Impact of Distributed Generation on Network Voltage Levels

Transcription

1 EEE8052 Distributed Generation Taster Material Impact of Distributed Generation on Network Voltage Levels Steady-state rise in network voltage levels Existing practice is to control distribution voltage levels at two places in the distribution network, the 33 kv bus in the bulk supply substation and the 11 kv bus at a primary substation [1]. Beyond this point the voltage falls due to the flow of load current through system impedances. Usually, the 33 kv voltage would be held near the statuary maximum limit of +6% and the 11 kv voltage would be regulated within pre-set limit using on-load tapchanging transformers and line drop compensation (LDC), a technique used to boost the substation voltage levels as the load current increases [2]. This is achieved by using a voltage relay to monitor the transformer output voltage and automatically initiate a tap change operation if the voltage strays outside the pre-set limits. This process is referred to as Automatic Voltage Control or simply AVC. Power infeeds from distributed generators tend to increase the local voltage level. This is because generators are usually operated at a lagging power factor. This reduces the amount of reactive power that needs to be transmitted down the network, pushing up voltage levels at the point of the connection. This can cause a problem with the voltage levels exceeding standard voltage levels, especially if the generator is connected to a voltage regulated 11 kv circuit. Small generators associated with renewable schemes will also tend to push the local voltage level up, especially if the amount of power generated exceeds the local demand. Methods for countering network voltage rise (i) Reducing primary substation voltage Lowering the 11 kv voltage at the primary substation is one obvious method for limiting the voltage rise produced by the connection of a generator to the 11 kv circuit. However, the DNO must ensure that all other customers supplied by all the other feeders connected to the primary substation or teed off the line will not be adversely affected and their voltage levels will not drop below the minimum statuary limit. The DNO must also consider how this voltage depression be addressed at times when the generator is not exporting power. This may not always be practical or possible.

2 (ii) Generator voltage control Generators may also be used to control system voltage levels by operating at a leading power factor (absorbing VArs). This however, could lead to poor power factors and stability problems, as we ll discuss in later Sections. It may also not be practical or possible since the network operator does not usually own or control the generator site. (iii) Use of additional buck/boost transformers The use of booster transformers to change the voltage at an intermediate point in the line between the generator and substation is very common in radial distribution networks in areas of low load density. The line voltage can be increased (or decreased) to give an in-phase voltage boost (or reduction) of about 10% on full load. An advantage of this form of voltage control is that the booster transformer need only be rated for full load current and the injected voltage, and is therefore only about 10% of the rating of a main power transformer. (iv) Automatic tap-changing of 11kV/400V transformers Most 11kV/400V transformers in use have off-line tap changers that can be set when the transformer is isolated. These are usually set according to some standard company policy and could be adjusted up or down in response to voltage regulation problems during a scheduled shutdown. These transformers could be converted for on-load tap-changing. If thyristor assisted tapchanging techniques could be employed, this could be achieved without the need to replace any of the existing transformers making it an economically viable option. An example of such a scheme [3] employing gate turn off thyristors (GTOs) as diverter switches and vacuum bottles as selector switches is shown in Fig. 1.

3 A C B S 5 S 6 11 kv line end S 1 S 2 S 3 S 4 Fig. 1 Single-phase power circuit of tap changer scheme employing vacuum switches (v) Reactive power injection The voltage levels at any point in the network may of course be controlled by injecting the right amount of reactive power at the node in question (lagging VArs to reduce the local voltage levels or leading VArs to increase the voltage). Thyristor controlled reactors (Fig. 2) acting as a variable reactor, where the inductive VArs supplied can be quickly varied by varying the firing angle of the two anti-parallel thyristors, could be used to reduce the local voltage levels at a node where a generator is connected although the size and cost of the equipment may prove to be a disadvantage. Alternatively, a fully controlled, PWM converter may be used to provide instantaneous static reactive power control (both leading and lagging) by controlling the magnitudes of the three injected ac line currents as well as their phase shift with respect to the ac line voltages, as shown in Fig. 3. The filter is needed to remove the high frequency current components from the as line current waveform. The disadvantage of this system is again the cost of the equipment.

4 line current 11 kv bus Fig. 2 VAr injection using thyristor controlled reactors line current 11 kv bus injected current PWM converter filter Fig. 3 Static VAr injection (vi) Increasing conductor size on overhead lines Replacing the conductor on an 11 kv overhead line with a larger conductor will significantly reduce its resistance, and to a lesser extent its inductance, and will smooth the voltage profile along the line. For example, a 70 mm 2 copper conductor has around 30% of the resistance and 90% of the inductance of a 16 mm 2 conductor of the same length. This gives a very effective, if expensive, method of controlling the line voltage rise problem. (vii) Line series compensation In this technique, line impedance is altered by simply connecting a capacitor or inductor (or a number of switched capacitors/inductors) in series with the line to modify the overall impedance and hence reduce/increase voltage regulation, as shown in Fig. 4. This may

5 provide a relatively cheap solution to the problem but detailed studies are needed to determine the effectiveness of the scheme in terms of the size and rating of the capacitors and their impact on line voltage profiles. Other problems include the cost of losses and possible LC resonances. Careful consideration needs also to be given to circuit transient behaviour, short-circuit withstand capability, capacitor protection and the possibility of ferroresonance. Fig. 4 Capacitive series compensation References 1. Methods to accommodate embedded generation without degrading network voltage regulation, 2001, DTI Publication URN 01/1005, UK 2. C. L. Masters, Voltage rise; the big issue when connecting embedded generation to long 11 kv overhead lines, IEE Power Engineering Journal, Feb. 2002, pp R. Shuttleworth, X. Tian, C. Fan and A. Power, New tap changing scheme, IEE Proceedings on Electrical Power Applications, vol. 143, no. 1, pp , 1996