new synthetic test circuit for testing thyristor valve in hvdc converter
DESCRIPTION
Thyristor Valve in Hvdc ConverterTRANSCRIPT
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5:10PM New Synthetic Test Circuit for Testing Thyristor Valve in HVDC ConverterByung-Moon Han, Myongji University, Korea (South)
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1
New Synthetic Test Circuit for Testing
Thyristor Valve in HVDC Converter
Hyun-Jun Kim, Do-Hyun Kim, Byung-Moon Han
Depart. Of Electrical Engineering
Myongji University
Seoul, Korea
Jae-Hun Jung, Eui-Cheol Nho
Depart. Of Electrical Engineering
Pukyung National University
Busan, Korea
Abstract— This paper proposes a new synthetic test circuit to
confirm the switching behavior of thyristor valve in HVDC
converter. The proposed circuit consists of a 2-phase chopper
with IGBT switch and auxiliary circuit to supplies low-voltage
high current to the test valve during on-state. The operation of
proposed circuit was verified through theoretical approach and
computer simulation. Based on computer simulation results, a
hardware scaled model for the proposed circuit was built and
tested to confirm the feasibility of implementing a real test
facility. The proposed synthetic test circuit could be widely used
to test the thyristor valve with synthetic manner.
Index Terms— Synthetic Test Circuit, Thyristor Valve, HVDC
(High Voltage DC) Converter, 2-Phase Chopper, Auxiliary
Circuit
I. INTRODUCTION
The power converter in HVDC system is the most
important element, which converts AC power to DC power or
vice versa. The HVDC power converter can be classified into
current source type and voltage source type, based on the type
of switching valves. Thyristor-based current source converter,
which was first installed in the beginning of 1980, is still
continuously under construction all over the world [1].
Voltage source HVDC power converter was first
commissioned in Gotland, Sweden in 1999. The number of
installation is continuously increased because of many
advantages over current source type HVDC converter.
However, the power rating and operation voltage of voltage
source HVDC power converter are still much lower than that
of current source HVDC power converter. So, its application
is focus on the grid connection of large off-shore wind farm
[2].
The HVDC power converter operates in extremely high-
voltage and high power condition. So, many units of thyristor
are connected in series to comprise one valve in the power
converter. The normal or abnormal operation of power
converter should be tested before it is commissioned in the
site. A huge test facility is required when the power converter
is tested under full power and voltage, which consumes a
huge amount of power and expense. And safety is also a big
issue due to extremely high testing voltage. In order to solve
this problem an effective test scheme was derived, in which
the turn-on and turn-off characteristic of thyristor valve is
tested under operation current and voltage when the power
converter operates under full voltage and current. The test
circuit to implement this scheme is called the STC (Synthetic
Test Circuit). [3,4,5]
The STC consists of a low-voltage high-current source to
provide the forward current during turn-on state, and a low-
current high-voltage source to provide the reverse voltage and
forward voltage during turn-off state. The STC is a critical
item for the manufacturer to build and install a current source
converter in HVDC system. The world leading manufacturer
in HVDC power converter such as ABB, Siemens, or Alstom
has operated its own STC.[6,7,8]
This paper proposes a new STC which provides turn-on
current through a low-voltage large-current source composed
of a 2-phase chopper with auxiliary circuit. The operation of
proposed STC is analyzed using computer simulations. Based
on the simulation results a hardware scaled model was built
and tested to confirm the simulation results.
II. OPERATION OF SYNTHETIC TEST CIRCUIT
Fig. 1 shows a general configuration of STC. It is
composed of a low-voltage high-current source which
provides a large current to the test valve VT during turn-on
state, and a low-current high-voltage source with a resonant
circuit which provides the reverse voltage and forward
voltage to the test valve. The resonant circuit changes the
polarity of voltage applied to the test valve by properly
turning on or off the auxiliary valve.
This work was financially supported by the 2nd Brain Korea 21 Project of MEST and the advanced human resource development program of MKE
through the Smart Grid Center in Myongji University (20114010203030).
2
Figure. 1. Configuration of Synthetic Test Circuit
The resonant circuit is comprised of three auxiliary
thyristor valves Va3-Va5, a resonant capacitor Cs, and two
reactors L1 and L2. The auxiliary thyristor Va1 applies turn-on
current to the test valve, and the auxiliary valve Va2 provides
high voltage to the resonant circuit.
Several structures are available to configure the low-
voltage high-current source. ABB proposes a back-to-back
converter with two six-pulse thyristor bridges, while Alstom
proposes a thyristor converter with series-connected
STATCOM (Static Synchronous Compensator). Siemens also
proposes a similar current source with ABB’s, in which the
resonant circuit has a different structure.
Whatever structure is selected, the STC should have a
current source to provide enough turn-on current and
maintain proper commutation overlap by setting the
commutation inductance as small as possible. Also the STC
can continuously provide the high voltage with reverse and
forward polarity, and the voltage level can be properly
adjusted during test. The required high voltage is normally
obtained from the rectified voltage in distribution system. If
the test voltage is over this level, a Cockcroft-Walton bridge
is utilized to raise it up to the proper voltage level.
The operational principle of STC can be explained with
the voltage-current waveform of test valve in Fig. 2, the
equivalent circuits for each mode in Fig. 3, and the operation
sequence in table I.
Figure. 2. Voltage-Current Waveform of Test Valve
At first it is assumed that the capacitor Cs is charged with
the polarity shown in Fig. 3a. When the auxiliary valve Va1
and the test valve VT turn on, the current source provides the
turn-on current. If the auxiliary valve Va3 turns on, the
auxiliary valve Va1 turns off and the current path Cs-Va3-L1-
VT forms a resonant circuit. The polarity of capacitor Cs is
reversed, and the auxiliary valve Va3 and the test valve VT
turn off.
If the auxiliary valve Va4 turns on, the reversed voltage of
capacitor Cs is applied to the test valve VT as shown in Fig.
3d. If the auxiliary valve Va5 turns on, the current path Cs-
Va5-L2 forms another resonant circuit as shown in Fig. 3e.
After half period of resonance, the voltage polarity of
capacitor Cs is reversed and the auxiliary valves Va5 and Va4
turn off as shown in Fig. 3f. If the auxiliary valve Va3 turns
on, a forward voltage is applied to the test valve VT.
The forward voltage is lower than the initial value due to
the losses of components. In order to compensate this
reduction, the auxiliary valve Va2 turns on and the voltage
across capacitor Cs rises as shown in Fig. 3g. If the voltage
across capacitor Cs is fully charged, the auxiliary valve Va2
turns off automatically as shown in Fig. 3h
(a) to-t1 (b) t1-t3
(c) t2-t3 (d) t3-t4
(e) t4-t5 (f) t5-t6
(g) t7-t8 (h) at t9
Figure. 3. Equivalent Circuit for Each Operation Mode
3
TABLE I. OPERATION SEQUENCE OF AUXILIARY VALVE
Time Valve Path
t0 ~ t1 t0
Va1
VT
ON
ON
Va1 - VT
t1 ~ t3
t1 Va3 ON
t2 Va1 OFF
t3 Va3 OFF
OFF
Cs - Va3 - L1 - VT
t3 ~ t4 t3 Va4 ON
Cs - VT - L1 – Va4
t4 ~ t6
t4 Va5 ON
t5 Va3
Va4
ON
OFF
t6 Va5 OFF
Cs - Va5 – L2
t7 ~ t8 t7 Va2 ON
t8 Va2 OFF
Va2 – L2 - VT
t9 t9 Va1
VT
ON
ON
Va1 - VT
III. NEW SYNTHETIC TEST CIRCUIT
A new STC was devised through detailed operation
analyses of the existing circuits developed by ABB, Siemens,
and Alstom. The newly proposed STC has a low-voltage
high-current source which is comprised of 2-phase chopper
and auxiliary switching circuit. The resonant circuit to supply
the reverse and forward voltage in turn-off period has same
structure as the circuit explained in the previous chapter.
Fig. 4 shows a configuration of proposed STC, in which
the current source is described in detail. In this figure the left
side shows the current source for supplying turn-on current to
the test valve, while the right side shows the resonant circuit
and the high voltage source.
Figure. 4. Configuration of Proposed Synthetic Test Circuit
The current source to supply the turn-on current consists of
a 6-pulse thyristor bridge, unidirectional 2-phase chopper,
and an auxiliary valve turn-off circuit. The rising rate of test
valve current is controlled by adjusting the output voltage of
6-pulse phase-controlled rectifier. The circuit to turn off the
auxiliary valve Va1 consists of a 6-pulse diode rectifier, two
IGBT switches, and capacitor C.
The operational principle of new STC can be explained
using the gate-pulse diagram shown in Fig. 5, the equivalent
circuit for each mode in Fig. 6, and the operation sequence of
each switching element in table 2.
Figure. 5. Valve Voltage and Test Valve Current
Fig. 5 shows timing diagram of the gate pulses for each
switching element, the test valve voltage, and the voltage
across the auxiliary valve Va1. At t0 the test valve VT,
auxiliary valve Va1, chopper switches IGBT1 and IGBT2, and
the auxiliary switch IGBT3 turn on and the supplying current
to the test valve starts to build up. This current stops rising at
t1, after then keeps constant value until t2 by the PWM switch
operation of 2-phase chopper. At t2 the chopper switches
IGBT1 and IGBT2 turn off to decrease the test valve turn-on
current. The rising or falling rate of valve turn-on current is
determined by the chopper input voltage and the inductance
of La and Lb.
The falling current ends at t4 when the switch IGBT3 and
the auxiliary valve Va1 turn off. At t3 a resonant current
composed of the capacitor Cs and inductor L1 occurs, which
ends at t5. When the switch IGBT4 turns on at t4, the voltage
across the auxiliary valve Va1 has a negative value.
Fig. 6 shows equivalent circuits classified according to
according to the operation mode, in which the 6-pulse phase-
controlled rectifier and the 6-pulse diode rectifier is
represented by equivalent voltage source Vin1 and Vin2. The
commutation inductance determines the current increasing
rate di/dt when IGBT1, IGBT2, IGBT3, Va1, and VT turn on.
It is assumed that the capacitors Cs and C are initially charged
with the polarity shown in Fig. 6a. When the turn-on current
reaches the desired value, both switches IGBT1 and IGBT2
operate in PWM switching pattern to maintain the constant
current. When the PWM switching is over, the magnetic
energy stored in La and Lb produce free-wheeling current
through the path composed of Diode1(Diode2)-La(Lb)-
IGBT3-Va1, and VT. The free-wheeling current keeps flowing
until it decreases down to zero. Before it reaches zero, the
auxiliary valve Va3 turns on to simulate the turn-off di/dt
characteristic of VT by injecting the resonant current whose
equivalent circuit is shown in Fig. 5c. So, the current through
VT has a shape of overlapping the supplying current and the
resonant current. Even though the supplying current reaches
zero, the auxiliary valve Va1 cannot be perfectly off. For
confirming the perfect turn-off, reverse voltage is applied to
the Va1 by turning on IGBT4. The reverse voltage of Vin2-Vin1
is applied through the path composed of C-IGBT4-La(Lb)-
4
IGBT1(IGBT2)-Vin1 as shown in Fig. 5d. When the resonant
current reaches zero, the test valve VT and auxiliary valve Va3
turn off. And the voltage polarity across the capacitor Cs is
reversed through the path Cs-Va3-L1-VT as shown in Fig. 6e.
When the auxiliary valve Va4 turns on, this reverse voltage is
applied to the test valve VT as shown in Fig. 6f. Other
operation modes for supplying the forward voltage across the
test valve are same as the operation mode described in
Chapter II.
(a)
(b) (c)
(d)
(e) (f)
Figure. 6. Equivalent Circuit for Each Operation Mode
TABLE II. OPERATION SEQUENCE OF EACH SWICHING ELEMENT
Time Component Path
t0
VT
IGBT1
IGBT2
IGBT3
Va1
IGBT4
ON
ON
ON
ON
ON
OFF
IGBT1(IGBT2) - La(Lb)
-IGBT3-
Va1 - VT
t1 IGBT1
IGBT2
PWM ON
PWM ON
IGBT1(IGBT2) - La(Lb)
-IGBT3-Va1 - VT
t2 IGBT1
IGBT2
PWM OFF
PWM OFF
Diode1(Diode2) - La(Lb)
-IGBT3-Va1 - VT
t3 Va3 ON
Diode1(Diode2) - La(Lb)-IGBT3
-Va1 - VT & Cs - Va3 - L1 - VT
t4
IGBT3
Va3
IGBT4
OFF
OFF
ON
Cs - Va3 - L1 - VT
t5
VT
Va3
Va4
OFF
OFF
OFF
t6 IGBT4
Va4
OFF
ON
IV. SIMULATION ANALYSIS
Computer simulation with PSCAD/EMTDC software was
carried out to verify the operation and performance of
proposed STC. The proposed STC considered in simulation
has a scale down rating with 30A forward current and 200V
reverse voltage in order to compare the simulation results
with the experimental results of scaled hardware model. The
circuit parameters were properly determined in order to
simulate the actual phenomena in the real-size STC. Table III
shows circuit parameters which was used in the scaled
hardware model. The current source, voltage source, IGBT
switches, and thyristor switches were represented using the
built-in model in PSCAD/EMTDC, while the gate pulse
generator and switching control were represented by a
separate user-defined model.
TABLE III. CIRCUIT PARAMETER FOR SCALED MODEL OF STC
Component Value Unit
Vin1 25 V
Vin2 50 V
Voltage Source 200 V
La , Lb 0.5 mH
C 1000 uF
L1 9 mH
L2 2 mH
Cs 15 uF
Fig. 7 shows simulation results when the test valve turns
on and off in the proposed STC. Fig. 8a shows the supplying
current from the 2-phase chopper. Both IGBT currents have a
ripple due to the chopper switching, in which the current
ripple through IGBT1 has 180o phase shifted from that
through IGBT2. It is clear that the phase interleave scheme
works perfectly to reduce the current ripple. In Fig. 8b the
first graph in black shows the current wave form when the
test valve turns off and on. And the first graph in red shows
the current wave form of the auxiliary valve Va1. The test
valve current has a wave form with superposition of the
auxiliary valve Va1 current and the resonant current. The
second graph shows the voltage across the test valve. At the
5
instant when the test valve current is zero, the reverse voltage
is applied to the test valve. After half period of resonance the
voltage polarity across the resonant capacitor changes to be
positive. The voltage across the test valve gets to zero.
The third graph shows the voltage across the auxiliary
valve va1, in which this voltage has negative value while the
resonant circuit operates. After then this voltage has similar
shape with reverse polarity to the voltage across the test valve.
Therefore, it is clear that the voltage-current characteristic of
simulation results is identical to the graph in Fig. 2.
(a) Interleaved current waveform of 2-phase chopper
(b) Current and voltage waveform of VT and Va1
Figure. 7. Simulation Results for Proposed Synthetic Test Circuit
V. SCALED MODEL EXPERIMENT
Based on simulation results, a scaled hardware model was
built using same circuit parameters as in the simulation. Fig.
8a shows a picture of the scaled hardware model and its test
set-up. Three DC power supplies were used to supply the
turn-on current and the reverse voltage for the test valve, and
the reverse voltage for the auxiliary valve. Fig. 8b shows an
enlarged picture of DSP controller, sensing bode, and gate
drives for IGBT and Thyristor switches. Fig. 8c shows a
power circuit including the test valve, auxiliary valves, IGBT
switches, and inductors and capacitors for the resonant circuit.
The main controller composed of DSP TMS320F28335 has
enough A/D and D/A ports, and CAN communication port to
send gate signal and control signal.
(a) Scaled hardware test set-up
(b) Controller and Gate drive
(b) Power circuit
Figure. 8. Test Set-up for Proposed Synthetic Test Circuit
Fig. 9 shows experimental results of the scaled hardware
model for the proposed STC. Fig. 9a shows the current
waveform from the 2-phase chopper, in which the superposed
current is displayed on Ch2, while the current waveforms
through each IGBT switch are displayed on Ch3 and Ch4.
Through the experimental results it is confirmed that the
superposed current has negligible ripples even though each
IBGT current has large amount of ripples.
Fig. 9b shows the turn-off characteristic of the auxiliary
6
valve Va1 and the test valve VT. The test valve current is
displayed on Ch1 and the voltage across the auxiliary valve
Va1 is displayed on Ch2. The voltage across the test valve is
displayed on Ch3 and the gate pulse for IGBT4 is displayed
on Ch4. Through these waveforms, it is confirmed that the
reverse voltage is properly applied to the auxiliary valve Va1.
Fig. 9c shows the voltage across the test valve and current
through the test valve. These waveforms are identical to the
theoretical waveforms in Fig. 2 and the simulated waveforms
in Fig. 7. Through these experimental results, it is confirmed
that the proposed STC can be utilized to test the three kinds
of operation, forward conduction, reverse sustain, and
forward sustain of thyristor valve.
(a) Current waveform of 2-phase chopper
(b) Turn-on and Turn-off characteristic of Val and VT
(c) Turn-on and Turn-off characteristic of VT
Figure. 9. Scaled-Hardware Experimental Results
VI. CONCLUSION
In this paper a new STC was proposed which can be
utilized to test the switching operation of thyristor valve in
HVDC power converter. The proposed STC consists of a 2-
phase chopper with IGBT switch and auxiliary circuit to
supplies low-voltage large current to the test valve during on-
state. The resonant circuit consists of inductor, capacitor, and
auxiliary valves which can supply the reverse voltage and the
forward voltage during off-state.
The operation feasibility of proposed STC was first
verified through theoretical analysis and computer
simulations. Based on computer simulation results, a scaled
hardware model for the proposed STC was built and tested to
confirm the feasibility of implementing a real test facility.
The proposed STC could be widely used for testing the
thyristor valve in HVDC Converter if it is built in real size.
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