Malcolm Barnes. Practical Variable Speed Drives and Power Electronics

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Power electronic converters 109

are best suited to high frequency switching applications. A typical performance

characteristic of a field effect transistor is shown below.

Figure 3.39:

Typical characteristic of a field effect transistor

Initially, high speed switching was not an important requirement for AC converter

applications. With the development of pulse width modulated (PWM) inverters, high

frequency switching has become a desirable feature to provide a smooth output current

waveform. Consequently, power FETs were not widely used until recently.

At present, FETs are only used for small PWM frequency converters. Ratings are

available from about 100 amp at 50 volt to 5 amp at 1000 volt, but for VSD applications

MOSFETs need to be in the 300–600 volt range. The advantages and disadvantages of

MOSFETs are almost exactly the opposite of BJTs.

The main advantages of a power MOSFET are

• High speed switching capability (10 nsec to 100 nsec)

• Relatively simple protection circuits

• Relatively simple voltage controlled gate driver with low gate current

The main disadvantages of a power MOSFET are

• Relatively low power handling capabilities

• Relatively high forward voltage drop, which results in higher losses than

GTOs and BJTs, limits the use of MOSFETs for higher power applications

3.8.5 Insulated gate bipolar transistor (IGBT)

The insulated gate bipolar transistor (IGBT) is an attempt to unite the best features of the

bipolar junction transistor and the MOSFET technologies. The construction of the IGBT

is similar to a MOSFET with an additional layer to provide conductivity modulation,

which is the reason for the low conduction voltage of the power BJT.

The IGBT construction avoids the MOSFET’s reverse conducting body diode but

introduces a parasitic thyristor, which could give spurious operation in early devices. The

110 Practical Variable Speed Drives and Power Electronics

IGBT device has good forward blocking but very limited reverse blocking ability. It can

operate at higher current densities than either the power BJT or MOSFET allowing a

smaller chip size.

The IGBT is a three terminal device. The power terminals are called the emitter (E) and

collector (C), using the BJT terminology, while the control terminal is called the gate (G),

using the MOSFET terminology.

SYMBOL:

IDEAL: Forward conduction: Resistance (less)

Forward blocking: Loss (less) (no leakage current)

Reverse blocking: Loss (less) (no leakage current)

Switch on/off time: Instantaneous

The electrical equivalent circuit of the IGBT, shown in Figure 3.40, shows that the

IGBT can be considered to be a hybrid device, similar to a darlington transistor

configuration, with a MOSFET driver and a power bipolar PNP transistor. Although the

circuit symbol above suggests that the device is related to a NPN transistor, this should

not be taken too literally.

Figure 3.40:

The equivalent circuit of an IGBT

The gate input characteristics and gate drive requirements are very similar to those of a

power MOSFET. The threshold voltage is typically 4 V. Turn-on requires 10 V to 15 V

and takes about 1 µs. Turn-off takes about 2 µs and can be obtained by applying zero

volts to the gate terminal. Turn-off time can be accelerated, when necessary, by using a

negative drive voltage. IGBT devices can be produced with faster switching times at the

expense of increased forward voltage drop.

An example of a practical IGBT driver circuit is shown in Figure 3.41 below. This

circuit can drive two IGBTs, connected to a 1000 V supply, at a switching frequency of

10kHz with propagation times of no more than 1µs.

Power electronic converters 111

Figure 3.41:

Circuit diagram of semikron SKHI 20 hybrid double IGBT or double MOSFET driver

IGBTs are currently available in ratings from a few amps up to around 500 A at

1500 V, which are suitable for 3-phase AC VSDs rated up to about 500 kW at

380 V/415 V/480 V. They can be used at switching frequencies up to 100kHz. bipolar

junction transistors (BJTs) have now largely been replaced by IGBTs for AC variable

speed drives.

The main advantages of the insulated gate bipolar transistor (IGBT) are:

• Good power handling capabilities

• Low forward conduction voltage drop of 2 V to 3 V, which is higher than for

a BJT but lower than for a MOSFET of similar rating

• This voltage increases with temperature making the device easy to operate in

parallel without danger of thermal instability

• High speed switching capability

• Relatively simple voltage controlled gate driver

• Low gate current

Some other important features of the IGBT are:

• There is no secondary breakdown with the IGBT, giving a good safe

operating area and low switching losses

• Only small snubbers are required

• The inter-electrode capacitances are not as relatively important as in a

MOSFET, thus reducing miller feedback

• There is no body diode in the IGBT, as with the MOSFET, and a separate

diode must be added in anti-parallel when reverse conduction is required, for

example in voltage source inverters

112 Practical Variable Speed Drives and Power Electronics

3.8.6 Comparison of power ratings and switching speed of gate controlled

power electronic devices

Figure 3.42:

Performance limits of gate controlled devices

3.9 Other power converter circuit components

Inductance

SYMBOL:

IDEAL: Inductance: constant (linear)

Resistance: zero (no losses)

EQUATIONS:

L = V

L fj =

Capacitance

SYMBOL:

Power electronic converters 113

IDEAL: Capacitance: constant (linear)

Resistance: infinity (no losses)

EQUATIONS:

C = I

C fj

Resistance

SYMBOL:

IDEAL: Resistance: constant (linear) and free of inductance and capacitance

EQUATIONS:

I R = V

Transformer

SYMBOL:

IDEAL: Magnetizing current negligible

Free of losses and capacitance

EQUATIONS:

××

Electromagnetic compatibility (EMC)

4.1 Introduction

Interference in electrical circuits refers to the presence of unwanted voltages or currents

in electrical equipment, which can damage the equipment or degrade its performance.

Electromagnetic interference (EMI) is a fairly broad term that covers a wide range of

undesirable electrical voltages and currents with a frequency spectrum from DC up to

the GHz range. EMI may be introduced into an electric circuit through the following

paths:

• Conducted over the power cables or signal cables

• Radiated as an electric or magnetic field from one circuit, which is the source

of the interference, and then coupled into another electric circuit, which is the

victim

Electromagnetic interference (EMI) includes frequencies in the radio spectrum

(100 kHz to 100 MHz) which are known as radio frequency interference (RFI). RFI is the

old terminology for the more modern and more general term EMI.

There are two main sources of electromagnetic interference (EMI):

• Natural events such as lightning, electrostatic discharges (ESD) and cosmic

discharges

• Man-made interference, which is mainly generated by electrical equipment

used for industrial and domestic power supply, communications and control

applications

This chapter concentrates on the man-made sources of EMI and mainly those present in

the industrial environment. Every electrical circuit should be considered to be a potential

source of electrical interference, particularly those where switching of inductive or

capacitive circuits takes place. Fortunately, most electrical interference is of a sufficiently

low level that it has no noticeable effect on other items of electrical equipment.

Electromagnetic compatibility (EMC) 115

Electromagnetic compatibility (EMC) refers to the ability of equipment to function

satisfactorily without producing emissions that degrade the performance of other

equipment and also are not affected by emissions from other equipment.

Electromagnetic interference (EMI) covers the following main groups:

• Conducted low frequency (LF) interference (up to about 10 kHz)

− Voltage dips and power interruptions

− Voltage sags and swells

− Voltage unbalance

− Power frequency variation

− DC in AC circuits and vice versa

− Harmonics in AC networks (up to approx 3 kHz)

− Coupled LF voltages and currents

• Radiated low frequency (LF) interference (up to about 10 kHz)

− LF, electric fields. Radiated from circuits with a high dv/dt

− LF, magnetic fields. Radiated from circuits with a high di/dt

• Conducted high frequency (HF) interference (from 10 kHz to 1 GHz)

− Transient over-voltages due to lightning or switching

− Oscillating transients due to resonance

− Coupled HF voltages and currents

• Radiated high frequency (HF) interference (from 10k Hz to 1 GHz)

− HF, electric fields. radiated from circuits with a high dv/dt

− HF, magnetic fields. radiated from circuits with a high di/dt

The rapid increase in the use of ‘non-linear’ power electronics devices, such as AC and

DC variable speed drives has increased the overall level of electro-magnetic interference

(EMI) in industry. To compound the problem, there has been a rapid increase in the

number of electronic control and communications devices, which operate at low voltages

and high speeds and are susceptible to this high level of interference.

A simple, but effective, way to understand interference problems is to remember that

there are always three elements to every interference problem:

• There must be a source of interference energy

• There must be a receptor or victim that is upset by the interference energy

• There must be a coupling path between the source and the receptor

The management of EMI and EMC in industrial environments falls into two categories:

• The establishment of standards for the containment of EMI by setting

maximum limits on EMI emissions from electrical equipment.

• The establishment of standards for the susceptibility (or immunity) of

electronic devices through good design and shielding of electronic equipment,

which will enable them to operate within certain levels of interference.

116 Practical Variable Speed Drives and Power Electronics

A number of EMC standards have been used in industry over the years. In recent times,

the international electrotechnical commission (IEC), through technical committee TC77

and its sub-committees, has established the new IEC 1000 series of standards to cover

EMC requirements. These standards were introduced in 1996 and have become the basis

of EMC standards in a number of countries, including Australia.

Some sections of IEC 1000 are re-issues of earlier IEC standards. For example, sections

of IEC 1000 Part-3 replaced the IEC 555 series. Sections of IEC 1000 Part-4 replaced the

IEC 801 series. While this re-numbering is an inconvenience in the short term, it will

bring the majority of EMC standards into a logical framework, which should facilitate the

development of a harmonized set of EMC standards for international use.

The new IEC 1000 series electromagnetic compatibility (EMC) has the following broad

structure:

Part-1: General considerations, definitions and terminology

Part-2: The environment

Part-3: Limits for harmonics and voltage variations for equipment connected to AC

supplies (replaces IEC 555: 1982)

Part-4: Testing and measurement techniques (replaces IEC 801: 1984)

Part-5: Installation and mitigation guidelines

Part-9: Miscellaneous EMC issues

IEC 1000-1.1 Application and interpretation of definitions and terms

IEC 1000-2.1 Description of environment for LF disturbances

IEC 1000-2.2 Compatibility levels for LF power disturbances

IEC 1000-2.3 Environment - Radiated and conducted phenomena

IEC 1000-2.4 Industrial low frequency conducted disturbances

IEC 1000-3.1 Replaces IEC 555-1: Definitions

IEC 1000-3.2 Replaces IEC 555-2: Harmonics

IEC 1000-3.3 Replaces IEC 555-3: Voltage fluctuations

IEC 1000-4.1 Testing and measurement – Overview of immunity tests

IEC 1000-4.2 Testing and measurement – Electrostatic discharge immunity tests

IEC 1000-4.3 Testing and measurement – Immunity to radiated radio frequency

electromagnetic fields

IEC 1000-4.4 Testing and measurement – Electrical fast transient (burst) immunity test

IEC 1000-4.5 Testing and measurement – Surge immunity tests

IEC 1000-4.6 Testing and measurement – Conducted RF disturbance immunity tests

IEC 1000-4.7 Testing and measurement – Harmonic measurement and instrumentation

for power supply systems and connected equipment

IEC 1000-4.8 Testing and measurement – Damped power (50 Hz) magnetic field

immunity test

IEC 1000-4.9 Testing and measurement – Pulse magnetic field immunity test

IEC 1000-4.10 Testing and measurement – Damped oscillatory magnetic field immunity

test

IEC 1000-4.11 Testing and measurement – Voltage dips and short voltage variations

immunity tests

4.2 The sources of electromagnetic interference

It is not practical to completely eliminate the electrical interference. The main objective is

to minimize their effect on other electronic equipment.

Electromagnetic compatibility (EMC) 117

The main sources of EMI in the industrial environment are:

• Any circuit which produces arcs

• Circuits which generate non-sinusoidal voltages, produce electric fields

• Circuits which generate non-sinusoidal currents, produce magnetic fields

AC variable speed drives use power electronic techniques to convert AC to DC

(rectifier) and then to convert DC to AC (inverter) to provide a variable voltage variable

frequency (VVVF) output. The overall efficiency and performance of the electric motor

depends on the quality of the current to the motor. Over the past decade, a smooth

sinusoidal current waveform has been achieved through the use of pulse width

modulation (PWM) and high frequency switching (10 kHz to 20 kHz). Unfortunately, the

AC converter has become a major source of both conducted and radiated electromagnetic

interference (EMI).

The two main areas of EMI generation are:

• Supply side (mains)

The switching frequency of a 6-pulse diode bridge is 300 Hz on a 50 Hz

power supply system. The harmonics generated by the rectifier fall into the

frequency spectrum up to about 3kHz and are conducted back into the power

system. The radiated EMI from the rectifier is of relatively low frequency

(low di/dt).

• Motor side

Due to the high inverter switching frequencies (typically between 2 kHz to 20

kHz), high frequency harmonics up to 10 MHz (RFI) are generated by the

inverter and conducted along the cable to the motor. The EMI radiated from

this cable is therefore of relatively high frequency often with high dv/dt.

Supply side harmonic interference is a continuous distortion (up to 3 kHz) of the

normal sinusoidal current waveform. The distortion frequencies are multiples of the

fundamental 50 Hz frequency.

Harmonic interference comprising mainly

low order odd harmonics

Motor side interference is a continuous high frequency distortion (above about 10 kHz)

superimposed on top of the normal sinusoidal waveform.

118 Practical Variable Speed Drives and Power Electronics

High frequency (RFI) superimposed

on a sinusoidal waveform

AC converters do not themselves radiate a high level of EMI energy. The

electromagnetic fields in the immediate vicinity (<100 mm) of the converter can be quite

high, but these diminish quite quickly according to the inverse square law and are

insignificant at a distance of about 300 mm. When AC converters are mounted in metal

enclosures, the electromagnetic radiation is largely eliminated. The main mechanism of

propagation of the EMI is through the supply cables, the cables to the motor and most

importantly through the earth connections. The supply cable is the most important route

for the transfer of EMI. Conduction along the control and communications cables is fairly

rare because these cables are usually well shielded and their source impedance is high.

4.3 Harmonics generated on the supply side of AC converters

AC converters use non-linear devices, such as diodes and thyristors, to convert the AC

supply voltage to a DC voltage. Rectifiers draw a non-sinusoidal current and distort the

AC voltage in the power supply system. They cause additional losses in other items of

plant and are the major source of electromagnetic interference. Harmonic distortion can

be looked upon as a type of electrical pollution in a power system and is of concern

because they can affect other connected equipment. As with other types of pollution, the

source and magnitude of the harmonic distortion should be clearly understood in order to

effectively deal with this problem.

4.3.1 Definitions

The fundamental frequency of the AC electric power distribution system is 50 Hz. A

harmonic frequency is any sinusoidal frequency, which is a multiple of the fundamental

frequency. Harmonic frequencies can be even or odd multiples of the sinusoidal

fundamental frequency.

The multiple, that the harmonic frequency is of the fundamental frequency, is called the

harmonic order. Examples of harmonic frequencies of the 50 Hz fundamental are:

Even Harmonics Odd Harmonics

2nd harmonic = 100 Hz 3rd harmonic = 150 Hz

4th harmonic = 200 Hz 5th harmonic = 250 Hz

6th harmonic = 300 Hz 7th harmonic = 350 Hz

8th harmonic = 400 Hz 9th harmonic = 450 Hz

etc etc

A linear electrical load is one, which draws a purely sinusoidal current when connected

to a sinusoidal voltage source, e.g. resistors, capacitors, and inductors. Many of the