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Fault-Tolerant Control of Five-Phase Current Source Inverter forMedium-Voltage Drives

M. A. Elgenedy *, A. S. Abdel-Khalik *, A. Elserougi *, S. Ahmed , and A. Massoud *,**

* Alexandria University, Egypt, Texas A&M University at Qatar, Qatar, **Qatar University, Qatar.

M.Atef.Elgenedy@gmail.com

Keywords: current source inverter (CSI), multiphase machine,mapping circuit, space vector pulse width modulation(SVPWM), sinusoidal pulse width modulation (SPWM).

Abstract

Multiphase Machines offer a promising solution to many practical challenges due to their advantages over the three phase counterparts, as fault-tolerant capability and lowertorque ripple magnitude. Among different multiphasemachines, five-phase machines correspond to a relatively

practical selection in industrial applications. Multiphasemachines are conventionally fed from voltage source inverter(VSI) as it facilities the operation under open-phase condition

but with a sophisticated controller. Among different powerconverter topologies, current source inverter (CSI) features asimple inverter structure, a lower switching dv/dt , and areliable short circuit protection. However, the control ofmultiphase CSI under open phase has not been yet considered

in the literature. Additionally, the space vector PWM(SVPWM) of multiphase CSI is still challenging.Alternatively, the mapped sinusoidal PWM can offer simplercontrol over SVPWM. In this paper, a fault-tolerant controllerfor a five-phase CSI inverter is introduced and proposed formedium voltage drive applications. The proposed gating signalgenerator provides a controllable linear modulation index withthe availability of over modulation. To verify the proposedfault-tolerant gating signal generator, a five-phase CSI feeds astatic R-L load is simulated using MATLAB/SIMULINK as acase study exploring the healthy and the opened-phase cases.

1 Introduction

It is well known that the base of the electric drive was the DCmotor. After the development of the variable speed electricdrive systems the DC motors were replaced with three-phaseinduction motors. These are a superior electric motors, simpleand robust in structure, easy to maintain and very reliable;moreover, the supply device for this type of motors, the voltagesource inverter, have been already generalized [1]. However,when the machine is connected to a modular power electronicconverter, such as a (voltage source inverter) VSI or a (currentsource inverter) CSI, then the need for AC supply with aspecific number of phases disappears since simply adding oneleg increases the number of phases. The development ofmodern power electronics makes it possible to consider thenumber of phases as a degree of freedom, i.e., an additional

design variable in electrical machines [2]. Main advantages ofmultiphase machines over their three-phase counterparts are:(i) lower torque ripples hence they reduce the mechanicalstresses. (ii) lesser cogging torque which is a characteristic of

permenant magnet syncrounous motor, PMSM. (iii) greaterfault tolerance. (iv) better torque-speed characteristics at lowspeed operation than the three-phase induction motors [2]. (v)lesser acoustic noise. (vi) higher efficiency. In addition, owingto their redundant structure, multiphase machine convertersimprove system reliability [3]-[6].

As a consequence, the use of multi-phase inverters togetherwith multi-phase AC machines has been recognized as a viableapproach to obtain high power ratings with current limiteddevices [7]. Among different multiphase machines, five-phasemachines correspond to a relatively practical selection inindustrial applications. The fault-tolerant property and thelower torque ripples of a five-phase system makes it a strongcandidate for safety critical applications such as defense,hospitals, ship propulsions, traction drive and aircraftapplications, etc. [4].

Multiphase machines are conventionally fed from voltagesource inverter (VSI) as it facilities the operation under open-

phase condition but with a sophisticated controller [8]-[13].Among different power converter topologies, current sourceinverter (CSI) features a simple inverter structure, a lowerswitching dv/dt , and a reliable short circuit protection [14].However, the control of multiphase CSI under open phase hasnot been yet considered in the literature.

Accordingly, many PWM techniques such as selectiveharmonic elimination PWM (SHE-PWM), carrier basedsinusoidal PWM (SPWM), and space vector PWM (SVPWM)have been developed for controlling the gating signals of thethree phase converters [14,15]. SVPWM is a digitalmodulation technique wherein a sampled reference vector issynthesized by time-averaging of a number of appropriateswitching state vectors. The reference and the switching statevectors are represented in a complex plane by a transformationfrom abc to - coordinates. The SVPWM techniques haveadvantages in terms of more control flexibility, low harmonic,and better dynamic performance. However, in a multiphaseconverter, each discrete state of conduction produces a vectorin an n-dimensional space, adding significant complexity to the

SVPWM of multiphase converters, where the n-dimensionalvector space can be decomposed into (h-1) 2 mutually

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orthogonal planes (where h is the number of phases) [16]. Themultiphase SVPWM for VSI was extensively addressed in theliterature [17]-[19], while SVPWM of multiphase CSI is stillchallenging. Alternatively, the mapped sinusoidal PWM canoffer simpler control over SVPWM.

Generally, the CSI required gating signals must fulfill twomain constraints. First, the input DC-link current to theconverter must be continuous, as any sudden loss of the currentresults in a large dv/dt due to the DC-link inductor. Second,only two switches must conduct at the same time [20], [21].These constrains represent the main challenge with carrier

based SPWM-CSI; mapping solutions are proposed to solvethe sophistication of modulation strategy compared to the VSIwhile keeping the working conditions of the CSI valid. Inreferences [22, 23] on-line generation of carrier based gatingsignal for three phase SPWM-CSI is introduced. With suchapproach, however, the duality between VSI and CSI is

extended, the mapping technique is implemented by mixedanalog/digital circuitry and a decoupling circuit stage is neededsince reference line currents are generated from line-to-neutralvoltage signals. In [24], the concept applied in [22, 23] wasgeneralized to cover any number of phases, although theintroduced method is digitally implemented, the author did notintroduce a clear formulation between the modulation index(m), the peak-output AC-current ( I ac) and the DC-link current( I dc).

In literature, mapping for SPWM-CSI is recognized for balanced case only. In case of open phase, the system can beconsidered as an unbalanced four-phase system. Hence, in this

paper the mapping for balanced four-phase SPWM-CSI isfirstly introduced, the unbalanced case is then deduced

In this paper, a fault-tolerant controller for a five-phase CSIinverter is introduced and proposed for medium voltage driveapplications. A five-phase CSI feeds a static R-L load issimulated using MATLAB/SIMULINK as a case study at bothhealthy and phase-open conditions.

The Mapping SPWM technique used for the CSI for healthycondition will be detailed in section II and the open phase casewill be introduced in section III, while the proposed systemconfiguration and simulation results are given in section IV.

2 SPWM-CSI mapping for healthy condition

The core of proper mapping for a five-phase SPWM-CSI is thegating signal generator, its inputs are basically the referencemodulating signals at fundamental frequency and the carriersignal waveform, while the output are the ten gating signals.Fig.1 shows the power topology of a five-phase CSI, theswitches shown in Fig.1 can be either IGBT with series diodefor increasing the reverse voltage blocking capability, IGCT,or GTO types.

The gating signal generator mainly consists of four stages that

satisfy the required constraints of CSIs, namely:

1) Switching pulse generator ( S pg ), which represents theregular carrier based SPWM by comparing 5modulating signals with the carrier signal as in Fig.2,

2) Complementary pulse generator ( C pg ), which ensuresthe DC-link current continuity by generating a pulse

output to the upper and lower switches in the same legat the same time when the 5-upper or 5-lowerswitches are all zero as shown in Fig.3.

3) Complementary pulse distributor ( C pd ) is responsiblefor ensuring equal distribution of the load current

between switches. It creates a pulse of (72/2 ) everyhalf cycle for each phase to ensure an equaldistribution. The input to this block is the five modulating signals with the arrangement shown inFig.4. The output is 5 signals located in the center ofthe conduction interval for a given switch.

4) Pulse combinator ( P c), In this part shown by Fig.5 thecreated complementary pulse generator C pg signal isequally distributed between the 5-legs using thecomplementary pulse distributor signals ( C pd1 toC pd5).

C i

Five-phaseLoad

I dc

10987

54321

6

Fig.1. Power circuit topology of five-phase CSI.

>s1 to s5

s1c to s5c

s1s2cs2

s3c

s5s1c

spg1

spg2

spg5

s1cs2

s2cs3

spg6

spg7

spg10s5cs1

Carrier Signal

Modulating Signals(m1 to m 5)

72

Fig.2. Switching pulse generator.

In order to define the relation between the modulation index(m), the peak-output AC-current ( I ac) and the DC-link current( I dc), a PI-controller is used as in Fig.6, such that themathematical relationship can be defined as in (1). Equation(1) defines the relation between the DC and the AC currents

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clearly and linearly as in conventional SVPWM used for three- phase CSIs.

(1)

spg1

spg5

spg6

spg10

cpg

Fig.3. Complementary pulse generator.

m 1m 5 sd15>

>sd15c

sd52sd52c

sd21>>

sd21c

sd13sd13c

m 5m 2

m 2m 1

m 3m 1

sd15sd52

sd15csd52c

cpd1

sd32>>

sd32c

sd24sd24c

sd43>>

sd43csd35sd35c

m 2

m 5

m 4

sd54>>

sd54csd41sd41c

m 5m 4

m 4m 1

m 2

m 3

m 4

m 3

m 3

sd21sd13

sd21c

sd13c

cpd2

sd32sd24

sd32csd24csd43sd35

sd43csd35csd54sd41

sd54csd41c

cpd5

cpd4

cpd3

Fig.4. Complementary pulse distributor.

g1

g6

spg1

spg6

cpd1

cpg

g5

g10

spg5

spg10

cpd5

Fig.5. Pulse combinator.

PIcontroller

mI dc

I ac

+

-

Modulaing Signals

With unity peak

ModulaingSignals Gating

SignalGenerator

Fig.6. Modulation index controller for SPWM-CSImapping.

3 SPWM-CSI mapping under one phase open3.1 Optimum phase currents for one phase open

When one or two phases are open, due to a device failure or afault in the phase windings, a forward rotating field can be stillobtained by setting the currents in the faulted phases to zero,and keeping the MMF and torque unchanged. During the fault

conditions, a new set of currents for the healthy phases isapplied. The control current strategies are chosen so as to have

a zero-sequence current equal to zero ( i=0) and a reasonableaverage torque. In order to maintain the same torque as thehealthy-mode operation, the current in the healthy phasesshould be modified to maintain equal current peak for eachhealthy phase [25]. If phase-A is open, the current ia is zero and

the currents in the remaining phases should satisfy thecondition in (2); (2)

Whose current phasor diagram is show in Fig.7. To maintainan undisturbed rotating MMF in a five-phase machine with one

phase open, the fundamental current peak of the healthy phasesshould increase about 1.382 times the initial value when all five

phases are functional as in (3) and (4) [25].

(3) (4)

72 A

ED

CB

36-

36

E_fault D_fault

C_fault B_fault

(a) (b)

Fig.7. Current phasor diagram: (a) healthy mode operationand (b) one-phase open.

A_normal

E_fault D_fault

C_fault

B_fault

D_normal

C_normal

B_normal

- - - - - normal four phase _______five phase when one phase opened

54

36

54

36

Fig.8 Delaying the conventional four phases to generate agating signals for one phase open five-phase system.

3.2 SPWM-CSI modified mapping for one phase open

Although the aforementioned SPWM-CSI was introduced forhealthy five-phase operation, it can be generalized to operatefor any number of healthy phases [24]. Whereas a healthyoperation of a four-phase SPWM-CSI requires, modulatingsignal angles of [0, -90, -180 and -270], eight switching

pulse generator and four complementary pulse distributers inorder to generate the eight gating signals. Since the phase-angles of the remaining four phases when one phase opened ina five-phase system are [-36, -144, 144 and 36], unequal

phase-angle displacement between any two adjacent phases iscertain. Applying modulating signals of these angles to the

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gating signal generator when one phase opened will lead toimproper operation of the mapping circuit. As a remedy to this

problem, a four-phase modulation signals of [0, -90, -180and -270] are used in the gating signal generator, where therequired phase-angles can be acquired by applying a phase-

angle delay to the output gating signals using the mannershown in Fig.8.

4 Simulation results

To verify the proposed fault-tolerant gating signal generator, afive-phase CSI feeds a static R- L load is simulated usingMATLAB/SIMULINK as a case study. The DC-link current isadjusted to 500 A ripples free. The capacitor filter is adjustedto 250 F. The value of the load resistance and inductancecombined with the output filter are 10 and 50 mHrespectively. The modulating signal fundamental frequencyand the switching frequency are 50 Hz and 1.5 kHzrespectively. Fig. 9 shows the proposed system configuration.

To investigate the system dynamic response, the referencemodulation index ( m) is changed such that the healthy and theopened-phase cases are explored. The reference m is changedduring three intervals, the first two intervals are dedicated tohealthy case with two different values of m, while in the thirdinterval phase-A is opened. Under open-phase case, the AC-

peak current should be 1.382 times its initial value when allfive phases are functional. The modulation index of the threeintervals is summarized as follows:

Interval I: Five healthy phases, m = 0.8 , for 0 < t 2s.

The simulation results are shown in Figs. 10, 11, 12 and 13.Fig. 10 shows that the modulation index controller forces theAC-peak current ( I ac) to follow the desired ( m I dc) value. Thefive-phase load currents at healthy operation are shown inFigs .11 and 12 for m equals 0.8 and 1 respectively. The load

peak-current value is 400 A, for m = 0.8, while it increases to500A for m = 1. When phase-A is opened, the proposed gating

signal generator method is applied such that phase-A currentdrops to zero, the load peak-current of the remaining four phases is 691 A (5001.382) with phase-angles of [-36, -144,144 and 36] as in Fig.13. During interval III, the switchingcurrent (current before the filter capacitor), , the output loadvoltage and FFT analysis of output load current are all recordedfor phase-B and are shown in Figs.14, 15 and 16 respectively.

5 Conclusion

In this paper, a new fault tolerant five-phase mapping forSPWM-CSI is recognized for balanced and open phase cases.

The proposed CSI provides a controllable linear modulationindex with the availability of over modulation. To verify the

proposed fault-tolerant gating signal generator, a five-phaseCSI feeds a static R-L load is simulated usingMATLAB/SIMULINK as a case study exploring the healthyand the opened-phase cases. The simulation results prove thatCSI can be effectively used to provide a fault tolerant operation

for multiphase drive system without the sophisticatedcontroller used with conventional VSI front end converter.Since the introduced system was mainly based on multiphaseMachines and CSIs, it has many advantages such as loweringtorque ripples leading to lessening the mechanical stresses onthe shaft; reducing the cogging torque in PMSM; loweringmaintenance rate; increasing system efficiency and toleratingfault conditions. These advantages are promising for mediumvoltage drives applications where multiphase generators can beused as an alternative to multi-level converters. In multiphasemachines, by dividing the required power between multiple

phases, more than the conventional three, higher power levelscan be obtained and power electronic converters with limited

power range can be used to operate with these multiphasemachines.

R-L Load

5-legged CSI

I ci

I inv I LL f , R f

C i

Five phase mapping logic[normal operation]

Four phase mapping logic[open phase operation]

Leg_A

g1

g6

I dc

Gating Signalgenerator

Open phase detected

Fig.9. Proposed system configuration.

Fig.10. Modulation index controller response.

0 0.5 1 1.5 2 2.5 30

100

200

300

400

500

600

700

800

900

Time seconds

C u r r e n t

( A )

M*Idc Iac

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Fig.11. Steady state five-phase healthy load currents, m=0.8.

Fig.12. Steady state five-phase healthy load currents, m=1.

Fig.13. Steady state five-phase load currents when phase-Aopened.

Acknowledgment

This publication was made possible by NPRP grant # [4- 941 -2 - 356] from the Qatar National Research Fund (a member ofQatar Foundation). The statements made herein are solely the

responsibility of the authors.

Fig.14. Steady state phase-B load voltage when phase-Aopened.

Fig.15. Phase-B switching current when phase-A opened.

Fig.16. Phase-B FFT load current analysis when phase-Aopened.

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