Power quality and EMC is an increasingly important area of research. In Europe it is estimated that poor power quality results in annual costs of €150 billion. [1] Increasing demands are also being imposed on power distribution systems, not only to provide better efficiency and functionality but also to accommodate new uses and generation techniques such as renewable energy and hybrid cars. New paradigms for power systems are appearing, such as smart grids, micro grids, the more electric ship, more electric aircraft and hybrid cars. These new paradigms are being created to improve the performance of power distribution systems and to meet the demands imposed by these modern users. Power electronics is a vital enabler for these new systems and has led to the widespread use of power electronic converters for energy efficient loads, flexible transmission systems and advanced power generation. This article discusses the opportunities and challenges created for power quality and EMC by the wider deployment of electronic power converters and the development of new power electronic devices.
Electronic power converters are widely used in electrical generators and in many loads. They are being deployed in distribution systems because they provide substantial benefits to power quality. The power electronic devices can be used to convert from one AC frequency to another AC frequency or just to decouple two AC systems. Electronic power converters can also be used to convert from AC to DC (rectifiers) or from DC to AC (inverters). Power converters have many design types and topologies but tend to have two main applications; as a series buffer device controlling current or power between two systems, or as a shunt device providing reactive power or harmonic currents to improve voltage quality (as shown schematically in Figure 1). Through these power electronic controlling devices, improved power quality can be achieved in such ways as:
- more flexible control of power flow around a distribution system
- improved control of voltage levels and increased voltage stability
- increased loading capability of equipment to its maximum thermal capabilities
- increase of the transient stability margin
- damping of power oscillations and limitation of short circuit currents
- reduction of reactive power flows and reduced current loop flow.
Figure 1: Typical converter arrangements for improved power quality
Power electronics controllers are thus the essential enabler that underpins the latest generation of power systems based on the concepts of smart grid and more electric vehicles. A power electronic supply or power converter is also an essential part of modern energy efficient loads and power sources. For instance, a typical energy efficient lighting ballast comprises an AC to AC converter changing the mains frequency to a high frequency to supply a fluorescent lamp, whereas a battery storage device has a DC to AC inverter converting a DC battery supply to the mains frequency. It can also be argued that, since most modern loads incorporate an electronic power converter buffering the connection to the supply system, the quality of modern supplies can be reduced without a significant detrimental effect on the users. For instance in aircraft frequency, wild (variable supply frequency) power systems are being considered. Figure 2 shows a typical smart grid with a range of renewable generation and energy efficient loads, all of which are connected to the power system via power converters. These power converters can be closely controlled to match load to total generation and also buffer the components from system power quality problems such as voltage dips or harmonics. Figure 3 shows a proposed, more open electric power system for commercial aircraft which also makes full use of electronic power converter technology to control and improve power quality in a more energy efficient system. The actuators can present a constant power load to the system or can even regenerate power back into the system. Figure 4 shows part of the power electronic substation being developed by the UNIFLEX research program. [2] Such a system offers far more flexibility control than the usual transformer-based substation, with complete independent bi-directional power flow operation and control of each connected network, and reactive power support (voltage control) with active harmonic filtering to comply with grid standards.
Figure 2: Diagram of a typical terrestrial microgrid
Figure 3: A generalized aircraft power system based on More Open Electrical Technologies (MOET) system for a one engine generator
Figure 4: 300 kVA prototype universal and flexible power management substation [2]
Although, power electronic converters are a very necessary component of the more advanced power systems with a potential to greatly improve overall power quality, they also represent challenges for power quality and EMC compliance. The main problems are that:
- converters are inevitably less reliable than directly connecting the loads and generators to the power system
- converters contain switching devices, which lead to increased harmonics or conducted emissions
- constant power control or bidirectional power flow from converters can effect power system stability.
Reliability can be improved through redundancy and by making full use of a modular approach for ease of maintenance. [2] Harmonics in power systems have been known about and studied for nearly a century, however, standards were set when a significant amount of linear or resistive loads dampened the harmonics. Modern power converters using high frequency, fully controlled switching devices can virtually eliminate generation of harmonics but at the expense of higher frequency conducted emissions and greater expense. Modern power converters can also very precisely control the load power so that the loads behave as constant power loads, but this leads to stability problems for the power distribution system. In order to address these opportunities and concerns, an holistic approach to power quality and EMC is required; one that will identify the most efficient or convenient way to provide complete compliance to standards and improved performance for the end users.
The simplest form of power converter is an uncontrolled rectifier made up of diode bridges to transform the AC power supply into DC. This is a common input stage for power converters. It has the disadvantage that it generates harmonics on the AC side. Figure 5 shows the typical harmonic currents generated by a capacitively smoothed rectifier. The triplin harmonics (3rd, 6th, 9th etc) are mostly zero sequence harmonics (identical on all phase) and therefore their return current is in the grounding system. The presence of large tripling current harmonics therefore requires a more substantial ground return conductor. Often present practice is for the ground conductor to have a reduced cross-section compared with the phase conductors, but this practice will need to be reviewed. The relationship of the voltage and current harmonics with supply admittance is typically as shown in Figure 6 for most rectifier circuits. The exact behavior of the harmonics from a rectifier circuit depends on whether a capacitor or inductor is used to smooth the DC side. From Figure 6 it can be seen that as the supply admittance increases, the current harmonics increases but voltage harmonics decreases. This represents the fundamental problem when trying to control power system harmonics. By simply adjusting the equipment rating (hence adjusting the system impedance) it is difficult to reduce or control simultaneously both the voltage and current harmonics. Usually some compromise is needed or additional filtering is required. The harmonic voltages are of concern to the users or loads as they could affect their operation. The harmonic currents are of concern to the utility or power supply as they can lead to excess losses in equipment and overheating. In the past, harmonics were dampened by the presence of linear loads (i.e. resistive loads) since the harmonic level is proportional to the ratio of non-linear loads to linear loads. But the proportion of non linear loads is rapidly increasing. It is estimated that by 2012, 60% of the loads on the power systems in the USA will be nonlinear loads. [3] If loads such as direct online motors are susceptible to this increase in voltage harmonics, then the power systems will need to be upgraded to a higher power rating (reduced system impedance) and the earth wires strengthened to accommodate the increase in current harmonics.
Figure 5: Typical harmonic currents (as a percentage of the fundamental rms) for a capacitively smoothed rectifier load
Figure 6: A typical variation in total harmonic voltage cand current distortion with the inverse of the system impedance for rectifier loads
More sophisticated converters have a much more reduced harmonic current output. On three phase systems, it is possible to use 12 or 18 pulse rectifier units. [3] Figure 7 illustrates the conducted emissions from a six-pulse, thyrister-controlled rectifier unit, showing that for some switching conditions high frequency conducted emissions can exceed standards. Converters using fully controlled switching devices such as GTOs or IGBTs can produce almost pure sinusoidal interfaces. These are far more expensive devices and, although the harmonics are reduced, the high frequency switching leads to higher frequency conducted emissions. Although, these higher frequency conducted emissions do not propagate as far as the lower harmonics, special electromagnetic interference filters have to be fitted to prevent interference with other users at the point of common connection and even for radiated emissions. The latest matrix converters making use of Silicon Carbide(SiC)-based diods can achieve 20 kVA/litre energy densities and over 50 kHz switching frequencies with 10 ns current risetimes. An example matrix converter designed for the aircraft industry is shown in Figure 8. Such devices present extreme EMC challenges, with not only conducted emissions but potential radiated emissions in the GHz frequency range. New EMI filter designs are needed, as well as enclosures with good shielding effectiveness.
Figure 7: Typical conducted emissions measured from a six-pulse thyrister-controlled rectifier unit
Figure 8: Photograph of a silicon carbide JFET-based matrix converter developed to reach a target of power density 20kW/dm3 with forced air cooling [5]
The complete ballast circuit topology for compact fluorescent lights with full filtering and power factor correction is shown in Figure 9. With such systems it is possible to provide a very compliant load that behaves as a good neighbor to all other users, but this is an expensive solution. Due to cost and size limitations, not all lighting ballasts are as complete as the one shown in Figure 9. At present there are no harmonic limits for LED light bulbs or compact fluorescent lights of less than 25 W. This has been shown to give problems if whole buildings are fitted with energy efficient lighting comprising bulbs of less than 25 W. [5] In this case, the local distribution system has to incorporate suitable filtering or be upgraded to accommodate the increased harmonics.
Figure 9: A schematic diagram of the principle components of ballast for compact fluorescent lights
Not only can modern power electronic systems degrade the voltage and current waveforms, but they can also degrade the stability of distribution systems. Fully controlled converters can provide complete power control to the loads. Thus the load current will increase if there is a supply voltage dip or will decrease during a voltage swell. Such loads essentially exhibit a negative impedance to voltage fluctuations which then can significantly degrade the stability margin for small power systems. Care has to be taken in choosing the control bandwidth for the converters to avoid loss of stability or passive damping has to be incorporated, but this increases power losses and negates the use of the high performance power converter. [6]
In conclusion, the technology for generation and distribution of electrical energy is going through a stage of rapid development as electrical energy is increasingly used to improve efficiency in the transport industry, renewable generation is being developed and society as a whole is becoming more dependent on electronic technology. Power electronics devices are an essential part of these developments, as they provide new opportunities for improved power quality and EMC. However, power electronic devices are controlled nonlinear devices that also in themselves lead to power quality and EMC challenges which must be addressed. This article has outlined some of the opportunities and challenges that are currently being researched. 
References
- “Strategies for development and diffusion of Energy Efficient Distribution Transformers”, SEEDT Project Report, July 2008, Project website: http://seedt.ntua.gr.
- Bifaretti, S., Zanchetta, P., Watson, A., Tarisciotti, L, Clare, J. “Predictive control for universal and flexible power management”, Proceedings of the IEEE Energy Conversion Congress and Exposition (ECCE), Atlanta, GA, 12-16 September 2010.
- Dugan, R.C., McGranaghan, M.F., Santoso, S., Beaty, H.W., Electrical Power Systems Quality, 2nd ed., McGraw-Hill, 2002, ISBN: 0-07-138633-X.
- de Lillo, L., Empringham, L., Schulz, M., Wheeler, P., “A high power density SiC-JFET-based Matrix Converter”, Proceedings of the 2011 14th European Conference on Power Electronics and Applications, EPE, 2011.
- Timens, R.B., Buesink, F.J.K., Ćuk, V., Cobben, J.F.G., Kling, W.L., Leferink, F.B.J., ”Harmonic Distortion in a New Building due to a Multitude of Electronic Equipment”, Proceedings of the IEEE Conference on EMC 2011, Long Beach, CA, 2011.
- Areerak, K-N., Wu, T., Bozhko, S.V., Asher, G.M., Thomas, D.W.P., “Aircraft Power System Stability Study Including Effect of Voltage Control and Actuators Dynamic” IEEE Transactions AES, vol. 47, no. 4, October 2011, pp. 2574-2589.
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David W. P. Thomas MIET SMIEEE CEng He received the B.Sc. degree in Physics from Imperial College of Science and Technology, the M.Phil. degree in Space Physics from Sheffield University, and the Ph.D. degree in Electrical Engineering from Nottingham University, in 1981, 1987 and 1990, respectively. In 1990 he joined the Department of Electrical and Electronic Engineering at the University of Nottingham as a Lecturer where he is now an Associate Professor and Reader. His research interests are in electromagnetic compatibility, electromagnetic simulation, power system transients and power system protection,. He is a member of CIGRE and convenor for Joint Working Group 4.207 “EMC of communication circuits, low voltage systems and metallic structures in the vicinity of power systems”. He is also Secretary to the IEEE EMC Technical committee T7 on Low Frequency EMC. |
