Recent Researches in System Science
Increasing Energy Efficiency by improving Power Quality JOSEPH F.G. COBBEN, VLADIMIR ĆUK, WIL L. KLING Electrical Engineering Department, Electrical Energy Systems Eindhoven University of Technology Den Dolech 2, PO Box 513, 5600 MB Eindhoven THE NETHERLANDS
[email protected] Abstract: - Different power quality phenomena influence also the efficiency of the electricity usage. Harmonic currents and unbalance in the currents will directly have a negative influence on the efficiency. A low power factor (PF) also contributes to a lower efficiency and can be partly related to harmonic distortion. Furthermore, these distorted currents will influence the quality of the voltage which, indirectly, can decrease the efficiency further. In this paper is the relation between these phenomena and electrical efficiency described and placed in perspective. Also, some additional information is given about the relation between voltage level and energy saving. Key-Words: - power quality, electrical efficiency, energy-saving, harmonics, unbalance, voltage level there will also be a current in the neutral conductor which will result in additional losses. Furthermore, due to the skin- and proximity effect the resistant of the conductors will increase for higher frequencies which will increase the losses. Nevertheless, the additional losses greatly depend on the amplitude and harmonic spectrum of the harmonic currents flowing in the circuit. For two situations, an indication of fundamental and additional losses in a cable is given.
1 Introduction Energy efficiency initiatives brought a lot of development, also in the area of power quality. Several companies are claiming a lot of energy savings by placing power factor compensators or passive or active filters. Also voltage regulators are used to reduce the voltage and in this way save some energy. Of course, all these activities will reduce the usage of energy but mostly not in promised amounts. To get a more realistic indication of the possible energy savings, additional losses due to distorted currents. unbalance in the currents and a low PF is studied. However, PF is not directly a power quality phenomena, due to harmonic distortion the PF can be low, leading to additional losses.
Situation 1: Cable, loaded with the nominal current and with 30% of third harmonic current, 30% of 5th harmonic, 20% of 7th harmonic and 10% of 11th and 13th harmonic current. Situation 2: Cable, loaded with the nominal current and with 20% of third harmonic current, 50% of 5th harmonic, 40% of 7th harmonic and 20% of 11th and 13th harmonic current.
2 Harmonics and losses Harmonic currents will result in additional losses in mostly all components. For the cables and transformers these additional losses are analyzed. The losses in a cable can be calculated using the formulae: n
h
1
1
Figure 1 is showing in both situations the fundamental losses (around 2.6%) and the additional losses in the phases and the neutral due to the harmonic distortion. Be aware, that these losses appear during the maximum load condition and in practice mostly there will be another load pattern, with lower losses as a result. In the situation described the additional losses are a proximally 2%. For the losses in the neutral the amplitude of the 3rd harmonic current (or other multiples of 3) are very important. High amplitude of these harmonics could also result in the need to choose a cable with a higher cross section area (which would reduce the losses).
PV I h2 Rh Where: n = the number of conductors h = the harmonic order I = the r.m.s.-value of the (harmonic) current R = the resistant at the calculated frequency There will be additional losses in all phases, but in the case of 3rd (or multiple of 3) harmonic currents
ISBN: 978-1-61804-023-7
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Recent Researches in System Science
Figure 2: Transformer losses due to non-linear load Figure 1: Indication of losses in a full-loaded cable.
In the situation without harmonic distortion the losses are approximately 1.4% and this increases to 1.62% or 1.88% of the apparent power can be used. Nevertheless, also for the transformer the additional losses will depend on the harmonic content of the current and will differ from one situation to another. The energy losses will also depend on the load profile and will not be constant and vary over time. Most of the time the energy savings will be less in percentage, since in the calculations the maximum loading is taken as reference.
Also in the transformer, there will be additional losses. Transformers losses can be divided in noload losses and load losses. In the case of no load losses, distortion of the voltage could give some increase in these losses, but in general these additional losses can be neglected. Load loss is subdivided into I2R loss and “stray loss”. Stray loss” can be defined as the loss due to stray electromagnetic flux in the winding, core, core clamps, magnetic shields, enclosure or tank walls, etc [1]. Transformers can be designed to tackle these additional losses or should be derated in nominal power to prevent overheating. The formula to calculate the derating-factor K is: 2 e I 1 h max q I h K 1 h 1 e I I 1 h2
3
1
2
PF
2
P S
P P Q2 D2 2
Where: P = active power S = apparent power Q = fundamental reactive power D = distortion reactive power
Where: e = ratio of fundamental eddy current loss to the resistive loss h = the harmonic order q = dependent on winding type & frequency
The distortion reactive power can be the result of the harmonic voltages and currents present in the system. Additional losses due to these harmonic currents are already explained. Due to the fundamental reactive power also additional losses will occur. By improving the displacement power factor (phase shift between fundamental voltage and current) the apparent power (and therefore the total current) can be reduced with reduction of losses as result. The achievable reduction in power losses can be calculated with the formulae:
With practical values as resp. e=0.1 and q=1.7 the derating of the transformer can be calculated. Assuming that after derating the total losses will be the maximum losses again, the increase of the losses due to harmonics can be calculated. For the situation 1 and 2 these calculations are made and shown in figure 2. The total amount of losses (for the 630 kVA transformer, used in this calculation) can be divided in 800 W no load losses and 8000 W load losses which are taken constant. The difference in the two situations is the derating factor, which causes the change in “nominal power” and therefore another percentage of losses.
ISBN: 978-1-61804-023-7
Power factor and losses
The total PF can be calculated with:
K PV
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cos 2 1 100% 1 cos 2
Recent Researches in System Science
Where: Cosφ1 = displacement power factor before improvement Cosφ2 = displacement power factor after improvement Figure 3 shows the possible reduction of the losses depending on the displacement power factor (DPF) before and after the improvement.
Figure 4: Additional losses due to unbalance in the currents The additional losses, depends of course on the amount of unbalance but could in extreme cases result in 1% of the total energy use.
5 Voltage level Another way to save not only energy losses but also (and mostly used for that purpose) energy usage is the reduction of the voltage level. The effect of this measure is depending of the type of device. When the device is a constant power source (for example a television) the reduction of the voltage level will result in a higher current. This will not reduce the energy use and certainly will not reduce the energy losses. It will result in a little increase of the losses. Nevertheless, when the load is more reacting as a constant impedance (most of the lighting) then there will be a reduction of energy use and also a small reduction in the energy losses. In figure 5 several devices with their dependency to voltage level are shown.[2]
Figure 3: Possible reduction of energy losses Before installing power factor correction, using capacitors or filters first of all, it should be analyzed what part of the reactive power is due to distortion and which part due to lagging (or leading!) fundamental current. Be aware, that distortion reactive power cannot be compensated, only by using capacitors. Furthermore, it is not wise to install capacitors (without protection) in a vicinity with high voltage distortion.
4 Unbalance in current The most effective way to deliver electricity is in a balanced three phase system. In that way the three phase currents will be equal and the neutral current will be theoretically zero. An unbalance in the current (by delivering the same power) will result in additional losses in the phases and a current in the neutral with the losses belonging to this current. Increasing the unbalance in current will result in additional losses as depicted in figure 4. Figure 5: Dependence active power to voltage level
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Recent Researches in System Science
It will depend from the type of devices, their behavior in time and their usage if the energy saving is reached as expected. When the voltage level is reduced in lighting installations, it will result in energy savings but the question is if the level of luminance is still acceptable. For a coffee machine for example the power can be reduced but still the same amount of energy is needed to get a cup of coffee. In that case power is reduced but is needed for a longer time, resulting in zero energy savings.
For example, in the case of a centralized solution for PF-compensation only the feeding transformer will benefit from this solution.
7 Conclusions There is a relation between efficient use of energy and several power quality phenomena. The quality of the current (harmonics, unbalance, current due to reactive power) could influence the energy losses. The amount of these additional losses are difficult to estimate and depending on the level of distortion. Nevertheless, as an indication additional energy losses between 2-4% could occur in a vicinity where there is a high level of distortion. These losses cannot be eliminated completely because it is almost impossible to remove all distortion in all parts of the installation. Also the energy use of mitigation device has to be taken in consideration. This however, does not result in a conclusion that mitigation of power quality phenomena is not useful or not cost-effective. By making a cost-benefit analysis of the application of these mitigation devices also the influence on the quality of the voltage should be considered. Furthermore, and perhaps even more important, the reduction of apparent power has to be considered. This could result in postponing of investment (for example if more power is needed) or lower tariff from the network operator when the current capacity of the connection could be reduced. Also the increasing life-time of components not overheated due to harmonics could be a good reason to invest in mitigation devices. Only energy savings mostly do not result in a positive cost-benefit analysis.
6 Mitigation of PQ-distortion As described before, there is some energy saving possible by improving the power quality, so by reducing the harmonic currents, controlling the voltage level, balancing the phase currents and increasing the PF. This could be reached by placing for example an active filter. Preferably a type which can improve all these phenomena at the same time. A drawback of the application of whatever device to improve power quality, is their own energy use. The energy savings that are wanted will be partly reduced due to the energy use of this application. Therefore, the expectation of the energy savings should not be too ambitious. In practice several manufacturers are promising a reduction of the energy use of 10 to 35%! By using active filters, power factor compensators, DC-coils and several other applications. In practice this amount of energy saving will not be reached. An additional energy saving will be reached for example by reducing the harmonic currents, as result of a better quality of the voltage. Harmonic distortion in the voltage will also result, even in the case of linear loads to harmonic currents and to additional stress on components. Motors, for example, will have some additional mechanical stress on the bearings and harmonic voltages will have a disadvantageous influence on the torque. This will also result in some additional heating of the motor and therefore some additional losses. On the other hand, by placing mitigation devices, not all power quality phenomena in all components can be improved. There will always a be part of the total installation with some harmonic currents, low PF or unbalance.
ISBN: 978-1-61804-023-7
References: [1] S.P. Kennedy and C.I. Ivey, Application design and rating of transformers containing harmonic currents, Conf Rec 1990 IEEE Pulp Ind. Tech. Confl, page 19-31. [2] Maryam Mohammadzadeh Sarab -Impact of Voltage Changes on Load Behavior and Energy Consumption, Traineeship University of Technology Eindhoven, 2011.
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