Power Quality for variable speed drives

Industries make use of a large amount of energy to power a diverse range of manufacturing and resource extraction processes. The industrial machines, especially those that are motor-driven, account for approximately 60% of electrical energy use and are ubiquitous in industrial facilities worldwide. These motors are the workhorses of industries. They include pumps that move fluids, fans that move air, compressors, conveyors, and every type of machine that depends on rotational force to get the work or process done. It is these motors that are the key to reduce energy use as well as CO2 emissions.

Electric motors run at fixed speeds and are ideally suited for applications where a constant motor speed is required. However, there are certain applications where varying motor speeds are preferred because they can meet the requirements of the varying load appropriately. Historically, various control methods have been employed to enhance the flexibility and consistency of manufacturing processes such as controlling the speed of the equipment, changing gear ratios or pulleys, and using hydraulic drives. In many cases, motors are also controlled by means of a valve or damper that regulates the flow, while the speed of the motor itself remains unchanged. These conventional methods are now-a-days considered inefficient ways that waste a huge amount of energy. This make motor drives, especially Variable Speed Drives (VSDs) a boon for today’s industry pursuing Energy Efficiency (EE) goals.

While selecting the right fit is important to get the best out of VSDs, it is equally important to understand the Power Quality (PQ) impact that it can have. This blog focuses on the key features of VSDs, their effect on PQ for industrial customers/equipment and their mitigation solutions.

VARIABLE SPEED DRIVES –KEY COMPONENTS AND APPLICATIONS

A Variable Speed Drive, also known as an Adjustable Speed Drive (ASD)is an electronic device that controls the characteristics (mainly frequency) of a motor’s electrical supply. Therefore, it is able to control the speed and torque of a motor, achieving a better match with the flow requirements of process or speed of the machine it is driving. Thus, in applications where variable control is desirable, slowing down a motor with a VSD does reduce energy use (Cube law depicting power is proportional to cube of flow dependant on speed needed) substantially. The output can also be changed to enable the motor to generate more or less torque as required. For example, using a VSD to slow down a fan or pump motors by 20% can save as much as 50% energy use.

The key components of VSDs are rectifiers to convert AC to DC, regulators to enable an exchange of data between VSD and peripheral devices, inverter to generate an AC supply with a semiconductor switch and control unit that gives and receives signals to other components to correctly operate the equipment.

VSDs provide continuous control by matching motor speed to the specific demands of work being performed. They are an excellent choice because they allow operators to fine-tune processes while reducing costs for energy and equipment maintenance. Some of the known applications of VSDs are:

  • Electrical Motors use approximately 2/3rdof the electrical energy consumption in an industrial sector. They have dominated fixed speed applications for many years but with the help of VSDs, they are also establishing themselves in controlled speed applications. The need for energy conservation in order to save the environment is a key driver for improving efficiency when employing VSDs. Improvement in motor efficiency offers major energy savings, reduces GHG emissions and decreases the payback time.
  • Fans – An on-off switch is a simple method to control the amount of time that fans are required to operate; however, they are not common ways to control a process since continuous on and off switching decrease the motor’s lifetime considerably. Varying a fan’s speed by VSDs allows it to match changing load requirements more closely, and because fan power draw is proportional to the cube of its speed, reducing speed can save a lot of energy.
  • Pumping Systems account for nearly 20% of the world’s energy consumption by electric motors and 25–50% of the total electrical energy usage in industrial facilities. A centrifugal pump runs with an AC induction motor which is a single speed device due to the fixed frequency of the applied power to the motor. VSD allows the frequency of the power to be controlled and the speed of the motor’s shaft to be adjusted.
  • Heating, ventilating and air conditioning (HVAC systems) – Typical HVAC systems run at partial-load conditions during most of operation time. VSDs are used to vary a pump and fan speed in HVAC systems. In these applications, speed control is used to regulate the flow of water or air because speed adjustment is an energy efficient method to control the flow. VSDs can save 15–40% of energy in HVAC application.

IMPACT ONPOWER QUALITY DUE TO VARIABLE SPEED DRIVES

While VSDs prove to be a medium of saving energy consumption to a considerable extent, but they are also responsible for creating an adverse effect on the power quality. Some of the key PQ issues caused by VSDs are:

  • Harmonics – Current and voltage harmonics in the AC supply are created by VSD (as a nonlinear load) connected to the power distribution system. There are significant changes in waveform distortion at different speeds and torque levels in the operation of VSDs. Harmonics pollute the electric system, which causes a problem if harmonic level increases beyond a certain level. Several problems such as increase in the electrical losses of the transformer, decreased efficiency of motor, nuisance tripping of circuit breakers, failure of electronic circuits, and decreased life expectancy of equipment are also caused due to harmonics generated from VSDs.
  • Voltage Sensitivity – If high-voltage distortion shows up as excessive flat-topping, it will prevent DC link capacitors from charging fully and will diminish the ride-through capability of the drive. Thus, a voltage sag which would not normally affect a drive will cause the drive to trip on under voltage. Improper grounding will affect the internal control circuits of the drive, with unpredictable results.
  • Capacitor Switching Transients – High-energy (relatively low frequency) transients that are characteristic of utility capacitor switching can pass through the service transformer, feeders, and converter front-end of the drive directly to the DC link bus, where it will often cause a DC link overvoltage trip. Input diodes could also be blown out by these transients.

POTENTIAL PQ MITIGATION SOLUTIONS AND THEIR BENEFITS

Thankfully, there are various ways of mitigating VSD generated PQ issues.

  • Harmonics – The following measures can be taken, if harmonic distortion due to VSDs is increased beyond acceptable limits:
    – Replacing the three-phase bridge rectifier by a Pulse-Width Modulation (PWM) controlled inverter bridge, which generates a nearly sinusoidal current
    – Installing passive harmonic filters tuned to the most important harmonics, which provides functions such as reducing neutral currents, transformer loading, peak phase or average phase current while increasing system protection and capacity
    – Replacing the six-pulse rectifier by a twelve-pulse rectifier inverter scheme of design to reduce the impact of distortion. One can opt for higher pulse rectifier for new designs too.
    – Installing active filters with the ability to compensate current harmonics
    – Lowering the impedance of the main distribution transformer

  • Voltage Sensitivity –
    – Additional energy storage–The aim is to store enough energy to supply the inverter when the power system doesn’t, due to a disturbance. Several methods of energy storage available are additional electrolytic capacitors, battery back-up, uninterruptible power supplies (UPS), super condensers, superconducting magnetic energy storage (SMES), etc.
    – Boost Converter– A DC-DC boost converter is a device that maintains the DC Bus voltage constant. It is connected to the output of the rectifier and before the capacitor filter. The converter can be integrated into the ASD supplied by the manufacturer, or it can be an add-on module in ASD with the accessible DC Bus.
    – Active Rectifier– This device is hardware equivalent to that of the PWM inverter but it performs the inverse function, i.e. it converts an AC fixed voltage into a DC adjustable voltage. Replacing the diode rectifier by an active rectifier has advantages like DC Bus voltage is controlled so it can remain constant in the presence of voltage dips.

  • Transients – Using reactors at the input of an ASDs or connected to its DC link is the most cost effective way to minimize the effect of capacitor-switching transients on ASDs.

CASE STUDY: MINIMIZATION OF HARMONIC DISTORTION OF INDUSTRIAL MOTOR DRIVES WITH ACTIVE POWER FILTER IN PAPER MILL

Problem Statement – Delta Paper Mill is one of the paper manufacturers in South India with three paper machines and a capacity of 51100 MT/annum of quality writing and printing papers. The paper machine section has a large number of VSDs installed and work continuously with multi drive control strategy. The plant was using 6-pulse thyristor rectifiers due to its well know advantages, however, it also emitted harmonic voltage and current.

Solution adopted – The data was recorded & measured using PQ analyser and it was observed that the considerable amount of harmonic distortion was present in both voltage and current waveforms of all the motor drives. In order to reduce the harmonic content of the motor drives, an Active Power Filter (APF) was implemented.

Benefits & Conclusion – Results were obtained for harmonic profile with APF as shown in below table:

 

% THD without filter

% THD with filter

Voltage

37.07%

1.87%

Current

26.54%

2.93%

It is clear that the harmonic content has been reduced to a great extent with Active Power Filter.

CONCLUSION

Conventional motors with fixed speeds are inefficient for many industrial applications. Instead, VSDs are reliable and cost effective means to control the speed of electrical motors. Installing VSDs on electrical motor applications improves the efficiency of the system and saves a huge amount of energy. They require little maintenance, provide the most energy efficient capacity control, have the lowest starting current of any starter type, and reduce thermal and mechanical stresses on motors and belts. However, when considering using a VSD to a motor, care must be taken to ensure that the drive is really needed, that it is used in the best place and that it is being used correctly with suitable PQ mitigations to provide the best energy saving advantages to the industry.

REFERENCES

(1) Variable speed drives

(2) Motor Systems Efficiency Supply Curves, December 2010

(3) Types of Variable Speed Drives – Ryan Chamberlin, June 11, 2011

(4) How adjustable speed drive affect power distribution– Fluke

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(6) Applications of variable speed drive (VSD) in electrical motors energy savings – R. Saidura,∗, S. Mekhilefb, M.B. Ali a, A. Safari b, H.A. Mohammed, Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia, Department of Mechanical Engineering, College of Engineering, Universiti Tenaga Nasional, Selangor, Malaysia

(7) A New Solution for Harmonics Generated by Variable Speed Drives

(8) Behaviour of variable speed drives under the influence of voltage

(9) Effect of Voltage Unbalance on Adjustable Speed drives and its mitigation using supecapacitor

(10) Adjustable speed drive – Richard Okrasa, P.Eng. Ontario Hydro, Fourth Edition, August 1997

(11) Minimization of Harmonic Distortion of Industrial Motor Drives with Active Power Filter in Paper Mill – A Case Study – Y.Kusumalatha, Ch.Saibabu, and Y.P.Obulesu, March 12-14, 2012

(12) Least Cost Planning: Should Utilities Invest in Energy Efficiency Rather Than in New Supplies? Ann Davison, Oxford Institute for Energy Studies, 1991

(13) Least Cost Planning in the Utility sector- Progress and Challenges

(14) Least Cost Planning Principles, Applications and Issues – USAID-SARI/Energy Program, October 2003

(15) Same Energy, More Power – Accelerating EE in Asia – ADB