VSD motors and Electromagnetic compatibility (EMC)

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 section 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) 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.

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 U.S..

Some sections of IEC 1000 are re-issues of earlier IEC standards. E.g., 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’s not practical to completely eliminate the electrical interference. The main objective is to minimize their effect on other electronic equipment.

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.

High frequency (RFI) superimposed on a sinusoidal waveform

AC converters don’t 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 ground 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.

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.

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 traditional devices connected to the power distribution system, such as transformers, electric motors and resistive heaters, have linear characteristics.

A non-linear electrical load is one, which draws a non-sinusoidal current when connected to a sinusoidal voltage source, e.g. diode bridge, thyristor bridge, etc. Many power electronic devices, such as variable speed drives, rectifiers and UPSs, have non linear characteristics and result in non-sinusoidal current waveforms or distorted waveform. An example of a periodic distorted waveform, which repeats itself 50 times a second.

The analysis of the harmonic distortion

The technique used to analyze the level of distortion of a periodic current waveform is known as Fourier analysis. The analysis method is based on the principle that a distorted (non-sinusoidal) periodic waveform is equivalent to, and can be replaced by, the sum of a number of sinusoidal waveforms, which are:

• A sinusoidal waveform at fundamental frequency (50 Hz)

• A number of other sinusoidal waveforms at higher harmonic frequencies, which are multiples of the fundamental frequency.

The process of deriving the frequency components of a distorted periodic waveform is achieved mathematically by a technique known as the Fourier transform. Microprocessor based test equipment, which is used for harmonic analysis, can do this very quickly using an on-line technique known as an FFT (fast Fourier transform). The example below illustrates a distorted voltage wave comprising a fundamental wave and a 3rd order harmonic wave, or simply the 3rd harmonic, which is a 150 Hz sinusoidal waveform (3 × 50 Hz). The total RMS value of the distorted current is calculated by taking the square root of the sum of the squares of the fundamental and harmonic currents.

++++ Distorted AC waveform - fundamental plus 3rd harmonic

Harmonic distortion of the current waveform is relatively easy to recognize as a distorted waveform, which is repetitive at the fundamental frequency of 50 Hz. Random noise does not have this repetition. The signature of odd and even harmonics is as follows:

• Odd harmonics are present when the negative half cycle is an exact repetition of the positive half cycle, but in the negative direction.

Alternatively, odd harmonics are present when the first and third quarters are similar and the second and fourth quarters are similar. Odd harmonics occur with rectifier bridges where the positive and negative half-cycles are symmetrical (even harmonics cancel)

• Even harmonics are present when the negative half cycle is NOT a repetition of the positive half cycle. Another characteristic of even harmonics is that the first and fourth quarters are similar and the second and third quarters are similar. It’s not common to find even harmonics in an industrial power system.

++++ Examples of typical distorted AC waveforms

(a) Distorted waveform containing only odd harmonics (b) Distorted waveform containing only even harmonics The level of the harmonic distortion generated by VSDs depends on a large number of variables, some of which are often difficult to quantify, such as:

• The magnitude of the current flowing through the converter

• The configuration of the power electronic circuit (6-pulse, 12-pulse, etc)

• The characteristics and impedances of the connected power supply system

The main reason why power electronic converters draw harmonic currents is that the current is discontinuous in each phase (refer to Section 3). From a harmonics point of view, it does not matter if the rectifier bridge comprises thyristors (controlled rectifier) or diodes (uncontrolled rectifier), they both behave similarly. In the rectifier bridge, only two thyristors (or diodes) are conducting at any one time and the periods of conduction pass from one thyristor (or diode) to the next. Over the period of one cycle of fundamental frequency, each of the 3 phases of the power supply carries a pulse of positive current for a period of 120 deg. and a pulse of negative current for a period of 120 deg.

++++ The sources of harmonic currents in a DC converter

These discontinuous phase currents combine on the DC side to result in a rippled DC current, which is usually smoothed by a choke in the DC circuit. Consequently, the rectifier can be considered to be a source of harmonic currents, which flow back into the power supply network impedance.

Power electronic converters don’t generate all the possible harmonics, only certain harmonic currents. The harmonic order and magnitude of the harmonic currents generated by any converter depends on 3 main factors:

• The pulse number (p) of the converter. The pulse number is the number of DC pulses produced at the output of the rectifier during one cycle of the supply voltage. The order of the harmonic currents that will be present can be predicted mathematically and is given by the formula:

1 ± k.p = n

where: n = order of the harmonics present

k = integers 1, 2, 3

p = pulse number of the converter

• The magnitude of the load current, ID on the DC side of the rectifier part of the converter affects the magnitude of the harmonic currents.

• The magnitude of the load voltage, VD on the DC side of the rectifier part of the converter affects the load current.

Effects of harmonics on other equipment

Harmonic currents cause distortion of the mains voltage waveform that affects the performance of other equipment and creates additional losses and heating. E.g., a total harmonic voltage distortion of 2.5% can cause an additional temperature rise of 4o C in induction motors. In cases where resonance can occur between the system capacitance and reactance at harmonic frequencies, voltage distortion can be even higher.

Capacitor banks (used for power factor correction) are particularly vulnerable. They present a low impedance path to high frequency harmonic currents. These increase the dielectric losses in the capacitor bank, which can lead to overloading and eventual failure.

Transformers, motors, cables, busbars and switchgear supplying current to converters should be de-rated (over-dimensioned) to accommodate the additional harmonic currents and the extra losses associated with the high frequency 'skin-effect'. Experience has shown that the current rating of transformers, cables, etc feeding 6-pulse converters must be de-rated by roughly 10% of the converter current and those feeding 12-pulse converters by roughly 5% of the converter rated current.

The electronic equipment used for instrumentation, protection, and control is also affected due to the interference coupled into the equipment or communications cables.

This affects the reliability and performance of the control system.

The mains supply current contains currents at the following harmonic frequencies:

f ) (k.p = f 1 n 1 × ±

where: fn = frequency of the nth harmonic component of current f1 = fundamental frequency of the supply voltage (n = 1)

k = integers 1, 2, 3, p = pulse number of the connected converter

The following table summarizes the harmonic currents that will be present in the following converter connections.

Converter Connection Pulse Number p Order of Harmonics n 1-phase, fullwave

3-phase, halfwave

3-phase, fullwave

Double 3-phase, fullwave one shifted 30 deg.

++++ Order of harmonics present for different converter connections

The magnitude of the harmonic currents depend on the active power drawn by the load, which is directly proportional to the DC current ID. E.g., for a 3-phase, 6-pulse converter, the fundamental current is given by:

The theoretical magnitude of the harmonic currents can be derived from the following simple formula, based on the assumption that the DC current ID is completely smooth (ripple-free). In practice, a ripple free DC current is not feasible, so the harmonic currents are invariably larger than the theoretical values.

….

where: In the nth harmonic component of current I1 the magnitude of the fundamental component of current n order number of the harmonic

E.g., the theoretical magnitude of the harmonic currents in the mains, generated by a 3-phase 6-pulse power electronic converter will be:

5th Harmonic (250 Hz): 20.0% of fundamental current 7th

Harmonic (350 Hz): 14.3% of fundamental current 11th

Harmonic (550 Hz): 9.1% of fundamental current 13th

Harmonic (650 Hz): 7.7% of fundamental current 17th

Harmonic (850 Hz): 5.9% of fundamental current 19th

Harmonic (950 Hz): 5.3% of fundamental current 23rd

Harmonic (1150 Hz): 4.3% of fundamental current 25th

Harmonic (1250 Hz): 4.0% of fundamental current.

Etc., etc.

The total RMS current drawn by a variable speed drive is the square root of the sum of the squares of the harmonic currents.

In a variable speed drive application, assume E.g. that the current drawn by the 3-phase 6-pulse rectifier at fundamental frequency (50 Hz) is 100 Amps. Using the theoretical values listed above, the following harmonic current values will be flowing:

20 amps (20%) at the 5th harmonic frequency (250 Hz)

14.3 amps (14.3%) at the 7th harmonic frequency (350 Hz)

9.1 amps (9.1%) at the 11th harmonic frequency (550 Hz), etc (ignoring harmonics above the 25th harmonic order)

Consequently, the magnitude of the total RMS current drawn by the VSD will be:

This illustrates that the total RMS current will be 4.1% greater than value of the fundamental current. This results in extra losses in the cables and transformers that feed the variable speed drive. It’s commonly accepted practice to de-rate the drive cables and transformers by 10%. These theoretical values are based on ideal commutation and a ripple free load current on the DC link. These ideal conditions don’t exist in practice and the magnitude of the harmonic currents depends on several factors, including:

• Power supply source impedance - inductance and short-circuit level

• Inductance of the supply side cables - are choke fitted

• Design of the DC link filter - is a DC link, choke fitted

• Type of rectifier - diode bridge or thyristor bridge

The table below illustrates how high the harmonic levels can be without some smoothing. However, this table should be treated with caution, it’s aimed at illustrating an example of the 'worst case' and does not necessarily represent any specific AC converter.

Standards gives some typical practical values of harmonic levels which are based on measurement. Reputable manufacturers of VSDs take great care to optimize the design of filters to keep harmonic currents in the supply side as low as possible. On DC drives, the 3 chokes are usually located on the supply side of the converter. This method is seldom used on AC drives, the main technique is to install a choke (inductance) in the DC link. On smaller drives where the actual level of current is quite small, chokes are usually omitted to save space and keep the cost down. This practice has been extended to larger drives by some manufacturers. On AC converters where little or no inductance is used, the level of the harmonic currents can be substantially higher than the theoretical values given in the formula above.

===

Diode Bridge Rectifier Harmonic Spectrum

Circuit Layout Phase Current | Waveform | 5th 7th 11th 13th

- No line choke

- No DC link choke

- Low source impedance

- No line choke

- No DC link choke

- High source impedance (e.g. Transformer)

- Line choke fitted

- No DC link choke

- Low source impedance

- DC link choke fitted

- High source impedance (e.g. Transformer)

====

++++ Example of harmonic spectrum with various types of filtering

The mechanical power of the variable speed drive is the product of the output torque and the rotational speed of the motor. This is reflected in the electrical input power that increases with speed.

• For a constant torque load, the active power increases in direct proportion to the speed.

• For a centrifugal fan or pump load, the active power increases as the cube of the speed.

The magnitude and phase angle of the fundamental current, and consequently the harmonic currents, changes as the speed changes. In this respect, PWM converters perform quite differently to DC drives.

In a PWM converter, the DC link voltage remains constant over the entire speed range and is derived from a diode bridge rectifier. As the speed increases, with a constant torque load, the active power increases and, therefore, the DC link current ID and the RMS value of the fundamental supply current increases in proportion to the speed. The harmonic currents in the supply also increase with speed from an initially low level.

In a DC drive, the DC voltage, which changes in proportion to the speed, is derived from a controlled rectifier bridge. As the speed increases, with a constant torque load, the active power and the DC voltage increase in direct proportion to speed. Therefore, the DC current ID and the RMS value of the fundamental supply current remains almost constant over the speed range. The DC current ID and the fundamental current are always slightly higher, compared to the PWM converter, because the firing angle of the controlled rectifier is never zero and the DC voltage is always slightly lower than that of the PWM converter.

++++ The difference in the supply current drawn by a DC converter and a PWM AC converter of the same capacity at full rated load torque.

Above illustrates these differences between the PWM and DC drives when driving constant torque loads at full rated load torque in the speed range 0 Hz to 50 Hz.

With the PWM drive, the harmonic currents decrease with speed reduction because the fundamental current decreases. With the DC drives, the harmonic currents remain roughly constant over the speed range because the fundamental current remains constant. If the load torque is reduced, the converter current will fall in the supply side of both the PWM and DC converter.

The figure below compares the 5th and 7th harmonic currents in an AC PWM drive with the equivalent harmonic currents in a DC drive.

++++ The 5th and 7th harmonic currents at rated torque generated by:

(a) DC converter

(b) PWM-type AC converter

Acceptable levels of distortion in the mains supply system

In the mains supply system, harmonic voltage distortion is the consequence of the flow of harmonic currents through the impedances in the power supply circuit connected to the converter. A typical power supply system at an industrial or mining plant consists of a source of AC power generation, which can either be a local generating station in a small system or a power station at the other end of a transmission line or transformer in a large system. The impedance between the 'ideal' generator and the main busbar is usually referred to as the source impedance Zs of the supply system. Additional impedance, usually comprising cables, busbars, transformer, etc exists between the main busbar and the converter busbar and is the cable impedance Zc.

The flow of current to a variable speed motor is controlled by the converter. The current is non-sinusoidal due to the non-linearity of the converter and the generation of harmonic currents. The flow of distorted current through the power distribution and supply system produces a distorted volt drop across the source and distribution impedances in series.

Other equipment, such as electric motors or even other consumers can be connected to the main busbar. Consequently, this busbar is referred to as the point of common coupling (PCC).

The voltage at the PCC will be distorted to an extent depending on the magnitude of the distorted current, the magnitude of the impedances and the ratio between them. The source impedance can easily be calculated from the system fault level and this is commonly used as the criteria for the permissible size of converter load. A high fault level means a low source impedance and vice versa. If the source impedance is low, then the voltage distortion will be low. The distribution impedance must be calculated from the design details of the distribution system.

A high distribution impedance will tend to reduce the voltage at the point of common coupling but increase it at the converter connection terminals. This voltage distortion can cause interference with the electronic trigger circuits of the converter and give rise to other problems if it becomes too high.

If the magnitude and the frequency of each harmonic current is known, a simple application of Ohm's law will give the magnitude of each harmonic voltage and the sum of them will give the total distorted voltage.

From law, the total harmonic distortion (THD) of voltage and current are given by the following formulae. Generally, it’s sufficient to use values of n up to 25.

… where: VT = Total harmonic voltage distortion

I_Thd = Total harmonic current distortion

V_1 = Fundamental voltage at 50 Hz

I_1 = Fundamental current at 50 Hz

V_n = nth harmonic voltage

I_n = nth harmonic current

The acceptable levels of harmonics in industrial power supply networks are clearly defined in Table 1 of the standard: disturbances in mains supply networks. Briefly, limits are set for the level of total harmonic voltage distortion, which are acceptable at the point of common coupling (PCC). The application of these standards requires the prior calculation of harmonic distortion at all points in the system before the converter equipment can be connected and, under certain circumstances, actual measurements of harmonic voltage to confirm the level of distortion.

Methods of reducing harmonic voltages in the power supply

The use of converters has many technical and economic advantages that will ensure their continued use in industrial and mining plants for many years ahead. In spite of the increase of harmonic distortion in power systems, their advantages far outweigh their disadvantages and their use will continue to grow.

As outlined above, harmonic voltage distortion at the point of common coupling is the result of the flow of harmonic currents through the source impedance. On a stiff power system, where the source impedance is low, the voltage distortion will be low. However, the fault level will be high and the short circuit protection equipment will have to be rated accordingly. On a smaller power system, where the source impedance is high, the voltage distortion will tend to be higher.

One of the most practical solutions is to install an inductance (choke) on the supply side of the AC converter to effectively increase the inductive impedance between the converter and the power supply. As shown, this effectively reduces the overall level of current distortion, particularly the 5th and 7th current harmonics. The choke can be located internally on the DC link (preferable) or connected externally at the input terminals of the converter. The line chokes need to be of special design to deal with the distorted current waveform. The inductance values of the choke are typically rated between 3% to 5% impedance at fundamental frequency based on the converter rating.

In general, there is not much that can be done to change the source impedance of a power system and, in difficult applications, the solution lies in the techniques to limit the source of the harmonic currents or to divert them to the system ground. There are two main methods of reducing harmonic currents: The use of multi-pulse converters: The use of converters of higher pulse numbers will greatly reduce the lower order harmonics. Alternatively, two converters of lower pulse numbers can be combined with a phase shift of 30 deg. to produce a system of higher pulse numbers. Theoretically, 12-pulse converters will generate harmonic currents of the order (12 k ± 1) and won’t contain the 5th, 7th, 17th, 19th, etc harmonics. In practice, these don’t disappear completely, due to slight differences in converter firing angles and unbalances, but are greatly reduced. The 5th harmonic current usually has the highest magnitude, so its elimination or reduction is desirable. This solution can be expensive.

When several similar converters, with controlled rectifiers, are connected to the same busbar, some cancellation of harmonic currents takes place due to phase shifts between the firing angle of converters running at different speeds. With PWM converters, with diode bridge rectifiers, very little cancellation takes place. The worst case should always be assumed for calculation purposes where the total current for each harmonic is the sum of the currents of the converters operating in parallel.

++++ Example of a 12-pulse rectifier bridge feeding a DC drive

The installation of a harmonic line filter close to the converter: The most common type of harmonic filters used in industry are series L-C filters with some damping resistance. These are usually connected to the busbar (PCC) supplying power to the variable speed drives. Filters may be of relatively simple single-tuned construction, but are usually the more sophisticated (expensive), 2nd or 3rd order filters to provide a wider frequency band. The filter is tuned to specific frequencies so that its impedance is at a minimum at the tuned frequency. The harmonic currents generated by the converter equipment are short-circuited by the filter. The harmonic filter is 'tuned' for a particular frequency when:

A typical line side filter comprises resistive, inductive and capacitive components as described below.

The main problem with harmonic filters is that they can become detuned over a period of time for any one of the following reasons:

• Changes in the filter capacitance due to age, temperature, or failure of capacitance units within the bank.

• Changes in the inductance due to temperature and current

• Small changes in the system frequency

Since the overall reactance of the filter becomes capacitive at frequencies below the tuned harmonic frequency, resonance can occur between the filter bank and the power system inductance at fundamental or other lower frequencies. This possibility should be considered in the design of harmonic filter equipment to avoid resonance.

Next: VSD motors and Electromagnetic compatibility (EMC)--part 2

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