CIRED
20th International Conference on Electricity Distribution
Prague, 8-11 June 2009 Paper 0150
THE ACTIVE USE OF DISTRIBUTED GENERATION IN NETWORK PLANNING Math H.J. BOLLEN, Fainan HASSAN, Mikael WÄMUNDSSON STRI AB – Sweden
[email protected]
Anders HOLM, Ying HE Vattenfall Research and Development – Sweden
Impact on voltage profile
This paper addresses ways in which distributed generation can be used to the advantage of network operators. A distinction is thereby made between “non-controlled generation” and “controlled generation”. Both types can contribute to an improved performance of the network, where the opportunities for controlled generation are obviously much bigger. It is shown that non-controlled generation can have a positive impact on losses, on undervoltages and on reliability. Controlled generation can further mitigate fast voltage fluctuations, voltage dips and harmonic distortion.
INTRODUCTION The introduction of generation to the distribution network is often a concern to network operators because the distribution network has been planned and designed for power transport in one direction [1][2][3].
The voltage magnitude at all load points has been calculated for a total load of 3288 kW and for 0, 850 and 1264 kW distributed generation at five different locations. The impact of non-controlled distributed generation on the voltage profile in the network is shown in Fig. 1. The figure shows the situation under back-up supply when the total load is equal to 3288 kW. 96 95 Voltage (%)
ABSTRACT
94 93 92 91 90 1
Distributed generation can also positively contribute to the performance of the network, for example by reducing the load and by improving the voltage profile. Based on the type of interface used to connect the generator, the reactive power exchange with the network may be controllable. This offers additional opportunities to improve the network performance. The network operator can actively use the distributed generation in planning and operation of the distribution network. A research and development project has therefore been started to quantify the ability of distributed generation to improve the performance of the distribution network and in that way allow the network operator to defer investments. Both controlled and non-controlled distributed generation have been considered. This paper summarizes some of the results of this project.
NON-CONTROLLED GENERATION System studied An existing Vattenfall rural distribution network has been studied in this project. The power-system analysis package NEPLAN has been used for the simulations. The network studied has a radial structure and consists of one primary substation and 38 secondary substations. 675 customers are connected to the system. The total length of the overhead lines along the feeder is approximately 36 km, divided over seven different types of overhead lines. No cables are used. The total demand of the network is on average about 2.35 MW.
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3
5
7
9 11 13 15 17 19 21 23 25 27 29 31 33 35 37
Load points
Fig. 1. Effect of DG on the voltage in the network: black solid: no DG; green dashed: 850 kW DG: red dotted: 1264 kW DG.
The injection of active power close to the consumption results in a reduced voltage drop and this in a better voltage profile during high load. In radial-operated meshed networks, the voltages are lowest during back-up operation. This is where DG will have the biggest impact.
Other impacts Distributed generation can positively impact the network in a number of other ways, next to the mitigation of undervoltages shown above. Producing the power closer to the place of consumption reduces the losses during both distribution and transmission. For the example network the losses in the 20-kV network reduced from 4.1 to 3.4% of the consumption. Introduction of DG will result in smaller annual losses. It will also reduce the risk of component overloading. Again this impact will be biggest during back-up operation. This has not been quantified for the example network as this network was voltage-drop limited. The improved voltage profile and the reduced risk of overload may also have a positive impact on the reliability if supply. During back-up operation it may not be possible to supply all load because of overcurrent and/or undervoltage restrictions. The presence of DG close to the load may allow all customers to be supplied even during back-up operation. This results in an improvement of the overall reliability of the network. For the example network the
CIRED
20th International Conference on Electricity Distribution
Prague, 8-11 June 2009 Paper 0150
expected energy not supplied (ENS) went down from 21.7 to 16.7 MWh/year despite a small increase in SAIFI from 6.1 to 6.5 interruptions/year.
Influencing factors It has been shown, by performing many simulations, that various factors can influence the way in which DG impacts the voltage profile, the reliability and the losses. Among those influencing factors are: the location of the units; the number of units; the size of the units; and the total amount of DG in relation to the total load. This implies that a detailed study is needed to quantify the exact impact. Most of these parameters are beyond the control of the network operator, but the location of the unit may be possible to impact.
Voltage Rise due to Active-Power Injection A simplified circuit-theory model has been used to estimate the amount of injected power needed to give 1% rise in voltage magnitude in 400-Volt and 20-kV networks. The results are given in Table 1 and Table 2. The currentcarrying capacity has been obtained for a three-phase cable in triangular configuration. For short feeders with a larger cross section a significant injection of active power is needed to obtain 1% rise in voltage magnitude.
power exhange with the grid can both be controlled without the need to reduce the amount of power coming from the energy source.
energy source
chopper
VSC inverter
filter
G R I D
Fig. 2. Principle of the converter-interfaced DG unit.
Voltage-dip mitigation The ability of controlled DG to mitigate voltage dips is illustrated in Fig. 3. The figure shows the estimated number of trips per year as a function of the PCC voltage for different values of the chopper size. For instance, with a voltage immunity of 0.8 pu, the DG unit would have twice the number of the estimated trips if ξ = 0.2 pu instead of 0.3 pu. But even a small chopper size (ξ = 0.1 pu) would significantly reduce the number of trips even without any improvement in the equipment immunity against dips.
Table 1. Active power needed to obtain 1% voltage rise, 400 Volt feeders of different length and cross-section. Size 50 mm2 95 mm2 240 mm2
Capacity 135 kW 199 kW 333 kW
50 m 93 kW 177 kW 447 kW
Feeder length 100 m 47 kW 88 kW 223 kW
500 m 9 kW 18 kW 45 kW
Table 2. Active power needed to obtain 1% voltage rise, 20-kV feeders of different length and cross-section. Size 50 mm2 95 mm2 240 mm2
Capacity 6.7 MW 9.9 MW 16.7 MW
1 km 12 MW 22 MW 56 MW
Feeder length 5 km 2.3 MW 4.4 MW 11 MW
20 km 0.58 MW 1.1 MW 2.8 MW
The main limitation is however that the voltage rise depends on the amount of injected active power. For units with a constant and predictable amount of injected power (bio-fuel, combined heat and power) the voltage rise can be used as a resource for the distribution company. The probability that the generation is not available during maximum load is sufficiently small. For strongly fluctuating generation (like solar and wind power) that probability would be too high and the worst cases should be considered in the planning process.
Fig. 3. Estimated number of trips with different chopper size. ξ= 0.1 pu (solid black), ξ = 0.2 pu (dashed blue), ξ = 0.3 pu (green with asterisk), and ξ = 0.4 pu (dash-dotted red that coincides with the curve ξ = 0.3 pu). The dotted red curve on the upper left corresponds to the case without compensation.
It is worth noting again, that the result in Fig. 3 is an estimate, highly dependent upon the location of the DG unit and the grid impedance as seen from the DG unit connection point. The curves shown in the figure have been calculated for 1 p.u. active power injection and a source impedance of 0.8 p.u.
CONTROLLED GENERATION
It is also important to note that the compensation method also allows better fault-ride through of the DG itself. This is advantageous for the quality and reliability of the supply.
Control principle
Mitigation of fast voltage-magnitude fluctuations
The control ability of DG can be extended by allowing the active power to be occasionally reduced for a short period of time. The basic principle of a converter configuration studied is presented in Fig. 2. A chopper is used to temporary store some of the energy coming from the source. This results in a reduction of the active-power flow through the inverter into the network. In this way active and reactive
The ability of controlled DG to mitigate fast voltagemagnitude fluctuations is illustrated in Fig. 4. An actual measurement of the voltage fluctuations close to an arc furnace has been used as the voltage at the remote bus. Both the active and the reactive powers are controlled to compensate for the voltage magnitude variations. The
CIRED2009 Session 4
Paper No 0150
CIRED
20th International Conference on Electricity Distribution
Prague, 8-11 June 2009 Paper 0150
oscillations have been significantly reduced.
Fig. 4. Voltage magnitude in pu during a 400-ms period at the remote bus (upper) and at the PCC (lower) using combined active and reactive power control.
Voltage Control by Variation of Reactive-Power Exchange The same simplified model as in the previous section has also been used to estimate the reactive power needed to achieve 1% change in voltage magnitude. The results are summarized in Table 3: in kvar and as a percentage of the transformer size. Table 3. Voltage-control capacity of controlled distributed generation at different locations in the distribution grid. Location
Transformer size
Needed for Percentage of 1% change transformer size
Urban MV, 2 km
40 MVA
1.4-5 Mvar
4-12%
Urban LV, 0 km
800 kVA
150 kvar
18%
Urban LV, 1 km
800 kVA
15 kvar
2%
Rural MV, 0 km
10 MVA
1000 kvar
10%
Rural MV, 5 km
10 MVA
27 kvar
0.3%
Rural LV, 0 km
100 kVA
13 kvar
13%
Rural LV, 1 km
100 kVA
1.7 kvar
2%
Voltage control in urban networks requires relatively large generation units, with the exception of locations further out on the low-voltage feeder. But as voltage drop is rarely a limitation on urban networks, voltage control is not needed. For rural networks voltage drop is often a limitation. The most convenient locations for voltage control are along the feeder in the MV or LV network. A distributed generator connected to a medium-voltage feeder could be used to control the voltage on the low-voltage side of a nearby distribution transformer. A unit with a reactive-power capacity of only 3% of the rating of the HV/MV transformer can compensate a voltage rise or drop of 10%. Voltage Coordination The use of controlled generation runs the risk of adverse interaction with the automatic tap changers on the HV/MV transformer. Different methods have been suggested to mitigate this: The use of different controllers at different time scales, where the generator takes care of the fastest and slowest variations. The generator (DG) takes care of the fast variations in voltage controlling both active and reactive power exchange. The automatic tap-changer on the
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HV/MV transformer controls the voltage on a time scale between a few seconds and a few minutes. The DG mitigates voltage magnitude variations that are not taken care of by the tap-changer within a few minutes, by using reactive-power control only. The use of measurements throughout the distribution network and a central controller that determines appropriate set points for the individual controllers. Compensating only the voltage variations due to the downstream load and the local production. The reactivepower controller can compensate these using a pre-set value of the source impedance. In this way the generator controller simply limits the voltage variations observed by the tap-changer controller as far as these are due to load variations downstream of the generator. Also voltage variations due to variations in injected active power can be compensated in this way.
Harmonic Distortion Generation with a power-electronics interface has the ability to mitigate harmonic distortion. Two types of control algorithms have potential application: Compensating the downstream harmonic emission. Introducing harmonic damping to mitigate or prevent high voltage distortion due to harmonic resonances. The first method of mitigation is shown in Fig. 5; the principle is the same as the one used in commercial active harmonic filters. The downstream harmonic currents are measured and a harmonic current is injected that compensates the harmonic current taken by the downstream load. The result is that the harmonic current through the feeder upstream of the generator is close to zero. Such filters are superior in performance compared to passive filters, however they are more expensive and may introduce higher losses. When integrating the filtering in the existing power-electronic converter of a generator, the additional costs and additional losses are expected to be significantly smaller.
G Igen Iload Fig. 5. Reducing harmonic distortion by compensating the downstream current.
By using the appropriate control algorithm the converter can emulate a resistance at harmonic frequencies that absorbs the energy present in the harmonic voltages and currents. Such a harmonic damping resistance could prevent and mitigate high harmonic levels due to resonances. The additional costs for generator units with a power-electronic converter, like DFIG wind turbines, is expected to be limited as the hardware is already in place.
CIRED
20th International Conference on Electricity Distribution
Prague, 8-11 June 2009 Paper 0150
DISCUSSION It has been shown that distributed generation has a certain potential in improving the performance of the distribution system. Only when the existing performance is insufficient or the future performance is expected to be insufficient (e.g. due to load growth) it will be worth to consider distributed generation as an option in the planning and operation of distribution and transmission systems. It is thereby very important to note that the expected growth in distributed generation could be the cause of the expected insufficient performance of the network. Limited amounts of non-controlled generation will reduce the risk of overloading, improve the voltage profile, and reduce losses by simply reducing the load flow. To be actively used in the planning of distribution system, the availability of the generation should be high. That makes that non-controlled wind-power installations have limited ability to improve the network performance. Combinedheat-and-power installations, biomass-based small-scale generation, small hydropower, and future wind or solar installations with storage capabilities are more suitable. The ability of distributed generation to improve the voltage profile and to mitigate overloads, makes that it can improve the supply reliability. The presence of distributed generation limits the need for load shedding when a back-up supply is used. The amount of improvement that can be achieved is again related to the availability of the active power by the distributed generation. The potential of controlled generation for improving the network performance is significantly bigger than for noncontrolled generation. Using the proper control algorithms voltage variations can be mitigated over a range of time scales, including voltage dips, fast voltage fluctuations (voltage flicker) and long-term voltage variations. Reactive power and voltage control requires a generator with a synchronous machine or with a power-electronics converter. It is worth mentioning that the reactive power is available for control purposes even when the generator does not produce any active power. It is however important to prevent adverse interaction with the automatic tap changers on the HV/MV transformer. For this it might be needed to only use limited control of the long-term voltage variations, like compensating the voltage drop due to the downstream load. This would still results in improvement of the voltage profile. A centralized controller could be used to coordinate between the different controllers and to create an optimal flow of reactive power through the distribution network within the limitations set by the TSO for the reactive-power exchange with the transmission system. Mitigation of harmonic distortion by using the powerelectronics interface is possible as well. No simulations of such algorithms have been performed within the project. Compensating the downstream harmonic current is the most promising application. It is based on algorithms used in commercially-available equipment and the additional costs
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seem to be low. Equipping all converter-based generation with a control algorithm resulting in harmonic damping could be a cheap and effective way of preventing high distortion levels due to harmonic resonances. Distributed generation also offers possibilities of contributing to the operational security of the transmission system. The possibilities are limited especially where it concerns active power contribution. Further replacement of large conventional generators by small units connected to the distribution grid, may result in a situation where their contribution to operational security becomes essential, even if less efficient. With any contribution of distributed generation to the network performance, a choice must be made on whether this contribution should be compulsory (e.g. as part of the connection agreement) or on a voluntary basis (e.g. as part of an ancillary services market).
CONCLUSIONS The growth in new sources of generation will most likely continue. Wind power is the most visible component of this growth, but other sources like combined-heat-and-power or small-scale hydro, may show further growth as well. These new sources of generation can contribute to the planning and operation of the power system in various ways. They should be treated as an asset during the planning and operation of distribution systems. Many proposals are presented in the scientific literature on how to use power electronics and advanced control algorithms to improve the voltage quality in distribution networks. What remains missing is a serious evaluation of the advantages of this from the network operator point of view. The project described in this paper is a first step towards such an evaluation.
ACKNOWLEDGEMENTS This work was funded as part of Vattenfall’s R&D program “Intelligent Networks”. The goal of this program is to ensure the satisfaction of today’s and tomorrow's network customers. The strategy is to take advantage of the rapid developments in information technology, and to combine these with the evolution of traditional power technology.
REFERENCES [1] N. Jenkins, R. Allan, P. Crossley, D. Kirschen, G. Strbac, “Embedded generation”, Institution of Electrical Engineers, London, 2000. [2] L. Freris, D. Infield, Renewable energy in power systems, Wiley, 2008. [3] M.H.J. Bollen, Y. Yang, F. Hassan, Integration of distributed generation in the power system – a power quality approach, Int Conf Harmonics and Quality of Power (ICHQP), September 2008.
