8
A Guide to Standard Medium Voltage
Variable Speed Drives
Part 2:
Choosing a motor control platform
and drive system
Rectifier
Intermediate
DC Link
Inverter
M
3~
ABB
A guide to medium-voltage
standard AC drives
Who should read this guide
This Technical Guide is available in six parts from the ABB address
given on the back cover.
It is aimed at the key decision makers engaged in the specification,
selection, purchasing, installation and/or commissioning of medium-
voltage AC variable speed drives, as a standard solution.
It is therefore aimed at electrical, mechanical and plant engineers
as well as managers, consultants and technicians.
There is a new thinking within industry. Standard, 'off-the-shelf'
medium-voltage AC drives can often be a more cost effective solution
than traditional 'engineered' drive systems, which are tailor made and
consequently more costly.
This Technical Guide series, therefore, aims to give a basic
understanding of the technologies and practices presently available to
those considering purchasing 'standard' medium-voltage AC drives.
However, in a Technical Guide of this nature it is not possible to
give an in-depth analysis of all aspects of selecting, purchasing,
installing and commissioning medium-voltage AC drives. The reader
is advised to consult ABB for more detailed information.
Contents
Introduction
Page 1
Selecting a Medium Voltage AC drive
Page 1
The solution
Page 6
Drive System Topologies
Page 7
Voltage-Source Inverter (VSI)
Page 7
Multi-level VSI
Page 9
Current-Source Inverter (CSI)
Page 10
Load Commutated Inverter (LCI)
Page 11
Cycloconverter
Page 12
Cascade/Kramer/SER
Page 13
Motor Control Platform
Page 14
Scalar Control
Page 15
Flux Vector
Page 15
Direct Torque Control (DTC)
Page 17
Summary
Page 19
Introduction
What is a variable speed drive?
A power electronic device that controls the speed and torque of
an electric motor is often called a Variable Speed Drive (VSD)*.
It takes energy from the mains and controls the energy flow to
the motor creating different motor speeds and torques as
required.
In this way, the drive can control the variables of a process,
such as flow, by controlling the speed of a pump.
When controlling torque, the load determines motor speed;
when controlling speed, the load determines motor torque.
Initially, DC drives and motors were used because speed and
torque could be controlled without the need for sophisticated
electronics. However, high maintenance requirements of DC
motors have led to a decrease in their popularity.
AC drives and motors are the most common in industry as
AC motors are inexpensive and need little maintenance. AC
drives have been developed to the extent where their torque and
speed control performance is as good as DC systems.
* Footnote: Beware of the conflicting jargon that is often used to describe a Variable Speed
Drive. A Variable Speed Drive is often referred to as a VSD, an AC drive, a converter, an
inverter or quite simply a drive. Other alternatives include VVVF = Variable Voltage
Variable Frequency; VFD = Variable Frequency Drive; and ASD = Adjustable Speed Drive
What to consider when selecting a medium-voltage
AC drive
Supply side
VARIABLE SPEED DRIVE -
Converter.
Rectifier
Intermediate
DC Link
Inverter
Motor
3~
LOAD
Figure 1: A simple
representation of a
drive system
When choosing a medium-voltage AC drive, considerations
need to be given to each element shown in the above schematic:
• The supply side
• The medium-voltage AC drive
(alternatively called 'converter')
• The motor
Supply side
Supply side
Converter
Motor
M
3~
Rectifier
Intermediate
DC Link
Inverter
Harmonics: The main concern on the input side is the
presence
of harmonics and the need to ensure that the AC drive conforms
to harmonic regulations such as IEEE 519.1992 and the UK's
G5/3. Furthermore, the drive should comply with these local
harmonic regulations without the need for additional harmonic
filters.
Harmonics that are the highest in amplitude, and therefore,
normally the most problematic in medium-voltage systems, are
the 5th and 7th harmonics. These can be removed by using, for
example, a 12-pulse uncontrolled diode bridge rectifier. A 24-
pulse unit can be used for weaker networks or where more
stringent harmonic requirements apply.
Input Power Factor: Ideally, the higher the power factor the
greater the cost savings will be as no extra reactive power
compensation equipment is needed and cables and transformers
can be dimensioned for lower current. This also avoids penalties
from utilities.
A fundamental power factor better than 0.97 and a total
power factor better than 0.95 should be the goal. Additionally,
the power factor should be constant over the entire speed range
without the need for additional power factor correction
equipment because the goal is to run the drive at other than full
speed and so power factor needs to be constant. With some drive
topologies, this is not possible (see page 7).
Input isolation transformer: It should be possible to position
the transformer both inside the electrical room, or, if conditions
dictate, outside the electrical room.
However, the ability to locate a transformer anywhere other
than the electrical room, depends on the choice of drive system
or topology (see page 7). For example, a three-level Voltage
Source Inverter topology, substantially reduces the number of
cables between the input isolation transformer and the
converter.
Other types of multi-level topology can need anything from
2
3
27 to 45 cables, making it problematic to place the transformer
away from the converter.
The freedom to choose the transformer location brings cost
savings, firstly from a smaller drive size, as the large transformer
does not need to be sited in the electrical room. Secondly, the
losses from the transformer are not being dissipated into the
room. Therefore, the cooling requirements for the electrical
room can be greatly reduced. This is especially important in
locations that have a high ambient temperature.
Converter
Supply side
Converter
8ZZ
Rectifier
Intermediate
DC Link
Inverter
Motor
M
3~
There are many considerations to be taken before purchasing a
medium-voltage AC drive, but the principal requirements from
any drive should be:
• Small overall dimensions - This is especially important in
industries such as offshore and the oil and
gas sector, where the
cost of real estate is high. Today, medium-voltage AC drives can
measure as small as 3 - 4.5 metres long, only 900mm deep and 2
metres high.
Small size has been mainly achieved by new technological
developments, particularly in the field of power semiconductor
switching devices.
See Part 3 of this Technical Guide series to discover the secrets to
small size.
• Low audible noise - Health and Safety legislation in many
countries is demanding noise levels that do not subject personnel
to harmful or irritating noise.
Fully compliant to the necessary EMC regulations - While
drives must not pollute the environment with high levels of
electro magnetic radiation, of equal importance is the need to
ensure that the installed drive is immune from the effects of
radiation being emanated from other equipment.
• Higher efficiency - is important if energy costs are to be
reduced.
Supply side
Motor
824
Converter
Rectifier
Intermediate
DC Link
Inverter
Motor
M
3~
Careful consideration should be given to the following:
• Compatibility with standard squirrel cage induction motors.
There are three main concerns:
1. Derating: Because harmonics cause additional
heating in a motor, this leads to the need to derate
the motor. When purchasing a motor for use on
medium-voltage drive, enquire, from the drive
supplier, as to whether derating is necessary.
2. Voltage stress: This can damage motor
insulation. Drives which incorporate fast
switching power semiconductors can have a high
voltage rate of change and it is this which can
damage the motor. An output filter which gives
a sine wave output can overcome this problem.
3. Common mode voltages. These are high
frequency voltages that can also damage motor
insulation. To be able to retrofit a drive onto an
existing motor, it is essential that the converter
does not subject the motor to high common.
mode voltages.
Common mode voltages can be overcome,
depending on the drive system (topology)
selected (see page 7). For example, a three-level
Voltage Source Inverter with an output filter
arrangement, can avoid common mode voltages
by earthing the star point of the output filter. This
simple solution eliminates the dangerous voltages
and provides one less concern when carrying out a
retrofit installation.
5
Voltage reflections: This is a specific concern at medium-
voltages, especially when retrofitting a drive onto an existing
motor as the condition or the quality of the insulation of that
motor may not be known. If left unchecked voltage reflections
can seriously damage motor insulation.
Torque pulsations: If a drive produces high torque pulsations
this can excite mechanical resonances and can damage the motor
shaft, the gear box (when used) and the load.
• Motor noise: Some converters, when retrofitted on motors,
produce extra motor noise. The motor runs louder and this extra
noise should not be tolerated.
Torque and speed performance: The demand, today, is for the
same level of torque and speed performance in medium-voltage
drives that is possible with some of today's low-voltage drive
equipment. The solution rests with the choice of motor control
platform (see page 14).
Typical common mode voltages are a series of voltage spikes superimposed on an
AC waveform
f
The solution
The solution to all the above considerations for supply,
converter and motor lies in the choice of converter (also)
referred to as medium-voltage AC drive).
It is important, therefore, to have an understanding of the
basic blocks which make up a medium-voltage AC drive, namely
the type of motor control platform and the topology on which
the drive is based.
The topology: is the name given to the various types of
electrical configuration for AC drive systems, employing
synchronous and induction motors of all types for a variety of
applications.
The motor control platform: lies at the heart of the drive
system. It is to be found within the converter element of the AC
drive topology. It is the motor control platform that controls the
flow of energy to the motor, which ultimately delivers the
desired torque and speed accuracy.
6
Drive System Topologies
Introduction
This section looks at the various arrangements - often referred
to as topologies - for medium-voltage AC drive systems.
It looks at the configuration, applications, performance and
speed/power limits of each type commonly in use by industry
today.
Voltage-Source Inverter (VSI)
The system consists of a Voltage Source Inverter with a constant DC voltage in the DC
link. A 12 or 24 pulse rectifier is used on the supply side, to ensure harmonics to the
network are kept to a minimum. The capacitor in the DC link smoothes the DC voltage
and supplies reactive power to the motor. The self-commutated inverter unit uses Gate
Turn-Off thyristors, High Voltage IGBTs or IGCTs, with both two or three level
Neutral Point Clamping (NPC) types available.
An NPC connected DC link can be used to improve motor loading capacity. The
constant DC voltage is applied to the motor terminals using pulse-width-modulation
or DTC. In the case of PWM this means
that the frequency and amplitude of stator
voltages can be controlled independently,
while with DTC motor flux and motor
torque are controlled directly.
ON
Rectifier
D.C. Link
Capacitor
Inverter
Output filter
Some manufacturers provide a sine wave
output filter. The combination of the
NPC and sine wave filter can give an
excellent output wave form.
7
Output filter
M
3~
Cage induction
motor
Typical output voltage and current of a 4.16kV medium-
voltage AC drive running at a frequency of 60 Hz.
ww
With this waveform, problems of voltage reflections can be eliminated. Voltage
reflections are a real concern at medium-voltage levels, because when a drive is
retrofitted onto an existing motor, the condition or the quality of the insulation of that
motor may not be known.
A voltage reflection occurs when a steep wave pulse is sent from a converter to the
motor along the motor cable. Due to an impedance mismatch between the motor
cables and the motor, that voltage can double in magnitude and it can damage the
motor insulation with disastrous consequences.
Voltage reflections are avoided due to the near pure sine wave output provided by
the filter. There are no steep wave pulses therefore voltage reflections are not a problem.
This means there is no limitation on the cable length, the only limitation being that of
voltage drop.
This makes drives with such output waveforms perfect for installations with long
cable runs between motor and converter, for example, downhole pumping where there
may be several kilometers of cable.
The output filter can also completely eliminate common mode voltages from the
motor due to the earthing arrangement of the filter.
ADVANTAGES
Operates on the robust cage induction
motor
For drives with sine wave output can
retrofit onto existing motors without
derating
Full torque even at standstill and very
low speed
Input power factor near unity for
entire speed range
• Reduced voltage stress on the motor
insulation
• Virtually unlimited cable length
Elimination of variable speed drive
induced torque pulsations
Quieter motor operation
Low network harmonics
• High dynamic performance
• High drive system efficiency
• Small footprint
TYPICAL TECHNICAL DATA
TWO-LEVEL INVERTERS
Power range:
up to 3400 kW
2.3 to 4.16kV
Frequency range:
0 to 200 Hz
Motor voltage:
THREE-LEVEL INVERTERS
Power range:
Motor voltage:
Frequency range:
Speed control range:
Converter efficiency:
APPLICATIONS
up to 8000 kW
2.3 to 6.9 kV
0 to 200Hz
0 to 100%
typically > 98%
Standard drives for pumps, fans,
compressors and conveyors (see part 1)
High performance drives for mills,
winches, cranes and other drives
requiring high control accuracy or
dynamic response
High speed motor drives
• Extruders
Multi-level Voltage-Source Inverter (derivative of VSI)
This system consists of multiple series connected low voltage cells which utilise low
voltage IGBTs, fed from a multi winding input isolation transformer.
Low Voltage Module
ADVANTAGES
Low network harmonics
Quasi sine wave output although not
sinusoidal
3 Phase Low
Voltage Supply
Multilevel Topology
Power output
of Module
Low Voltage Module
DISADVANTAGES
Very high parts count gives low
inherent reliability
Additional low voltage cell by-pass
arrangement may be required to
ensure availability
Has a large footprint compared to
standard drives
A A A
人
АА
Input
power
3 phase
MV AC
MV Induction motor
9
Current-Source Inverter (CSI)
The system consists of a Current Source Inverter with a DC current link. It contains a
controlled (line commutated) rectifier on the line side, a DC link with a reactor, and a
self-commutated inverter on the motor side which converts the direct current to
adjustable frequency three phase current. The amplitude of the motor current is
adjusted by the controlled rectifier, whereas the frequency, and thus motor speed, is
controlled by the inverter.
ADVANTAGES
Four quadrant operation
Cost effective
ж
+4
本本
Line commutated
converter
Reactor
Self-commutated
converter
M
3~
Cage induction
motor
DISADVANTAGES
Continuous operation at very low
speed is not always possible
Power factor is not constant over
entire speed range (poor
power factor at low speed)
⚫ DC link reactor introduces additional
losses as well as a larger drive and
higher cost
APPLICATIONS
Pumps
Blowers and fans
TYPICAL TECHNICAL DATA
Power rating:
up to 8000kW
Output voltage:
2.3 to 6.9kV
Output frequency:
Converter
0-± 75 Hz
Efficiency:
Speed control range:
97%
2 - 100%
10
Load Commutated Inverter (LCI)
A DC current source is formed from the line commutated controlled rectifier and a
reactance. The Load Commutated Inverter (LCI) operates with variable machine
voltage and frequency, switching the DC current to the machine winding. In this
system, the synchronous machine behaves like a DC machine. The inverter operates as a
static commutator. The control prevents
the rotor from falling out of step. Thus,
the machine is fully self controlled.
Transformer
Controlled
rectifier
Reactor
Inverter
Synchronous
M
3~
motor
TYPICAL TECHNICAL DATA
Typical power
1- 80MW
range: (depending
on power rating)
Maximum speed:
Speed control
range:
Motor frequency:
Converter efficiency:
7500 rpm
(0) - 10 - 100%
0 - 125 Hz
>99%
99-99.4%
depending on
power
ADVANTAGES
Single motor drive for medium and
high power ratings
• Suitable for synchronous machine
with brushless or slipring excitation
Inherent 4-quadrant operation
Wide speed and power range
DISADVANTAGES
Not suitable for use on standard
squirrel cage induction motors
May not be cost-effective for simple
applications
Large footprint compared to standard
drives
APPLICATIONS
Suitable for continuously operating
drives for:
Fans and pumps
High speed compressors
Reciprocating compressors
• Wind tunnel fans
Rolling mills
Extruders
⚫ Coupling of variable speed generators
to constant frequency utility network
"
11
Cycloconverter
The drive is usually fed from a supply network via transformers. Each motor phase has
its own converter, which consists of two anti-parallel six-pulse converters.
The cycloconverter operates as a three-phase current source. The stator and a
cycloconverter converts the AC voltage, at line frequency, to an alternating voltage at
load frequency, without any intermediate DC circuit. However, the load frequency is
limited to 40% of the line frequency.
Stator and excitation currents are
controlled to ensure optimal performance
of the drive, both statically and
dynamically, over the entire frequency
range.
о
с
3
M
23
D
Transformers
Cycloconverter
Synchronous
motor
TYPICAL TECHNICAL DATA
Typical power range:
Low voltage
High voltage
Max. speed:
Max. frequency:
Speed range
Motor frequency:
1.5-15 MW
5.0-30 MW
600/720 rpm
for 50/60 Hz supply
20/24 Hz for
50/60 Hz supply
± 0-100Hz
20 - 24Hz
ADVANTAGES
4-quadrant operation
Motor cos phi = 1.0 is possible
High stall and holding torque
available
Excellent performance at low speeds
• High dynamic overload capacity
Field weakening range 1:3
DISADVANTAGES
Limitation on maximum output
frequency
Power factor not constant over entire
speed range
May not be cost-effective on standard
applications
Has a large footprint compared to
standard drives
APPLICATIONS
Rolling mills
Propulsion drives for ships
Ore and cement mills
Mine hoists
Wind tunnel fans
12
Cascade /Kramer/Slip Energy Recovery (SER) drive
The DC link converter consists of a three-phase diode rectifier bridge, which operates
at slip-frequency and feeds rectified slip-power to a smoothing reactor and a line
commutated inverter back to the AC supply network. The speed of the motor is
M
3~
сое
Feedback
transformer
Inverter
Reactor
Diode/thyristor
rectifier
Wound Rotor
Induction Motor
Starting resistor
TYPICAL TECHNICAL DATA
Power range:
Max. speed:
Speed control:
0.5-5 MW
(20) MW
1500 rpm (50 Hz)
1800 rpm (60 Hz)
Typically 50-98%
of synchronous
speed
controlled by adjusting the DC current.
The circuit without the DC reactor
but with thyristors in the rotor-side
rectifier gives a compact, efficient
solution which is insensitive to supply line
disturbances.
ADVANTAGES
Cost effective because converter only
needs to be dimensioned for feedback
power
• High total efficiency
Inherent by-pass (the motor can be
started and run at constant speed,
independent of the converter)
Suitable for retrofitting of existing
slip-ring motors
DISADVANTAGES
Cannot be used on standard squirrel
cage induction motors.
⚫ Power factor not constant over speed
range
APPLICATIONS
Fans and pumps
⚫ Retrofit installation
(existing slip-ring motor drives)
Crushers and mills
Wood chippers
Compressors and blowers
13
Motor Control Platform
Introduction
Here, the different types of motor control platforms presently
available are examined together with an insight into the
advantages and disadvantages of each.
Pulse Width Modulation (PWM)
Also known as Scalar or V/F Control, PWM drives were one of
the first to be developed. They supply frequency and voltage to
the motor to produce torque and speed.
A PWM drive takes mains AC voltage and frequency,
converts it to DC and then uses a Pulse Width Modulator to
simulate a sine wave.
The sine wave is supplied to the motor, the voltage and
frequency of the wave determining the torque and speed
produced.
As torque and speed are controlled indirectly and the actual
motor shaft position is not taken into account, speed and torque
control is not as accurate as DC drives.
This makes producing high torque at low speeds (less than
10Hz) virtually impossible for PWM drives.
Also the electronics that produce the sine wave are
complicated and the modulator reduces torque and speed
response times.
14
Scalar Control
V
Frequency
reference
V/F
Modu-
AC
ratio
lator
f
FEATURES
• Control in short steps to avoid overshooting
• Modulator introduces additional delay in the control loop
AC motor control theory goes back to Scalar Control where a
constant voltage and frequency ratio was used to keep the motor
flux constant.
Also, Scalar Control needs an additional modulator that has
a pre-defined switching pattern to control the switching of the
power semiconductors.
A disadvantage of Scalar Control is that control of torque is
not possible. The load dictates torque. Another disadvantage is
that the modulator introduces a delay in the response time of the
control system.
Flux Vector
V
Speed
control
Torque
control
Modu-
AC
lator
f
T
15
FEATURES
Tachometer feedback required
• Modulator introduces additional delay in the control loop
With the advent of Flux Vector control, indirect control of
torque became possible. With Flux Vector control, motor flux
and motor torque are controlled separately.
Flux Vector drives give a higher performance than PWM
models. A disadvantage of Flux Vector control is that a separate
speed feedback device is always needed between the motor and
the control system and this adds to the drive's cost and reduces
reliability. This is because the control system always needs to
know the relative position of motor flux and data flux.
'Sensorless' or open-loop Flux Vector technology, developed
to operate without an encoder, significantly reduces the cost of
this type of drive while maintaining the high performance.
Open-loop models measure motor current as the motor
operates and translates it into torque and flux, producing values
using an in-built flux/torque look-up table. A control algorithm
estimates motor slip and hence the speed and position of the
motor shaft. The drive then calculates the voltage and frequency
necessary to produce the required torque and speed from the
motor slip and look-up values.
Sensorless Flux Vector drives give more accurate control of
motor torque and speed than PWM models and can produce
high torque at low speed.
At very low speeds (less than 5Hz), motor slip estimates are
inaccurate. The voltage and current values required to produce
the torque are therefore also inaccurate limiting the torque
performance at very low speeds.
However, Flux Vector drives produce full torque at much
lower speeds than PWM models - typically 3 to 5Hz. Their
torque response and speed accuracy is several orders of
magnitude greater than PWM.
16
17
Direct Torque Control (DTC)
Speed
control
Torque
control
AC
FEATURES
• Torque controlled by a single step
• No risk of overshoot with fast 25us control loop
•
Torque step rise time less than 10ms
DTC is the newest technology to reach the market and controls
motor speed and torque directly. This gives accurate speed and
torque control, even at low speeds.
With DTC, motor flux and motor torque are used as primary
control variables and direct torque control can be achieved with
no speed feedback device. Thus, there is one less component to
buy and one less component that can go wrong.
DTC features a highly accurate motor model, which
produces signals that represent actual torque, flux and shaft
speed. These actual values are compared with reference values in
the model during a 25µs cycle. Only if the actual values exceed
pre-set limits are the signals supplied to the motor.
As the drive only supplies energy to the motor when
necessary and a modulator is not used, torque and speed
response are ten times faster than other open loop technologies.
Also, the just-in-time' switching means that less energy is used.
For medium-voltage AC drives, DTC provides the ability to
control torque in the fastest way known today. This is major
advantage for systems that require high dynamic performance
and ensures that the system is always under control.
DTC brings some major performance benefits.
• No modulator, therefore no fixed switching pattern - this
reduces motor audible noise.
• Static speed control error is only from 0.1 to 0.5% of
nominal speed. This compares to 3% for Scalar Control.
