Using a frequency converter to control harmonic distortion
On three separate occasions, over a two year period, OMG Harjavalta Nickel Oy experienced unexplained damage to equipment on its nickel production line.
On each occasion the equipment damage was confined to a 690V system that was being fed by a 3.15MVA transformer at the production plant. Over 2MW of the load on this system is controlled by frequency converters and there were suspicions that the culprit could be these non-linear loads and the variations they were producing in the network power quality.
Pertti Sihvonen, OMG’s electrical and automation manager, is not the first end-user to experience the effects of non-linear loads on a power network.
End-users and power companies are increasingly concerned about a phenomenon called harmonics. Harmonic distortion can manifest itself in some serious ways and may disturb or even damage sensitive equipment connected to the same electrical network.
You may not be able to see, smell or hear a harmonic but you can detect its damage by way of excessive heating of conductors, motors or transformers through to spurious tripping of circuit breakers, damage to lighting and interference with communications equipment.
Non-linear loads connected to the network, such as rectifiers and regular motor drives, produce harmonic components in the network current and, via the current, in the network voltage.
The harmonic content of the network current is generally described in terms of total harmonic distortion (THD), which expresses the magnitude of the harmonic currents relative to the 50Hz fundamental. The THD of regular frequency converters is approximately 40–70percent, for example.
Checking the levels
In order to check the power quality, Pertti Sihvonen asked Nokia Capacitors and ABB to measure the harmonic distortion levels. The two sets of results were similar: voltage distortion was over 7percent, while the relevant standards specify a figure of less than 3percent.
"We thought about purchasing an active filter as a basic upgrade, but it turned out that a filter with the power rating we needed would cost more than a frequency converter," Sihvonen says.
It was at this point that ABB suggested the use of a new type of frequency converter which it had recently launched in the market, namely a low harmonic drive. Significantly lower current distortion levels – even less than 5percent – can be achieved with today’s frequency converter technology. ABB’s low harmonic drives have an active supply unit to keep the network current almost sinusoidal.
Frequency converters
So what are the alternatives if choosing a frequency converter and what makes the low harmonic drive so special?
Nearly all frequency converters in the market place today are voltage source PWM designs and consist of three parts, the rectifier, the dc link and the inverter.
The rectifier often seems the simplest item and the majority of rectifiers are in practice simple diode bridges. These are simple and robust, but have one main drawback: the current taken from the network is not sinusoidal and large levels of harmonic distortion will
be present. This means that line or dc link chokes may well be needed and diode rectifiers can be used in multiples with special transformers (12 pulse, 18 pulse, 24 pulse rectifiers).
A different solution can be to use a low harmonic ‘active rectifier’.
The basic technology has been on the market for around 15years. As component costs have reduced and performance requirements have increased it has become more common.
In this solution, an active converter, usually consisting of the same components as the motor converter, is used to generate the dc voltage.
The operating principle of the ‘active rectifier’ is that the switches control connection to the network in a pattern intended to produce sinusoidal current without the harmonic components associated with conventional rectifiers. The solution uses a switching rate substantially higher than the network frequency, which will effectively reduce the low frequency harmonics.
This will generally still leave some higher frequency noise, due to the switching rate, which can be filtered by LCL filters.
The ABB solution is specifically designed to use the principles found in motor control namely Direct Torque Motor Control (DTC), taking the references from the network, rather than the motor.
The control includes four main controllers:
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Torque and flux hysteresis controllers.
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Dc voltage and reactive power controllers.
On the basis of measurements, the following four parameters are calculated:
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Actual value for flux.
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Actual value for torque.
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Actual value for reactive power.
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In some implementations of the active rectifier it is possible to allow bi-directional power flow, so the unit becomes regenerative, and in others uni-directional power flow will allow optimal use of the semiconductors, and enhanced ratings.
Other advantages of the active rectifier include:
Estimate for frequency.
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Ability to control the net side power factor. Use of ‘spare’ rectifier capacity for site power factor correction
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Ability to control the dc bus voltage. Eliminates need to limit motor output because of converter system volt drop.
The use of active rectifiers has been claimed by some to be less efficient than the use of a simple diode rectifier bridge, but in practice the question should always be: "What happens to the string efficiency?"
Lower harmonic content reduces losses in the supply network ie in the cabling and the transformer. And in any evaluation, the effects and losses of other harmonic mitigation equipment must also be taken into account.
So did it work for OMG?
Meanwhile back at OMG, the ABB low harmonic drive was installed. The voltage distortion was rechecked and found to be a little over 4percent, which according to experts is an acceptable situation.The frequency converters are operating as planned and producing the desired results.
According to Sihvonen, OMG cannot simply ignore the disturbances because there are around 500 frequency converters at the plant.
The chemical plant, which was constructed in 2003, uses a Profibus fieldbus-based control network/system which has 150 frequency converters connected.
During the past two years OMG has acquired more frequency converters than control valves, for control purposes. As a result there are more frequency converters than control valves in the new control loops at the plant.
With ABB’s low harmonic drives the network current is almost harmonic-free and the plant’s power factor can be corrected. Low harmonic levels and reactive power compensation boost power distribution efficiency and decrease network loading.
At the same time the need for
over-dimensioning of power plant and distribution network capacity is reduced.
Enter 66 or at www.engineerlive.com/ede
Kari O. Kovanen is with ABB Oy, Helsinki, Finland. www.abb.com/drives
