FAULT ANALYSIS OF HVDC TRANSMISSION SYSTEMS

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International Journal of Electrical Engineering & Technology (IJEET)
Volume 7, Issue 3, May–June, 2016, pp.106–116, Article ID: IJEET_07_03_009
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FAULT ANALYSIS OF HVDC
TRANSMISSION SYSTEMS
MUJIB J. PATHAN
ME Student, Government College of Engineering Aurangabad,
Maharashtra, India
Dr. V. A KULKARNI
Associate Professor, Government College of Engineering Aurangabad,
Maharashtra, India
ABSTRACT
This paper analyzes the behaviour of a Voltage Source Converter Based
HVDC system under DC pole to ground fault & AC faults for 2-level VSC-
HVDC & 12-pulse VSC-HVDC system in order to better understand the
system under such faults. DC line faults on HVDC systems utilising Voltage
Source Converters (VSC) are a major issue for HVDC systems in which
complete isolation of the faulted system is not a viable option. The occurrence
of pole-to-ground faults on DC link is the most common fault in HVDC system.
It was observed that with the occurrence of DC pole to ground faults leads to
substantial over current in the AC grid system which may lead to damage of
the converter valve. Simulation of 2-level VSC-HVDC under AC fault is
carried out. The fault current magnitude is attempted with the mathematical
analysis & which was found to be the same as the simulated result. This paper
also compares the performance of the conventional 12-pulse (CSC) HVDC
system with the PWM based 2-level VSC-HVDC & 12-pulse VSC-HVDC
system.
Key words: CSC-HVDC, Fault Analysis, IGBT, PWM, THD, VSC
Cite this Article: Mujib J. Pathan and V. A Kulkarni, Fault Analysis of
HVDC Transmission Systems. International Journal of Electrical Engineering
& Technology, 7(3), 2016, pp. 106–116.
http://www.iaeme.com/ijeet/issues.asp?JType=IJEET&VType=7&IType=3
1. INTRODUCTION
In India, with the rapid increase in energy demand, grid integration of renewable
energy sources has become very essential [1], [2]. In the different renewable energy
sources, wind energy benefits from HVDC technology for power transmission to
enhance the system performance [3].There is two technologies of HVDC transmission

Fault Analysis of HVDC Transmission Systems
systems: the conventional (CSC) HVDC system which uses the line commutated
thyristor valve and VSC based IGBT using PWM technique makes it economically
feasible to connect small scale, renewable power generation plants to the main AC
grid [4].Voltage source converter-based-HVDC (VSC-HVDC) systems are considered
to be the technology of choice for efficient grid integration which provides the fast
and independent control of active and reactive power flow in both directions [5]–[8] ,
low harmonic generation which enhances the power quality and stability of the
system[9].
The most dominant and frequent faults on the HVDC system are DC Pole-ground
fault on DC link & AC faults such as L-G, L-L & LLL .These faults are analyzed in
this paper. When DC pole-ground fault occurs, substantial over current generated due
to the rapid decrease in the DC voltage. The high AC grid current contribution into
DC fault passes through the freewheeling diodes which may damage the converter
valve due to high fault current so some protection is needed to overcome the DC side
problems in the HVDC Converters [10]. The behaviour of 2-level VSC-HVDC & 12-
pulse VSC-HVDC under AC faults on sending end of transformer are analyzed.
This paper is organized in six sections. Following the introduction, Section II
presents HVDC system model. Three models have been developed for 2-level VSC,
12-Pulse (CSC) Conventional HVDC & 12-Pulse VSC-HVDC. The simulation result
shows the superiority of VSC-HVDC with the CSC-HVDC system. Section III
analyses the behaviour of 2-level VSC- HVDC system & 12-pulse VSC-HVDC
systems under DC pole-ground fault. Section IV analyzes the behaviour of 2-L VSC
HVDC & 12-pulse VSC-HVDC under AC fault. Simulation of 2-level VSC-HVDC is
carried out under AC faults e.g. L-G, L-L & LLL faults. The magnitude of fault
current is attempted. Section V compares the performance of the conventional 12-
pulse (CSC) HVDC system with the PWM based 2-level VSC-HVDC & 12-pulse
VSC-HVDC system using FFT analysis. In the last, Section VI consists of conclusion.
2. HVDC SYSTEM MODEL
Figure 1 Basic HVDC transmission system model for Simulation
The block diagram for basic HVDC transmission system model for Simulation is
shown in Fig.1.The parameter of the HVDC system are given in Table I [11],
[12].Three simulation models have been developed: 2-level VSC, 12-pulse VSC &
12-pulse conventional HVDC. It consists of the main components such as
transformer, AC filter, DC-link capacitor, DC filter & converters. The key parts of
HVDC are converters which can realize the conversion from AC to DC bi-
directionally.

Mujib J. Pathan and V. A Kulkarni
TABLE I Parameter For HVDC System Model
3. DC POLE-GROUND FAULT ANALASIS
The behaviour of 2-level VSC-HVDC & 12-pulse HVDC under DC pole-ground
faults is analyzed. When DC pole-ground fault occurs, substantial over current
generated due to rapid decrease in the DC voltage .The high AC grid current
contribution into DC fault passes through the freewheeling diodes which may damage
the converter valve due to high fault current [11].
The basic layout of 2-level VSC-HVDC system under the DC pole-ground fault
on the DC transmission line shown in fig.2
Figure 2 Basic layout of VSC-HVDC under DC pole-ground fault
The simulation of 2-level VSC-HVDC is carried out & corresponding voltage &
current waveform during the normal operation is shown in Fig.3 (a) & fig.3 (b)
respectively.(a)
2-Level VSC-HVDC
Rated base power 450 MW
Grid ph voltage(Peak)(AC) 100KV
3-ph transformer ratio 1:1
Rated (base) DC voltage 200KV
Rated (base) DC current 2000 A
DC cable resistance 14 mΩ/km
DC cable inductance 0.1mH/Km
DC link capacitor 100µF
12-Pulse HVDC
Grid line-line voltage(AC) 400KV
Converter Transformer 298.6MVA
Smoothing Reactor 360mH
Rated DC Voltage 500 KV
Rated DC Current 1500A

(a) (b)
(c) (d)
(e)
Figure 3 Simulation results for 2-level VSC-HVDC System (a) DC link voltage during
normal operation, (b) DC link current during normal condition, (c) DC current during pole-
ground fault,(d) DC voltage during pole-ground fault, (e) AC grid current during fault
when a single pole-ground fault occurs at t=0.08s on the sending end of the DC
line, the DC voltage of the DC link capacitor voltage rapidly decreases, resulting in a
significant rise in DC fault current & after 0.01s isolates the fault so system comes to
normal condition as shown fig.3(c) &fig .3(d) respectively. Fig.3 (e) shows the AC
grid current contribution during DC Pole-ground fault increases the magnitude of
fault current in AC side.
Simulation of 12-pulse VSC –HVDC is carried out the corresponding voltage &
current waveform under normal & faulty condition shown in fig.4

(a) (b)
(c) (d)
Figure 4 Simulation results for 12-pulse VSC-HVDC System (a) DC link voltage during
normal operation, (b) DC link current during normal condition, (c) DC current during fault,
(d) DC voltage during fault, (e) AC grid current during fault
Fig.4 (a) & (b) shows the voltage & current waveform during normal operation for
12-pulse VSC-HVDC respectively. when a single pole-ground fault occurs at t=0.08s
on the sending end of the DC line its DC link voltage rapidly decreases & fault
current increases in magnitude& at t=0.09s isolates fault so the system comes to
normal condition as shown in fig.4(c) & fig.4(d) respectively. Fig.4 (e) shows the AC
grid current contribution during the DC Pole-ground fault increases the magnitude of
fault current in AC side.
4. AC FAULT ANALYSIS
The behaviour of 2-level VSC-HVDC & 12-pulse VSC-HVDC transmission systems
under the AC faults like L-G; L-L & LLL faults are analyzed. Simulation for 2-level
VSC–HVDC is carried out for fault on AC side. AC faults like L-G, L-L, and LLL
fault are created and variations in current magnitude are observed.

A] Simulation
 Single line to ground (LG) fault: The single line to ground fault occurs on the primary
side of the converter transformer at rectifier end. Fault occurs on phase A at 0.08s and
last for 0.01sec, the current magnitude of phase A increases and voltage across it
decreases rapidly to zero. When fault isolates at 0.09sec the waveform comes to
normal value as shown in fig 5(a)
 Line- line (LL) fault: The double line fault occurs on the primary side of the converter
transformer at rectifier. Fault occurs on phase A & phase B at 0.08s and lasts for
0.01sec, the two phase currents are same in magnitude and system becomes
unbalanced as shown in fig.5 (b)
 Three phase to ground (LLL) fault: A three phase is the most severe fault compared
to the other two faults. Fault occurs on 3-phases at 0.08sec, the fault current
magnitude is found to be more compare to other two faults as shown in the fig.5(c)
(a) (b)
(c)
Figure 5 Simulation results for 2-level VSC-HVDC System (a) fault current during, (b) Fault
current during L-L, (c) fault current during LLL
B] Mathematical Analysis
To validate the result of 2-level VSC–HVDC under the AC fault with the simulated
result the mathematical analysis has done, the single line block diagram under such
condition is shown in fig.6

Figure 6 Diagram indicating AC fault location
The mathematical analysis of same system during L-G, L-L & LLL faults has
been attempted; the fault current magnitudes during L-G, L-L & LLL fault are
calculated [13].
The magnitude of AC fault current during L-G fault is given by
The magnitude of AC fault current during L-L fault is given by
The magnitude of AC fault current during L-L-L fault is given by
It is observed that the magnitude of fault current is calculated mathematically is
found to be nearly equal with the simulated result. As the calculation is done
neglecting the magnetising components the little variation is observed. The table II
shows the fault current magnitude with simulated case result & mathematical
calculated results.
Table II Fault current magnitude
Simulation of 12-pulse VSC-HVDC under AC faults L-G, L-L, LLL faults has
been carried out & corresponding fault current magnitude are shown in fig.7
(a) (b)
Type of
fault
Simulated
Result
Mathematical
calculated Result
L-G 4×10 6
A 3.24×10 6
A
L-L 4.9×10 6
A 4.54×10 6
A
LLL 6×10 6
A 5.4×10 6
A

Figure 7 Simulation results for 12-pulse VSC-HVDC System (a) fault current during L-G, (b)
Fault current during L-L, (c) fault current during LLL
5. THD CMPARISON
The performance of the 12-pulse conventional (CSC) HVDC system with the PWM
based 2-level VSC-HVDC & 12-pulse VSC-HVDC system is done using FFT
analysis. The THD comparison for 2-level VSC-HVDC, 12-pulse conventional (CSC)
HVDC & 12-pulse VSC-HVDC has been done using FFT analysis as shown in the
Fig.8.
(a)

(b)
(c)
Figure 8 FFT analysis (a) 12-pulse VSC-HVDC, (b) 12-pulse conventional (CSC) HVDC, (c)
2- level VSC-HVDC

Table III Comparison of total harmonic distortion
Converter THD
2-L VSC 15.39%
12-Pulse CSC 9.21%
12-Pulse VSC 4.21%
It is observed that the output waveform for 12-Pulse VSC-HVDC nearer to sin
wave & its THD is least as compared to the 2-level VSC & conventional HVDC
Transmission systems. It also ensures that as the no. of pulses of the converter
increases the THD decreases which minimizes the filter requirement & enhances the
power quality of the system.
CONCLUSION
This paper analyses the behaviour of HVDC systems under DC pole-ground fault &
AC fault for both 2-level VSC-HVDC system & 12-pulse VSC-HVDC technology. It
is observed that the occurrence of the DC pole to ground faults leads to substantial
over current in the system which may lead to damage of the converter valve.
Simulation of 2-level VSC-HVDC is carried out under AC faults like L-G, L-L, LLL
faults. The magnitude of fault current is verified by mathematical analysis. This paper
also compares the performance of the 12-pulse conventional (CSC) HVDC system
with the PWM based 2-level VSC-HVDC system and
12- Pulse VSC-HVDC system using FFT analysis. It is observed that the output
waveform for
12-Pulse VSC-HVDC nearer to sin wave & its THD is least as compared to the 2-
level VSC-HVDC & conventional HVDC Transmission systems which minimizes the
filter requirement & enhances the power quality of the system.
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FAULT ANALYSIS OF HVDC TRANSMISSION SYSTEMS

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