ISSN: 1693-6930
TELKOMNIKA Vol. 15, No. 1, March 2017 : 36
– 47 38
Table 1. Operating Status of the Switch and Pole Voltage in the NPC inverter
Switching state
Device Switching Statusx=a,b,c Pole
voltage S
x1
S
x2
S
x3
S
x4
V
xz
P On
On Off
Off V
DC
2 O
Off On
On Off
N Off
Off On
On -V
DC
2
S
X,1
S
X,2
S
X,3
S
X,4
C
2
C
3
Z i
x
S
X,1
D
X,1
D
X,2
D
X,3
D
X,4
D
1
D2 S
X,1
S
X,2
S
X,3
S
X,4
C
2
C
3
Z i
x
S
X,1
D
X,1
D
X,2
D
X,3
D
X,4
D
1
D2 S
X,1
S
X,2
S
X,3
S
X,4
C
2
C
3
Z i
x
S
X,1
D
X,1
D
X,2
D
X,3
D
X,4
D
1
D2 S
X,1
S
X,2
S
X,3
S
X,4
C
2
C
3
Z i
x
S
X,1
D
X,1
D
X,2
D
X,3
D
X,4
D
1
D2 S
X,1
S
X,2
S
X,3
S
X,4
C
2
C
3
Z i
x
S
X,1
D
X,1
D
X,2
D
X,3
D
X,4
D
1
D2 S
X,1
S
X,2
S
X,3
S
X,4
C
2
C
3
Z i
x
S
X,1
D
X,1
D
X,2
D
X,3
D
X,4
D
1
D2
state P state O
state N state P
state O state N
ia0 ib0
ic0 ia0
ib0 ic0
Figure 2. Shows the Current Paths According to the Switching State and Phase Current Direction
In Figure 3 the proposed closed-loop control scheme is shown. The inverter control is based on a decoupled control of the active and reactive power. The P
ref
is set by a PI controller that compares the actual DC bus voltage and the reference generated by the MPPT, and
provides i
d
active current reference in a synchronous reference frame attached at grid voltage vector. The other component of current vector i
q
represents the reactive current. It can be fixed at zero to maintain almost unity power factor, also it can be compensated the reactive power of
the load. The reactive power of the load is calculated using the αβ reference frame. i
dref
and i
qref
can be calculated with power control. After that, the V
dref
and V
qref
can be obtained from the current PI control. By applying the inverse Park transformation to αβ vector components, the
desired references V
αref
and V
βref
are obtained. These are passed to inverter control which gives outputs of pulses to drive the multilevel inverter switches. The switching pattern
generation is achieved using Space Vector Pulse Width Modulation SVPWM technique [36, 37].
+ -
PI
+ -
abc
dq
PLL Power
control
dq
r ef
P
r ef
Q
d
u
q
u
d
i
q
i
dref
i
qref
i
qref
u
dref
u
, ,
a b
c
i i i ,
,
a b
c
u u u
DC
V Filter
Grid
SVPWM
Figure 3. Block Diagram for the Proposed Control Scheme
2.2. O-C Fault Analysis
The phase current includes information on the switching states in the NPC inverter. If an O-C fault occurs, the switching state does not reach the desired state. This causes a change in
V
xz
and the phase current i
x
. Therefore, the fault condition of an inverter can be diagnosed through the analysis of i
x
[11]. The possible and impossible switching states that can be achieved as a result of faulty switches are listed in Table 2.
TELKOMNIKA ISSN: 1693-6930
Open-circuit Fault Diagnosis for Grid-connected NPC Inverter B ased on… Xiaofeng Wan
39 Table 2. Switching States in Faulty Mode
Faulty switch Possible state
Impossible state S
x1
O,N P
S
x2
N O,P
S
x3
P O,N
S
x4
O,P N
In this section, the operation of the NPC inverter under the open-circuit fault is analyzed considering the open-circuit in two semiconductor elements simultaneously at most, as it is rare
that more than two switching device fails at the same time. 1. Open fault in switch S
x1
When the switching state is “P,” the open-switch fault occurs in S
x1
, D1 becomes reverse-biased. The grid phase voltage E
xs
is the voltage between the grid and ground. The current path is opened, and the positive phase current becomes zero. If E
xs
is higher than the dc-link voltage V
DC2
, the current path is formed through diodes D
x1
andD
x2
. Since this period is very short and the potential difference is very small, the negative phase current is almost zero.
2. Open fault in switch S
x2
When the open-switch fault occurs in switch S
x2
wh ile the switching state is “P,” the
phase current does not flow, because the phase current path is opened. Even when the switching state is “O,” the phase current path is not formed, because the switch S
x2
is opened. If E
xs
is higher than V
xz
, the phase current is formed through switch S
x3
and diode D
2
. However, the negative phase current does not flow, because this period is very short.
3. Open fault in switch S
x3
When the switching state is “N,” the phase current path is opened because of the occurrence of the open-switch fault in switch S
x3
. The negative phase current becomes zero. Even when the switching state is “O,” the phase current path is not formed, because switch S
x3
is opened. The path is formed through diode D
1
and switch S
x2
when V
xz
is higher than E
xs
. However, the positive phase current does not flow because of the abovementioned reasons.
4. Open fault in switch S
x4
When the open-switch fault occurs in switch S
x4
while the switching state is “N,” diode D
2
becomes reverse biased. The current path follows the open circuit, and the phase current becomes zero.
a b
Figure 4. One switch with O-C fault in phase a
a b
Figure 5. Two Switch With O-C Fault in Phase a
0.08 0.09
0.1 0.11
0.12 0.13
0.14 -6
-4 -2
2 4
6
Ia Ib
Ic
Normal Sa1 Fault
0.08 0.09
0.1 0.11
0.12 0.13
0.14 -6
-4 -2
2 4
6
Ia Ib
Normal Ic
Sa2 Fault
0.08 0.09
0.1 0.11
0.12 0.13
0.14 -6
-4 -2
2 4
6
Ia Ib
Normal Ic
Sa2 Sa4 Fault
0.08 0.09
0.1 0.11
0.12 0.13
0.14 -6
-4 -2
2 4
6
Ia Ib
Normal Ic
Sa1 Sa3 Fault
ISSN: 1693-6930
TELKOMNIKA Vol. 15, No. 1, March 2017 : 36
– 47 40
a b
Figure 6. Two Switch with O-C fault in Phase a and Phase b As the closed-loop control of the system, three phase currents interact with each other.
Figure 4a and b show the output phase current i
x
for faults in switches S
a1
and S
a2
respectively; Figure 5a, b denote the phase current i
x
for two switches faults in leg a, respectively; Figure 6a, Bindicate the phase current i
x
for two switches faults in leg a and leg b respectively. As in the previous analysis, the phase current waveform is distorted. The fault
feature is integrated in the three phase currents waveforms, and it is difficult to diagnosis with the experience.
3. Fault Diagnosis of NPC Inverter 3.1. Structure of Fault Diagnostic System
