://doi.org/10.1109/TPEL.2008.2008441. [2] Van Hertem D, Ghandhari M. Multi-terminal VSC HVDC for the European supergrid: obstacles. Renew Sustain Energy Rev 2010;14(9):3156–63. https://doi. org/10.1016/j.rser.2010.07.068. [3] Feng W, Tjernberg LB, Mannikoff A, Bergman A. A new approach for benefit evaluation of multiterminal VSC–HVDC using a proposed mixed AC/DC optimal power flow. IEEE Trans Power Delivery 2013;29(1):432–43. https://doi.org/ 10.1109/TPWRD.2013.2267056. [4] Sadovskaia K, Bogdanov D, Honkapuro S, Breyer C. Power transmission and distribution losses–A model based on available empirical data and future trends for all countries globally. Int J Electr Power Energy Syst 2019;1(107):98–109. https:// doi.org/10.1016/j.ijepes.2018.11.012. [5] Lin YK, Chang PC, Fiondella L. A study of correlated failures on the network reliability of power transmission systems. Int J Electr Power Energy Syst 2012;43 (1):954–60. https://doi.org/10.1016/j.ijepes.2012.06.060.
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