Transformers and Substations Handbook 2014

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so that customers receive as close to ideal voltage as is possible. The total load on the busbar is calculated by summating transformer currents I CT,1 and I CT,2 (see Figure 3 ) and this is

I CT,2 , are summated, the network power factor can be found. The true load on each transformer and its contribution to circulating CT,1 and I

used to calculate a bias to apply to the voltage control. These two simple elements together achieve the main aims of voltage control. Other benefits of this system are that: • The system is extremely simple • Transformers and tap-changers on a site do not have to be iden- tical • Incoming voltages can be differ- ent • Transformers can be paralleled across networks. Although the actual power factor at a particular time may not be the speci- fied power factor pf sys , as long as the

Source

VCR

Load A

100 L

400 L

Load B

100 L

Interconnected Load C

200 L

34.3 kV

Figure 5: Example network for embedded generation scenarios.

deviation is not large the voltage control will be satisfactory. If the ac- tual power factor varies greatly from the set-point, the effect will be an error in the controlled voltage, as the load current will be considered as circulating current by the TAPP scheme. Varying power factors In circumstances where the load power factor can vary substantially, the TAPP scheme with its power factor set-point may not be a viable option. An alternative scheme, known as the true circulating current scheme, is described below and can be used in these circumstances. Figure 4 shows the current seen by two AVC relays I CT,1 and I CT,2 , with respect to their phase voltages V VT (when the transformer LV circuit breakers are closed the measured voltages will be identical). The load currents, I load,1 and I load,2 , have the same power factor. Transformer 1 is on a higher tap position than Transformer 2, hence a circulating current, represented by I circ in the diagram, will flow. If the measured currents,

current can be established. Therefore LDC error is eliminated.

Embedded generation For this discussion, an example network is used and is shown in Figure 5 . For the purpose of explanation a single transformer is shown supplying load to a nominal 33 kV busbar and the load is assumed to be unity power factor. Three circuits are supplied from the busbar. Load C is interconnected to a remote substation, and, for operational flexi- bility, the voltage control to the transformer tap changer is configured for reactive control (TAPP). If Load C is not interconnected to another site, true circulating current control can be implemented. The basic voltage level is set to 33 kV and, at the transformer load shown (400 L), the load drop compensation (LDC) applied at 4% in- creases the busbar voltage to about 34,3 kV. These figures are used for the purpose of explanation only. A number of scenarios involving generation embedded in this network are discussed.

Transformers + Substations Handbook: 2014

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