Three-Level NPC Inverter Fault Diagnosis by the Average Current Park’s Vector Approach Mohsen Bandar Abadi, A. M. S. Mendes, S. M. A. Cruz Abstract -- This paper presents a diagnosis method for the detection and location of open-circuit faults in 3-Level NPC inverters with SVPWM modulation scheme based on evenorder harmonic elimination. The performance of the NPC inverter under normal and faulty conditions is firstly investigated both by simulation and experiments. Afterwards, a method for inverter diagnosis is proposed, based on the Average Current Park’s Vector. This method proved to be capable of detecting and locating the phase and the semiconductor pair where the faulty IGBT is located.
Index Terms--Fault diagnosis, 3-Level NPC inverters, opencircuit Faults, Average Current Park’s Vector.
I. INTRODUCTION inverters are highly recommended for the THREE-level next generation of medium voltage and high power
applications. The main advantages of the multilevel inverters when compared to conventional two-level inverters are their higher voltage capability, reduction of output harmonic content, lower switching losses, accomplishing of higher power quality waveforms and lower dv/dt [1]. Nowadays, important aspects for power converters, particularly the ones used in the high power range and in critical applications are fault detection and protection. For 3-level Neutral Point Clamped (NPC) converters, fault detection is even more critical because their complex topology, associated to the large number of devices and other components used in the converter, increase the probability of occurrence of a failure. The growing demand for continuous service in industrial and transport applications has stimulated in recent years research efforts towards the realization of fault-tolerant multilevel conversion structures [2], [3]. Several diagnostic methods for inverters can be found in the literature. The slope method is based on the slope of the diameter of the current space vector trajectory and is used for fault detection in [4]. The Average Current Park’s Vector approach (ACPV) allows, in 2-level inverters, to detect and locate the faulty switch through the analysis of the current vector magnitude and the current vector angle [5]. In [6], the examination of the amplitude of the current deviation allows the detection of an open semiconductor in a 2-level inverter. Another fault diagnosis method based on the normalized dc current is presented in [7] for voltage source active rectifiers and in [8] for voltage source
inverter. In [9], a real-time inverter fault diagnosis method is proposed, based on the subtractive clustering analysis of the stator current vector and a quick mean current vector calculation. Another method found in literature for 2-level inverters is based on the difference between the measured output voltages of the converter and their respective reference values [10]. In [11] has been introduced an approach for fault detection and finding faulty switch in 3-level inverter by measuring the clamp-diode legs current. This method needs to have 6 extra sensors in each clamp-diode leg in order to detect the fault type. Until now, the Average Current Park’s Vector has been applied for the diagnosis of faults in 2-level inverters. This paper extends its use for the case of 3-level NPC inverters, by allowing the diagnosis and the location of an opencircuit semiconductor. II.
Three-level NPC inverter firstly introduced in [12] is one of the most well-known multilevel configurations explored in the literature [13]-[15]. Fig. 1 shows the topology of a 3level NPC inverter. The inverter has three legs each one has four active switches with antiparallel diodes. The dc bus has 2 capacitors, thus providing the middle point “O”. At any time, only two of the four switching devices in each leg are turned ON. Hence, the output voltages of the converter (vaO, vbO, vcO) will only assume three possible values: +VDC /2, 0, -VDC/2. The operation of each inverter leg can be represented by three switching states [P], [O], and [N], according to the information provided in Table 1 for leg A [14]-[16].
The authors wish to ackowledge the financial suport of the Portuguese Science Foundation (FCT - Fundação para a Ciência e Tecnologia) under project number PTDC/EEA-EEL/100156/2008, titled “Fault Diagnosis in High Power Drives Based on Multilevel Converters” Mohsen Bandar Abadi(1), A. M. S. Mendes(2) and S. M. A. Cruz(3) are with the University of Coimbra and Instituto of Telecomunicações; Department of Electrical and Computer Engineering, Pólo II - Pinhal de Marrocos, P - 3030-290 Coimbra, Portugal; (1)
[email protected]; (2)
[email protected]; (3)
[email protected].
978-1-4673-0142-8/12/$26.00 ©2012 IEEE
THREE-LEVEL NPC INVERTERS
1893
Fig. 1. Schematic representation of a three-level NPC inverter.
Several modulation schemes for 3-level NPC inverters have been reported in literature. Space Vector Pulse Width Modulation (SVPWM) is one of the preferred real-time modulation techniques and is widely used for digital control of voltage source inverters [13], [14]. Fig. 2 shows the space vector diagram and the division of the complex plane in sectors and regions, underlying the use of the SVPWM modulation technique in a 3-level NPC inverter [13]-[17]. As can be seen, there are 6 sectors (S1 to S6), each one with 6 regions (R1 to R6).
The SVPWM algorithm for 3-level inverter is based on volt-second balancing principle, that is, the product of the reference voltage V ref and sampling period Ts equals the sum of the voltage multiplied by the time interval of chosen space vectors. In the NPC inverter, the reference vector Vref can be synthesized using the three nearest stationary vectors. For instance, when V ref falls into region 2 of sector 1, as shown in Fig.2, the three nearest vectors are V2, V8 and V14, from which [14]:
Vref Ts V2Ta V8Tb V14Tc
TABLE I SWITCHING STATES OF LEG A.
POWER SEMICONDUCTORS STATES S1 S2 S3 S4
SWITCHING STATE
OUTPUT VOLTAGE (VA0)
P (1)
+VDC /2
ON
O (0)
0
N (‐1)
‐VDC /2
ON
OFF
OFF
OFF
ON
ON
OFF
OFF
OFF
ON
ON
Taking all three phases into account, the inverter has a total of 27 possible combinations of switching states. The 27 states correspond to 19 voltage vectors whose space vector diagram is given in Fig. 2. Based on their magnitude (length), the voltage vectors can be divided into four groups [14], [16]:
(1)
where Ta, Tb and Tc are the dwell times for V2, V8 and V14, respectively. Table II shows the dwell time of each seven-part switching sequence for the 3-level NPC inverter with evenorder harmonic elimination [13], [14]. In this model, a lookup table with 3 inputs (Sector number, Region number and Part number) is used. As an example, Table III shows the legs states when V is in sector 2 [15]. TABLE II DWELL TIME OF EACH PART FOR THE 3-LEVEL NPC INVERTER.
Zero vectors (V0), providing three zero states [PPP], [OOO], and [NNN]. Small vectors (V1 to V6): each small vector is associated with two states, all having a magnitude of 1/3VDC. Medium vectors (V7 to V12), with a magnitude of √3/3 ( 3 / 3) VDC .
Part Number Type of Region odd
even
Large vectors (V13 to V18), with a magnitude of 2/3VDC.
1
2
3
4
5
6
7
Ta
Tb
Tc
Ta
Tc
Tb
Ta
4
2
4
Tc
Tb
2 2 2 2 Ta Tc Ta Tb
4
2
2
2
2
2
Tc 4
TABLE III PHASES STATES IN SECTOR 2.
The inverter RMS output voltage can be adjusted by the magnitude of V ref V ref e j shown in Fig. 2.
Sector 2 Pn
Rn
1
2
3
4
5
6
1 2 3 4 5 6 7
OON OOO OPO PPO OPO OOO OON
OPO OOO OON NON OON OOO OPO
OON OPN OPO PPO OPO OPN OON
OPO OPN OON NON OON OPN OPO
OON OPN PPN PPO PPN OPN OON
OPO OPN NPN NON NPN OPN OPO
III.
THREE-LEVEL NPC INVERTERS UNDER OPENCIRCUIT FAULTS
Fig. 2. Space vector diagram for a 3-level NPC inverter.
Open-circuit (OC) fault means the power semiconductor device remains in off-state permanently. For example, this situation appears when the gate driver unit or the gate firing hardware circuit fails [18]. In Table IV, are listed all possible output voltages for va0 after the occurrence of one OC fault in one semiconductor located in leg A, while Table V shows all unavailable voltage vectors for the same faulty situation [18]. 1894
TABLE IV POSSIBLE OUTPUT VOLTAGE IN LEG A AFTER AN OC FAULT.
Condition of semiconductors located in leg A S1 S 2 S 3 S 4
Possible output phase voltage va0 ‐VDC/2 , 0 , VDC/2 ‐VDC/2 , 0 ‐VDC/2 VDC/2 0 , VDC/2
OK
OK
OK
OK
FAULTY
OK
OK
OK
OK
FAULTY
OK
OK
OK
OK
FAULTY
OK
OK
OK
OK
FAULTY
Fig. 3. ACPV angle when an open-circuit fault occurs in one of IGBTs belonging to upper pair of leg A (pair 1).
TABLE V UNAVAILABLE SWITCHING STATES UNDER FAULT CONDITIONS IN LEG A. Unavailable switching states Small Medium Vectors Vectors PPO, POO, POP PON, PNO
Faulty switch Zero Vectors S1 PPP
As can be observed, the value of ACPV angle assumes an almost constant value after the occurrence of the fault, this given the indication of the fault. The value obtained in this situation is comprised between 150º and 210º, meaning that the faulty IGBT belongs to pair 1.
S2
OOO, PPP
OPO, PPO, POO, POP, OOP, OON, ONN, ONO
OPN, PON PNO, ONP
Large Vectors PPN, PNP, PNN PPN, PNP, PNN
S3
OOO, NNN
OPO, OPP, OOP OON, NON, NOO, NNO, ONO
OPN, NPO NOP, ONP
NPN, NPP, NNP
S4
NNN
NON, NOO, NNO
NPO, NOP
NPN, NPP, NNP
IV.
V.
DIAGNOSIS APPROACH BASED ON ACPV APPROACH
According to the ACPV approach, an open-circuit fault detection and defect semiconductor localization are accomplished by analyzing the length and position of the Park’s vector of the mean value of each inverter output current over one period [5], [19]-[21]. Firstly, the average value of each inverter output current ( iaav , ibav , icav ) is calculated by (2) [5]:
Ij av
1 N i j ,k N k 1
,
j=a,b,c
(2)
A simulation model of a 3-level inverter feeding a 3phase induction motor was developed and implemented in Simulink environment. The induction motor was controlled by a Rotor Field Oriented Control (RFOC) strategy. The main parameters of the simulated motor are shown in Table VII. Several simulation tests were conducted with a rotor speed reference of 600 rpm and with the motor running at no-load and with a load torque of TL=5 N.m. All these results were obtained for steady state conditions. The transient effects were not taken into account. Under healthy inverter operating mode, the current waveforms for the motor no-load and 5 N.m load operation are shown in Fig. 4(a) and (b), respectively. The current Park’s Vector locus, for the same operating conditions, is shown in Fig. 5. The results obtained are the typical ones for a system of this kind.
where N is the amount of samples, k is the current sample number and a, b and c are the indices of the phases. Afterwards, the Park’s vector transformation is applied to those values, in order to obtain the magnitude (Īsav) and phase angle (θ ) of a complex vector, calculated by (3) [5].
I sav I dav jI qav I sav sav 2 1 1 I aav I bav I cav I dav 3 6 6 1 1 I bav I cav I qav 2 2
SIMULATION RESULTS
TABLE VI LOOKUP TABLE TO FIND THE FAULTY IGBT PAIR. Faulty Faulty Faulty Angle of ACPV switch pair phase 150º
