Tài liệu tiếng anh Điện tử công suất mạch MERS A new AC current switch called MERS with low on state voltage IGBTs for renewable energy and power saving applications

Luận văn, Điện tử công suất, đề tài tốt nghiệp, đồ án, thực tập tốt nghiệp, đề tài

A New AC Current Switch Called MERS with Low On-State Voltage IGBTs (1.54 V) for Renewable

Energy and Power Saving Applications

Ryuichi Shimada∗, Jan A Wiik∗, Takanori Isobe∗, Taku Takaku†, Noriyuki Iwamuro†,

Yoshiyuki Uchida‡, Marta Molinas§ and Tore M Undeland§

∗Tokyo Institute of Technology, N1-33, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan,

Email: rshimada@nr.titech.ac.jp

†Fuji Electric Device Technology Co., Ltd, 4-18-1, Tsukama, Matsumoto, Nagano 390-0821, Japan

‡Curamik Electronics KK, Assorti Takanawa, 3-4-13 Takanawa, Minato-ku, Tokyo 108-0074, Japan

§Norwegian University of Science and Technology, Institutt for elkraftteknikk, 7491 Trondheim, Norway

Abstract— Emergence of new power electronics configurations

have historically been one of the important drivers for

improve-ment of the IGBT technology Developimprove-ment of new IGBTs is

said to be a trade-off between saturation voltage, short-circuit

capability and switching losses With the common applications

requiring high switching frequency and short-circuit capability,

the saturation voltage performance has not been fully optimized.

This paper describes a new configuration called the Magnetic

Energy Recovery Switch (MERS) It is characterized by using

simple control and low switching frequency, where saturation

voltage is the main contributor to losses The semiconductor

requirements of this configuration have led to the development of

a new low on-state voltage IGBT Application in the area of wind

power conversion shows potential for efficiency improvements.

Additionally, due to the soft-switching nature of the MERS

application, series connection of the new IGBTs in variable

frequency induction heating application is shown to be easy

without voltage sharing problems.

Emergence of new power electronics configurations have

historically been one of the important drivers for improvement

and development of the IGBT technology Since the

introduc-tion of the IGBT in the early 1980s have need for higher

power and reduced losses been given main attention Several

technologies have resulted, such as various trench structures

and field stop layer

In a majority of the application areas, high frequency

switching and need for short circuit capacity have been

impor-tant requirements In motor drive applications, usually there is

no internal output impedance, meaning that a short-circuit at

the inverter terminals is a direct short-circuit of the inverter

transistors [1] As a result, turn-off of the IGBTs must be

managed in the case of a short circuit without being destructed

Several trade-offs exist in the development of IGBTs, some

of them being switching losses, short circuit capability and

on-state losses With the typical existing applications, low

switching losses and high short circuit capability have been

prioritized

This paper looks at a new power electronics configuration

called the Magnetic Energy Recover Switch (MERS) The

Fig 1 Configuration of the MERS.

configuration is characterized by low switching speed, reduced need for short circuit capability and simple control The special features of the configuration have led to the development

of a new type of IGBT with lower conduction losses The characteristics of the MERS are first discussed This is fol-lowed by a description of the newly developed low saturation voltage IGBT Application examples of the MERS are then given, where the advantages of the new IGBT compared to the existing ones are discussed

II MAGNETICENERGYRECOVERYSWITCH(MERS)

A Configuration

The configuration of the Magnetic Energy Recovery Switch (MERS) is shown in Fig 1 and consists of 4 forced com-mutated switches and a small dc-capacitor The configuration

is similar to that of a single phase full bridge, but the control is different and the size of the capacitor is several times smaller The configuration was first suggested as a bi-directional current switch with snubber re-generation using power-MOSFETs [2] The ability to control the flow of reac-tive power and the phase of the current was further investigated and the name Magnetic Energy Recovery Switch (MERS) proposed [3] [4] By putting the MERS in series between an inductive load and the power source can the reactive power (or magnetic energy) be ”recovered” to the load

May 18-22, 2008 Oralando, FL

(a) (b) (c)

Fig 2 Possible current paths through the MERS when current is going in

upward direction.

Fig 3 Example of resulting voltage and current curves when controlling

the MERS.

B Operation principle

The operation principle of the MERS is based on controlling

the path of the current through the device (Fig 2) By doing

this can either the phase of the current or the size of the

injected voltage be controlled Examples of resulting curves

are shown in Fig 3 Three modes of operation are indicated

The not-continuous and the dc-offset mode results by turning

two and two switches on/off in pairs while the active

by-pass mode results when actively by-by-passing the capacitor for

a part of the period The resulting current waveform depends

on the application; however, it will always be 90 degrees to

the injected series voltage

In principle, the MERS can act as a variable capacitor The

size of the capacitive injected series voltage can be varied from

zero to rated voltage within the current rating of the device

This also means that the series voltage injection capability

stays constant even with varying frequency Another important

feature is the simplicity of the control When operated in

dc-offset mode can a phase shift of the gate signals directly

control the phase of the current [5]

C Applications

Several types of applications have been investigated from

low power to high power range as shown in Fig 4 With

the ability to control the phase of the current, the size of the

(a)

(b)

(c) Fig 4 Example of MERS applications (a)Light intensity control of discharge lamps (b)Voltage and power factor control of induction motor (c)Series compensation in transmission system.

load voltage or the power factor can also easily be controlled This is attractive in cases where only voltage control is needed and variable frequency is not important One such application is light intensity control of discharge lamps such

as fluorescent and high intensity discharge lamps [6] With a high efficient configuration, energy savings can be achieved with low cost Voltage and power factor control of induction motors is another application [7] The voltage of the induction motor can be controlled to improve efficiency Additionally, the flow of reactive power to other loads can be controlled

In the high end power scale of MERS applications is the se-ries compensation in transmission systems By controlling the injected series voltage, the flow of power can be controlled and increased Several technologies already exist; however, MERS has advantages such as large operating range, simple control

as well as good characteristics seen from a semiconductor perspective

Further two promising applications are that of wind power conversion and induction heating These applications will be discussed further in section IV

D Device characteristics

To facilitate the success of the MERS application, it is important to consider the special characteristics seen from a semiconductor device perspective Device related characteris-tics are:

• Line frequency switching: one switch is only turned on and off once during a fundamental cycle meaning low

Fig 6 Cross-section diagram of a conventional trench FS-IGBT.

switching losses

• Soft-switching: Turn-on is performed at zero current

Zero voltage turn-off is achieved when operating in

not-continuous mode

• Losses are similar to as if the current continuously goes

through one diode and one active switch, meaning

on-state voltage of the devices are of great importance

• High short circuit capability of the devices is not needed

due to having one MERS device in each phase, shorting

of input or output terminals will not lead to a short of

the transistors Additionally, the capacitor is smaller than

a normal voltage source converter

Based on these characteristics, a new low on-state voltage

IGBT has been developed, as described in the next section

III DEVELOPMENT OF NEW LOW ON-STATEIGBTS

A new IGBT with low saturation voltage has been developed

for the MERS configuration [8] IGBT development concerns

the trade-offs shown in Fig 5 The conditions can be changed

if the IGBT is applied to new circuit configurations

For the MERS configuration, high speed switching

charac-teristics are not required because of line frequency switching

(up to 50 or 60 Hz in typical) On the other hand, a

ma-jority of IGBT applications are commonly used with high

frequency switching with PWM control technique (several

kHz) Moreover, large short circuit capability is not required

Consequently, saturation voltage can be prioritized, as well as

the forward voltage of FWD (free wheeling diode)

Fig 7 Appearance of 1200 V/150 A trench FS-IGBT chip for MERS.

The IGBT and FWD chips were designed optimally for the MERS applications based on the chips used for a conven-tional 1200 V Field-Stop (FS) IGBT module with trench gate structure [9] Fig 6 shows cross-section diagram of the FS-IGBT A distinctive feature of the conventional IGBT is low resistance and high speed switching due to thin silicon wafer with less than 150 μm for 1200 V The trench gate structure also contributes to low resistance and high switching speed For the MERS applications, design optimization was applied

to the conventional IGBT as follows:

1) Improvement of bipolar characteristics by increasing minority carriers injection from the p-collector region 2) Reduction of the resistance of n- region by improving placement of electrodes

For the FWD, lifetime control was reduced and this resulted

in decreasing forward drop voltage

Fig 7 shows appearance of the newly designed IGBT, which has 150 A current rating and dimension of 12.8 mm

× 12.8 mm Comparison of the VCE(sat) -IC characteristics

of the conventional trench FS-IGBT and the new IGBT for MERS are shown in Fig 8 Low saturation voltage (1.54 V)

at rated current was achieved, where the conventional IGBT with same rating has 2.10 V saturation voltage The FWD voltage was also improved to 1.2 V from 1.75 V

1200 V - 150 A IGBT module for MERS using the developed IGBT and FWD chips were fabricated and tested Test results confirmed isolation voltage and switching charac-teristics

IV APPLYING LOW ON-STATEIGBTS TOMERS

A Loss reduction of wind power conversion system

With the trend going toward full-converter solutions for wind power applications, a lot of attention is given to reducing cost and improving the efficiency of the power electronics converter MERS has been suggested as a solution combined with a diode bridge rectifier to achieve a good generator utilization even with a high efficiency [10] [11] [12] [13] The conventional solution today uses an active rectifier with hard-switching and resulting high switching losses Due to the high frequency harmonics in the voltage output waveform, a filter is needed Several configurations exist, with 2-level and

Fig 8 VCE(sat) - IC characteristics of the conventional trench FS-IGBT

and the trench FS-IGBT for MERS.

3-level topologies being common A 3-level configuration is

shown in Fig 9(a)

Permanent magnet generators applied to wind power

gen-eration have typically a high synchronous reactance, (in the

area of 1 per unit) With a pure diode bridge solution, the

terminal voltage of the generator will drop as the current

increases As a result, the maximum output power is limited

and the armature loss is increased for a given power output

By inserting a capacitor in series between the generator and

the diode bridge, the voltage drop across the synchronous

reactance can be cancelled and the output power increased

A requirement for such a compensator is that it shall be able

to operate during variable speed and not cause any resonant

problems The configuration of the system when using the

MERS is shown in Fig 9(b)

The concept can be further explained by the use of phasor

diagrams, as shown in Fig 10 There is a large voltage drop

across the synchronous reactance,XS Without any

compen-sation, the output voltage will drop and the power output is

limited (Fig 10(b) On the other hand, the MERS can cancel

the voltage drop across the synchronous reactance and control

the phase of the current along the internal voltage such that

maximum output power results (Fig 10(c)) By controlling the

size of the MERS voltage, the output power can be adjusted

for a given speed and dc-voltage

Experiments were conducted on a 50 kW permanent magnet

generator designed in the image of a large scale wind power

generator The specifications are given in Table I and a picture

of the generator and MERS is shown in Fig 11 The MERS

in the experiments used the new IGBTs

Time trends for a case with 39kW output are shown in Fig

12 There is some distortion in the generator terminal voltage;

however, the distortion in the current is significantly lower due

to the large synchronous reactance

(a)

(b) Fig 9 Wind power conversion system configurations (a)Conventional 3-level PWM rectifier (b)MERS and diode rectifier.

TABLE I

S PECIFICATIONS OF EXPERIMENTAL SET - UP

Synchronous inductance 4.5 mH Armature resistance 0.18 Ω

IGBT current rating 150 A

The losses of the experimental system have been analyzed The left part of Fig 13 (case 1) shows the loss distribution

of the generator side converter using the low loss IGBTs By using the new IGBTs, the MERS losses were improved with 23.5 percent compared to using conventional IGBTs Due to the low voltage utilization of the devices in the experiment, the losses are comparatively high

The relative losses for a real scale system have been esti-mated and also included in the figure By increasing the dc-link voltage, and as a result the no-load voltage of the generator, the efficiency can be improved High voltage utilization of the MERS device should be possible due to switching the current off when the voltage across the capacitor is low This means the surge voltage will be low compared to that of a traditional PWM converter Two cases were investigated In case number

2, the dc-link voltage was set to 1100 V, which is a suitable

(b) (c) Fig 10 Phasor diagram illustrating the purpose of the MERS (a) Equivalent

circuit of the power conversion system with MERS (b) Phasor diagram

when the MERS is not activated (c) Phasor diagram when the MERS is

compensating the whole voltage drop across the synchronous reactance.

Fig 11 Picture of PM-generator and MERS used in experimental system.

voltage level for grid side inverter with 1700 V IGBTs In

case number 3, the dc-link voltage was further increased such

that the peak MERS voltage reached 1000 V, meaning close

to optimal voltage utilization of the MERS IGBTs Significant

loss reductions can be found in both cases

The conventional solution using a PWM rectifier can in

principle perform the same operation as that of the MERS

system The optimization of such a system is complex due to

trade off between switching frequency and size of generator

side filter Several efficiency estimations and specifications

exist, with one manufacturer specifying a recent converter

system efficiency of 97.7 percent [14] By assuming equal

loss sharing between active rectifier and grid side inverter, the

resulting losses of active rectifier is 1.15 percent (included as

Fig 12 Time trends for a case with 355 V dc-link voltage and 39 kW power output.

case 4 in Fig 13) The losses can be found to be significantly higher than that of the MERS case In summary, this indicates that the MERS configuration can contribute to energy savings and promote renewable energy conversion

B Induction heating 1) Controllable frequency induction heating: Induction

heating is widely used for industrial heating especially in the steel industry Many high power induction-heaters, up to several mega watts, are installed to hot strip mills in the steel industry [15] This heating method is efficient compared to other heating methods like gas furnace, because the substance

is heated directly by electric power Moreover, induction heating is a promising heating method to produce value added products because it has high potential for high performance heat control

In general, induction heating uses a high frequency ac power supply and a capacitor is connected in shunt or series with the load to compensate reactive power because of low load power factor as shown in Fig 14(a) Therefore this type of converter can reduce ratings of power electronics components; however, frequency can not be controlled Moreover, high power in-duction heating often uses natural commutated current source inverter using thyristors and parallel connected capacitors to turn off thyristors In this case, it is challenging to operate the inverter when the load condition is changed dynamically or under no load condition

By using the MERS as an ac power supply to induction heating, the frequency can be controlled This new feature adds another controllable property to induction heating, making this heating method more attractive for various industrial fields

2) High frequency inverter using MERS configuration:

The basic configuration of the proposed converter is shown in Fig 14(b) A diode rectifier is connected to the dc capacitor

Fig 13 Relative losses for the experimental system and estimated large

scale system (1) Experimental system (2) 1100V dc-link voltage (3) 1000V

peak MERS voltage (4) Conventional system.

of MERS through a dc inductor The MERS configutration is,

in this case, the same as a full-bridge inverter; however, the dc

capacitor size is small and this results in the capacitor voltage

being changed dynamically from zero to peak voltage as for

the other MERS applications Dc power is supplied from the

diode rectifier through the dc inductor to the capacitor

Fig 15 shows operating modes of the MERS type inverter

and Fig 16 shows schematic waveforms From the point of

MERS, current flowing to the load is the same as for the

other MERS applications; however, there is no power supply

connected to the load side ac circuit Active power consumed

at the load is provided by the dc current link to the capacitor

Fig 16 also shows the voltage across and current flowing

through a switch The current starts to flow in the reverse

diode when the voltage across the IGBT is zero The current

changes polarity with zero voltage naturally, and the current

is shut down immediately with zero voltage Therefore, every

switching is performed under zero voltage and/or zero current

condition This means the switching losses and EMI can be

reduced Moreover, this soft-switching realizes the following

advantages:

1) Surge voltage caused by turning off appears with almost

zero static voltage, while voltage source type converter

needs to carry the surge voltage on top of a full rated

dc link voltage This gives some advantages regarding

device voltage rating

2) Series connection of devices can be achieved with

com-paratively small snubber circuit and/or not complicated

(a)

(b) Fig 14 Circuit configuration of MERS inverter with diode rectifire for controllable frequency induction heating.

(c) Fig 15 Operational states of MERS inverter.

gate control technique because of low dv/dt characteris-tics

3) Optimized IGBT for soft-switching can be used This can reduce conduction losses due to low saturation voltage

To use this converter in the soft-switching condition, zero voltage period of the capacitor is needed This is realized under the not-continuous mode

3) Development of a 90 kVA induction heating power supply: To demonstrate and investigate the proposed method, a

90 kVA 150 - 1000 Hz controllable frequency power supply for steel strip induction heating was developed Circuit diagram

Fig 16 Schematic waveforms of load voltage and load current Applied

voltage and flowing current of V-arm device are also shown.

and overview of this power supply are shown in Fig 17 The

optimum designed IGBTs were used Three IGBTs were

con-nected in series to evaluate performance of series connection

of IGBTs

Experiments using test facilities for induction heating of

steel were conducted Capacitor of the MERS was selected

based on typical electrical parameters and a maximum

fre-quency of 1000 Hz for achieving soft switching condition

Experiments confirmed that frequency can be controlled

dy-namically

Fig 18(a) shows time-trends for the case of 1000 Hz and

100 A Waveforms of applied voltage and current of the coil

included some harmonic distortions; however, there are no

obvious problems related to heating characteristics One of

the important characteristics of this configuration is series

connection of IGBTs Voltage sharing of three connected

IGBTs as shown in Fig 18(b) was mesured For canceling the

effect of different stray capacitance, small capacitors (0.47μF)

were connected in parallel to each IGBT Voltages across three

IGBTs are also shown in Fig 18(a) Good voltage sharing can

be seen even though no special technique is applied to the gate

drivers This indicates the potential for simple series

connec-tion of IGBTs in the MERS circuit, enabling construcconnec-tion of

large scale variable frequency induction heating

(b) Fig 17 90 kVA prototype power supply for controllable frequency induction heating (a)Circuit diagram (b)Overview of the power supply cabinet installed

in a test facility.

A new power electronics configuration called the Magnetic Energy Recovery Switch (MERS) has been discussed Im-portant characteristics are simple configuration, typically low switching frequency and simple control, where the saturation voltage is the main contributor to losses The special semi-conductor requirements of this application have led to the development of a new low saturation voltage IGBT (1.54 V) Experiments using the new IGBT have been performed for

a wind power application and a variable frequency induction heating application It is shown that the losses in the wind power application can be reduced significantly when applying the MERS configuration and the new IGBTs Additionally, simple series connection of the devices is demonstrated in the induction heating application

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