Title:

Variable Speed Drive To Offer Premium Power Quality in Single Package

Author:

Princeton Power Systems

Date:

February 13, 2004

More About Princeton Power and Power Quality & Drives

A new variable speed drive (VSD) under development at Princeton Power Systems is designed to solve power quality problems before end users have to think about them.

Princeton Power Systems produces electrical power conditioners - a crucial link in the electrical system for both facilitating the adoption of clean power sources, and for allowing precise control of industrial processes.

Princeton Power's core products are 1) Motor controllers, 2) Wind turbine converters, and 3) Grid-tied inverters. Understanding how these solutions work is crucial to understanding the important role of a power conditioner in the electrical infrastructure.

Variable Speed Drive To Offer Premium Power Quality in Single Package

Power quality can mean a lot of different things for different applications. Because of that, VSD customers have many factors to consider when they purchase a new drive. Is its harmonic distortion low enough? Can it control power factor? Will it damage the motor with rapid changes in voltage? Efficiency and overall reliability must be looked at as well. It is often the case that the end user has neither the expertise nor budget to research these problems and implement an appropriate solution.

The engineers at Princeton Power Systems, in Princeton, NJ, are addressing all of these concerns with their patented AC-link™ technology, and hope to market a unique category of VSDs: drives with all around premium power quality.

A Growing Problem

Power quality has become a serious issue in recent years. The rapid spread of digital technology through industry and the home is a large part of the cause. Digital systems are highly sensitive to poor power quality, and yet are sources of it themselves, because they are typically nonlinear loads. In the motor drive world, the culprit is the VSD, which has become extremely popular because of the flexibility and huge energy savings it provides. However, without expensive filtering, the rapid switching of VSDs injects severe current harmonics into the power grid. The results are malfunctioning digital equipment, trips in upstream transformers, and a general lack of reliability of local electrical systems. To make things worse, utilities have become savvier about penalizing users for poor power quality.

AC-link™

Under development for the last several years at PPS, AC-link™ is a new and innovative method for power conversion, and substitute for the Pulse Width Modulation (PWM) used by all standard VSDs. The technology has advantages in both cost and reliability over PWM converters because it replaces insulated gate bipolar transistor (IGBT) switches with cheaper, more dependable thyristors (silicon controlled rectifiers, or SCRs). The key to achieving precision power control with simpler active components is AC-link’s™ advanced control algorithms. Princeton Power aims to solve power quality problems with software, and its working prototype for a 30kW VSD suggests that the concept is on track.

Total Harmonic Distortion (THD)

Harmonics, the main factor in premium power quality, are important primarily because of the havoc they wreak with the local grid and the extra costs they incur with the utility when certain standards are not met. They also cause energy inefficiency, which results in dangerous overheating, as well as further increases in utility bills.

AC-link™ achieves low harmonics by controlling current instead of voltage. It dictates the size and frequency of charge pulses passed through a central capacitor, rather than applying voltages directly between each side of the device. The small capacitors on the input necessary for circuit operation allow AC-link™ to meet the relevant standards for harmonics, such as IEEE 519, with only minor inductive filtering on each phase.

While motor-side waveforms are not typically included within a conventional definition of power quality, it should be noted that the symmetrical topology of AC-link™ creates equally low harmonics on the output. Drives with poor output harmonics often suffer from jittery motor operation in the form of torque ripple and speed fluctuations.

Power Factor Control

Because AC-link™ directly controls current, it is capable of running at any power factor specified by the user. This functionality allows AC-link™ to be used for compensating undesirable power factors of other loads in same system. Simple operation at unity power factor is possible as well. Like harmonics, power factor is an important part of meeting utility requirements. Because operating at low power factors causes more current to be used at a given power level, it results in inefficiencies in transmission lines, the costs of which are added to the user’s utility bill. These expenses can sum to thousands of dollars for an industrial facility.

Efficiency

As stated above, low harmonics and precise power factor control are both good for efficiency. However, AC-link™ also has inherent efficiency advantages in its simpler hardware. Thyristors are not capable of active shutoff during operation; instead, they must be soft-commutated by a circuit in which voltages routinely force currents to zero. The AC-link™ algorithms enable these soft commutations, and the absence of hard turn-offs like those performed by IGBTs make AC-link™ more efficient than PWM systems. Greater than 98% efficiency can be achieved with AC-link™.

dV/dt

One of AC-link’s™ most unique performance advantages comes from reduced rates of change in voltage, known as dV/dt. Recent research has identified high dV/dt, such as that created by a PWM drive, as a much larger factor in motor failure than had previously been thought. High voltage spikes through a motor cause arcing currents through the motor bearings, wearing them down by a mechanism identical to micromachining. According to motor maintenance statistics, 40% of motor failures are caused by bearing failures, and 25% of bearing failures result from high voltage derivatives caused by the rapid switching frequencies of PWM devices (see Figure 3).

Because AC-link™ is inherently a current control device, as opposed to a voltage control PWM system, it is capable of limiting dV/dt to a few volts per microsecond, while PWM devices typically experience dV/dt’s on the order of 1500-3000 V/us. The result is near total elimination of bearing currents. To achieve the same result, PWM users must buy expensive filtering components that often complicate control of the system. The result is that most customers operate unfiltered drives, and thus risk costly motor repairs and downtime.

Problems with dV/dt can affect other aspects of facility operation as well. In particular, plant layout and design can depend on dV/dt. Large installation distances between VSDs and their loads always exacerbate dV/dt levels because of wire inductance. A plant engineer may observe that a working PWM drive operating close to its load becomes completely dysfunctional when powering a motor farther away.

Finally, rapid changes in voltage can generate unwanted electromagnetic interference (EMI). The vastly smaller dV/dt produced in the motor lines by AC-link™ can offer satisfactory performance for applications where low EMI is required.

Development

To date AC-link™ has been demonstrated in a working 40hp pump motor drive, operating at frequencies from 0-60 Hz, and in a 250kW AC-AC converter under a contract with the Office of Naval Research. Princeton Power Systems is currently in the development and design stages of a 100hp pump motor drive, and a 5MW ship propulsion drive.

AC-link™ is revolutionary in the world of motor drives because it has the ability to solve all major power quality problems with a single device. This feat is made possible by the competitive advantage it gains from its innovative circuit topology and unique control algorithms. End users of VSDs should benefit greatly if AC-link™ technology fulfills its potential.

DrivesMag.com
Title:	Variable Speed Drive To Offer Premium Power Quality in Single Package
Author:	Princeton Power Systems
Date:	February 13, 2004
More About Princeton Power and Power Quality & Drives A new variable speed drive (VSD) under development at Princeton Power Systems is designed to solve power quality problems before end users have to think about them. Princeton Power Systems produces electrical power conditioners - a crucial link in the electrical system for both facilitating the adoption of clean power sources, and for allowing precise control of industrial processes. Princeton Power's core products are 1) Motor controllers, 2) Wind turbine converters, and 3) Grid-tied inverters. Understanding how these solutions work is crucial to understanding the important role of a power conditioner in the electrical infrastructure.	Variable Speed Drive To Offer Premium Power Quality in Single Package Power quality can mean a lot of different things for different applications. Because of that, VSD customers have many factors to consider when they purchase a new drive. Is its harmonic distortion low enough? Can it control power factor? Will it damage the motor with rapid changes in voltage? Efficiency and overall reliability must be looked at as well. It is often the case that the end user has neither the expertise nor budget to research these problems and implement an appropriate solution. The engineers at Princeton Power Systems, in Princeton, NJ, are addressing all of these concerns with their patented AC-link™ technology, and hope to market a unique category of VSDs: drives with all around premium power quality. A Growing Problem Power quality has become a serious issue in recent years. The rapid spread of digital technology through industry and the home is a large part of the cause. Digital systems are highly sensitive to poor power quality, and yet are sources of it themselves, because they are typically nonlinear loads. In the motor drive world, the culprit is the VSD, which has become extremely popular because of the flexibility and huge energy savings it provides. However, without expensive filtering, the rapid switching of VSDs injects severe current harmonics into the power grid. The results are malfunctioning digital equipment, trips in upstream transformers, and a general lack of reliability of local electrical systems. To make things worse, utilities have become savvier about penalizing users for poor power quality. AC-link™ Under development for the last several years at PPS, AC-link™ is a new and innovative method for power conversion, and substitute for the Pulse Width Modulation (PWM) used by all standard VSDs. The technology has advantages in both cost and reliability over PWM converters because it replaces insulated gate bipolar transistor (IGBT) switches with cheaper, more dependable thyristors (silicon controlled rectifiers, or SCRs). The key to achieving precision power control with simpler active components is AC-link’s™ advanced control algorithms. Princeton Power aims to solve power quality problems with software, and its working prototype for a 30kW VSD suggests that the concept is on track. Total Harmonic Distortion (THD) Harmonics, the main factor in premium power quality, are important primarily because of the havoc they wreak with the local grid and the extra costs they incur with the utility when certain standards are not met. They also cause energy inefficiency, which results in dangerous overheating, as well as further increases in utility bills. AC-link™ achieves low harmonics by controlling current instead of voltage. It dictates the size and frequency of charge pulses passed through a central capacitor, rather than applying voltages directly between each side of the device. The small capacitors on the input necessary for circuit operation allow AC-link™ to meet the relevant standards for harmonics, such as IEEE 519, with only minor inductive filtering on each phase. While motor-side waveforms are not typically included within a conventional definition of power quality, it should be noted that the symmetrical topology of AC-link™ creates equally low harmonics on the output. Drives with poor output harmonics often suffer from jittery motor operation in the form of torque ripple and speed fluctuations. Power Factor Control Because AC-link™ directly controls current, it is capable of running at any power factor specified by the user. This functionality allows AC-link™ to be used for compensating undesirable power factors of other loads in same system. Simple operation at unity power factor is possible as well. Like harmonics, power factor is an important part of meeting utility requirements. Because operating at low power factors causes more current to be used at a given power level, it results in inefficiencies in transmission lines, the costs of which are added to the user’s utility bill. These expenses can sum to thousands of dollars for an industrial facility. Efficiency As stated above, low harmonics and precise power factor control are both good for efficiency. However, AC-link™ also has inherent efficiency advantages in its simpler hardware. Thyristors are not capable of active shutoff during operation; instead, they must be soft-commutated by a circuit in which voltages routinely force currents to zero. The AC-link™ algorithms enable these soft commutations, and the absence of hard turn-offs like those performed by IGBTs make AC-link™ more efficient than PWM systems. Greater than 98% efficiency can be achieved with AC-link™. dV/dt One of AC-link’s™ most unique performance advantages comes from reduced rates of change in voltage, known as dV/dt. Recent research has identified high dV/dt, such as that created by a PWM drive, as a much larger factor in motor failure than had previously been thought. High voltage spikes through a motor cause arcing currents through the motor bearings, wearing them down by a mechanism identical to micromachining. According to motor maintenance statistics, 40% of motor failures are caused by bearing failures, and 25% of bearing failures result from high voltage derivatives caused by the rapid switching frequencies of PWM devices (see Figure 3). Because AC-link™ is inherently a current control device, as opposed to a voltage control PWM system, it is capable of limiting dV/dt to a few volts per microsecond, while PWM devices typically experience dV/dt’s on the order of 1500-3000 V/us. The result is near total elimination of bearing currents. To achieve the same result, PWM users must buy expensive filtering components that often complicate control of the system. The result is that most customers operate unfiltered drives, and thus risk costly motor repairs and downtime. Problems with dV/dt can affect other aspects of facility operation as well. In particular, plant layout and design can depend on dV/dt. Large installation distances between VSDs and their loads always exacerbate dV/dt levels because of wire inductance. A plant engineer may observe that a working PWM drive operating close to its load becomes completely dysfunctional when powering a motor farther away. Finally, rapid changes in voltage can generate unwanted electromagnetic interference (EMI). The vastly smaller dV/dt produced in the motor lines by AC-link™ can offer satisfactory performance for applications where low EMI is required. Development To date AC-link™ has been demonstrated in a working 40hp pump motor drive, operating at frequencies from 0-60 Hz, and in a 250kW AC-AC converter under a contract with the Office of Naval Research. Princeton Power Systems is currently in the development and design stages of a 100hp pump motor drive, and a 5MW ship propulsion drive. AC-link™ is revolutionary in the world of motor drives because it has the ability to solve all major power quality problems with a single device. This feat is made possible by the competitive advantage it gains from its innovative circuit topology and unique control algorithms. End users of VSDs should benefit greatly if AC-link™ technology fulfills its potential. Copyright ©1997-2004, Princeton Power Systems and DrivesMag.com. All rights reserved.