a simple snubber configuration for three-level gto inverters-jeong-hyoun

7/28/2019 A Simple Snubber Configuration for Three-Level GTO Inverters-Jeong-Hyoun

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246 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 14, NO. 2, MARCH 1999

A Simple Snubber Configuration forThree-Level GTO Inverters

Jeong-Hyoun Sung, Student Member, IEEE, and Kwanghee Nam, Member, IEEE

AbstractA simple snubber configuration for three-level gateturn-off thyristor (GTO) inverters is proposed. The proposedsnubber has a single resistor per arm for stored energy dissipa-tion, while the conventional RLD/RCD snubber contains six. Thisimplies that the proposed snubber needs only one chopper circuitper arm for snubber energy recovery. This helps reduce the size,cost, and number of components. Besides the single resistor, theproposed snubber requires two less diodes per arm than theRLD/RCD snubber. Furthermore, the proposed snubber resolvesthe voltage imbalance problem between inner and outer GTOswithout additional components. We have analyzed the proposedcircuits and proven its performance through simulations andexperiments.

Index TermsEnergy recovery circuit, snubber circuit, three-level GTO inverter.

I. INTRODUCTION

IN HIGH-POWER systems, such as a steel mill drive, gate

turn-off thyristors (GTOs) are widely used. Due to the

high-power rating (6 kV, 6 kA) and the turn-off capability,

GTOs are much more attractive than conventional thyristors

for sophisticated applications. GTOs, however, require a

snubber circuit which limits the current and voltage rising rate

at the time of turning on and off, respectively. When turning on

a GTO, the current rising rate must be restricted below a

specified value to prevent an excessive initial current loading.When turning off a GTO, the voltage rising rate also

must be restricted below a specified value to avoid a sudden

heat pulse generation and to prevent retriggering by an internal

capacitance.

A number of snubber configurations have been proposed for

two-level GTO voltage source inverters [1][3]: the RLD/RCD

snubber, Undeland snubber, and McMurray snubber [1], [2].

The Undeland and McMurray snubbers are modified from

the RLD/RCD snubber, minimizing the number of snubber

circuit components. Specifically, they have a single resistor

per arm for energy dissipation. This is particularly useful in

constructing an energy recovery circuit. Holtz has investigated

an energy recovery circuit for the McMurray snubber that didnot employ a chopper [3].

The three-level inverter, often called a neutral point clamped

(NPC) inverter, is suitable for high-voltage applications since

it guarantees equal voltage sharing between serially connected

Manuscript received July 10, 1997; revised July 17, 1998. Recommendedby Associate Editor, L. Xu.

The authors are with the Department of Electrical Engineering, POSTECHUniversity, Pohang 790-784, Korea.

Publisher Item Identifier S 0885-8993(99)01825-6.

power devices. Furthermore, the three-level configuration con-

tributes to reducing voltage harmonics [4][8].

For the three-level system, the RLD/RCD snubber could be

constructed as shown in Fig. 1(a), but is impractical for energy

recovery, since it requires six discharging resistors in separate

locations on each arm. Furthermore, such a snubber would

cause a voltage imbalance between serially connected GTOs

when either or turns off, since the middle snubber

capacitors do not find their discharging paths due to

the blocking action of the clamping diodes This

imbalance imposes higher voltage stress on the inner GTOs

and may lead to a destruction of inner GTOs. Okayama et

al. [6] proposed a snubber circuit which is able to locate

energy recovery choppers at points of fixed voltage, such as

at the dc-link side or the neutral point. Hence, the chopper

bank capacitors can be connected in parallel among armsyielding a suitable structure for energy recovery. It also has

some noticeable characteristics such as a guaranteed voltage

balancing mechanism between serially connected GTOs and

a reduced capacitance of turn-off snubbers. However, it has

more components, compared with the RLD/RCD snubber. On

the other hand, Suh et al. extended the Undeland snubber to

the three-level system with overvoltage clamping capability.

This paper presents a new and efficient snubber configu-

ration for three-level GTO inverters. The proposed snubberconfiguration can be regarded as an extension of the Mc-

Murray snubber [1] to the three-level system. Advantages

of the proposed scheme are: 1) small number of parts; 2)

suitable structure for snubber energy recovery; 3) second-

order current dynamics in the period of the snubber capacitor

discharging; and 4) no voltage imbalance between serially

connected GTOs. We have analyzed the proposed circuit, and

proven its performance through simulations and experiments.

II. NEW SNUBBER CONFIGURATION

FOR THREE-LEVEL GTO ARM

A. Structure

The RLD/RCD snubber and the proposed snubber circuits

for a single arm appear in Fig. 1. The proposed snubber circuit

includes four shunt capacitors ( ), two se-

ries inductors ( ), four diodes ( ),

and a single resistor ( ). Table I compares the number of

components for the RLD/RCD snubber and the proposed

snubber. The proposed snubber includes a single resistor, while

the RLD/RCD snubber has six resistors. When a need to

construct an energy recovery system occurs, the advantage

08858993/99$10.00 1999 IEEE


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SUNG AND NAM: SIMPLE SNUBBER CONFIRMATION FOR THREE-LEVEL GTO INVERTERS 251

(a) (b) (c) (d)

Fig. 6. Commutation paths during the transition from S0

to S0 1

: (a) Initial state. (b) Step 1. (c) Step 2. (d) Step 3.

(a) (b)

(c)

Fig. 7. Equivalent circuits during the transition from S0

to S0 1

:

limit the voltage rising rate of At the final state,

the load current flows through the freewheeling diodes

and Commutation sequences and the equivalent circuits

are shown in Figs. 6 and 7.

Step 1): When is turned off, the load current rejected

by is absorbed by both and as shown in Fig. 6(b).

During the turn-off operation, the voltage over remains

while the voltage over increases. In Step 1,

decreases linearly from to zero during the period, i.e,

We obtain from Fig. 7(a) that

(32)

(33)

Note that the derivation process of (33) is similar to that of

(20). We obtain from (32) and (33) that

(34)

which is identical to (22). Hence, its solution is the same

as (24). This step ends at when reduces to zero.

Step 2): The equivalent circuit of Step 2 is shown in

Fig. 7(b). Note that this equivalent circuit is identical to

Fig. 5(b), since is assumed to be constant. Therefore,

the solution is given by

which is the same as (28). This step ends at

when reaches

Step 3): Due to the current flowing through , a voltage

overshoot is generated over This voltage overshoot will

disappear soon in the current loop We obtain

from Fig. 7(c) that

(35)

For simplicity, we assume that is equal to zero. Then, we

obtain from (35) that

(36)

(37)

Applying KCL at point P in Fig. 7(c) and using (36), (37), and

obtained from ,

we obtain

(38)

With the initial conditions and

, the solution and corresponding values

are obtained such that

(39)

(40)

where

IV. ENERGY RECOVERY CIRCUITS

In high-power inverter systems, it is common to recover the

energy stored in the snubber circuits [6], [12], [13]. To recover

the energy into the dc-link capacitor, a chopper circuit is used

in place of a dissipation resistor. Note again that our proposed

snubber has only one dissipation resistor on each arm. Thus

only one chopper per arm can handle the energy recovery as

shown in Fig. 8(a). Note that the number of choppers can be

reduced further to one for an inverter by using the isolation

transformers as shown in Fig. 8(b). In Fig. 8(a), the snubber


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(a) (b)

Fig. 8. Energy recovery circuit. (a) For an arm. (b) Three-arm integration to a single chopper.

resistor is replaced by a capacitor to which chargesstored in are transferred whenever the

discharging paths are formed. If the capacitance of is

sufficiently larger than (for example, 100 times), can

be regarded as a voltage source. If the voltage level over

exceeds a threshold level, then the two insulated gate bipolar

transistors (IGBTs) begin to switch alternately. Energy can

then be delivered to the dc link during both turn-on and turn-

off periods. Energy transfer during the on period, however, is

undesirable because the current may go beyond the limits of

IGBT or diode rating. Hence, the turn ratio of the transformer

is chosen such that the energy is transferred during the turn-

off period (flyback chopper) [10], [11]. Using the two IGBTs

prevents the magnetization of the transformer [1], [3].We calculate the total energy transfer from the snubber

inductors and capacitors to storage capacitor in a

pulsewidth modulation (PWM) cycle. The stored energy in

inductor is transferred to the storage capacitor when

turns off from a conducting state [see Figs. 4(d) and 10]

and is equal to

(41)

In calculating the transferred energy from the snubber

capacitors to , we neglect since they are small

compared with There are two operations in the

change: when changes from to zero

and when changes from to The

amount of transferred energy for the two cases is different. In

the first case, the transferred energy is equal to

(42)

In the second case, the transferred energy is equal to

(43)

Therefore, for the PWM sequence

, neglecting the diode recovery current, the total energy

transferred to per arm is equal to

(44)

Thus, the chopper size or the power rating of dissipation

resistor must be estimated based on (44).

V. DESIGN PROCEDURE AND SIMULATION RESULTS

Based on the circuit analysis, we may summarize the design

procedure of the proposed snubber components.

A. Design Procedure

We assume that a proper GTO is selected for an inverter,

with a specified dc-link voltage and maximum load current

From the GTO manufacturers specification, we obtain

the critical rate of rise of off-state voltage and a

maximum rate of rise of on-state current

Step 1) (Capacitor): Choose capacitance such thatand

The voltage rating of capacitors

should meet and

Step 2) (Inductor): Choose inductance such that

Obviously, current rating of inductor

should be larger than

Step 3) (Resistor): To ensure safe discharging during

turn-on period, it is normal to choose such that

, where is the minimum

turn-on time of the GTO. The power rating of the resistor


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Fig. 9. Simulation results ( S0

to S1

) : (a) iL S 1

; i

D C 1

; (b) vL S 1

; (c)v

C S 1

; v

C S 2

; and (d) vC S 3

; v

C S 4

:

must be larger than , where

is the PWM switching frequency [see (44)].

For the experiment, we chose a GTO (ABB 5SGA

1028F0001) with the following ratings: A,

V, V/ s, and

A/ s, where is the repetitive peak off-state voltage and

is the maximum controllable turn-off current. For a

safe turn off, we limit V/ s. Additionally, the

maximum load current is assumed to be A.

We then obtain from Step 1) that F. Hence,

we choose F and F. For

a safe turn-on, we choose A/ s. Conforming

to Step 2), we choose H. Since s,

we let according to the inequality in Step 3).

B. Simulation Results

All the simulations were carried out for a three-level single

arm using the commercial simulation tool SABER with exam-

ple parameters which are described in the previous subsection.

We further assume that V and

A. Voltage and current transitions in the three basic

commutations are shown inFigs. 911. Fig. 9 shows the inductor current clamping

diode current inductor voltage and capacitor

voltages during the commutation from

to off, on). One can notice from Fig. 9(a) and

(b) that the presence of inductor helps reduce the current

growing rate of Note from Fig. 9(a) that is 260

A/ s, and the peak turn-on current of is almost 1000 A

while the load current is 300 A. This relatively large current

overshoot is caused, in part, by the charging current of

and and results in a voltage overshoot over as

shown in Fig. 9(d).

Fig. 10. Simulation results(

S

1

toS

0

) :

(a)i

L S

1

; i

D C

1

;

(b)v

G

1

;

(c)v

C S 1

; v

C S 2

; and (d) vC S 3

; v

C S 4

:

Fig. 11. Simulation results( S

0 toS

0 1

) :

(a)i

L S 2

; i

D C 1

;

(b)v

G 2

;

(c)v

C S 1

; v

C S 2

; and (d) vC S 3

; v

C S 4

:

TABLE IIICOMPARISON OF DATA OBTAINED FROM ANALYSIS AND

SIMULATION ( ED

= 2 6 0 0 V, IL O A D

= 3 0 0 A)


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(a) (b)

Fig. 13. Experimental plots during the transition from S0

to S1

: (a) iL S 1

; v

C S 1

; v

C S 2

: (b) vL S 1

; v

G 1

:

(a) (b)


to S0

: (a) iL S 1

; v

C S 1

; v

C S 2

: (b) vC S 1

; v

G 1

:

Fig. 14(a) shows and when is turning

off during the transition from to The current decreasing

rate of is 15 A/ s under 85-A load current. Fig. 14(b)

shows the capacitor voltage and the corresponding

anodecathode voltage of Fig. 14(b) shows that the

rising rate of is controlled at 38 V/ s by the action of

The corresponding simulation result, although in different

condition, is shown in Fig. 10.


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(a) (b)


to S0 1

: (a) iL S 2

; v

C S 1

; v

C S 2

: (b) vC S 2

; v

G 2

:

TABLE IVCOMPARISON OF DATA OBTAINED FROM ANALYSIS AND

EXPERIMENTS ( ED

= 1 0 0 0 V, IL O A D

= 8 5 A)

Fig. 15(a) shows and when is turning

off during the transition from to Fig. 15(b) also shows

and the anodecathode voltage of The rising rate

of is about 42 V/ s, which is about the same value as that

of shown in Fig. 14(b). Note further that from Fig. 15(b),

the anodecathode voltage of reaches V after

overshoot, but in the RLD/RCD snubber circuit, it can stay at

the maximum overshoot value, without returning to , due

to the blocking action of The corresponding simulation

result, although in a different condition, is shown in Fig. 11.

From all experimental figures, we can see that the voltage

rising rates of are the same, which verifies (2).Table IV shows data obtained from analytical solutions and

experimental results. We can see that both results agree, within

a small range for error or minor adjustments.

VII. CONCLUDING REMARKS

A simple snubber circuit is proposed for a three-level GTO

inverter/converter. In the proposed snubber circuit, all snubber

capacitors in an arm participate in limiting the voltage rising

rate of a GTO. Furthermore, an inductor is also integrated into

the capacitor circuit, thus there is no extra inductor energy

discharging circuit. From these facts, the proposed snubber

can be looked upon as an extended version of the McMurray

snubber for the three-level system. The characteristics of the

proposed snubber are:

1) small number of parts, especially a reduced number of

resistors;

2) suitable structure for snubber energy recovery;

3) second-order current discharging dynamics relieving the

large initial current loading to GTOs;4) guaranteed voltage balancing mechanism between seri-

ally connected GTOs;

5) reduced capacitance of snubber capacitors.

Simulation and experimental works have demonstrated the

validity of the proposed snubber.

REFERENCES

[1] W. McMurray, Efficient snubbers for voltage-source GTO inverters,IEEE Trans. Power Electron., vol. PEL-2, pp. 264272, July 1987.

[2] T. Undeland, F. Jenset, A. Steinbakk, T. Rogne, and M. Hernes,A snubber configuration for both power transistor and GTO PWMinverters, in IEEE PESC Rec., 1984, pp. 4253.

[3] J. Holtz and K. H. Werner, A nondissipative snubber circuit for high-power GTO inverters, IEEE Trans. Ind. Applicat., vol. 25, no. 4, pp.620626, 1989.

[4] A. Nabae, I. Takahashi, and H. Akagi, A new neutral-point-clampedPWM inverter, IEEE Trans. Ind. Applicat., vol. 17, no. 5, pp. 518523,1981.

[5] S. Tamai et al., 3 level GTO converter-inverter pair system for largecapacity induction motor drive, in EPE Annu. Meeting Conf. Rec., vol.13, Sept. 1993, pp. 4550.

[6] H. Okayama, M. Koyama, S. Tamai, and T. Fuji, Large capacity highperformance 3-level GTO inverter system for steel main rolling milldrives, in Conf. Rec. IAS Annu. Meeting, 96, pp. 174179.

[7] J. H. Suh, B. S. Suh, and D. S. Hyun, A new snubber circuit forhigh efficiency and overvoltage limitation in three-level GTO inverters,

IEEE Trans. Ind. Applicat., vol. 44, no. 2, pp. 145156, 1997.[8] B. S. Suh and D. S. Hyun, A circuit design for clamping an over-voltage

in three-level GTO inverters, in IECON 94, pp. 651656.


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[9] G. Seguier and F. Labrique, Power Electronic Converters. New York:Springer-Verlag, 1993.

[10] J. G. Kassakian, M. F. Schlecht, and G. C. Verghese, Principles of PowerElectronics. Reading, MA: Addison-Wesley, 1991.

[11] N. Mohan, T. M. Undeland, and W. P. Robbins, Power Electronics.New York: Wiley, 1995.

[12] J. A. Taufiq and Y. Shakweh, New snubber energy recovery schemefor high power traction drive, in IPEC-Yokohama 95, pp. 825830.

[13] S. Irokawa, T. Kitahara, F. Kchikawa, and T. Nakajima, A new snubberenergy recovery method for voltage source self-commutated converters,

in IPEC-Yokohama 95, pp. 15721577.

Jeong-Hyoun Sung (S96) was born in Pusan,Korea, on August 6, 1970. He received the B.S.and M.S. degrees in electrical engineering fromPOSTECH University, Pohang, Korea, in 1995 and1997, respectively. He is currently working to-wards the Ph.D. degree in electrical engineering atPOSTECH University.

His main interests are ac motor control and powerinverter/converter systems.

Kwanghee Nam (S83M86) was born in Seoul,Korea, on September 26, 1956. He received theB.S. degree in chemical technology and the M.S.degree in control and instrumentation engineering,both from Seoul National University, Seoul, in1980 and 1982, respectively, and the M.A. degreein mathematics and the Ph.D. degree in electricalengineering from the University of Texas, Austin,in 1986.

He is currently an Associate Professor in the

Department of Electrical Engineering, POSTECHUniversity, Pohang, Korea. His main interests are ac motor control, high-power drives, power converters, and nonlinear systems analysis.

a simple snubber configuration for three-level gto inverters-jeong-hyoun

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