Active network management

The transformation of the distribution network from passive to active operation has already started with demonstrations and early examples of the use of active network management. Although some of these techniques may seem rather obvious and straightforward, they allow considerable increase in the distributed generation that can be connected. The penalty is some increased complexity in distribution net-work control but this is manageable for individual schemes. A more serious diffi-culty comes with the dramatically increased complexity when a number of these individual active network management schemes are installed in the same section of network. The individual, ad-hoc active network management solutions are not coordinated with each other and their combined behaviour becomes difficult to predict. Then a system-wide solution becomes necessary. A number of trails and demonstrations are being developed [8–10] but so far there is no generally agreed approach to system-wide active network management.

7.2.1 Generator output reduction and special protection schemes

Figure 7.1 illustrates the significant benefit that can be obtained with a simple active network management scheme.¹Two circuits, each of 10 MW capacity, are used to connect a distributed generator to the power system. The load at the busbar to which the generator is connected varies between 2 MW and 10 MW.

Under ‘fit-and-forget’ the generator must be able to export its full output at any time. Hence, assuming one circuit can be out of service at any time, the maximum rating of generation that could be connected is 12 MW (10 MW export to the network plus 2 MW minimum load).

If the generator is a wind farm, it will only operate at its rated output for less than, say, 30% of the time, depending on the wind speeds. These times of full output

1This illustration is based on MW flows with no consideration of voltage or power factor.

are unlikely to coincide often with times of minimum load and so a wind farm capacity of greater than 12 MW, up to 20 MW, can be installed and the generation managed actively. The generation is then operated with either the power flows in the connecting circuits or in the load monitored. If excessive flow in the connecting circuits (more than the 10 MW firm capacity) is detected, the wind farm output is reduced. The choice of wind farm size is based on a cost-benefit calculation to determine the most cost-effective capacity of wind farm considering the wind resource and load, both of which can be estimated over a long period with confidence.

A development of this generator output control concept is to monitor the state of the connecting circuits and to use more of their thermal capacity. Here it is assumed that normally both circuits are in service, giving a capacity of 20 MW. This would then allow a wind farm of between 22 MW and 30 MW to be connected, depending on the degree of curtailment of generator output that is considered acceptable. If one of the circuits trips or is out of service, the wind farm output is reduced to a maximum of 10 MW plus the load. The special protection scheme trips the circuit for a fault and immediately reduces the output of the wind farm.

It may be seen through these simple examples that the increase in capacity of distributed generation which may be considered for connection is significant. Of course, issues such as voltages, stability and protection need detailed study and may well become limiting factors. However, experience has shown that significantly increased capacities of generation may be connected, particularly on MV circuits of 33 kV and above, if active management of this type is used.

A key administrative barrier is the basis on which the offer of connection is made to the developer of the generation scheme. Once the fit-and-forget philosophy is abandoned, the generator no longer has access to the network for all of its output, all of the time. The connection of a larger generator, with some restriction on its operation, may well be in the developer’s commercial interest, but there is a degree of risk introduced as to how much energy it will be able to export. This may lead to difficulties in financing the project. It is also likely that electrical losses in the connecting circuits will be increased as the circuits become more heavily loaded.

7.2.2 Dynamic line ratings

Another active network management approach is to monitor the environmental conditions of the overhead lines and increase their rating when possible. In Northern Europe, high wind speeds, and hence full output of wind farms, tend to be during the

10 MW

10 MW

Load 2–10 MW

Generator 0–? MW

Figure 7.1 Illustration of simple active network management

winter months when the thermal capacity of overhead line circuits is increased by the low ambient temperatures and increased wind speeds over the conductors. These ambient conditions are monitored and used to calculate the capacity of the overhead line (particularly the sag of the conductors) to allow increased current to flow.

7.2.3 Active network voltage control

On medium voltage overhead networks, particularly 11 kV in the United Kingdom, the limiting factor for the connection of generation is often steady-state voltage rise.

This is caused by the real power generated acting on the resistance of the circuit, while its low reactance means that absorbing reactive power is not an effective way of controlling the voltage rise. Hence, ways to control the tap changers of the 33 kV/

11 kV transformer in order to increase the amount of distributed generation that may be connected have been investigated.

Figure 7.2 shows one approach [11]. Measurements of the network are taken (voltage magnitudes and power flows) and supplied to the distribution management system controller (DMSC) to determine the optimum operating points of both the tap changers and the generator outputs. The value of the energy from the various generators is also communicated to the DMSC, which in this example is located at a 33 kV/11 kV substation. Although control of the generators is considered by the controller, either by varying their power factor or reducing their real power output, the most cost-effective control action is likely to be changes in the set-point of the transformer tap changers. Loads are only shed in extreme situations given the much higher costs of shedding load compared to operating a tap changer.

Figure 7.3 shows how real-time measurements from the network may be combined with historical load data in an under-determined distribution state esti-mator to give a representation of the voltage magnitudes within the network. These voltages and power flows are then used by a controller, either using a simple voting system or an optimal power flow algorithm, to determine the best control action to be taken [11,12].

DMSC

P,Q,V,£

P,Q,V,£

P,Q,V,£

A P,-Q

CHP S P ± Q

PV P ± Q

Figure 7.2 Active distribution management

7.2.4 Integrated wide-area active network management

More comprehensive wide-area active network management schemes have been investigated in a number of research and development projects [13]. An example is shown in Figure 7.4 where a controller is located at each 33 kV/11 kV substation not only to control its 11 kV network, but also to communicate with adjacent substations and to the higher voltage levels.

7.2.5 Smart Metering

The introduction of Smart Metering to all customer loads with data available in real time has the potential to increase dramatically the visibility of voltages and power flows within the distribution system. At present, there are very limited real-time

State

Figure 7.3 Distribution management system controller

Bulk

Figure 7.4 Wide-area active network management [13]

measurements on the distribution network and so its state can only be estimated using historical load data and the limited number of measurements that are present.

Once the data from the Smart Meters becomes available it will, in principle, be possible to use the type of state estimator found on the transmission network (over-determined with more measurements than states) to give a much more robust and accurate picture of the distribution system.