Titel: Power quality and reliability in distribution networks with increased levels of distributed generation, Elforsk report 08:39
Författare: Yongtao Yang, Math Bollen
Utgivare: Elforsk, Vindforsk
Årtal: 2008
Ämnesord: infrastruktur
Sökord/Keywords: EU-DEEP, CODGUNet, DG, voltage dips
Rapport
Sammanfattning/Abstract: This report describes the relations between distributed generation and power quality. The information is obtained from available reports and literature, with focus on the direct impact of increased wind power penetration on the power quality and reliability in distribution networks.
The survey is carried out based on the following information sources:
1. Reports from the EU-DEEP project.
2. Reports from the CODGUNet project.
3. Latest published literature within the area that are not covered in the
above studies.
The impacts of the increased penetration of distributed generation investigated in the report are summarized with a focus on the Nordic systems as follows:
Overloading and Losses
For small amounts of DG the risk of overloading reduces. Only when the maximum production exceeds the sum of maximum and minimum load downstream of any location along a distribution feeder, the risk of overload may increase. This could be the case for wind-power installation connected to a rural network.
For small amounts of DG the losses in the distribution-system will decrease. When the maximum amount of DG exceeds twice the minimum load, the losses at some locations and some moments in time will increase. The losses are expected to increase significantly only when the average production (over time and over a larger area) exceeds twice the average load.
Voltage variations and flicker
The introduction of DG will result in an overall increase of the voltage in the distribution system. This is a positive development, as it will mitigate undervoltages for remote customers.
For larger amounts of DG, the connection of DG will result in voltages with the end-users that are above the normally-acceptable limit. This is expected to become a problem first for the connection of larger DG units to remote parts of distribution feeders. The risk is highest for wind power connected to rural areas.
The exception is formed by DG units with induction-machine interface where the reactive-power consumption could actually result in a reduction of the voltage. As the voltage drop often determines the length of rural distribution feeders, already a small amount of DG could become a problem here.
The variations in power production associated with wind power could result in high switching frequency of the transformer tap changers. This normally does not adversely impact the customers, but could result in premature failure of the tap changers.
The impact of wind-power installations on the flicker level is small but cannot be neglected. An increase in the flicker level (Pst) by 0.2 to 0.4 due to wind power installations is expected. This will only be a concern for locations where the flicker level already is close to the acceptable limit. Overall, the impact of DG on the flicker level is not an issue requiring further research.
Harmonic distortion
The emission of low-frequency harmonics by DG units is small compared to existing emission and no serious problems are to be expected. The additional capacitance due to the connection of DG units will introduce new resonance frequencies and cause a shift to lower frequencies of existing resonances frequencies. The main risk is in new resonance frequencies occurring near the 17th and 19th or 23rd and 25th harmonics. The initial studies show a risk of high distortion levels in the frequency band above 2 kHz due to powerelectronic interfaces as in DFIG or full-converter units. More detailed studies
are needed to quantify the severity of this.
Protection coordination
In general, operation and protection of distribution networks appears to develop to become more similar to transmission systems. This is particularly true for MV distribution systems. Solutions generally applied in transmission systems will solve many of problems
encountered in distribution networks when the penetration of DG increases, island operation is introduced, multiple in-feed is allowed or distribution networks become meshed.
For large DG units with synchronous machine interface, incorrect operation of the protection becomes unavoidable when the unit size becomes larger than about 50% of the rating of the HV/MV transformer. Some coordination problems may occur already for much smaller levels of penetration, but these can be easily solved by changing the settings of the overcurrent protection.
Reliability
The reliability of the power supply as experienced by the end users is not strongly impacted by the introduction of small amounts of DG. The impact is mainly positive, as the loading of the system will be reduced when production takes place closer to the load. DG with controlled islanding capability will significantly improve the supply reliability of the customers close to DG, however a number of challenges remain and further study is needed.
Possible negative impacts for larger DG penetration include an increased risk of incorrect protection operation. With massive DG employment, the closure of conventional power stations or the lack of building new conventional stations will increase the risk of generation shortage. Models of the electricity market in combination with probabilistic models of the production are needed to quantify this impact.
Transmission-system operational security
The impact of DG on transmission-system operation is complex. The different phenomena are qualitatively understood but no numbers are available on the actual quantitative impact. The two main impacts appear to be the unpredictable behaviour of DG units during major disturbances at transmission level (experienced as voltage or frequency deviations at the terminals of the DG installation) and the overall weakening of the transmission system when large conventional power stations are replaced by DG. Quantifying this impact requires among others the development of a set of performance indicators for transmission-system security.
Impact of DG on the transmission-system operation will occur only with larger penetration levels. As long as the amount of DG is less than the uncertainty in load prediction, its impact on transmission-system operation will likely be small. Of course such a statement requires a quantitative study to be confirmed.
At this stage the lack of solid knowledge of the impact of especially largescale wind-power integration on the operation of transmission and subtransmission systems has resulted in connection requirements that may be overly restrictive. This increases the costs of connecting DG and forms a barrier against wide-scale use of environment-friendly sources of energy.
Voltage dips
DG units with synchronous machine interface (like medium-sized CHP or biofuel units) will reduce the amount of voltage dips experienced by customers electrically close to these units. This assumes that the units remain connected for at least a large part of the fault-clearing time. Also DG units with induction-machine interface have some positive influence but significantly less.