2022 2nd International Conference on Power Electronics & IoT Applications in Renewable Energy and its Control (PARC) | 978-1-6654-3215-3/22/$31.00 ©2022 IEEE | DOI: 10.1109/PARC52418.2022.9726546
2022 2nd International Conference on Power Electronics & IoT Applications in Renewable Energy and Its Control (PARC)
GLA University, Mathura, India. Jan 21-22, 2022
Frequency Spectral Analysis of 6-Pulse LCC-HVDC
in Single Conductor Ground Return Configuration
Using FFT
Ravi Shankar Tiwari
Department of Electrical Engineering,
GLA University,
Mathura, UP-281406, India,
e-mail: ravishankar.tiwari@gla.ac.in
Om Hari Gupta
Department of Electrical Engineering,
National Institute of Technology
Jamshedpur, JH-831014, India,
e-mail: omhari.ee@nitjsr.ac.in
which operated then have either been scrapped or upgraded to
semiconductor converter technology[6]. Semiconductors
devices have been in use since the 1970s and are still a growing
technology because of the high switching capacity and ability
to withstand high current rating. Examples are the diode, diac,
triac, thyristors, MOS-controlled thyristors (MCTs) [10],
insulated-gate bipolar transistors (IGBT), and integrated gatecommutated thyristors (IGCTs), etc. [7]. This paper looks
critically into the two dominant HVDC converter technologies
taking into consideration. In [8], explains the modeling control
and design of a 12-pulse VSC-HVDC transmission system.The
detailed modeling of ± 500 kV LCC-HVDC and simulation
using PSCAD/EMTDC is performed in [9]. The model
includes an AC system, AC/DC filters, converter transformers,
smoothing reactors, and transmission line – appropriate for
various studies in HVDC transmission systems.
Abstract – HVDC is one of the viable options for electrical
power transmission and integration of renewable energy into the
existing grid. To make an economic, controlled and efficient
exchange of power HVDC uses current source converter (CSC)
and voltage source converter (VSC) technologies. The 6-pulse
LCC is the basic building block of CSC. The realization of
flexible power flow control, asynchronous interconnection, and
higher line loading is possible only because of HVDC
transmission using LCC or VSC. However, the use of LCC or
VSC causes the injection of harmonics in both the AC and DC
sides of the system. Thismay further lead to higher losses and
complexity in operation. Therefore, it is important to analyze and
eliminate the dominant frequency components generated. This
work performs the spectral analysis of line parameters on both
AC and DC sides of each rectifier station to detect the dominant
harmonic component present using FFT. The test system was
developed using 6-pulse LCC-HVDC at sending station and
constant load inversion mode at receiving station. The length of
the HVDC transmission line is 300 km having distributed
parameters supplying power to the constant load at receiving
end. The MATLAB/ Simulink environment is used to develop the
test system and digitalized at the sampling interval of 43μs.
The spectrum analysis of the HVDC system is performed
due to various reasons such as fault analysis, small signal
stability analysis and to figure out the oscillatory phenomenon
generated in between wind firmsand HVDC systems. The
widely popular signal processing tools are Wavelet transforms
(WT), discrete Fourier transforms (DFT), Fast Fourier
transform (FFT), Neural Network (NW), Support vector
machines (SVM), and Machine learning, etc. Stockwell
transforms and mathematical morphology are the other multiresolution and non-linear signal shape-based techniques. A
synthetic signal is used to facilitate didactic analysis of various
transforms in[10] with the sampling frequency of 128 samples
frequency of 7.268 kHz.Reference [11] reveals that harmonicbased and FFT techniques are faster and providea detailed
frequency spectrum than those transforms based on timefrequency and WL. In [12], realizes dominant frequencies in
DC line parameters using FFT to implement the distance
protection scheme in HVDC lines.
Keywords: LCC-HVDC, VSC, FFT, Graetz circuit, IGBT
I.
INTRODUCTION
HVDC transmission makes use ofpower electronic
converters for changing AC supply to DC at the sending station
and convertingback to AC at the receiving station[1], [2].
These converters usually have a two 6-pulse arrangement
connected in series, forming a 12-pulse converter connected in
a star-star, star-delta, to the AC networks. A DC capacitor, AC
filters, and inductor and are also included in the converter
circuitry. The two convertersstations are linked via submarine
or overhead cables at the same location for back-to-back
HVDC connection. Thecontinuous advancement in HVDC
systems is associatedwithadvances in the design of efficient
power electronics converters[3]. Here are the two prevailing
methods used for AC to DC or vice versa conversion. These
methods are voltage source converter (VSC) and Line
commutated converter (LCC). These technologies are
successful because of the development in power electronics
devices [4], [5]. Previous to the LCC or VSC, there are several
attempts executed for AC to DC or vice-versa conversion but
was unsuccessful due to safety reasons. The discovery of
mercury-arc valves gavetransitory success to this conversion
which afterward became obsolete. The mercury-arc valve
978-1-6654-3215-3/22/$31.00 ©2022 IEEE
Salauddin Ansari
Department of Electrical Engineering,
National Institute of Technology
Jamshedpur, JH-831014, India,
e-mail: 2019rsee006@nitjsr.ac.in
II.
TEST SYSTEM DESCRIPTION AND DESIGN
The layout in Fig. 1 shows the interconnection of the test
system. It is operating to supply 250 kV, 2 kA, 500 MW DC
supply between the rectifier andinverter stations. The sending
end rectifier station is fed through a 5000 MVA AC grid
operating a line voltage of 315 kV. The transformer interfacing
AC system to the converters is of 6000 MVA, 315/210 kV
rating. The universal bridge rectifier based on 6-pulse LCC is
designed for the AC to DC conversion. A diode for
unidirectional supply together with a 24 kV Dc is used to
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degree in electrical and the 3rd and 2nd valves are in conduction,
the instantaneous output voltage is given as (2).
model the inverter station of the HVDC transmission system.
The two stations are connected via a 300 km overhead
transmission line of distributed parameters. Smoothening
reactors and AC filters are the integral parts of either station.
AC filters serve the purpose ofeliminatingdominant harmonics
as well as supplying reactive power at the converter stations.
The smoothing rectors of 0.5 H to 0.2 H are connected at the
terminal of DC lines. For, AC filters rated at 300 MVA, tuned
for 5th and 7th harmonics connected before converter
transformer.
Converter
Station 1
Ir, Vr
Pole 1
Power
system 1
Vdc
Converter
Station 2
Ii, Vi
AC filters
Vh
6-PULSE (GRAETZ CIRCUIT) CONVERTER TOPOLOGY
AND HARMONICS ANALYSIS
In
eb
Va
Vdc
Vb
ec
Vc
Fig. 2. Converter transform with Graetz rectifier circuit
The desired voltage and current ratings are achieved by
properly selecting the number of valves connected in series or
parallel combinations of valve strings. If “s” valve are
connected in series in a string, “r” be the number of strings
connected in parallel, and “q” valves are present in a
commutation group than, the total valves are given by “qrs”.
Thus, the converter average DC voltage is given as (1)
Vd 0
sq
S
Vm .Sin
S
q
D 60
³
D
(eb ec ).dZt
3 2
S
VLLCosD
(3)
Vd 0
2
[1 (h 2 1)sin 2 D ]1/2
h 1
2
(4)
6
nS
.I p
(5)
WhereIp is the peak current flowing through converter
valves transformer primary windings.There are various signal
processing tools available to analyzethe AC/DC system
parameters. Wavelet is one of the techniquesthat give
information related to harmonics contents of a non-stationary,
non-periodic signal generated. It provides variable window
sizes without the risk of time-frequency resolution.
However,the Fourier transform givesrelatively poor timefrequency resolution. The wide data window of FFT provides
fine frequency and mediocretime resolution. The FFT is
suitable for developing protection schemes based on the THD,
DC component, or magnitude of fundamental frequency
components, etc. This does not require the fine frequency
resolution and provides a certain degree of flexibility in the
classification of different operational states[13], [14].
Lsr
ea
(2)
The smoothing reactor maintains the ripple-free DC line
current, thus free from harmonic contents during normal
operation. However, the harmonic component present in
alternating current circulating through the converter valves and
transformer primary windingisgiven by (5).
Fig. 2 shows the schematic diagram of the Graetz circuit
producing a 6-pulse output voltage. The circuit is widely used
as the basic building block of HVDC converters. It provides the
most use of converter unit and transformer and maintainsthe
low voltage across the valves not participating in conduction.
This low voltage across non-conducting valves is equal to the
peak inverse voltage which valve can withstand.
3 phase
AC
supply
6
2S
AC filters
Fig. 1. six-pulse LCC-HVDC Test System
III.
2VL sin(Zt 600 )
The DC output voltage (Vdc) hasa ripple frequency of six
times the fundamental. Fourier series analysis technique is used
to extract the order of harmonic contents existing in (3) and it
would be expressed as (4).
P-G
fault
F1
eb ec
The average DC output at the terminals of HVDC lines are
givens as (3)
Lsi
Lsr
A1
Vd
To analyze the voltage and current signal It has been widely
recognized that wavelet transform is agood technique to
analyze the non-periodic and non-stationary signal, such as
voltage and current with fault transient in the power system. It
offers variable window sizes without the risk of compromising
time-frequency resolution. On the other hand, Short Time
Fourier Transform (STFT) is relatively not known as a
favorable candidate due to its poor time-frequency resolution;
wide window gives finer frequency resolution but mediocre
time resolution, and vice versa. Even so, STFT can still be
used, granted, a certain degree of compromise is allowed. A
protection system does not need a highly precise measurement
of harmonics to perform effectively. Hence, for STFT to be
used for the application of fault detection, it is worth paying the
price for sacrificing frequency resolution in exchange for better
(1)
The Graetzcircuit includes the six thyristors or IGBT valves
strings arranged in bride type and a converter transformeris
provided with a tapping arrangement for voltage control.The
transformer primary windings are connected in star with neutral
grounded fed from AC. However, secondary windings may
eitherbe connected in star or delta with ungrounded neutral
supplying AC power to the converter bridge. When the six
valve strings are fired in a definite order produces a DC supply
that hassixpulses per cycle of AC. For firing angle delays by α
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response of various AC system parameters during steady-state,
dynamic and transient operation states are shown in Fig. 6.
time information, which is far more significant. The STFT in
(4) is computed by taking the Discrete Time
Fourier Transform (DTFT) of each windowed block and the
process illustrated in Fig. 3
0.5
-0.5
0.45
0.55 0.6 0.65
(a) Time (s)
alpha degree
Vdr p.u.
0
-1
0.45
0.5
2
2
1
VdL
Id
1
2
-0.5
1
0.9
0.5
α0
1
0
0.5
0.6
(a) Time (s)
0.65
1
0.55
0.6
(b) Time (s)
0.65
0.55
0.6
(d) Time (s)
0.65
150
100
50
-1
0.55
30
0.6
(c) Time (s)
0.65
0
20
Fig. 5. Response of (a) DC line current (Idc), (b) DC line voltage (Vdc), (c)
rectifier terminal voltage and (d) firing angle α0, after fault clearance and
due to the HVDC regulator action
10
0.3
0.4
(c) Time (s)
0.55
0
50
40
0
-1
2
0.3
0.4
(b) Time (s)
0.55 0.6 0.65
(d) Time (s)
50
0
0.45
0.55 0.6 0.65
(c) Time (s)
Alpha (α)
0.5
0.5
100
1.5
Vdr
0.3
0.4
(a) Time (s)
0.55 0.6 0.65
(b) Time (s)
Fig. 4. Close-in fault (F2), response of (a) DC line current (Idc), (b) DC line
voltage (Vdc), (c) rectifier terminal voltage and (d) firing angle α0, at
rectifier
1.1
0.8
0.2
0.5
150
1
0
VdL p.u.
Idc , Pref p.u.
0
0.5
0.5
1
0.5
2
1
Vdr p.u.
0.5
0
0.45
A. DC-AC line parametric response during various
operational states
Case (a) - The steep rise in reference power from 250 MW
(0.5 p.u.) to 375 MW (0.75 p.u.) causes the DC line current
changes from 1 kA (0.5 p.u.) to 1.5 kA (0.75 p.u.). This is
achieved by the action of the HVDC regulator, which reduces
the firing angle (α) from 300 to 150 using PI control and current
regulator. The response time of the current regulator is of the
order of .1 s or 6 cycles of AC fundamental.
-1
0.2
1
VdL p.u.
SIMULATION RESULTS AND DISCUSSION
The simulation results are obtained by taking the sampling
interval of 43.4 μs. A dynamic response result of the test
system is obtained by applying a step-change in DC line
current at 0.3 s and a DC line fault is applied at 0.5 s. The
response of current regulator time is plotted for each casesuch
as a) step rise inreference power of 0.25p.u, and b) close-in
short circuit fault in HVDC line[15], [16]. The results of the
two cases are shown in Fig. 3 and Fig. 4, respectively
0
0.2
1.5
Idc , P r ef p.u.
IV.
2
1.5
0
0.2
0.3
0.4
(d) Time (s)
0.5
2
Iabc p.u.
Fig. 3. Response of (a) Dc line current (Idc), (b) DC line voltage (Vdc), (c)
rectifier terminal voltage and (d) firing angle α0, at the rectifier
Cases (b) - The short circuit close-in fault is simulated at
0.5 s and the fault persists for a duration of 50 ms. The
response of DC line parameters during close-in fault is shown
in Fig. 4. A close-in short circuit at the DC line leads to a rise
in fault current up to 4 kA (2 p.u.) and a rise in Id up to 2.96 kA
(1.48 p.u.) within 10 ms. The regulator action activated after 10
ms to limit the fault current up to the nominal value of 2 kA.
After confirmation that the rise in current is due to fault
current, the protection system leads to a rise in firing angle by
300 to 1650. Thus, the operational mode of the rectifier changed
into inversion mode. The DC line transient energy started to
feedback into the AC network to recover system normal
operation. The response of the HVDC regulator along with
other DC line parameters is shown in Fig. 5. Similarly, the
1
Change in Pref
0
-1
-2
0.2
Vabc p.u.
2
Inception of F2
HVDC Regulator action
0.3
Steady state
Response
0.4
0.5
(a) Time (s)
0.6
0.7
0.3
0.4
0.5
(b) Time (s)
0.6
0.7
1
0
-1
-2
0.2
Fig. 6. AC side response during steady-state, dynamic, and transient
conditions with (a) three-phase current (Iabc) (b) three-phase voltage
(Vabc)
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B. Frequency Spectral Analysis During Steady-state,
Dynamic and Transient operation of HVDC system
The magnitude of fundamental and harmonic components
is expressed as the percentage of DC componentsthat exists.
The FFT analysis uses the sampled data of two cycles with the
fundamental frequency of 60 Hz. The maximum frequency is 2
kHz and the maximum frequency selected for THD
computation is Nyquist. Fig. 7 represents the steady-state FFT
of AC phase voltage measured on the rectifier side of the
HVDC system. The fundamental signal magnitude is 14.6e-5 %
of DC components (7.007e-05). Fig. 8, represents the FFT of
AC line currents (Iabc) during steady-state operation. This
analysis can effectively utilize to detect the operating mode of
the HVDC link using local measurements of AC-DC sides.
Similarly, the FFT analysis using AC line current during the
dynamic operational state of the HVDC link is shown in Fig. 9.
The FFT during the transient state of HVDC lines generated
due to close-faults at sending end is plotted in Fig. 10 and Fig.
11. Fig. 10, shows the FFT of line current (Iabc), while Fig. 11,
indicates phase voltages. Fig. 12, indicates the FFT analysis of
DC line current during the transient operational state of the
HVDC line. It indicates thatvarious high-frequency
components exist and can be detected based on FFT
magnitude. Thus, the magnitude of the fundamental frequency
component, THD, and order of harmonics can be significantly
utilized to indicate variousoperational states of the HVDC
system.
DC = 0.02024 , THD= 4220.90%
4000
3500
Mag (% of DC)
3000
2500
2000
1500
1000
500
0
200
400
600
800
1000
Frequency (Hz)
1200
1400
1600
1800
2000
Fig. 9. Magnitude versus frequency spectrum of AC side phase current Iabc
during dynamic operation state (i.e. change in power demand)
DC = 0.2041 , THD= 604.69%
X = 60
Y = 545
Mag (% of DC)
500
400
300
200
100
0
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Frequency (Hz)
Fig. 10. Magnitude versus frequency spectrum of AC side phase current (Iabc)
during close in fault at rectifier terminals
DC = 7.007e-05 , THD= 1469058.81%
16x105
0
x 105
DC = 0.0004209 , THD= 229882.82%
14
2
Mag (% of DC)
Mag (% of DC)
12
10
8
6
4
1.5
1
0.5
2
0
0
0
200
400
600
800
1000
1200
Frequency (Hz)
1400
1600
1800
2000
200
400
600
800 1000 1200
Frequency (Hz)
1400
1600 1800
2000
Fig. 11. FFT of AC phase voltages (Vabc) during close-in fault at HVDC line at
rectifier terminal
Fig. 7. Magnitude versus frequency spectrum of AC side phase voltages Vabc
during steady-state operation
DC = 0.000171 , THD= 356589.53%
x 105
0
DC = 0.04977 , THD= 1214.18%
600
500
3
Mag (% of DC)
Mag (% of DC)
3.5
2.5
2
1.5
1
300
200
100
0.5
0
400
0
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Frequency (Hz)
0
200
400
600
800 1000
1200
Frequency (Hz)
1400
1600
1800
2000
Fig. 12. FFT of DC line current (Idc) during close-in fault at HVDC line near
sending end
Fig. 8. Magnitude versus frequency spectrum of AC side phase current Iabc
during steady-state operation
4
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V.
CONCLUSION
[5]
The FFT analysis of LCC-HVDC line used for
interconnecting remort thermal power plants and on-shore and
off-shore wind generations are performed. The spectral analysis
of AC and DC-side line parameters are performed using the
data window of 2 cycles. The variation of line parameters along
with the frequency spectrum are used to classified the
operational state of the HVDC system.The magnitude of
frequency spectrum obtained is expressed relative to the
magnitude of the DC component present in the line parameters.
Results indicate the strength of fundamental frequency
components during both steady-state and dynamics operation is
much compared to the transient state operational state. It is
concluded that the FFT of AC and DC line currents are more
suitable than that of line voltages for the classification of
various operational states such as short circuit, change in load
or normal operation. During dynamic and transient states, the
magnitude of DC components in line currents rises
significantly compared to steady-state. Thus, FFT analysis of
AC line currents or DC line current or voltage and THD
calculated can clearly distinguish the operation states of the
HVDC system.
[6]
[7]
[8]
[9]
[10]
[11]
[12]
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