Effects of Inverter Control Parameters on
Commutation Failure for a HVDC Link and on
Torsional Vibrations for a Turbine Generator
Chi-Hshiung Lin

Abstract--In the paper, we analyze the commutation failure
events caused by ac network faults happened at the rectifier and
inverter sides of a HVDC link. It is found the inverter controls are
significant in affecting the commutation behavior, whereas the
rectifier controls are almost ineffective. For a rectifier side ac
fault, the control parameters of the inverter current regulator are
crucial to the commutation failure. As to an inverter side ac fault,
the control parameters of the inverter voltage regulator play a
non-negligible role. Further, it is found the single-phase-toground fault is the most likely one to cause the commutation
failure. The three-phase-to-ground fault is less likely to cause the
commutation failure than a single-phase-to-ground fault, but
when it does, a repetitive commutation failure is very likely to
take place and the HVDC l
i
nk wo
n’
t recover smoothly. In
addition, it is studied the torsional vibrations induced on the
inverter side turbine generator due to a rectifier side ac fault. It is
found there may be up to several times difference in the torsional
torques induced, depending on the commutation behavior. And on
certain settings of the inverter current regulator parameters, a
single-phase-to-ground fault might cause a more serious impact
on a turbine generator than a three-phase-to-ground fault.
Index Terms—HVDC Link; Turbine Generator; Commutation
Failure; Inverter; Torsional Vibration
I. INTRODUCTION
C
OMMUTATION failure is a phenomenon that the transfer
of current from the off-going valve to the on-going valve
is not successful for a thyristor converter. For a HVDC system,
it is a serious event and a lot of works have been done to
understand such a phenomenon [1, 2]. It is shown that the
basic reason for commutation failure is that the extinction
angle during system disturbance is too small. When a thyristor
valve is going to turn off, the internal excess charges must be
removed before it can establish a forward blocking capability.
If forward voltage is imposed across the turning-off valve
before the storage charges have been removed completely, the
thyristor valve cannot turn off successfully. As a result,
commutation failure happens. Therefore, it requires a
sufficient negative voltage-time area (or a sufficiently large
extinction angle) for a valve to ensure the completion of a
commutation process.
Also, a lot of works have been done to know the cause and
consequence of the commutation failure [3, 4, 5]. It is shown
that commutation failure is mainly caused by the ac network
C. H. Lin is with the Department of Electrical Engineering, Kao Yuan
University, Kaohsiung, Taiwan. (e-mail: lin_chi_hshiung@hotmail.com)
faults. When there are disturbances in the ac systems, a sudden
transient reduction in the commutation voltage or a sudden rise
in the dc current might be induced. And reports have revealed
that commutation failures may happen during an ac system
disturbance, where the voltage reduction is only as small as
10%. Once commutation failure happens, it generally causes
stresses on valves, delay in restart time and, even, interruption
of transmitted power. Fortunately, many approaches have been
found to be helpful for alleviating the problem. For examples,
larger smoothing reactors may limit the rate of rise of dc
current, increasing in short circuit ratio of the inverter bus may
reduce the voltage dip during faults, a greater margin angle for
an inverter may provide more time for commutation, etc.
Moreover, there are control approaches proposed for
decreasing the commutation failure frequency [6, 7, 8]. They
are generally to continuously monitor the ac system
disturbances. Occasionally a dangerous situation is detected,
the control system advances the firing time of thyristors.
According to the survey above, the studies about the
effect of control parameters of HVDC converters on
commutation failure have never been found. We deem that this
effect is significant because the transient response of network
is crucial to the commutation time available, so studies are
made in this paper. However, it is a challenge because there
are interactions between the rectifier and inverter units as well
as between the voltage and current regulators. The interactions
are too complicated to be analyzed by analytic approach, so
just the simulation analyses are adopted here. It is found that
the inverter current control parameters are crucial to
commutation failure when subjecting to the rectifier side ac
faults. As to the inverter side ac faults, it is the inverter voltage
control parameters to play a significant role. Rectifier controls
are found to be ineffective, so no further discussions will be
made.
In another aspect, we have further studied the torsional
vibrations induced at turbine generator units. For the
operations of a HVDC link to impact the turbine generator
units, many have been done. For examples, the torsional
interactions between HVDC link and turbine generators have
been studied in [9, 10]. The HVDC line faults to stress the
turbine generator shafts were also studied in [11]. In [12], the
impacts on rectifier side turbine generators causing by rectifier
station faults were investigated. However, the paper didn’
t
involve the commutation failure. In [13] and [14], the impacts
of the inverter station faults on the rectifier side turbine
generators and on the inverter side turbine generators were
investigated, respectively. These two papers have taken into
consideration the commutation failure. According to the
論文獎勵成果彙編P31
survey above, it can be seen that the impacts of rectifier side
faults on inverter side turbine generators haven’
t been studied.
So, the studies in the paper would make up this deficiency. It is
found that, on certain settings of the inverter current regulator
parameters, a single-phase-to-ground fault might cause a more
serious impact on a turbine generator than a three-phase-toground fault.
II. SYSTEM STUDIED
A. System Descriptions
The system studied is an asynchronously linked HVDC
system, as shown in Fig. 1. A 1,000MW (500kV, 2kA) dc
transmission line is used to transmit power from a 5,000MVA
(500kV, 60Hz) network to a 10,000MVA (345kV, 50Hz)
network. The converter configuration is typical, including a
12-pulse rectifier and a 12-pulse inverter. The dc transmission
line is 300km long, with a 0.5H of smoothing reactor on each
side. Each converter transformer, in a typical YY-and-YD
winding configuration, equips with a tap changer. In normal
operation, the primary voltage of the rectifier transformer is
keeping at 0.90pu and the one of the inverter transformer is
keeping at 0.96pu. The reactive power required by the
converters is provided by a set of filters (capacitor bank plus
11th, 13th and high-pass filters, total 600Mvar on each side).
depending on the firing pulse as well as the thyristor voltage
and current. The switching dynamics (e.g. commutation failure)
of a converter could be accurately represented by using such a
model.
The control strategy of the HVDC link studied is the
Constant Current and Constant Voltage (CC&CV) scheme
instead of the Constant Current and Constant Extinction Angle
(CC&CEA) scheme. Both the rectifier and inverter have the
same control structure. A detailed converter control model is
adopted, of which the schematic diagram is shown in Fig. 2,
with the parameters listing in Table 1. The controller has a
voltage and a current regulator operating in parallel. All the
regulators are typical PI controllers. Four control modes are
available for the converters: 1- current control; 2- voltage
control; 3- alpha minimum limitation; 4- alpha maximum
limitation.
I dc ref
Vdc meas
VDCOL
1
Vdcm arg
I dcref lim
1 sTV 
1 sTF 


Function
Vdcmeas

1
 
I dc m arg
1 sTC I dc meas
I dc
dV
 Vdc ref
 
max
max
k
Current Regulator k p  i
s
min
Vdc
k
Voltage Regulator k p  i
s
min
ord CC
ord VC
min
ord
Tr. YY
50Hz
Network
to Pulse Generator
Reactor
Rectifier
Voltage Dependent Current Order Limit Function
Rectifier
Controller
AC
Filter
60Hz
Network
Tr.YY
I dc ref lim ( pu )
(DC Tx. Line)
YD
1.0 I dc ref
I dc ref
0.3 I dc ref
Vdcmeas ( pu )
Inverter
0.18
0.6
(Tx. Line)
Reactor
YD
Tr.
AC
Filter
B2R
GEN
B2F
LP2R LP2F
Fig. 2 Schematic diagram of the converter control model
Inverter
Controller
B1R
Table 1 Parameters of the Converter Control Model
B1F
LP1R LP1F
HP
Turbine generator for torsional vibration studies
Fig. 1 System studied
B. System Modeling
A graphic program using the Matlab/Simulink/PowerSystem-Blockset has been constructed for the entire HVDC
system in order for transient simulations. The packaged
models are used if available, to which we don’
t give unnecessary details.
The dc transmission line is modeled as a circuit model
for considering the capacitive effects. The rectifier and
inverter units are composed of thyristors, and an individual
thyristor is modeled as a switch in series with a resistor, an
inductor and a voltage drop. The switch will be turn on/turn
off (according to the preset switching characteristics)
Rectifier control
I dc ref 1 pu
Inverter control
I dc ref 1 pu
Vdc ref 2 pu
V dc ref 1 pu
max 165 deg
max 165 deg
min 5 deg
min 92 deg
I dc m arg 0.0 pu
I dc m arg 0.1 pu
Vdc m arg 0.0 pu
Vdc m arg 0.05 pu
Current regulator
k p 45 deg/ pu
Voltage regulator
k p 35 deg/ pu
k i 4500 deg/ pu / sec
k i 2250 deg/ pu / sec
Measuring time constant
TC 0.0003 sec
TV 0.01 sec
T F 0.0001 sec
For the master control of a rectifier, the current order is
論文獎勵成果彙編P32
first determined, with constrains being imposed to keep the
current within the maximum and minimum limits. The
maximum current is determined by the Voltage Dependent
Current Order Limiter (VDCOL) that is implemented for
helping to recover from faults. The VDCOL automatically
reduce the reference current set point when dc line voltage
decreases (e.g. during a severe ac fault). The PI type of current
regulator accounts for current regulating. If the current
regulator doesn’
t hit any limits, the rectifier is operating in the
current control mode. In case it hits the low limit the rectifier
turns to operate in the alpha minimum limitation mode. In case
it hits the high limit the firing angle will be larger than the one
computed by the voltage regulator, thus the control mode will
be changed to the voltage control mode.
For the master control of an inverter, the action is
reversed. Normally, the inverter operates in the voltage control
mode with a PI controller to regulate the voltage. It will switch
to the current control mode when the rectifier regulator touch
the low limit and unable to maintain the desired dc current.
instance of fault applying, with the firing angle (alpha-rec)
falling down to 5 degrees. Later on, it changes positions
between the current control mode and the alpha minimum
limitation mode. As to the operation mode of inverter (modeinv), it changes from the voltage control mode to the current
control mode during the faulty and recovery period, with the
firing angle (alpha-inv) oscillating between 110 degrees and
140 degrees. It is significant to note that the current flow of
HVDC link doesn’
t cease.
YD side
YY side
3
3
2
2
1
0
0.6
3
1
0.7
0.8
0.9
0
0.6
3
1
2
1
0
0.6
3
0.8
0.9
0
0.6
3
1
0.8
0.9
0
0.6
3
1
0.8
0.9
1
0.7
0.8
0.9
1
0
0.6
3
1
0.7
0.8
0.9
1
0.7
0.8
0.9
1
0.8
0.9
1
2
1
0.7
0.8
0.9
0
0.6
3
1
2
2
1
1
0.7
0.8
0.9
0
0.6
1
time (sec)
0.7
time (sec)
4
4
2
2
idc-inv
vdc-inv
Fig. 3 Inverter valves currents (in pu) without commutation
failure (in sequence of 1 to 6 from top to bottom)
0
-2
0.4
0.6
0.8
0
-2
0.4
1
160
0.6
0.8
1
0.6
0.8
1
0.8
1
5
mode-inv
alpha-inv
4
140
120
3
2
1
100
0.4
0.6
0.8
0
0.4
1
30
5
mode-rec
alpha-rec
4
20
10
3
2
1
0
0.4
0.6
0.8
time (sec)
A. Non-commutation-failure Case
For the setting of (25 deg/pu, 4500 deg/pu/sec), the
currents flow through the twelve inverter valves are shown in
Fig. 3. It can be seen that neither the YD side nor the YY side
valves occur the commutation failure.
The resulting rectifier and inverter responses are shown
in Fig. 4. In the steady state, the rectifier is operated in the
current control mode, and the inverter in the voltage control
mode. The firing angles are about 18 degrees and 141 degrees,
respectively, for the rectifier and inverter. When the fault is
applied, the inverter side dc voltage (vdc-inv) and current
(idc-inv) start to oscillate. The operation mode of the rectifier
(mode-rec) changes to the alpha minimum limitation at the
0.9
1
0
0.6
0.8
1
0.7
2
0
0.6
3
0.7
2
1
For ac faults happened in the rectifier side network, it is
found the inverter current regulator will govern the
commutation failure behavior. The reason is that time is a
crucial factor. Too fast response may make the negative
voltage-time area required by an inverter valve become insufficient for commutation to complete. So slightly differing in
the settings of the proportional constant (kp) and/or the
integral constant (ki) may result in wholly different
commutation behaviors. Depending on the success or failure of
commutation, the HVDC link will present wholly different
performance in responding to the fault. We shall examine such
phenomena for a single-phase-to-ground fault in the following
two cases. It is assumed a ground fault occurs in the phase-A
of the rectifier side network at 0.6 s and is cleared at 0.7 s. For
the current regulator parameter of (kp, ki), it is set at (25
deg/pu, 4500 deg/pu/sec) in one case, and at (45 deg/pu, 4500
deg/pu/sec) in another case. In both cases, the voltage
regulator parameter (kp, ki) is kept at (35 deg/pu, 2250
deg/pu/sec). For both the settings, transient simulations have
confirmed the well operations of the HVDC link under the
situations of load changing and reference changing. In a
temporary dc line fault, the link can also recover successfully.
1
1
0.7
2
III. SIMULATION STUDIES OF THE RECTIFIER SIDE AC FAULTS
0.9
2
1
0
0.6
3
0.8
1
0.7
2
0
0.6
3
0.7
2
1
0
0.4
0.6
time (sec)
Fig. 4 Converter responses without commutation failure
B. Commutation-failure Case
For the setting of (45 deg/pu, 4500 deg/pu/sec), the currents
flow through the twelve inverter valves are shown in Fig. 5. It
can be seen that, at 0.72 s or so, a commutation failure occurs
in the valves 2 and 3 of YD side, and in the valves 3 and 6 of
YY side.
The responses for the rectifier and inverter are shown in
Fig. 6. It is important to see that the HVDC link current
ceases to flow (note: it is called “
collapse”in this paper) due
to the commutation failure, and needs to be re-established
when recovery.
論文獎勵成果彙編P33
The operation mode of converters could explain the
reason for the HVDC link to collapse. It can be seen that the
control action is disturbed by the commutation failure, leading
to the rectifier be pushed to the inverter operation (with the
firing angle changing to 130 degrees), and the inverter to the
alpha maximum limitation mode (with the firing angle
changing to 165 degrees). Thus, the HVDC link fails to
function normally.
YD side
YY side
3
3
2
2
1
1
0.7
0.8
0.9
0
0.6
3
1
2
2
1
1
0
0.6
3
0
0.6
3
0.7
0.8
0.9
1
2
0.8
0.9
0
0.6
3
1
1
0.7
0.8
0.9
1
0.7
0.8
0.9
1
0.7
0.8
0.9
1
2
1
1
0.7
0.8
0.9
0
0.6
3
1
2
2
1
1
0
0.6
3
0
0.6
3
0.7
0.8
0.9
1
2
0.7
0.8
0.9
1
0.8
0.9
1
2
1
0
0.6
0.9
1
0.7
2
0
0.6
3
0.8
2
1
0
0.6
3
0.7
1
0.7
0.8
0.9
0
0.6
1
time (sec)
0.7
2
2
0
1
0
0.8
-1
0.4
1
0.6
0.8
4
160
140
120
3
2
1
0.6
0.8
0
0.4
1
150
0.6
0.8
1
0.8
1
5
mode-rec
alpha-rec
4
100
50
3
2
1
0
0.4
0.6
0.8
time (sec)
C
B
1p-g
2p-g
3p-g
3000
A
4000
ki (deg/pu/sec)
5000
6000
1
0
0.4
Fig. 7 Parameter borderlines for the inverter current regulator
IV. TORSIONAL VIBRATIONS
1
5
mode-inv
alpha-inv
180
100
0.4
D
To study the torsional vibrations induced at the shafts
and blades of a turbine generator, a 950MW turbine generator
unit is especially added to the 60Hz network. And it is
assumed power is transmitted reversely from the 50Hz
network to the 60Hz network.
3
idc-inv
vdc-inv
4
0.6
100
90
80
70
60
50
40
30
20
10
0
2000
time (sec)
Fig. 5 Inverter valves currents (in pu) with commutation
failure (in sequence of 1 to 6 from top to bottom)
-2
0.4
kp (deg/pu)
0
0.6
3
a double-phase-to-ground fault or a three-phase-to-ground
fault, yet it will collapse when subjecting to a single-phase-toground fault. If the parameters are set at the district A, the
HVDC link could wholly prevent from collapsing. That is the
single-phase-to-ground fault is the most likely one to collapse
the HVDC link, the three-phase-to-ground fault is the next and
the double-phase-to-ground fault is the least. That is an unusual order. Thereby, it is the single-phase-to-ground fault
rather than the three-phase-to-ground fault that should be
examined to ensure the satisfactory setting of the inverter
current regulator parameters.
0.6
time (sec)
Fig. 6 Converter responses with commutation failure
C. Criterion
We have further examined the responses for various
combinations of kp and ki parameters under the single-phaseto-ground (1p-g), double-phase-to-ground (2p-g) and threephase-to-ground (3p-g) faults. It is found there is a borderline
in the parameter plane for each fault. The lower-and-left side
of a borderline is a district not to cause the collapse of the
HVDC link. So a smaller value of the kp and ki parameters is
preferred.
The borderlines for the three types of fault are shown in
Fig. 7. The parameter plane is divided into the A, B, C and D
districts by the borderlines. If the parameters are set at the
district B, the HVDC link will not collapse when subjecting to
A. Turbine generator
The generator is a 4-poles/1800rpm machine with
voltage rating of 23.75kV, which is driven by a steam turbine
unit and supplies power to the converter bus via a step-up
transformer (with power rating of 1057MVA) and two
parallel transmission lines. An IEEE type-1 AVR regulates
the generator terminal voltage. Again, this sub-system is
constructed by the Matlab/Simulink/Power-System-Blockset
packaged models. The steam turbine unit, including a highpressure stage (HP) and two low-pressure stages (LP1, LP2)
steam turbines, is a close-coupled and cross-compound reheat
unit that operates at a rotational speed of 1800 rpm. Each lowpressure turbine has F and R spindles, including eleven rows
of blades with twisting structure and serrated type of root.
B. Turbine-generator-exciter-blade model
It is very difficult to characterize a complex turbine
generator structure, yet it has been commonly accepted to use
the lumped mass-damping-spring model for the studies on
torsional vibrations. In Fig. 8, it is shown such a model for the
turbine-generator-exciter-blade mechanism studied, with
parameters listing in Table 2. One could refer to [15, 16] for
details of how the model and its parameters were derived.
According to the lumped mass-damping-spring model,
dynamic equations of motion can be derived. Here, we use a
論文獎勵成果彙編P34
very particular approach, electromechanical analogy [17], to
derive them. For an individual section (e.g. Rotor-j section) as
shown in the upper part of Fig. 9, it can be transformed into an
equivalent circuit as in the lower part of Fig. 9. By the loop
analyses, one could find easily the state space equations as the
followings.
DB2F
DLP2R/GEN
DLP2R
KREC/EXC
JEXC
JHP
DHP/LP1F DLP1F/LP1R DLP1R/LP2F DLP2F/LP2R
DLP1R
DLP2F
DHP
DLP1F
KGEN/REC
JGEN
JLP1R
KLP1F/LP1R
JLP1F
KHP/LP1F
JB2F
JLP2R
KB1R
KB1F
DB2R
JB2R
KB2F
KB2R
KLP1R/LP2F KLP2F/LP2R KLP2R/GEN
JB1R
JLP2F
JB1F
JREC
DB1R
DB1F
DGEN/REC
DGEN
DREC/EXC
DREC
DEXC
d B
j Bj
dt
d B
j Bj K Bj * j K Bj * jB D Bj * Bj / J Bj
dt

(3)

(4)
where J, K, D represent respectively the inertia, stiffness and
damping of rotor or of shaft. The  and  are angular
displacement and angular velocity respectively. For turbine
rotors,  is the torque delivered by the turbine. The torque is
determined by the opening of the main control valves (CVs)
and reheater/intercepter valves (IVs) that is dependent on
turbine governor action. For the generator rotor,  is the
electromagnetic torque delivered to power system. As to the
ones with superscript “
B”
, they are the blade variables or
parameters.
Fig. 8 Mass-damping-spring model of the turbine-generatorexciter-blade mechanism
Table 2 Parameters of the Mass-damping-spring model
Rotor
section
HP
LP1F
LP1R
LP2F
LP2R
GEN
REC
EXC
B1F
B1R
B2F
B2R
Inertia constant
(MW-s/MVA)
0.1787
0.6462
0.6410
0.6499
0.6602
1.1616
0.00344
0.00236
0.0344
0.0344
0.0344
0.0344
D Bj
Bj
Damping coefficient
(MW-s/MVA-rad)
0.00180
0.00023
0.00021
0.00021
0.00021
0.00012
0.00000
0.00000
0.00004
0.00004
0.00004
0.00004
J jB
K jB
Bj
j
K j , j 1
K j 1 , j
j-1
j1
j
Jj
D j 1 , j
D j , j1
Dj
1
K jB
j1
j
Dj
Jj
LOOP 2
Table 2 continued
Shaft
section
HP/LP1F
LP1F/LP1
R
LP1R/LP2
F
LP2F/LP2
R
LP2R/GEN
GEN/REC
REC/EXC
LP1F/B1F
LP1R/B1R
LP2F/B2F
LP2R/B2R
Stiffness coefficient
(MW/MVA-rad)
144.15
1595.0
Damping coefficient
(MW-s/MVA-rad)
0.00
0.00
206.0
0.00
1584.9
0.00
325.28
117.16
1.61
36.2
36.2
36.2
36.2
0.00
0.00
0.00
0.00
0.00
0.00
0.00

B
j

1
K j , j 1
LOOP 1

Eigen-values
-1.95620262425 +/- 1806.76898690731i
-0.04277156682 +/- 719.08407897187i
-0.03615391816 +/- 698.19047239941i
-0.06628545682
-0.06272706389 +/- 122.00368899893i
-0.07436986263 +/- 232.13115405894i
-1.33455883572 +/- 252.58777166885i
-0.36759195240 +/- 289.60578342518i
-0.08216900537 +/- 333.30197487371i
-0.06376719130 +/- 326.51259771547i
-0.05532705174 +/- 312.89871647952i
-0.05653688675 +/- 313.05339760415i
(2)
LOOP 2 equations:
j1
B
j
K j 1, j
D j 1 , j
Table 3 Eigen-values of the Mass-damping-spring Model


D
B
j
Fig. 9 Rotor-j section of the mass-damping-spring model and
its equivalent circuit
d
j j
(1)
dt
d
j j D j D j , j 1 D j 1 , j j D j , j 1 * j 1 D j 1, j * j 1 
dt
K j , j 1 K j 1, j K Bj j K j , j 1 * j 1 K j 1, j * j 1 K Bj * jB / J j

B
j
D j , j 1
LOOP 1 equations:
 
-+
J
1
-+
j
and
Frequency (Hz)
287.55
114.44
111.12
19.42
36.94
40.20
46.09
53.05
51.96
49.80
49.82
By rewriting the torque equations to the power equations
further transforming the equations into the
論文獎勵成果彙編P35
0.5
0
-0.5
6
7
8
9
0.5
0
-0.5
10
6
LP2R/GEN
0.5
0
-0.5
6
7
8
9
9
10
8
9
10
8
9
10
0
-0.5
6
7
0.05
B2F
torque (pu)
B2F
torque (pu)
8
LP2R/GEN
0.5
-1
10
0.05
0
-0.05
7
1
torque (pu)
torque (pu)
1
-1
E/M torque (pu)
E/M torque (pu)
1
0.5
0
-0.5
6
7
8
9
0.5
0
-0.5
10
1
LP2R/GEN
torque (pu)
0.5
0
-0.5
-1
6
7
8
9
7
8
9
10
8
9
10
8
9
10
LP2R/GEN
0.5
0
-0.5
-1
10
0.1
6
7
0.1
B2F
torque (pu)
0.05
0
-0.05
-0.1
6
1
6
7
8
9
10
B2F
0.05
0
-0.05
-0.1
6
time (sec)
7
time (sec)
Fig. 11 Turbine generator responses for a three-phase-toground fault (left: without commutation failure, right: with
commutation failure)
Table 4 Maximum-to-minimum Values of Torsional Torques
Fault type
Parameter district
LP2R/GEN torque (pu)
B2F torque (pu)
1p-g
A
B
0.42
1.2
0.017
0.09
3p-g
B
0.8
0.03
C
2.2
0.22
1
E/M torque (pu)
E/M torque (pu)
1
1
torque (pu)
C. Transient simulations
As indicating in the introduction, it has been studied the
impact for the fault and turbine generator to be at the same
side of a HVDC link. So just only the situation for an ac fault
at rectifier side to stress the turbine generator at inverter side
is studied here.
Since the system configuration has been changed due to
the addition of a generator, the inverter current regulator
parameter borderlines for the three types of faults would be
shifted. Nevertheless, it is the same for the parameter plane to
be divided into the A, B, C and D districts.
The transient simulations for a single-phase-to-ground
fault and for a three-phase-to-ground fault are made here. It is
assumed the ground fault is taken place in the rectifier side
(50Hz side) network at 7 sec, lasts for 5 cycles, and then is
cleared automatically. Neither the inverter nor the rectifier
will be blocked in the simulations. The inverter current
regulator parameters are set at the districts A and B for the
case with a single-phase-to-ground fault, and at the districts B
and C for the case with a three-phase-to-ground fault. Thus,
both the HVDC link non-collapsing and collapsing situations
could be achieved for the two faults.
than the one induced under the non-collapsing situation, and
the blade torque is about 6 times. Thus, it can be known the
inverter current regulator parameters are so significant to
affect the stressing of the turbine shafts and blades.
torque (pu)
Matlab/simulink model, the turbine-generator-exciter-blade
model can thus be incorporate with the electric power system.
By making an eigen-analysis on the turbine-generatorexciter-blade model, the eigen-values can be obtained as in
Table 3, which reveals the natural modes of the mechanism.
6
7
8
time (sec)
9
10
0
-0.05
6
7
time (sec)
Fig. 10 Turbine generator responses for a single-phase-toground fault (left: without commutation failure, right: with
commutation failure)
In Figs. 10 and 11, it is shown the turbine generator
responses, including the generator electromagnetic torque
(E/M torque) as well as the torsional torques at the
LP2R/GEN shaft and B2F blade sections, for the two cases.
For understanding easily, we tabulate the maximum-tominimum values of torsional torque in Table 4. It can be seen
that there is a remarkable difference in torsional vibrations
induced at the turbine shaft and blade. The shaft torque
induced under the collapsing situation is about 3 times larger
D. Discussions
It is significant to make a comparison between the
single-phase-to-ground and three-phase-to-ground faults in
Table 4 for the parameters to be set at the district B. It is
surprised to find that a single-phase-to-ground fault causes a
more serious impact on the turbine generator than a threephase-to-ground fault does. The reason is that the HVDC link
is collapsed for the single-phase-to-ground fault, yet it is not
for the three-phase-to-ground fault. So it can be known that an
un-satisfactory setting of the inverter current regulator
parameters may make the stressing on a turbine generator
even worsen.
Furthermore, it is also significant to pay attention to the
E/M torque in Fig. 11. One can observe that there occurs a
repetitive disturbing phenomenon for the commutation failure
situation. This is caused by the repetitive collapse in the
HVDC link due to the repetitive commutation failure. A lot of
cases have been examined for that, and the same results are
obtained. It seems that a three-phase-to-ground fault is less
likely to collapse the HVDC link than a single-phase-toground fault, but when it does, it can not recover smoothly. In
practice, the converters will be blocked if more than two
successive collapses in the HVDC link were detected.
V. FURTHER STUDIES ON THE INVERTER SIDE AC FAULTS
It has been known that the operation of a HVDC link is
very sensitive to the ac network faults in inverter side. If the
論文獎勵成果彙編P36
fault is close to the inverter station, commutation failure
would occur and the HVDC link would collapse without
exception. Thus, in order for studies, it is assumed the fault
applied to the inverter side ac network is a line-to-resistor-toground one. Here, the grounding resistance may be imagined
as the distance from the inverter station to the fault.
After many case studies, it is found the inverter voltage
regulator, instead of the inverter current regulator, plays a
significant role in affecting the link’
s commutation behavior.
For a single-phase-to-ground fault with 55 ohms of grounding
resistance, the relationship between commutation failure and
inverter voltage regulator parameters is shown in Fig. 12.
Again, it can be found that there is a borderline, dividing the
parameter plane into two districts. The borderline is very nonlinear. Parameters in district A would cause the commutation
failure (and the collapse in HVDC link), whereas in district B
would not. So a larger value of the kp and ki parameters is
preferred, that is just opposite to that for the inverter current
regulator.
is likely to happen and the HVDC link won’
t recover
smoothly.
Furthermore, it is summarized below the findings about
the torsional vibrations induced on an inverter side turbine
generator due to a rectifier side ac fault.
1. Depending on the commutation behaviors of the HVDC
converter, there may be up to several times of difference
in the shaft and blade torsional torques induced.
2. For certain settings of the inverter current regulator
parameters, a single-phase-to-ground fault might cause a
more serious impact on a turbine generator than a threephase-to-ground fault.
REFERENCES
[1]
[2]
[3]
ki (deg/pu/sec)
3200
3000
[4]
2800
2600
B
2400
2200
2000
A
[5]
[6]
1800
1600
140
150
160
170
180
kp (deg/pu)
190
200
Fig. 12 Parameter borderline for the inverter voltage
regulator
[7]
[8]
[9]
VI. CONCLUSIONS
The collapsing of a HVDC link caused by the
commutation failure is a very complicated phenomenon
because the rectifier unit and the inverter unit would interact
each other, and so as to the voltage regulator and the current
regulator of a converter. Thus, it is hard to sophisticatedly
analyze that, and there is usually not a general rule could be
obtained. For the system studied, the following conclusions
are found to be noteworthy for the commutation failure.
1. For the rectifier side ac faults, the control parameters of
the inverter current regulator are crucial in inducing the
commutation failure.
2. For the inverter side ac faults, the control parameters of
the inverter voltage regulator could affect the
commutation behavior.
3. It is the single-phase-to-ground fault rather than the threephase-to-ground fault to be the most likely one to cause
the commutation failure.
4. A three-phase-to-ground fault seems less likely to cause
the commutation failure than a single-phase-to-ground
fault, but when it does, a repetitive commutation failure
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