HANDLING HARMONICS WITH AC DRIVES

AC motors are abundant in the most of the industry. Every fan and pump application requires an AC motor. For efficient and reliable control, adjustable-speed drives are becoming a common companion for these motors. However, as drives are boosting production, they might also be contributing to electrical noise problems, such as line current harmonics.

Pumping and mixing motors are becoming abundant in industrial wastewater facilities as systems expand to keep up with population growth and demand for more water, distributed over larger areas. To maintain water pressure and control flow, adjustable speed drives are a valuable companion for these AC motors.

Drives are an efficient alternative to throttling values, as they match power needs and motor speed to the demands of varying flow. Drives can save energy and electrical costs, increase pressure reliability, reduce cumbersome pump changes and lessen damage to mechanical equipment. However, as drives are boosting efficiency and reliability, they might also be contributing to power quality problems, such as line current and voltage harmonics.

Harmonics are deviations from the sinusoidal fundamental AC line voltage and current. Most electrical power in North America operates at a frequency of 50 hertz. A harmonic frequency is an integer multiple of this fundamental frequency. So in a 50-hertz system, the second harmonic would be 100-hertz, the third would be 150-hertz, and so on. The addition of any harmonic to the sinusoidal fundamental current or voltage will create distortion. The greater the amplitudes of the harmonics present, the greater the distortion in the electrical waveform. Very simply, whenever a voltage or current does not look like a perfect sinusoidal waveform, it contains harmonics.

Compared to DC motor drives, AC motor drives cause very few problems. However, poorly designed applications can result in power line voltage distortions. These voltage distortions can cause problems for other equipment connected to the same power lines – resulting in erratic operation of controls, dimming of lights, audible noise on telephone systems, and static on AM and FM radios. The distribution transformers and cables feeding these drives will also experience additional heating, which reduces the power utilization of those components.

What Causes Harmonics?

Unlike an AC motor operating across the power line, the current drawn from a distribution transformer feeding a typical AC drive is far from a sinusoidal waveform. This occurs because the drive is taking current from the transformer only during certain times of the cycle to convert the AC line voltage to a fixed DC voltage within the drive. The drive then pulse-width-modulates this fixed DC voltage into variable frequency, variable voltage for the motor. The AC-to-DC conversion is what causes the harmonics. Current flows only during part of the cycle and is off during other parts of the cycle, creating the odd-looking current waveform. It is this distorted current that creates the voltage distortion.

As the number of drives employed in automation systems grows, so too does the misconception that drives are the only piece of the puzzle to harmonic problems. While they may comprise a sizable portion of non-linear loads, they are only one element in the harmonic equation. That's why it's important to analyze all electrical loads that could potentially cause problems for a system before making any final conclusions.

What Affects Harmonic Current?

The harmonic current created by the drive can be affected by the presence of a DC link choke, the DC bus capacitance and the line inductance between the drive input and its source of voltage; such as transformer inductance, cable inductance and additional line reactors.

The closer you are to the drive, the greater the voltage harmonic distortion. This is why the voltage distortion is greater at the drive than at the transformer. If other equipment needs to be connected to the same transformer as the drive, connect the other equipment as close to the transformer as possible. This will minimize the effect of the current harmonics produced by the drive on the voltage waveform, which can cause problems with the other equipment.

Reducing Harmonics

There are several methods and products to reduce the line current harmonics created by drives. Cost is not the only factor that distinguishes one solution from another. Even though the addition of line reactors or passive filters can help reduce the current harmonics, they will also reduce the DC bus voltage within the drive at full speed, full load conditions. This will prevent the drive from being able to provide full power to the motor, limiting the power out of the motor to about 95% of its nameplate rating.

Why would you install a 100 hp motor only to have its capability limited to 95 hp, especially under peak demand conditions? This is why multi-pulse solutions are a better fit for most situations, since no derating is necessary and it is less expensive than other mitigation methods.

Harmonic Mitigation Methods

For a 250 hp drive, the chart above details the typical harmonic current, I(THD), and relative installed costs associated with various harmonic mitigation methods.

(For simplicity, we will say that the 250hp drive is connected to a supply with a short circuit capacity of 60,000A, will draw 260Arms of fundamental current for its load, and will have a cost of 100%.)

The goal for those specifying harmonic mitigation solutions is to combine the most cost effective solution for the particular drive and power distribution system. Since several factors come into play in determining the harmonics in the drive system, Emerich Power Quality Analysis  needs to consider all factors before arriving at a final recommendation.

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Effects Arises due to Voltage Unbalance

Unbalanced systems indicate the existence of a negative sequence that is harmful to all poly phase loads, especially three-phase induction machines.

The main effect of voltage unbalance is motor damage from excessive heat. Voltage unbalance can create a current unbalance 6 to 10 times the magnitude of voltage unbalance. In turn, current unbalance produces heat in the motor windings that degrades motor insulation causing cumulative and permanent damage to the motor. 

This scenario would result to expensive facility downtime due to motor failures.

The graph below shows the relationship between voltage unbalance and temperature rise, which approximately increases by twice the square of the percent of voltage unbalance.

1. Increased current loading and losses in the network.
2. With equal load power the phase currents can attain 2 to 3 times the value, the losses 2 to 6 times the value. It is then only possible to load lines and transformers with half or one third of their rated power.
3. Increased losses and vibration moments in electrical machinery.
4. The field built up by the negative sequence component of the currents runs against the phase sequence of the rotor and therefore induces currents in it, which lead to increased thermal loading.
5. Rectifiers and inverters react to unbalance in the power supply with uncharacteristic harmonic currents.
6. In three-phase systems with star connection, current flows through the neutral conductor.

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Basics of Voltage Unbalance - Causes & Sources

The electrical power issues that most frequently affect industrial plants include voltage sags and swells, harmonics, transients, and voltage and current unbalance.

Voltage Unbalance (or Imbalance) is defined by IEEE as the ratio of the negative or zero sequence component to the positive sequence component. In simple terms, it is a voltage variation in a power system in which the voltage magnitudes or the phase angle differences between them are not equal. It follows that this power quality problem affects only poly phase systems (e.g. three-phase).

Voltages are rarely exactly balanced between phases. However, when voltage unbalance becomes excessive, it can create problems for poly phase motors and other loads. Moreover, adjustable speed drives (ASD) can be even more sensitive than standard motors.

Voltage unbalance is primarily due to unequal loads on distribution lines or within a facility. In other words, the negative or zero sequence voltages in a power system typically result from unbalanced loads causing negative or zero sequence currents to flow.

In a balanced three-phase system, the phase voltages should be equal or very close to equal. Unbalance is a measurement of the inequality of the phase voltages. Voltage unbalance is the measure of voltage differences between the phases of a three-phase system. It degrades the performance and shortens the life of three-phase motors.

Effects of Unbalance

Voltage unbalance can cause three-phase motors and other three-phase loads to experience poor performance or premature failure because of the following:

• Mechanical stresses in motors due to lower than normal torque output.
• Higher than normal current in motors and three-phase rectifiers.
• Unbalance current will flow in neutral conductors in three-phase systems.

Voltage unbalance at the motor terminals causes high current unbalance, which can be six to 10 times as large as the voltage unbalance. Unbalanced currents lead to torque pulsation, increased vibration and mechanical stress, increased losses, and motor overheating. Voltage and current unbalances could also indicate maintenance issues such as loose connections and worn contacts.

Unbalance can occur at any point throughout the distribution system.Loads should be equally divided across each phase of a panel board. Should one phase become too heavily loaded in comparison to others, voltage will be lower on that phase. Transformers and three-phase motors fed from that panel may run hotter, be unusually noisy, vibrate excessively, and even suffer premature failure.

How measure unbalance

You can make some basic phase-to-phase voltage unbalance measurements using a high-quality Digital Multi Meter and phase-to-phase current unbalance using a high-quality clamp meter. Accurate, real-time unbalance measurements need a three-phase power quality analyzer to enable solving unbalance problems. Open circuits and single-phase to ground faults are easier to correct than load balancing, which typically requires corrective system-level design changes..

In reality, voltage differences between phases vary as loads operate. However, motor or transformer overheating, or excessive noise or vibration, can merit troubleshooting for voltage unbalance. Monitoring over time is the key to capturing unbalance. In a three-phase system, the maximum variation in voltage between phases should be no more than 2 percent (the V neg % value on the analyzer), or significant equipment damage can occur.

Causes & Sources

General

The utility can be the source of unbalanced voltages due to malfunctioning equipment, including blown capacitor fuses, open-delta regulators, and open-delta transformers. Open-delta equipment can be more susceptible to voltage unbalance than closed-delta since they only utilize two phases to perform their transformations.  

Also, voltage unbalance can also be caused by uneven single-phase load distribution among the three phases - the likely culprit for a voltage unbalance of less than 2%. Furthermore, severe cases (greater than 5%) can be attributed to single-phasing in the utility’s distribution lateral feeders because of a blown fuse due to fault or overloading on one phase.

Motors

The facility housing the motor can also create unbalanced voltages even if the utility supplied voltages are well balanced. Again, this could be caused by malfunctioning equipment or even mismatched transformer taps and impedance. Similar to the utility, poor load distribution within the facility can create voltage unbalance issues.

The motor itself can also be the source of voltage unbalance. Resistive and inductive unbalances within the motor equipment lead to unbalanced voltages and currents. Defects in the power circuit connections, the motor contacts, or the rotor and stator windings, can all cause irregular impedances between phases in the motor that lead to unbalanced conditions.

References:

ANSI C84.1-2006

Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2^nd ed.). New York: McGraw-Hill.

IEEE 1159-1995. Recommended Practice For Monitoring Electric Power Quality.

National Electrical Manufacturers Association (NEMA) Publication No. MG 1-1998 Motors and Generators

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VFD - Variable frequency drive Advantages & Disadvantages

Variable Frequency Drive(VFD)is a system that regulates the rotational speed of a motor by controlling the input supplied to it. VFD’s can also be known Adjustable Frequency Drives(AFD) or Variable Voltage Variable Frequency Drives (VVVFD).

VFD’s Application:

VFD’s are used in many different applications,the most common include fan blower, and HVAC systems. A VFD can be attached to a fan/blower system to help conserve energy by supplying only the amount of air that the process requires. Variable frequency drives can also be used on pumps, conveyors, and machine tool drives. 

Advantages of Using a VFD

Advantages of a Variable Frequency Drives – The main reason/advantage for implementing a VFD into a system is to save money by reducing the overall system energy being consumed. Along with saving money in energy reduction here are some more advantages of using a VFD in a system.

• Able to control the processes of the system better, giving the operator a bypass option in the case of an inverter failure.
• System starts up “softer” than normal, meaning it will decrease the average 6    to 7 up to 20 times inrush current during start up.
• Saves money on the electricity costs of a system.
• By better controlling a motor’s speed, life of V-belt and or coupling devices is increased.
• No appurtenance loss for using this control device in a system.
• Device can include a breaking feature (Check with manufacturer)
• Reduces the costs of a system eliminating the need to buy an anti rotational device. 

Disadvantages of Using a VFD

Disadvantages of Variable Frequency Drives – VFD’s, though useful in making a system more efficient also bring with them some disadvantages.

· Upfront cost of a VFD can be relatively high depending on how large your system is.    

· Adding a VFD device may lead to a system resonance at certain speeds, leading to;        

1. Dramatically increased noise

2. Excessive vibration.

· VFD device have been known to shorten the life

· Can reduce the service factor on the motor it’s used on. 

3.Initial Drive Cost

   - Motor Heating at low Speeds

   - Maintenance

   - Induced Power Line Harmonics

However, these disadvantages can be mitigated to an extent by correct application and maintenance as outlined above. A simple calculation of energy savings versus the cost of the drive, its installation and maintenance can guide the decision to install a drive (or not).

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Metals sector inattentive of its power quality losses! Why?

Metals manufacturing plants have a high demand for electrical power, are electric motor intense, have high raw material costs & rely heavily on sophisticated automation and on a skilled labour force.Common to these characteristics is that they all lead to significant operating losses when the electric power is interrupted.

The European Copper Institute (ECI) Power Quality Survey has identified that the metals sector loses over 10% per cent of its net bottom line due to Power Quality operating issues.

The domino effect of production stoppages

Manufacturing plants in the metals sector make generally intense use of electric motors of different ratings and specifications, often integrated in one controlled system. A loss of control of one motor caused by an unexpected and sudden power interruption or power dip can upset the general control and bring the whole production system to a complete standstill. Such an event can, in turn, result in a wide range of financial losses:

Both manufacturing and administrative staff will be made idle when “the lights go out”,often for several hours.

Irregularities in power supply can shorten the useful operating life of expensive high tech machinery or even permanently damage them, resulting in high maintenance and equipment replacement costs.

Raw material that was being processed at the moment of the event can be irrecoverably wasted, or if not will require additional reprocessing to make it again fit for purpose.

Unexpected stoppages play havoc with production planning, resulting in delivery delays, loss of reputation for reliability and consequently loss of business.

Different cost centers blur the reality

The cumulative financial losses caused by power interruptions tend not to be assessed as a totality, relating as they do to different cost centers and occurring at different moments in time.

This might explain why, despite those high potential losses, metal manufacturing sites are on the average not as immune to poor power quality as might be expected. In many of the cases,the total financial waste caused by these interruptions was equivalent to the organisations’ annual electricity bill.

A high price for an under-designed process system

Because of an inadequate power system design, this metals company was being crippled by power interruptions and sudden power reductions causing machinery to fail. In many cases,the precise power quality issue that led to such a breakdown was not known due to a lack of monitoring equipment. The power quality events resulted in production stoppages of 2 hours on average, causing about 300 production staff to be idle. The cost of poor power quality for this company was further exacerbated by raw materials wastage, equipment damage, reduction of operating life time of heavier equipment and the additional maintenance required to get production up and running again.

" Metal industry loses the equivalent of over 50% of its annual electricity bill due to Power Quality losses"

3.4 % of company’s annual turnover being wasted Each time the lamination process in this company was interrupted due to power quality problems, it resulted in high financial losses, consisting mainly of wasted raw materials, low revenues, and staff downtime. Those losses were calculated as equivalent to 3.4% of this company’s annual turnover and to 30% of their net profit.

Understanding the problems – designing the solutions

Emerich Energy Power Quality Survey demonstrates that the majority of the PQ problems faced by the metals sector could be avoided by a more appropriate design of the factory’s own electrical installations. The solutions therefore lie very much in the industry’s own hands. Electrical design engineers involved in this survey recommend a holistic approach to review all the issues at hand, based on three operational pillars:

Correct measurement, to assess the full impact of power quality events, and why they are happening

Appropriate design for the electric installations, ensuring reliability and resilience

Considered investment justified by assessing system renovation cost set against the accumulated losses. 

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Electrical Enclosure IP Ratings - what they mean?

IP Ratings (Ingress Protection)

A two-digit number established by the International Electro Technical Commission, is used to provide an Ingress Protection rating to a piece of electronic equipment or to an enclosure for electronic equipment.

IP	6	8
“Ingress Protection”	First Digit: Solids Protection	Second Digit: Liquids Protection

The protection class after EN60529 are indicated by short symbols that consist of the two code letters IP and a code numeral for the amount of the protection.

Example: IP65 

The two digits represent different forms of environmental influence:

 • The first digit represents protection against ingress of solid objects.

 • The second digit represents protection against ingress of liquids.

The larger the value of each digit, the greater the protection. As an example, a product rated IP54 would be better protected against environmental factors than another similar product rated as IP42.

IP TABLE:

IP..	First digit: Ingress of solid objects	Second digit: Ingress of liquids
0	No protection	No protection
1	Protected against solid objects over 50mm e.g. hands, large tools.	Protected against vertically falling drops of water or condensation.
2	Protected against solid objects over 12.5mm e.g. hands, large tools.	Protected against falling drops of water, if the case is disposed up to 15 from vertical.
3	Protected against solid objects over 2.5mm e.g. wire, small tools.	Protected against sprays of water from any direction, even if the case is disposed up to 60 from vertical.
4	Protected against solid objects over 1.0mm e.g. wires.	Protected against splash water from any direction.
5	Limited protection against dust ingress. (no harmful deposit)	Protected against low pressure water jets from any direction. Limited ingress permitted.
6	Totally protected against dust ingress.	Protected against high pressure water jets from any direction. Limited ingress permitted.
7	N/A	Protected against short periods of immersion in water.
8	N/A	Protected against long, durable periods of immersion in water.
9k	N/A	Protected against close-range high pressure, high temperature spray downs.

IP protection of the PIP:

A PIP in the standard PIP housing is generally IP51 protected. Higher IP protection level with the standard PIP housing (up to IP54) can be reached with good positioning / orientation of the PIP. In other special PIP-housings, like a MIL-housing up to IP67 protection is possible.

IP protection of the PANEL-PIP:

The PANEL-PIP is available in various housings. Those allow a protection level of up to all around IP65.

Range

While we cover a huge range of electrical enclosures, our most common IP ratings are probably 65, 66, 67 and 68. So for quick reference, these are defined below:

• IP65 Enclosure - IP rated as "dust tight" and protected against water projected from a nozzle.
• IP66 Enclosure - IP rated as "dust tight" and protected against heavy seas or powerful jets of water.
• IP 67 Enclosures - IP rated as "dust tight" and protected against immersion.
• IP 68 Enclosures - IP rated as "dust tight" and protected against complete, continuous submersion in water.

IGBT & Its Application in Power Quality

An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch which, as it was developed, came to combine high efficiency and fast switching. 

It consists of four alternating layers (P-N-P-N) that are controlled by a metal-oxide-semiconductor (MOS) gate structure without regenerative action. Although the structure of the IGBT is topologically the same as a thyristor with a 'MOS' gate (MOS gate thyristor), the thyristor action is completely suppressed and only the transistor action is permitted in the entire device operation range. It switches electric power in many applications: variable-frequency drives (VFDs), electric cars, trains, variable speed refrigerators, lamp ballasts, air-conditioners and even stereo systems with switching amplifiers.

IGBT comparison table [1]
Device characteristic	Power bipolar	Power MOSFET	IGBT
Voltage rating	High <1kV	High <1kV	Very high >1kV
Current rating	High <500A	High > 500A	High >500A
Input drive	Current ratio hFE 20-200	Voltage VGS 3-10V	Voltage VGE 4-8V
Input impedance	Low	High	High
Output impedance	Low	Medium	Low
Switching speed	Slow (µs)	Fast (ns)	Medium
Cost	Low	Medium	High

IGBT Applications: Industrial

IGBT industrial applications are due to their use in driving motors. Availability of the IGBT in the early 1980s enabled development of cost-effective ASDs (Adjustable speed drives) for motors. These drives reduce energy consumption by more than 40%. Two-thirds of the electricity in the world is used to run motors, so IGBT technology has had a huge impact on energy consumption. 

These applications include:

Industrial Motor Drives

Adjustable Speed Drives for Motor Control

Pulse Width Modulated ASD

Factory Automation

Robotics

Welding

Induction Heating

Milling and Drilling Machines

Metal and Paper Mills

Electrostatic Precipitators

Textile Mills

Mining and Excavation

IGBT Optimization for Industrial Applications

IGBT Technology in Power Quality World

The versatile and adaptive design possibility of the IGBT system in all the areas where Power Quality is a mandatory requirement such as, 

• Power Factor Improvement using Static Condenser(AVG) using IGBT Technology
• Harmonics Mitigation by Active Harmonic Mitigator (AHM) - Voltage Source Converter using IGBT Technology
• Unbalance Compensation(AUG) (Negative Sequence) using IGBT Technology
• Active Front End DRIVES using IGBT Technology 
• Active Front End UPS (Uninterrupted Power  Supply) using IGBT Technology
• Voltage Dip/Sag & Surge Protection using IGBT Technology(Series Compensation)
• Industrial Power Automation solutions for Power Supplies using IGBT Technology
• Special DRIVES for LOCO applications using IGBT  Technology
• High Frequency and Higher Power Quality conversions for AVIATION Applications

And many more..

Conclusion

From the Power Factor improvement and Drive application to Voltage Dip / Surge correction, the IGBT Technology has it all. India is focusing on a Better Power Quality more than any other country in the world currently. 

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