Pricing of distribution networks with distributed generation
6.5 Illustration of the principles of evaluating DUoS charges in networks with distributed generation
6.5.2 Multi-voltage level example
We now consider a radial distribution network composed of a 132 kV/33 kV substa-tion with two transformers, two 33 kV out-going circuits (the rest of the 33 kV network is represented by a lumped load of 50 MW maximum demand), a 33 kV/11 kV sub-station with two 33 kV/11 kV transformers and two 11 kV feeders (the rest of the 11 kV network is represented by a lumped load of 10 MW maximum demand), each of which supplies four 11 kV/0.4 kV transformers with a maximum demand of 400 kW.
At the 33 kV busbar of the substation a 15 MW CHP plant is connected, and a wind farm of 1 MW is connected to one of the 11 kV circuits (Figure 6.5).
Table6.6Nodalnetworknodal,timeuseuse-of-systemchargesandannualuserpayments Winterpeaknodal price(£/kW) busbar2 Summeroff-peak nodalprice(£/kW) busbar2 Generatorpayment (pricegeneration inperiod)(£’000) Demandpayment (pricedemand inperiod)(£’000)
Annualnetpayment (goestopayfor asset)(£’000) Forthesystemshown inFigure6.27035280245 Forthesystemshown inFigure6.40742070350
4 x 0.4 MW = 1.6 MW (4 x 0.1 MW = 0.4 MW)
50 MW (12.5 MW)
10 MW (2.5 MW) 33 kV
33 kV
11 kV CHP
5 MW (15 MW)
Wind farm 0.2 MW (1 MW)
132 kV G
F
E
D
C 4 x 0.4 MW = 1.6 MW
(4 x 0.1 MW = 0.4 MW)
Figure 6.5 132/33 kV radial distribution system
A distribution network is typically designed to cope with the expected max-imum loading condition, which is likely to occur at a time of maxmax-imum demand with minimum local generation. With distributed generation, another extreme condition needs to be considered, i.e. the condition where the generators produce maximum output and demand is minimum. Therefore, in this example, these two critical loading conditions are considered. The first number (without brackets) is the loading or generation during maximum demand–minimum generation (this would typically be associated with winter daytime), and the second number (in brackets) is the loading or generation during minimum demand–maximum gen-eration (this would typically be associated with summer night time).
The design of the distribution network should take into account the contribu-tion of distributed generacontribu-tion to network capacity. This was discussed in Chapter 5.
For the sake of this illustrative example, an effective contribution of 5 MW (30% of installed capacity) is allocated to the CHP generator, and 200 kW (20% of installed capacity) to the wind turbine. In the context of DUoS pricing, it could be inter-preted simply that the CHP and wind turbine are capable of substituting 5 MW and 200 kW respectively, of a distribution circuit capacity.
Minimum demand is assumed to be 25% of the maximum demand.
The flows in both loading conditions can be obtained by simple inspection.
Critical flows are summarised in Figure 6.6. The arrows show the direction of the flows. The critical loading of items of plant can be seen to be the largest power flows of the two loading periods.3
It can be observed that critical loading for the 11 kV feeder, 33 kV/11 kV and 132 kV/33 kV transformers is driven by maximum demand, while critical loading of the 33 kV circuits is driven by maximum generation, and these occur at different periods (time of use). The assets whose capacity is driven by maximum demand–
minimum generation condition (winter daytime) such as 11 kV feeders, 33 kV/11 kV
1.6 MW
Figure 6.6 Computation of critical flows
3Note that the critical flows determine the reference (optimal) ratings of the associated plant. The reference rating of 11 kV and 33 kV circuits are 2 3 MW and 2 12.7 MW respectively. Due to the topology of 11 kV circuits, one feeder must cope with all 11 kV loads when one of the 11 kV feeders loses supply from the 33 kV/11 kV substation and the normally open point is closed. The optimal rating of the 132 kV/33 kV substation is 2 58 MW. These reference ratings of the individual network components (transformers and lines at various voltage levels) can be compared with the plant ratings of the existing network.
and 132 kV/33 kV transformers, are then classified as demand dominated (DD), while the capacity of 33 kV feeders are driven by minimum demand–maximum generation condition (summer night time) and are hence classified as generator dominated (GD).
Given the direction of the critical flows and knowing the direction of demand-and generation-driven flows (Figure 6.6), it is easy to see how demdemand-and demand-and gen-eration at various voltage levels will pay or get paid for the use of individual network circuits. For example, an incremental increase in load of the demand connected to an 11 kV feeder, during the maximum demand periods, will increase the loading on the 11 kV feeder, 33 kV/11 kV and 132 kV/33 kV transformers.
Therefore, this demand will be charged for the use of these circuits and the total charge will be based on maximum demand of 3.2 MW.
For the generation-dominated 33 kV circuit, the relevant critical period is determined by the coincidence of maximum generation and minimum demand.
Hence, demand connected at 11 kV will be rewarded for the use of this 33 kV circuit, based on the load during minimum demand of 0.8 MW and the corre-sponding reward to demand will be obtained during the summer night periods.
Consider now charges for the wind farm. The wind farm will be rewarded for the use of the 11 kV network and 33 kV/11 kV and 132 kV/33 kV transformers and will be charged for the use of 33 kV circuits. The rewards for using these circuits will be based on the generator effective contribution to network capacity, i.e. 0.2 MW and apply during winter daytime. On the other hand, the charges for the use of 33 kV circuit will be based on the maximum generation output (1 MW) and applied during summer night periods.
In order to evaluate network charges for individual users, per unit annuitised capacity costs (£/kW/year) are allocated to each item of plant in the network. For illustrative purposes, the estimate annuitised typical costs of 132 kV circuits, 132 kV/33 kV transformers, 33 kV circuits and 33 kV/11 kV transformers for typical urban network in the United Kingdom are used.
The system is presented again in Figure 6.7 with all critical loadings highlighted.
The typical annuitised costs of individual circuits are shown next to the net-work model in Figure 6.7. DUoS exit charges for demand customers connected at various points in the network are also listed. The polarity of charges is adopted to be positive for downstream and negative for upstream power flows respectively.
Consider now the 132 kV/33 kV transformer. This is demand-dominated plant since the direction of the power flow is downstream. Hence, all downstream demand and generation customers pay and are paid 5.2 £/kW/year respectively for the use of this particular plant during maximum demand conditions, while charges are zero during the minimum demand period.
The next plant to be considered is the 33 kV circuit. This is a generation-dominated plant since the direction of the critical power flow is upstream. Hence, all downstream generation and demand customers pay and are paid 6.7 £/kW/year respectively for the use of this plant during maximum generation condition, while zero is charged during the maximum demand period.
As shown in Figure 6.7, the total DUoS exit charges for demand customers connected to the 33 kV busbar of the 33 kV/11 kV transformer is 5.2 £/kW/year applied during the maximum demand period (5.2 £/kW/year for the use of the 132 kV/33 kV transformer and 0 £/kW/year for the use of the 33 kV circuit), and DUoS entry charges of 6.7 £/kW/year during minimum demand period (0 £/kW/year for the use of the 132 kV/33 kV transformer and 6.7 £/kW/year for the use of the 33 kV circuit use).
The 33 kV/11 kV transformer is demand-dominated plant since the direction of the critical power flow is downstream. Hence, all downstream demand customers are charged and all downstream generation customers are paid 4.3 £/kW/year for the use of this particular plant during maximum demand conditions, while the charges are zero during the minimum demand period.
Therefore, the total entry charges for the generation connected to the 11 kV busbar of the 33 kV/11 kV transformer are9.5 £/kW/year during maximum demand period (5.2 £/kW/year for the use of the 132 kV/33 kV transformer, 0 £/kW/year for the use of the 33 kV circuit and4.3 £/kW/year for the use of the 33 kV/11 kV transformer, and 6.7 £/kW/year during minimum demand period (0 £/kW/year for the use of the 132 kV/33 kV transformer) 6.7 £/kW/year for the use of the 33 kV circuit and 0 £/kW/
year for the use of the 33 kV/11 kV transformer).
Max. demand
Figure 6.7 Evaluation of DUoS exit charges
Finally, the 11 kV feeder is demand-dominated plant since the direction of the critical power flow is downstream. Hence, all downstream generation customers are paid 11 £/kW/year for the use of this particular plant during maximum demand conditions, while the charge is zero during the minimum demand period. Therefore, the total charge for generation customers connected to the 11 kV circuit is20.5
£/kW/year during the on-peak period (5.2 £/kW/year for the use of the 132 kV/33 kV transformer, 0 £/kW/year for the use of the 33 kV circuit,4.3 £/kW/year for the use of the 33 kV/11 kV transformer and11 £/kW/year for the use of 11 kV circuit) and DUoS entry charges of 6.7 £/kW/year during minimum demand period (0 £/kW/year for the use of the 132 kV/33 kV transformer, 6.7 £/kW/year for the use of the 33 kV circuit, 0 £/kW/year for the use of the 33 kV/11 kV and zero for transformer and for the use of 11 kV circuit).
The DUoS charges (assuming positive polarity for demand customers) and revenues collected from various users during peak demand and off-peak demand conditions are given in Tables 6.7 and 6.8. The connection point G corresponds to the balancing point. Note that 58 MW is imported under peak demand conditions while 0.2 MW is exported under minimum demand condition from the grid supply point (point G).
During the peak-load condition, the annual revenue is collected for all demand-dominated assets, while for the generation-demand-dominated plant revenue is recovered
Table 6.7 On-peak demand DUoS prices and revenues from demand and generation customer
C 20.5 3.2 0.2 65600 4100 61500
Total 420600 30100 390500
Table 6.8 Off-peak demand (peak generation) DUoS prices and revenues from demand and generation customers
during peak generation periods. The costs of the individual plant items for the reference rating are given in Table 6.9.
In this particular case, the total annual revenue received for the demand-dominated plant is £390500/year, as shown in Table 6.7. (This is exactly equal to the total costs of the individual plant items as shown in Table 6.9, i.e. £390500 =
£301600þ 55900 þ 33000.) On the other hand, the total annual revenue received from DUoS charges during the off-peak demand period is £85090, as shown in Table 6.8. This is exactly equal to the total cost of generation-dominated circuit.
The on- and off-peak demand DUoS related expenditure of individual users is presented in Table 6.10. The total annual DUoS revenue equals the total annuitised cost of the reference network.
References
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Table 6.9 Annuitised cost of individual plant items
Plant Unit cost (£/kW) Max. flow (MW) Cost (£)
Transformer 132 kV/33 kV 5.2 58 301600
Circuit 33 kV 6.7 12.7 85090
Transformer 33 kV/11 kV 4.3 13 55900
Circuit 11 kV 11 3 33000
Total 475590
Table 6.10 Annual DUoS charges for individual network users
User On-peak charge (£) Off-peak charge (£) Total charge (£)
Demand at F 260000 0 260000
Generator at E 26000 100500 74500
Demand at D 95000 16750 78250
Demand at C 65000 5360 60240
Generator at C 4100 6700 2600
Total 475590
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