A reliable DC fault protection system is essential

for the development of HVDC grids. Therefore, this paper deals

with the voltage derivative ROCOV scheme to locate and detect

DC faults. The algorithm is able to differentiate internal and

external faults considerably fast. The proposed algorithm is

analyzed in a HVDC grid with different fault case scenarios.

Finally, the ROCOV protection thresholds are discussed.The authors thank the support from the Spanish Ministry of

Economy, Industry and Competitiveness (project

ENE2016-79145-R AEI/FEDER, UE) and GISEL research

group IT1083-16), as well as from the University of the

Basque Country UPV/EHU (research group funding

PPG17/23)

Abarrategi, O.

Díaz, J.

Larruskain, D.M.

Perez-Basante, A.

Rubio, A.

Renewable Energy and Power Quality Journal

English

TECNALIA Publications

 
International Conference on Renewable Energies and Power Quality (ICREPQ’19) 
Tenerife (Spain), 10th to 12th April, 2019 
Renewable Energy and Power Quality Journal (RE&PQJ) 
ISSN 2172-038 X, Volume No.17, July 2019 
 
 
 
ROCOV scheme for Fault Detection and Location in HVDC systems 
 
J. Díaz1, O. Abarrategi1, D.M. Larruskain1,  A. Perez-Basante2 and A. Rubio2 
 
1 Department of Electrical Engineering 
University of the Basque Country UPV/EHU, Bilbao (Spain) 
Phone/Fax number:+0034 946014970, e-mail  oihane.abarrategui@ehu.eus 
2 Energy Unit, Tecnalia 
Parque Tecnológico de Vizcaya, Edificio 700, 48160 Derio (Spain) 
 
 
 
Abstract. A reliable DC fault protection system is essential 
for the development of HVDC grids. Therefore, this paper deals 
with the voltage derivative ROCOV scheme to locate and detect 
DC faults. The algorithm is able to differentiate internal and 
external faults considerably fast. The proposed algorithm is 
analyzed in a HVDC grid with different fault case scenarios. 
Finally, the ROCOV protection thresholds are discussed. 
 
Key words 
 
DC fault, Detection, HVDC Grid, Location, ROCOV.  
 
1. Introduction 
 
The effective protection of HVDC systems is essential. 
Nowadays, the main objective is the protection of Multi 
Terminal DC systems (MTDC) as they provide higher 
reliability through the interconnected network. 
Nevertheless, the protection of MTDC systems is even 
more complicated than point-to-point  HVDC systems. 
 
Plenty detection and location schemes for Multi Terminal 
HVDC systems have been proposed [1]; [2] and [3] use a 
current derivative based protection methodology but the 
DC link inductors limit the current rate. [4] presents a 
scheme based on voltage and presents extended simulations 
to classify the different thresholds. [5] presents a 
methodology based on voltage derivatives to locate DC 
faults but the methodology deteriorates if the sampling 
frequency decreases. Fast back-up protection based on 
linear discriminant analysis (LDA) is proposed in [6-8]. 
 
One of the main challenges is the identification of the line 
in which the fault occurs within an interval of a few 
milliseconds, because the target of the scheme besides the 
protection of the network components, is to be selective, 
i.e. when a fault happens only the conductor in which it 
occurs has to be isolated from the rest of the network.  
 
Thus, [9] addresses  the DC line protection by measuring 
Rate of Change of  Voltage (ROCOV) and the impact of 
fault location on ROCOV and of current limiting reactors 
on the fault current are analyzed. This paper proposes a 
ROCOV approach in a MTDC four terminal meshed 
system, where different fault cases have been applied to 
each of the links. 
 
 
2.  ROCOV 
 
As current rises rapidly in a highly capacitive DC network, 
detecting and locating the fault fast in order to know which 
protection to trip is very important. Because of it, it is not 
possible to use protection methodologies that need end-to-
end communication, as the propagation speeds varies 
depending on the transmission line parameters and it would 
delay the operation too much. It is important to measure the 
fault transient and locate the faulted line. For this purpose, 
overcurrent is a good indicator, so there is the possibility to 
use Rate of Change of Current (ROCOC). However, the 
ROCOC perceived by the Circuit Breaker (CB) is more or 
less proportional to the voltage change on the line side of 
the inductor, so does not give more information that the 
line-side voltage itself [7]. 
  
A possible solution is to use the Rate of the Change of 
Voltage. The variation in the ROCOV voltage change rate 
from an almost null steady state can be used to detect the 
occurrence of a fault very quickly, with only a few 
microseconds since the fault wave reaches the limiting 
inductance of the current derivative. In this way, it allows 
to differentiate the faults in the line that is wanted to be 
protected (internal faults) from other faults that are taking 
place in adjacent lines. This determines that the ROCOV 
values decrease as the fault "moves away” from the 
measure point. 
 
The calculation method has been based on considering the 
voltage differential measured and giving the result in 
kilovolts per millisecond. Thus, the general expression 
resulting from the calculation of the ROCOV is shown in 
eq 1.  
 
https://doi.org/10.24084/repqj17.336 429 RE&PQJ, Volume No.17, July 2019
ௗ௏್ೠೞ
ௗ௧
ൌ
௏మି௏భ
௧మି௧భ
ൌ
௏೔ି௏೔షభ
ଵ൉ଵ଴షఴ
  (1) 
 
The value of the step time considered for the simulations 
(dt=10-8ms) is included in this expression. The ROCOV 
will have a great application in the case of study since, if 
establishing a value of it as a threshold, it is possible to 
determine the minimum possible value measured for a fault 
within the primary protection zone for which the switch 
should turn on, and the maximum value for any fault for 
which the switch should not do so. 
 
Threshold setting determines the fault detection accuracy 
and it is an iterative process, as it depends on the time it 
takes for the fault to clear. Initially, the ROCOV due to fault 
occurrence is determined and used to choose a series of 
preliminary thresholds. Then, the protection scheme can be 
modelled and fault clearance simulated in order to establish 
the maximum ROCOV value. If needed the preliminary 
threshold values can be modified.   
 
 
3. Simulations 
 
In order to test the performance of the proposed ROCOV 
scheme for fault location and detection, a set of simulations 
have been done in a four terminal meshed DC network. The 
Multi Terminal HVDC network incorporates two offshore 
wind farms and two inland converters. These are connected 
to the main electrical network forming a four-terminal DC 
meshed system. For the HVDC test system, standard 
parameters based on current technology [10] are used. In 
addition, the network includes a modular multilevel 
continuous converter (MMC) model in each of its 
terminals, as well as a cable model that depends on the 
frequency and that covers the different possible routes 
between them.  
 
The used software is an EMT type, specifically, the PSCAD 
computer system. This model is a key to the study of 
electromagnetic flows of the DC energy system, such as 
those related to fault situations, protection studies or 
dynamic studies of networks and converters. In addition, 
this system guarantees a certain flexibility since it allows 
adapting the different components to the possible 
phenomena that may appear. The disposition of the study 
case model is shown in Figure 1. In the paper, the protection 
of the system is analysed, developing the basic ROCOV 
algorithm. 
 
Figure 1. - Proposed HVDC network system [11] 
 
The network consists on two links 200 km long (links 13 
and 14), a 150 km link (link 24) and two 100 km links (links 
12 and 34). Their parameters are included in Table I. A 10 
mH filter reactance in series with the MMC converter and 
the DC bus is connected at cable ends. In addition, switches 
are included at the end of each transmission line. The series 
reactor for the DC switches has a value of 100 mH, similar 
to the case proposed in [12]. The remaining parameters 
relating to the AC networks and the HVDC converters are 
shown in Table II. 
 
 
 
  
EXT 
RADIUS 
[mm] p [Ω·m]  ε rel [-] µ rel [-] 
Core 20 1,70E-08 - 1 
Insulation 49 - 2,3 1 
Sheath 52 2,20E-07 - 1 
Insulation 55 - 2,3 1 
Armour 59 1,8-7 - 10 
Insulation 64 - 2,3 1 
Table I. – Parameters of the cables 
 
 
  CONV. 1,2,3 CONV. 4   
Rated power 900 1200 [MVA] 
AC converter voltage 380 380 [kV] 
AC network voltage 400 400 [kV] 
Transformer reactance 0,2 0,2 [pu] 
AC network resistance 1,77 1,34 [Ω] 
AC network reactance 17,7 13,4 [Ω] 
Arm reactor 84,8 63,6 [mH] 
Arm capacitance 29,3 39 [µF] 
Arm resistance 0,885 0,67 [Ω] 
Bus filter reactor 10 10 [mH] 
Table II. - Parameters of the studied network 
 
 
 
3.1 Base case 
 
In the paper a general study is performed, considering 
different faults at the most demanding locations, which are 
the beginning of each of the cables that compose the HVDC 
network. The fault resistance is 0.01Ω, which is a minimum 
value that allows analyzing the worst possible fault 
situation in order to test the reaction of the system. This 
way, faults are applied at the beginning of each cable (as 
close as possible to the bus), since this is the point with a 
higher ROCOV value. Different types of faults have been 
simulated, nevertheless, in this paper, due to page 
limitation, only pole to pole faults are shown. Figure 2 and 
3 show the ROCOV values (kV/ms) for an internal and 
external fault, respectively, since fault inception. The fault 
is applied at link 12 near bus 1, this way; it can be 
considered as an internal fault for the protections of link 12 
https://doi.org/10.24084/repqj17.336 430 RE&PQJ, Volume No.17, July 2019
and as an external fault for the remaining links. The 
ROCOV values in link 12 are much larger than in the 
remaining links. Therefore, it is not possible to use just one 
figure for all links, as the results of the faulted link would 
overlap the remainder curves. In all links, various peak 
values can be noticed, where the highest ROCOV stands 
out. Anyway, the most remarkable one is the initial 
ROCOV peak as it will be the first value that can be 
detected by protections. 
 
Pole to pole faults have been applied in all links. Table III 
depicts the initial and maximum ROCOV peak values in all 
links, when faults are located at both endings of the links. 
These values are measured in both endings of the links. 
 
Table III shows that the initial peak value is approximately 
64.000 kV/ms for all internal faults in the bus located 
beside the fault. However, this number is reduced in the bus 
located in the other ending of the link. Moreover, the peak 
ROCOV is around 93.000 kV/ms in the faulted link. These 
figures are maintained in all fault cases, even though the 
length of the links is different. The measures in the 
remainder links are below 500. Accordingly, there is a 
significant difference between the ROCOV values of 
faulted links and remaining links. This fact is the basis 
proposed for differentiating internal and external faults for 
protection devices. A protection system based on ROCOV 
algorithms can discriminate the existence of an internal 
fault in a link depending on the initial ROCOV peak, this 
value can be used as a threshold to determine a fault 
situation.   
 
Besides, links that are contiguous to the faulted link show 
higher values than the remainder links. Thus, it is critical to 
analyse all cases and stablish an accurate threshold for a 
reliable protection system. The objective is that the CBs 
that are located at both endings of the faulted link trip when 
there is an internal fault. This way, the remaining system 
can continue operating safely. 
 
The measures in both ends of the links are similar to a great 
extent for peak values, but they are delayed 0.66 ms due to 
the traveling waves. The initial value is detected at 0.71 s 
and the peak value at 0.7125 after fault inception.  
 
 
 
 
  
                
 
 
 
 
Figure 2. – ROCOV values for link 12, fault in link 12 
https://doi.org/10.24084/repqj17.336 431 RE&PQJ, Volume No.17, July 2019
 
 
Figure 3. – ROCOV values for links 13, 14, 24 and 34 fault in link 12 
 
 
 
  ROCOV (kV/ms) IN MEASURING BUS 
FAULT RESPONSE DC BUS 1 DC BUS 2 DC BUS 3 DC BUS 4 
LOCATION LINK INITIAL PEAK INITIAL PEAK INITIAL PEAK INITIAL PEAK 
  Link 12 -64327,3 93242,0 -33731,1 91807,5 - - - - 
LINK Link 13 -84,6 82,7 - - -99,7 144,2 - - 
12 Link 14 -84,5 91,7 - - - - -99,7 138,7 
  Link 24 -147,8 406,4 -146,8 407,3 - - -196,2 423,2 
  Link 34 -67,8 90,1 - - -67,6 85,5 -60,2 99,6 
  Link 12 -84,5 275,0 -122,4 264,9 - - - - 
LINK Link 13 -64330,4 93683,8 - - -13821,9 93399,6 - - 
13 Link 14 -84,4 154,1 - - - - -99,4 -144,0 
  Link 24 -42,7 114,9 -43,3 114,8 - - -63,6 -94,3 
  Link 34 -120,6 310,5 - - -120,4 313,5 -180,6 311,5 
  Link 12 -84,3 268,4 -122,2 262,5 - - - - 
LINK Link 13 -84,5 134,3 - - -99,7 -139,6 - - 
14 Link 14 -64334,4 93581,5 - - - - -13822,4 93644,2 
  Link 24 -125,2 170,1 -125,1 171,9 - - -89,8 -150,6 
  Link 34 -136,8 220,0 - - -136,8 221,0 -89,4 224,8 
  Link 12 -147,5 332,2 -96,9 337,8 - - - - 
LINK Link 13 -41,0 -61,0 - - -38,9 62,9 - - 
24 Link 14 -124,8 146,8 - - - - -97,1 152,6 
  Link 24 -64383,1 93631,5 -64383,1 93631,5 - - -96,1 238,6 
  Link 34 -141,8 218,4 - - -144,1 218,4 -23073,3 92920,1 
  Link 12 -36,9 -84,6 -41,9 -78,4 - - - - 
LINK Link 13 -118,8 146,6 - - -101,5 102,8 - - 
34 Link 14 -135,3 139,0 - - - - -110,0 149,5 
  Link 24 -142,2 138,3 -143,1 138,3 - - -110,0 109,7 
  Link 34 -63873,0 93032,6 - - -63873,0 93032,6 -33492,9 92785,4 
 
Table III. – ROCOV (kV/ms) values for pole to pole faults. Base case. 
 
 
 
 
 
 
 
 
https://doi.org/10.24084/repqj17.336 432 RE&PQJ, Volume No.17, July 2019
3.2 Resistance variation 
 
In this section, a sensitivity analysis of the system response 
is shown when the resistance of the fault differs from 1 Ω to 
100 Ω. Table IV shows the initial and peak ROCOV values 
for pole to pole faults applied in link 12, beside bus 1.  
 
 
  ROCOV (kV/ms)  
FAULT RESP. DC BUS 1 DC BUS 2 
RESIST LINK INITIAL PEAK INITIAL PEAK 
  Link 12 -63404,8 93222,4 -33249,6 91803,1 
 Link 13 -83,4 -106,2 - - 
R=1Ω 
 Link 14 -83,3 -106,1 - - 
  Link 24 -145,8 496,1 -145,7 406,0 
  Link 34 -67,3 -100,8 - - 
  Link 12 -59932,3 93137,8 -31436,7 91784,1 
 Link 13 -79,1 -91,5 - - 
R=5Ω 
 Link 14 -79,0 -91,3 - - 
 Link 24 -138,1 414,3 -138,1 414,3 
  Link 34 -63,7 -92,6 - - 
  Link 12 -56092,3 93252,5 -29430,8 91836,3 
 Link 13 -74,2 -231,3 - - 
R=10Ω 
 Link 14 -74,2 -231,2 - - 
  Link 24 -129,7 433,8 -129,7 433,8 
  Link 34 -78,1 115,8 - - 
  Link 12 -49720,8 93121,8 -26100,1 92572,2 
 Link 13 -66,2 -233,0 - - 
R=20Ω 
 Link 14 -65,9 -232,0 - - 
  Link 24 -115,5 -126,0 -115,7 -126,0 
  Link 34 -53,3 -91,5 - - 
  Link 12 -46649,0 -46649,0 -23446,8 92362,0 
 Link 13 -59 7 -204,7 - - 
R=30Ω 
 Link 14 -59,6 -207,6 - - 
  Link 24 -104,7 -264,6 -103,9 -264,6 
  Link 34 -33,4 167,7 - - 
  Link 12 -40516,1 -40511,1 
-
2128207 92254,3 
 Link 13 -54,4 -177,9 - - 
R=40Ω 
 Link 14 -54,3 -184,1 - - 
 Link 24 -94,8 -221,2 -94,6 -217,6 
  Link 34 -29,9 -115,3 - - 
  Link 12 -37083,4 -37083,4 -19484,5 -19484,5 
 Link 13 -49,59 -172,4 - - 
R=50Ω 
 Link 14 -49,0 -178,4 - - 
  Link 24 -87,2 -180,7 -87,2 -180,7 
  Link 34 -28,6 -161,8 - - 
  Link 12 -26084,2 -26084,2 -13697,5 -13697,5 
 Link 13 -35,3 -87,7 - - 
R=100Ω 
 Link 14 -35,2 -79,4 - - 
  Link 24 -61,8 -105,8 -61,6 -106,4 
  Link 34 -21,6 -121,0 - - 
 
Table IV. – ROCOV (kV/ms) values for different fault 
resistances in pole to pole faults in link 12. 
 
 
 
Figure 4.- ROCOV development for different fault resistances. 
 
Finally, Fig. 4 outlines the initial ROCOV values of the 
pole to pole fault in link 12. All measures are taken in bus 
1. The figure shows that larger fault resistances lead to 
lower ROCOV peak values, as expected. Therefore, high 
resistance faults will have low ROCOV values. In 
consequence, these faults can be more challenging to 
detect. 
 
4.  Conclusions 
 
This paper proposes the use of a ROCOV algorithm to 
protect HVDC grids. A four-terminal MTDC has been 
used as a study case. An extensive analysis of fault cases 
show that the proposed algorithm is particularly fast 
discriminating internal and external faults. The paper 
results include the most demanding possible faults, 
consequently, pole to pole faults have been applied in all 
links. The ROCOV values for each fault case have been 
measured in all busses. The length of the cable is not 
considered a relevant factor in view of the fact that the 
values of ROCOV in each case have been very similar. 
 
The value that indicates the existence of a fault is the initial 
peak of the voltage derivative. This value can be measured 
0.71 ms after fault inception in all links. Therefore, the 
demanding protection requirements are fulfilled. As a 
conclusion, the ROCOV scheme can be determined to be 
fast enough for a reliable HVDC protection system.  
 
Moreover, the time in which the initial value appeared and 
the peak value of the fault has been the same in all fault 
cases. This is valuable for determining the exact fault 
inception time. Furthermore, the maximum values of 
ROCOV appear on the cable where the fault occurs, as 
expected. This is the basis for differentiating internal and 
external faults. 
 
 
 
 
 
 
 
 
 
 
https://doi.org/10.24084/repqj17.336 433 RE&PQJ, Volume No.17, July 2019
Acknowledgement 
 
The authors thank the support from the Spanish Ministry of 
Economy, Industry and Competitiveness (project 
ENE2016-79145-R AEI/FEDER, UE) and GISEL research 
group IT1083-16), as well as from the University of the 
Basque Country UPV/EHU (research group funding 
PPG17/23). 
 
References  
 
[1] M.J. Pérez Molina, D.M. Larruskain, P. Eguia Lopez, O. 
Abarrategi, M. Santos-Mugica, “A comparison of non-unit 
and unit protection algorithms for HVDC grids” AEIT 
HVDC International Conference 2019, Florence, Italy, May 
9-10, 2019. 
[2]  L. Tang, ‘‘Control and protection of multi-terminal dc 
transmission systems based on voltage-source converters,’’ 
Ph.D. dissertation, Dept. Elect.Eng., McGill Univ., 
Montreal, Canada, 2003 
[3]  J. Yang, J. Fletcher, J. O’Reilly, ‘‘Multiterminal DC 
wind farm collection grid internal fault analysis and 
protection design’’ IEEE Trans. Power Del., vol. 25, no. 4, 
pp. 2308–2318, Oct. 2010. 
[4]  K. De Kerf, K. Srivastava, M. Reza, ‘‘Wavelet-based 
protection strategy for DC faults in multi-terminal VSC 
HVDC systems’’ IET Generat., Transmiss. Distrib,.vol. 5, 
no. 4, Apr. 2011, pp. 496–503. 
 
 
 
[5]  W. Leterme, J. Beerten, and D. Van Hertem, ‘‘Nonunit 
protection of HVDC grids with inductive DC cable 
termination’’ IEEE Trans. Power Del., vol. 31, no. 2, pp. 
820–828, Apr. 2016. 
[6]  W. Leterme, S. P. Azad, and D. Van Hertem, ‘‘A local 
backup protection algorithm for HVDC grids,’’ 
IEEE Trans. Power Del., vol. 31, no. 4, pp. 1767–1775, 
Aug. 2016. 
[7] M. Abedrabbo, D. Van Hertem, ‘‘A primary and 
backup protection algorithm based on voltage and current 
measurements for HVDC grids’’ in Proc. Int. High 
Voltage Direct Current Conf., Oct. 2016, pp. 854–860. 
[8] S. P. Azad, W. Leterme, D. Van Hertem, ‘‘Fast breaker 
failure backup protection for HVDC grids’’ Electr. Power 
Systs. Res., vol. 138, pp. 99–105, Sep. 2015. 
[9] J. Sneath, A. Rajapakse, ‘‘Fault detection and 
interruption in an earthed HVDC grid using ROCOV and 
hybrid DC breakers’’ IEEE Trans. Power Del., 2014, DOI: 
10.1109/TPWRD.2014.2364547 
[10] M. Monterrubio. “Transporte de energía eléctrica en 
corriente continua. Enlaces HVDC-VSC”. Proyecto fin de 
carrera. Universidad de Valladolid. Marzo, 2013.  
[11] J. Häfner, B. Jacobson, “Proactive Hybrid HVDC 
Breakers: A key innovation for reliable HVDC grids,” in 
CIGRÉ Bologna Symp., Bologna, Italy, 13- 15 Sep. 2011, 
[12] N. Ahmed, S. Norrga, H.-P. Nee, A. Haider, D. Van 
Hertem, L. Zhang, L. Harnefors, “HVDC supergrids with 
modular multilevel converters the power transmission 
backbone of the future,” in Proc. IEEE SSD, 20-23 Mar. 
2012. 
 
 
 
 
 
https://doi.org/10.24084/repqj17.336 434 RE&PQJ, Volume No.17, July 2019


ROCOV scheme for Fault Detection and Location in HVDC sytems

 International Conference on Renewable Energies and Power Quality (ICREPQ’19) Tenerife (Spain), 10th to 12th April, 2019 Renewable Energy and Power Quality Journal (RE&PQJ) ISSN 2172-038 X, Volume No.17, July 2019    ROCOV scheme for Fault Detection and Location in HVDC systems  J. Díaz1, O. Abarrategi1, D.M. Larruskain1,  A. Perez-Basante2 and A. Rubio2  1 Department of Electrical Engineering University of the Basque Country UPV/EHU, Bilbao (Spain) Phone/Fax number:+0034 946014970, e-mail  oihane.abarrategui@ehu.eus 2 Energy Unit, Tecnalia Parque Tecnológico de Vizcaya, Edificio 700, 48160 Derio (Spain)    Abstract. A reliable DC fault protection system is essential for the development of HVDC grids. Therefore, this paper deals with the voltage derivative ROCOV scheme to locate and detect DC faults. The algorithm is able to differentiate internal and external faults considerably fast. The proposed algorithm is analyzed in a HVDC grid with different fault case scenarios. Finally, the ROCOV protection thresholds are discussed.  Key words  DC fault, Detection, HVDC Grid, Location, ROCOV.   1. Introduction  The effective protection of HVDC systems is essential. Nowadays, the main objective is the protection of Multi Terminal DC systems (MTDC) as they provide higher reliability through the interconnected network. Nevertheless, the protection of MTDC systems is even more complicated than point-to-point  HVDC systems.  Plenty detection and location schemes for Multi Terminal HVDC systems have been proposed [1]; [2] and [3] use a current derivative based protection methodology but the DC link inductors limit the current rate. [4] presents a scheme based on voltage and presents extended simulations to classify the different thresholds. [5] presents a methodology based on voltage derivatives to locate DC faults but the methodology deteriorates if the sampling frequency decreases. Fast back-up protection based on linear discriminant analysis (LDA) is proposed in [6-8].  One of the main challenges is the identification of the line in which the fault occurs within an interval of a few milliseconds, because the target of the scheme besides the protection of the network components, is to be selective, i.e. when a fault happens only the conductor in which it occurs has to be isolated from the rest of the network.   Thus, [9] addresses  the DC line protection by measuring Rate of Change of  Voltage (ROCOV) and the impact of fault location on ROCOV and of current limiting reactors on the fault current are analyzed. This paper proposes a ROCOV approach in a MTDC four terminal meshed system, where different fault cases have been applied to each of the links.   2.  ROCOV  As current rises rapidly in a highly capacitive DC network, detecting and locating the fault fast in order to know which protection to trip is very important. Because of it, it is not possible to use protection methodologies that need end-to-end communication, as the propagation speeds varies depending on the transmission line parameters and it would delay the operation too much. It is important to measure the fault transient and locate the faulted line. For this purpose, overcurrent is a good indicator, so there is the possibility to use Rate of Change of Current (ROCOC). However, the ROCOC perceived by the Circuit Breaker (CB) is more or less proportional to the voltage change on the line side of the inductor, so does not give more information that the line-side voltage itself [7].   A possible solution is to use the Rate of the Change of Voltage. The variation in the ROCOV voltage change rate from an almost null steady state can be used to detect the occurrence of a fault very quickly, with only a few microseconds since the fault wave reaches the limiting inductance of the current derivative. In this way, it allows to differentiate the faults in the line that is wanted to be protected (internal faults) from other faults that are taking place in adjacent lines. This determines that the ROCOV values decrease as the fault "moves away” from the measure point.  The calculation method has been based on considering the voltage differential measured and giving the result in kilovolts per millisecond. Thus, the general expression resulting from the calculation of the ROCOV is shown in eq 1.   https://doi.org/10.24084/repqj17.336 429 RE&PQJ, Volume No.17, July 2019ௗ௏್ೠೞௗ௧ൌ ௏మି௏భ௧మି௧భൌ ௏೔ି௏೔షభଵ൉ଵ଴షఴ  (1)  The value of the step time considered for the simulations (dt=10-8ms) is included in this expression. The ROCOV will have a great application in the case of study since, if establishing a value of it as a threshold, it is possible to determine the minimum possible value measured for a fault within the primary protection zone for which the switch should turn on, and the maximum value for any fault for which the switch should not do so.  Threshold setting determines the fault detection accuracy and it is an iterative process, as it depends on the time it takes for the fault to clear. Initially, the ROCOV due to fault occurrence is determined and used to choose a series of preliminary thresholds. Then, the protection scheme can be modelled and fault clearance simulated in order to establish the maximum ROCOV value. If needed the preliminary threshold values can be modified.     3. Simulations  In order to test the performance of the proposed ROCOV scheme for fault location and detection, a set of simulations have been done in a four terminal meshed DC network. The Multi Terminal HVDC network incorporates two offshore wind farms and two inland converters. These are connected to the main electrical network forming a four-terminal DC meshed system. For the HVDC test system, standard parameters based on current technology [10] are used. In addition, the network includes a modular multilevel continuous converter (MMC) model in each of its terminals, as well as a cable model that depends on the frequency and that covers the different possible routes between them.   The used software is an EMT type, specifically, the PSCAD computer system. This model is a key to the study of electromagnetic flows of the DC energy system, such as those related to fault situations, protection studies or dynamic studies of networks and converters. In addition, this system guarantees a certain flexibility since it allows adapting the different components to the possible phenomena that may appear. The disposition of the study case model is shown in Figure 1. In the paper, the protection of the system is analysed, developing the basic ROCOV algorithm.  Figure 1. - Proposed HVDC network system [11]  The network consists on two links 200 km long (links 13 and 14), a 150 km link (link 24) and two 100 km links (links 12 and 34). Their parameters are included in Table I. A 10 mH filter reactance in series with the MMC converter and the DC bus is connected at cable ends. In addition, switches are included at the end of each transmission line. The series reactor for the DC switches has a value of 100 mH, similar to the case proposed in [12]. The remaining parameters relating to the AC networks and the HVDC converters are shown in Table II.      EXT RADIUS [mm] p [Ω·m]  ε rel [-] µ rel [-] Core 20 1,70E-08 - 1 Insulation 49 - 2,3 1 Sheath 52 2,20E-07 - 1 Insulation 55 - 2,3 1 Armour 59 1,8-7 - 10 Insulation 64 - 2,3 1 Table I. – Parameters of the cables     CONV. 1,2,3 CONV. 4   Rated power 900 1200 [MVA] AC converter voltage 380 380 [kV] AC network voltage 400 400 [kV] Transformer reactance 0,2 0,2 [pu] AC network resistance 1,77 1,34 [Ω] AC network reactance 17,7 13,4 [Ω] Arm reactor 84,8 63,6 [mH] Arm capacitance 29,3 39 [µF] Arm resistance 0,885 0,67 [Ω] Bus filter reactor 10 10 [mH] Table II. - Parameters of the studied network    3.1 Base case  In the paper a general study is performed, considering different faults at the most demanding locations, which are the beginning of each of the cables that compose the HVDC network. The fault resistance is 0.01Ω, which is a minimum value that allows analyzing the worst possible fault situation in order to test the reaction of the system. This way, faults are applied at the beginning of each cable (as close as possible to the bus), since this is the point with a higher ROCOV value. Different types of faults have been simulated, nevertheless, in this paper, due to page limitation, only pole to pole faults are shown. Figure 2 and 3 show the ROCOV values (kV/ms) for an internal and external fault, respectively, since fault inception. The fault is applied at link 12 near bus 1, this way; it can be considered as an internal fault for the protections of link 12 https://doi.org/10.24084/repqj17.336 430 RE&PQJ, Volume No.17, July 2019and as an external fault for the remaining links. The ROCOV values in link 12 are much larger than in the remaining links. Therefore, it is not possible to use just one figure for all links, as the results of the faulted link would overlap the remainder curves. In all links, various peak values can be noticed, where the highest ROCOV stands out. Anyway, the most remarkable one is the initial ROCOV peak as it will be the first value that can be detected by protections.  Pole to pole faults have been applied in all links. Table III depicts the initial and maximum ROCOV peak values in all links, when faults are located at both endings of the links. These values are measured in both endings of the links.  Table III shows that the initial peak value is approximately 64.000 kV/ms for all internal faults in the bus located beside the fault. However, this number is reduced in the bus located in the other ending of the link. Moreover, the peak ROCOV is around 93.000 kV/ms in the faulted link. These figures are maintained in all fault cases, even though the length of the links is different. The measures in the remainder links are below 500. Accordingly, there is a significant difference between the ROCOV values of faulted links and remaining links. This fact is the basis proposed for differentiating internal and external faults for protection devices. A protection system based on ROCOV algorithms can discriminate the existence of an internal fault in a link depending on the initial ROCOV peak, this value can be used as a threshold to determine a fault situation.    Besides, links that are contiguous to the faulted link show higher values than the remainder links. Thus, it is critical to analyse all cases and stablish an accurate threshold for a reliable protection system. The objective is that the CBs that are located at both endings of the faulted link trip when there is an internal fault. This way, the remaining system can continue operating safely.  The measures in both ends of the links are similar to a great extent for peak values, but they are delayed 0.66 ms due to the traveling waves. The initial value is detected at 0.71 s and the peak value at 0.7125 after fault inception.                            Figure 2. – ROCOV values for link 12, fault in link 12 https://doi.org/10.24084/repqj17.336 431 RE&PQJ, Volume No.17, July 2019  Figure 3. – ROCOV values for links 13, 14, 24 and 34 fault in link 12      ROCOV (kV/ms) IN MEASURING BUS FAULT RESPONSE DC BUS 1 DC BUS 2 DC BUS 3 DC BUS 4 LOCATION LINK INITIAL PEAK INITIAL PEAK INITIAL PEAK INITIAL PEAK   Link 12 -64327,3 93242,0 -33731,1 91807,5 - - - - LINK Link 13 -84,6 82,7 - - -99,7 144,2 - - 12 Link 14 -84,5 91,7 - - - - -99,7 138,7   Link 24 -147,8 406,4 -146,8 407,3 - - -196,2 423,2   Link 34 -67,8 90,1 - - -67,6 85,5 -60,2 99,6   Link 12 -84,5 275,0 -122,4 264,9 - - - - LINK Link 13 -64330,4 93683,8 - - -13821,9 93399,6 - - 13 Link 14 -84,4 154,1 - - - - -99,4 -144,0   Link 24 -42,7 114,9 -43,3 114,8 - - -63,6 -94,3   Link 34 -120,6 310,5 - - -120,4 313,5 -180,6 311,5   Link 12 -84,3 268,4 -122,2 262,5 - - - - LINK Link 13 -84,5 134,3 - - -99,7 -139,6 - - 14 Link 14 -64334,4 93581,5 - - - - -13822,4 93644,2   Link 24 -125,2 170,1 -125,1 171,9 - - -89,8 -150,6   Link 34 -136,8 220,0 - - -136,8 221,0 -89,4 224,8   Link 12 -147,5 332,2 -96,9 337,8 - - - - LINK Link 13 -41,0 -61,0 - - -38,9 62,9 - - 24 Link 14 -124,8 146,8 - - - - -97,1 152,6   Link 24 -64383,1 93631,5 -64383,1 93631,5 - - -96,1 238,6   Link 34 -141,8 218,4 - - -144,1 218,4 -23073,3 92920,1   Link 12 -36,9 -84,6 -41,9 -78,4 - - - - LINK Link 13 -118,8 146,6 - - -101,5 102,8 - - 34 Link 14 -135,3 139,0 - - - - -110,0 149,5   Link 24 -142,2 138,3 -143,1 138,3 - - -110,0 109,7   Link 34 -63873,0 93032,6 - - -63873,0 93032,6 -33492,9 92785,4  Table III. – ROCOV (kV/ms) values for pole to pole faults. Base case.         https://doi.org/10.24084/repqj17.336 432 RE&PQJ, Volume No.17, July 20193.2 Resistance variation  In this section, a sensitivity analysis of the system response is shown when the resistance of the fault differs from 1 Ω to 100 Ω. Table IV shows the initial and peak ROCOV values for pole to pole faults applied in link 12, beside bus 1.      ROCOV (kV/ms)  FAULT RESP. DC BUS 1 DC BUS 2 RESIST LINK INITIAL PEAK INITIAL PEAK   Link 12 -63404,8 93222,4 -33249,6 91803,1  Link 13 -83,4 -106,2 - - R=1Ω  Link 14 -83,3 -106,1 - -   Link 24 -145,8 496,1 -145,7 406,0   Link 34 -67,3 -100,8 - -   Link 12 -59932,3 93137,8 -31436,7 91784,1  Link 13 -79,1 -91,5 - - R=5Ω  Link 14 -79,0 -91,3 - -  Link 24 -138,1 414,3 -138,1 414,3   Link 34 -63,7 -92,6 - -   Link 12 -56092,3 93252,5 -29430,8 91836,3  Link 13 -74,2 -231,3 - - R=10Ω  Link 14 -74,2 -231,2 - -   Link 24 -129,7 433,8 -129,7 433,8   Link 34 -78,1 115,8 - -   Link 12 -49720,8 93121,8 -26100,1 92572,2  Link 13 -66,2 -233,0 - - R=20Ω  Link 14 -65,9 -232,0 - -   Link 24 -115,5 -126,0 -115,7 -126,0   Link 34 -53,3 -91,5 - -   Link 12 -46649,0 -46649,0 -23446,8 92362,0  Link 13 -59 7 -204,7 - - R=30Ω  Link 14 -59,6 -207,6 - -   Link 24 -104,7 -264,6 -103,9 -264,6   Link 34 -33,4 167,7 - -   Link 12 -40516,1 -40511,1 -2128207 92254,3  Link 13 -54,4 -177,9 - - R=40Ω  Link 14 -54,3 -184,1 - -  Link 24 -94,8 -221,2 -94,6 -217,6   Link 34 -29,9 -115,3 - -   Link 12 -37083,4 -37083,4 -19484,5 -19484,5  Link 13 -49,59 -172,4 - - R=50Ω  Link 14 -49,0 -178,4 - -   Link 24 -87,2 -180,7 -87,2 -180,7   Link 34 -28,6 -161,8 - -   Link 12 -26084,2 -26084,2 -13697,5 -13697,5  Link 13 -35,3 -87,7 - - R=100Ω  Link 14 -35,2 -79,4 - -   Link 24 -61,8 -105,8 -61,6 -106,4   Link 34 -21,6 -121,0 - -  Table IV. – ROCOV (kV/ms) values for different fault resistances in pole to pole faults in link 12.    Figure 4.- ROCOV development for different fault resistances.  Finally, Fig. 4 outlines the initial ROCOV values of the pole to pole fault in link 12. All measures are taken in bus 1. The figure shows that larger fault resistances lead to lower ROCOV peak values, as expected. Therefore, high resistance faults will have low ROCOV values. In consequence, these faults can be more challenging to detect.  4.  Conclusions  This paper proposes the use of a ROCOV algorithm to protect HVDC grids. A four-terminal MTDC has been used as a study case. An extensive analysis of fault cases show that the proposed algorithm is particularly fast discriminating internal and external faults. The paper results include the most demanding possible faults, consequently, pole to pole faults have been applied in all links. The ROCOV values for each fault case have been measured in all busses. The length of the cable is not considered a relevant factor in view of the fact that the values of ROCOV in each case have been very similar.  The value that indicates the existence of a fault is the initial peak of the voltage derivative. This value can be measured 0.71 ms after fault inception in all links. Therefore, the demanding protection requirements are fulfilled. As a conclusion, the ROCOV scheme can be determined to be fast enough for a reliable HVDC protection system.   Moreover, the time in which the initial value appeared and the peak value of the fault has been the same in all fault cases. This is valuable for determining the exact fault inception time. Furthermore, the maximum values of ROCOV appear on the cable where the fault occurs, as expected. This is the basis for differentiating internal and external faults.           https://doi.org/10.24084/repqj17.336 433 RE&PQJ, Volume No.17, July 2019Acknowledgement  The authors thank the support from the Spanish Ministry of Economy, Industry and Competitiveness (project ENE2016-79145-R AEI/FEDER, UE) and GISEL research group IT1083-16), as well as from the University of the Basque Country UPV/EHU (research group funding PPG17/23).  References   [1] M.J. Pérez Molina, D.M. Larruskain, P. Eguia Lopez, O. Abarrategi, M. Santos-Mugica, “A comparison of non-unit and unit protection algorithms for HVDC grids” AEIT HVDC International Conference 2019, Florence, Italy, May 9-10, 2019. [2]  L. Tang, ‘‘Control and protection of multi-terminal dc transmission systems based on voltage-source converters,’’ Ph.D. dissertation, Dept. Elect.Eng., McGill Univ., Montreal, Canada, 2003 [3]  J. Yang, J. Fletcher, J. O’Reilly, ‘‘Multiterminal DC wind farm collection grid internal fault analysis and protection design’’ IEEE Trans. Power Del., vol. 25, no. 4, pp. 2308–2318, Oct. 2010. [4]  K. De Kerf, K. Srivastava, M. Reza, ‘‘Wavelet-based protection strategy for DC faults in multi-terminal VSC HVDC systems’’ IET Generat., Transmiss. Distrib,.vol. 5, no. 4, Apr. 2011, pp. 496–503.    [5]  W. Leterme, J. Beerten, and D. Van Hertem, ‘‘Nonunit protection of HVDC grids with inductive DC cable termination’’ IEEE Trans. Power Del., vol. 31, no. 2, pp. 820–828, Apr. 2016. [6]  W. Leterme, S. P. Azad, and D. Van Hertem, ‘‘A local backup protection algorithm for HVDC grids,’’ IEEE Trans. Power Del., vol. 31, no. 4, pp. 1767–1775, Aug. 2016. [7] M. Abedrabbo, D. Van Hertem, ‘‘A primary and backup protection algorithm based on voltage and current measurements for HVDC grids’’ in Proc. Int. High Voltage Direct Current Conf., Oct. 2016, pp. 854–860. [8] S. P. Azad, W. Leterme, D. Van Hertem, ‘‘Fast breaker failure backup protection for HVDC grids’’ Electr. Power Systs. Res., vol. 138, pp. 99–105, Sep. 2015. [9] J. Sneath, A. Rajapakse, ‘‘Fault detection and interruption in an earthed HVDC grid using ROCOV and hybrid DC breakers’’ IEEE Trans. Power Del., 2014, DOI: 10.1109/TPWRD.2014.2364547 [10] M. Monterrubio. “Transporte de energía eléctrica en corriente continua. Enlaces HVDC-VSC”. Proyecto fin de carrera. Universidad de Valladolid. Marzo, 2013.  [11] J. Häfner, B. Jacobson, “Proactive Hybrid HVDC Breakers: A key innovation for reliable HVDC grids,” in CIGRÉ Bologna Symp., Bologna, Italy, 13- 15 Sep. 2011, [12] N. Ahmed, S. Norrga, H.-P. Nee, A. Haider, D. Van Hertem, L. Zhang, L. Harnefors, “HVDC supergrids with modular multilevel converters the power transmission backbone of the future,” in Proc. IEEE SSD, 20-23 Mar. 2012.      https://doi.org/10.24084/repqj17.336 434 RE&PQJ, Volume No.17, July 2019

http://dsp.tecnalia.com/bitstream/11556/750/1/336-19-diaz.pdf

ROCOV scheme for Fault Detection and Location in HVDC sytems

Abstract

Similar works

Full text

Available Versions

TECNALIA Publications

TECNALIA Publications