By David Chapman & Bruno De Wachter
An overview of possible issues for the Distribution Network Operator (DNO)
The EU has set a target of 20% renewable energy by 2020. The majority of this renewable energy will be achieved by distributed generation (DG) units connected to the LV or MV network. As a result, the distribution network operators (DNOs) will face a number of difficulties. On one hand, they must maximise access for the connection of the DG generation units, but on the other, they must maintain the established levels of power quality, availability, and reliability of the distribution network at a reasonable cost.
The connection of DG is problematic because the distribution system was designed to feed power from a relatively few large generation stations to a very large number of consumers. It was not designed to accept the feed in of energy at medium and low voltage levels.
What follows is a brief overview of basic issues that can arise when DG units, such as PV panels, wind turbines, or cogeneration units, are connected to the network.
These issues can be classified as follows:
Voltage control and balance during normal operation
A single distribution substation typically serves several radial lines to which many consumers are connected. The voltage on these lines gradually decreases so that at the end of the line, the voltage is lower than that at the substation. The EN 50160 standard stipulates that the voltage at each user connection point should be:
In order to ensure that even the most distant consumer receives an adequate minimum voltage, even when consumption is high, DNOs normally set the voltage at the substation towards the high end of the permitted range.
But what happens when a distributed generation unit is installed near the remote end of such a radial line? Current is injected into the line, reducing or even reversing the flow of power drawn from the substation. If the DG unit is sufficiently powerful or if there are several of them, the voltage on sections of the line can rise above that of the substation and the DNO can no longer control the voltage. The problem is made worse by the fact that small DG units are non-dispatchable; meaning that they cannot be remotely commanded on or off by the DNO to match system load. PV generates when the sun shines, wind turbines when the wind blows, micro-CHP when the heating load is present. Moreover, on a single radial circuit, all the PV units of a particular type are likely to operate simultaneously, so there is little diversity of supply.
Where penetration of DG remains low, maintaining the statutory voltage limits while coping with the addition of DG is not too difficult. However, as penetration rises, local distribution networks will need to be designed differently. So-called ‘smart grids’, the subject of a great deal of discussion at present, will be needed if the contribution from DG is to be maximised while preserving the integrity of the network.
Another issue is that of balance between the phases. Suppose a small PV system is connected to one phase of the distribution line. When the PV system injects power, voltage level will become higher in this phase than in the other two. A simple solution, but one which increases the cost of small DG systems, is to require all DG systems to be three-phase connected. However, this is not appropriate in countries where only a single-phase service is provided to small consumers.
Stability in case of incidents on power lines or generation units
Before permitting the connection of a DG unit, the DNO must carefully consider what will happen in the event of a fault on the network.
Suppose a DG unit is connected at the end of a LV line, which is fed from a substation, and suppose a fault occurs on this line. Although the LV network is radial, there are interconnecting points that introduce a certain degree of redundancy into the system. This redundancy is used under fault conditions to maintain supply to otherwise isolated consumers. This emergency route can be much longer than the regular one, causing the voltage at the DG connection point to rise. Under fault conditions, the DNO is released from the limits of EN50160, but it must still consider the effects of any installed DG systems. The DG unit may have to be disconnected to ensure the safety of other equipment on the network.
The electricity supply industry also has to plan for sudden power plant breakdowns. The loss of a large generator unit means that more power is being consumed than is being generated which results in a reduction in the frequency of the supply. Large generating machines, because of their size, have a high inertia and can ride through the frequency drop. However, for most DG units, which have little or no inertia, a fast reduction in frequency indicates a serious problem and most will disconnect from the network. The transmission operator maintains a ‘spinning reserve’ of capacity that can be brought on line relatively quickly to arrest the reduction in frequency and rebalance power supply and demand. But if the loss of a large generator is not compensated quickly enough, it may be followed by the tripping of many DG units, thereby compounding the problem.
Network protection issues
As already mentioned in Point 2, DG units must automatically disconnect when the voltage at the network connection point becomes too high. But even more important is the opposite case: when the line is without power. In such a case, it is often the safety of the individuals that are working on the line which is at stake. The automatic disconnection device must therefore have a very high level of reliability.
Another network protection issue is that normal schemes and settings of overcurrent relays, earth fault relays, and short circuit breakers on the network may no longer be appropriate when DG units are connected.
Moreover, DG units complicate designing an adequate selectivity for the protection relays. Establishing a selective protection (disconnecting only those lines with problems) can be done by setting increasing time delays on the relays when going upstream in the network. However, this ‘chronological selectivity’ only works if the direction of the power flow is conventional, and DG units are capable of reversing this flow.
Power Quality issues
The DNO is not only responsible for maintaining the rms voltage level between limits; it must also ensure that quality of power remains within specified standards. This task is complicated by DG units since they can introduce voltage variation and harmonics.
Voltage variation and flicker
Even under normal working conditions, a PV panel, a wind turbine or a cogeneration unit can result in large and sudden changes in the current injected into the network. This results in variations in voltage. These variations can be caused, for example, by an errant gust of wind, a dark cloud passing in front of the sun, or a sudden stoppage of a cogeneration unit. Moreover, variations in power generation by DG units have a low diversity. If the wind stops blowing for one wind turbine, it will most probably stop blowing for all wind turbines in the local area. This is in direct contrast with the sudden variations in electrical consumption by individual electricity users, which have a much higher diversity. The intermittent nature of DG units can result in variations on the network.
A solution is to equip the intermittent power sources with a battery system that stores energy at moments of peak production and injects it into the net when the renewable power unit is not producing. A few such systems are already in use, but high quality, large-scale storage systems are generally still too expensive to be a cost-effective option. This may change in the future since battery technology is undergoing rapid development.
Apart from the intermittency factor that occurs even during normal working conditions, the high inrush currents of cogeneration generators units or DG unit connection transformers can also cause voltage dips.
Flicker is the apparent variation of luminous flux from a lamp due to small cyclic variations in supply voltage. Because of the way the human brain and eye interact, this effect is especially apparent at frequencies in the 5 to 20 Hz range and it occurs with even quite small voltage variations. Wind turbines can induce flicker by the variation in output caused when a turbine blade passes the support mast.
All these voltage variations are caused by changes in current flowing in the impedance of the supply. Reducing the supply impedance — which can be done by using a larger conductor — is a simple solution but is only viable in new networks or where a significant DG capacity is being installed.
The power-electronic interfaces of DG units contribute to waveform distortion on the distribution network. This results in frequency components at integer multiples of the grid frequency. Regulation of harmonics is complex; EN 50160 sets a maximum limit for the harmonic pollution at the point at which a consumer is connected while various national and international standards and codes limit the harmonic pollution caused by a DG unit. The harmonic distortion caused by a converter can be limited by converter design and by local filtering.
Power factor is a measure of the reactive power demand in relation to the active power demand of an installation. Reactive power does no useful work but causes excess losses in the distribution network. DNOs charge customers with high reactive power (low power factor) consumption. If these customers were to install, for instance, PV panels to serve some of the installation’s active power needs, less would be needed from the network, and the DNO would observe a lower power factor leading to a higher reactive power charge. From the DNO’s perspective, the presence of DG on the network changes the conventional pattern of consumption and the effective power factor of the load.
Integrity of the remote control signals
Remote control signals are distributed via the electricity network to control, for example, the switching of public lighting and the switching of day and night tariffs. Some DG units, especially wind turbines, can affect these signals, reducing their amplitude and making them undetectable. Some DNOs require DG units with the greatest potential for causing disturbance to be equipped with blocking filters to avoid this problem.
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