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Example Load Flow

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As discussed already, renewable energy generation can affect both line loadings and voltages throughout the system. Load flow is a technique that allows the flows of real and reactive current throughout the network to be calculated, based on the location of the loads and sources and the line impedances.

The network of Figure 6.1 is used here to illustrate the way in which load flow analysis is applied at a distribution level to assess the effect of connecting a renewable energy generator at a node. This network is redrawn in Figure 6.3 to include node numbering and a proposed embedded renewable energy generator at node 10. It is necessary to know whether such an embedded generator is likely to affect adversely the network voltage profile.

At the outset a fault analysis is undertaken to provide the short-circuit levels at all nodes and so indicate the acceptable rating of the proposed renewable energy generator. To embark on a load flow it is necessary to specify the parameters of the lines, transformers and the known node variables. Table 6.2 gives the line data in terms of line position, length and line type. Table 6.3 provides additional information for each line type in terms of impedance and current rating.

Section 5.6.5 described the analysis required to determine network fault levels. For the network of Figure 6.3 the results are shown in Figure 6.4 . As expected, the fault level is highest close to the secondary of the distribution transformer (node 1) and due to the cumulative impedance of the transmission lines declines moving away from this point. At node 10, where the renewable energy generator is to be connected, the fault level is 42 MVA. A wind turbine rated at 500 kW resulting in a short-circuit ratio of 0.5I42 = 12% is within the acceptable range.

In this example it is assumed that the embedded generator is a wind turbine consisting of a directly connected induction generator operating at a power factor of 0.9 at full output. At

33 kV

' Primary substation

11 kV Trifl-1

Distribution ¿3 transformer^

Distribution ¿3 transformer^

	400 V	\l0	V-
		Embedded §i
1 km		Generator	1
	Figure 6.3	Example load flow
Table 6.2	Line data (input to load flow)
From node	To node	Length (m)	Line type
1	2	1297	100AAAC
2	3	304	50AAAC
2	4	626	100AAAC
4	5	391	100AAAC
5	6	738	185AL
5	7	492	100AAAC
7	8	583	100AAAC
8	11	1110	50AAAC
11	12	539	50AAAC
12	16	1253	185 AL
8	9	1000	50AAAC
9	10	539	185 AL
9	13	1154	50AAAC
13	15	583	50AAAC
13	14	652	50AAAC
14	17	791	95AL

AAAC: All aluminium alloy conductor

Table 6.3 Line type data (input to load flow)

Line type	R (Q/km)	X (Q/km)	Rating (A)
50AAAC overhead line	0.550	0.372	219
100AAAC overhead line	0.277	0.351	345
95 AL underground cable	0.320	0.087	170
185AL underground cable	0.164	0.085	255

Figure 6.4 Example fault levels (in MVA) throughout a typical rural 11 kV feeder

Table 6.4 Node data (input to load flow)

Node number	P (kW)	Q (kVAR)
1
2
3	-238	- 71
4	-159	-48
5
6	-340	- 102
Q	-178	-53
9
10	500	- 250
11	-458	- 137
12	- 221	- 66
13
14	- 97	- 29
15	-386	- 116
16	-161	-48
17	-64	- 19

a rated output this provides an injection of 500 kW and the absorption of 250 kVAR at node 10. In order to determine the voltage profile of the network under these conditions there is a need to assess whether this proposal is feasible. To proceed with the load flow analysis it is required that two variables are defined at each node. This distribution network consists solely of consumer PQ nodes and one embedded generator that can also be treated as a PQ node. The input data to the load flow are shown in Table 6.4.

The results of the load flow analysis are shown in Figure 6.5 . The voltage profile at consumer nodes is easily within the acceptable limits of ±1%. At the PCC the rise of voltage due to the active power injection is in this case moderated by the extraction of reactive power from the renewable energy source. Of course, if capacitors are installed at the wind turbine

Figure 6.5 Load flow results: voltages (kV)

to correct the power factor towards unity, the voltage rise would have been significantly higher and perhaps unacceptable. A rerun of the load flow analysis would provide a wealth of information on system voltage sensitivity to a range of renewable energy source locations and operational conditions.

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