In this section the main technical impacts of embedded generation on the distribution system are reviewed. Later chapters of the book will deal with these topics in more detail, but the intention here is to provide an introductory overview of the issues.
1.5.1 Network voltage changes
Every distribution utility has an obligation to supply its customers at a voltage within specified limits. This requirement often determines the design and expense of the distribution circuits and so, over the years, techniques have been developed to make the maximum use of distribu-tion circuits to supply customers within the required voltages. The volt-age profile of a radial distribution feeder is shown in Figure 1.6.
The precise voltage levels used differ from country to country but the principle of operation of radial feeders remains the same. Table 1.4 shows the normal voltage levels used.
Figure 1.6 shows that the ratio of the MV/LV transformer has been adjusted using off-circuit taps so that at times of maximum load the most remote customer will receive acceptable voltage. During minimum load the voltage received by all customers is just below the maximum allowed.
If an embedded generator is now connected to the end of the circuit then the flows in the circuit will change and hence the voltage profile. The most onerous case is likely to be when the customer load on the network is at a minimum and the output of the embedded generator must flow back to the source. It will be shown in Chapter 3 that for a lightly loaded distribution network the approximate voltage rise (ΔV) due to the generator is given (in per unit) by
ΔV = (PR + XQ)/V (1.1)
Introduction 11
where P= active power output of the generator Q= reactive power output of the generator R= resistance of the circuit
X= inductive reactance of the circuit V= nominal voltage of the circuit
In some cases, the voltage rise can be limited by reversing the flow of reactive power (Q) either by using an induction generator or by underex-citing a synchronous machine and operating at leading power factor.
This can be effective on MV overhead circuits, which tend to have a higher ratio of X/R. However, on LV cable distribution circuits the dom-inant effect is that of the real power (P) and the network resistance (R) and so only very small embedded generators may generally be connected out on LV networks.
For larger generators a point of connection is required either at the LV Figure 1.6 Voltage variation down a radial feeder (after Reference 8)
A voltage held constant by tap-changer of distribution transformer A–B voltage drop due to load on MV feeder
B–C voltage boost due to taps of MV/LV transformer C–D voltage drop in MV/LV transformer
D–E voltage drop in LV feeder
Table 1.4 Voltage levels used in distribution circuits
Definition Typical UK voltages
Low voltage (LV) Medium voltage (MV) High voltage (HV)
LV <1 kV 1 kV <MV <50 kV 50 kV <HV <220 kV
230/400 V 33 kV, 11 kV 132 kV 12 Embedded generation
busbars of the MV/LV transformer or, for larger plants, directly to an MV or HV circuit. In some countries simple design rules are used to give an indication of the maximum capacity of embedded generation which may be connected at different points of the distribution system. These simple rules tend to be rather restrictive and more detailed calculations often show that more generation can be connected with no difficulties.
Table 1.5 shows some of the rules that are used.
An alternative simple approach to deciding if a generator may be connected is to require that the three-phase short-circuit level (fault level) at the point of connection is a minimum multiple of the embedded gen-erator rating. Multiples as high as 20 or 25 have been required for wind turbines/wind farms in some countries, but again these simple approaches are very conservative. Large wind farms have been success-fully operated on distribution networks with a ratio of fault level to rated capacity as low as 6 with no difficulties.
If system studies are undertaken to investigate the effect of embedded generators on the network voltage then these can either consider the impact on the voltage received by customers or may be based on permis-sible voltage variations of some intermediate section of the distribution network. Studies considering the effect of embedded generation on, for example, the 11 kV network voltage are simpler to carry out but tend to give more restrictive results than those actually considering the effect on the voltage received by network customers.
Some distribution utilities use more sophisticated control of the on-load tap changers of the distribution transformers including the use of a current signal compounded with the voltage measurement. One tech-nique is that of line drop compensation [8] and, as this relies on an assumed power factor of the load, the introduction of embedded gener-ation and the subsequent change in power factor may lead to incorrect operation if the embedded generator is large compared to the customer load.
Table 1.5 Design rules sometimes used for an indication if an embedded generator may be connected
Network location Maximum capacity of embedded
generator
out on 400 V network at 400 V busbars
out on 11 kV or 11.5 kV network at 11 kV or 11.5 kV busbars
on 15 kV or 20 kV network and at busbars on 63 kV to 90 kV network
1.5.2 Increase in network fault levels
Most embedded generation plant uses rotating machines and these will contribute to the network fault levels. Both induction and synchronous generators will increase the fault level of the distribution system although their behaviour under sustained fault conditions differs.
In urban areas where the existing fault level approaches the rating of the switchgear, the increase in fault level can be a serious impediment to the development of embedded generation. Uprating of distribution net-work switchgear can be extremely expensive and, under the charging policies currently used in the UK, this cost will be borne by the embed-ded generator. The fault level contribution of an embedembed-ded generator may be reduced by introducing an impedance between the generator of the network by a transformer or a reactor but at the expense of increased losses and wider voltage variations at the generator. In some countries explosive fuse type fault current limiters are used to limit the fault level contribution of embedded generation plant.
1.5.3 Power quality
Two aspects of power quality are usually considered to be important: (i) transient voltage variations and (ii) harmonic distortion of the network voltage. Depending on the particular circumstance, embedded genera-tion plant can either decrease or increase the quality of the voltage received by other users of the distribution network.
Embedded generation plant can cause transient voltage variations on the network if relatively large current changes during connection and disconnection of the generator are allowed. The magnitude of the cur-rent transients can, to a large extent, be limited by careful design of the embedded generation plant, although for single generators connected to weak systems the transient voltage variations caused may be the limita-tion on their use rather than steady-state voltage rise. Synchronous gen-erators can be connected to the network with negligible disturbance if synchronised correctly, and antiparallel soft-start units can be used to limit the magnetising inrush of induction generators to less than rated current. However, disconnection of the generators when operating at full load may lead to significant, if infrequent, voltage drops. Also, some forms of prime-mover (e.g. fixed speed wind turbines) may cause cyclic variations in the generator output current which can lead to so-called
‘flicker’ nuisance if not adequately controlled [9,10]. Conversely, however, the addition of embedded generation plant acts to raise the distribution network fault level. Once the generation is connected any disturbances caused by other customers’ loads, or even remote faults, will result in smaller voltage variations and hence improved power qual-ity. It is interesting to note that one conventional approach to improving 14 Embedded generation
the power quality of sensitive high value manufacturing plants is to install local generation.
Similarly, incorrectly designed or specified embedded generation plants, with power electronic interfaces to the network, may inject har-monic currents which can lead to unacceptable network voltage dis-tortion. However, directly connected generators can also lower the harmonic impedance of the distribution network and so reduce the net-work harmonic voltage at the expense of increased harmonic currents in the generation plant and possible problems due to harmonic resonances.
This is of particular importance if power factor correction capacitors are used to compensate induction generators.
A rather similar effect is shown in the balancing of the voltages of rural MV systems by induction generators. The voltages of rural MV networks are frequently unbalanced due to the connection of single-phase loads. An induction generator has a very low impedance to unbal-anced voltages and will tend to draw large unbalunbal-anced currents and hence balance the network voltages at the expense of increased currents in the generator and consequent heating.
1.5.4 Protection
A number of different aspects of embedded generator protection can be identified:
•
protection of the generation equipment from internal faults•
protection of the faulted distribution network from fault currents supplied by the embedded generator•
anti-islanding or loss-of-mains protection•
impact of embedded generation on existing distribution system protectionProtecting the embedded generator from internal faults is usually fairly straightforward. Fault current flowing from the distribution network is used to detect the fault, and techniques used to protect any large motor are generally adequate. In rural areas, a common problem is ensuring that there will be adequate fault current from the network to ensure rapid operation of the relays or fuses.
Protection of the faulted distribution network from fault current from the embedded generators is often more difficult. Induction generators cannot supply sustained fault current to a three-phase close-up fault and their sustained contribution to asymmetrical faults is limited. Small synchronous generators require sophisticated exciters and field forcing circuits if they are to provide sustained fault current significantly above their full load current. Thus, for some installations it is necessary to rely on the distribution protection to clear any distribution circuit fault and hence isolate the embedded generation plant which is then tripped on Introduction 15
over/undervoltage, over/under frequency protection or loss-of-mains protection. This technique of sequential tripping is unusual but neces-sary given the inability of some embedded generators to provide adequate fault current for more conventional protection schemes.
Loss-of-mains protection is a particular issue in a number of coun-tries, particularly where autoreclose is used on the distribution circuits.
For a variety of reasons, both technical and administrative, the pro-longed operation of a power island fed from the embedded generator but not connected to the main distribution network is generally con-sidered to be unacceptable. Thus a relay is required which will detect when the embedded generator, and perhaps a surrounding part of the network, has become islanded and will then trip the generator. This relay must work within the dead-time of any autoreclose scheme if out-of-phase reconnection is to be avoided. Although a number of techniques are used, including rate-of-change-of-frequency (ROCOF) and voltage vector shift, these are prone to nuisance tripping if set sensitively to detect islanding rapidly.
The neutral grounding of the generator is a related issue because in a number of countries it is considered unacceptable to operate an ungrounded system and so care is required as to where a neutral connec-tion is obtained and grounded.
The loss-of-mains or islanding problem is illustrated in Figure 1.7. If circuit breaker A opens, perhaps on a transient fault, there may well be insufficient fault current to operate circuit breaker B. In this case the generator may be able to continue to supply the load. If the output of the generator is able to match the real and reactive power demand of the load precisely then there will be no change in either the frequency or voltage of the islanded section of the network. Thus it is very difficult to detect reliably that circuit breaker A has opened using only local meas-urements at B. In the limit, if there is no current flowing though A (the generator is supplying all the load) then the network conditions at B are unaffected whether A is open or closed. It may also be seen that since the load is being fed through the delta winding of the transformer then there is no neutral earth on that section of the network.
Figure 1.7 Illustration of the ‘islanding’ problem 16 Embedded generation
Finally, embedded generation may affect the operation of existing dis-tribution networks by providing flows of fault current which were not expected when the protection was originally designed. The fault contri-bution from an embedded generator can support the network voltage and lead to relays under-reaching.
1.5.5 Stability
For embedded generation schemes, whose object is to generate kWh from new renewable energy sources, considerations of generator transient sta-bility tend not to be of great significance. If a fault occurs somewhere on the distribution network to depress the network voltage and the embed-ded generator trips, then all that is lost is a short period of generation.
The embedded generator will tend to overspeed and trip on its internal protection. The control scheme of the embedded generator will then wait for the network conditions to be restored and restart automatically. Of course if the generation scheme is intended mainly as a provider of steam for a critical process then more care is required to try to ensure that the generator does not trip for remote network faults. However, as the inertia of embedded generation plant is often low and the tripping time of distribution protection long, it may not be possible to ensure stability for all faults on the distribution network.
In contrast, if an embedded generator is viewed as providing support for the power system then its transient stability becomes of considerable importance. Both voltage and/or angle stability may be significant depending on the circumstances. A particular problem in some countries is nuisance tripping of rocof relays. These are set sensitively to detect islanding but, in the event of a major system disturbance (e.g. loss of a large central generator), may mal-operate and trip large amounts of embedded generation. The effect of this is, of course, to depress the system frequency further. The restoration, after an outage, of a section of the distribution network with significant embedded generation may also require care. If the circuit was relying on the embedded generation to support its load then, once the circuit is restored, the load will demand power before the generation can be reconnected. This is, of course, a common problem faced by operators of central generation/transmission networks but is unusual in distribution systems.
Synchronous generators will pole-slip during transient instability but when induction generators overspeed they draw very large reactive cur-rents which then depress the network voltage further and lead to voltage instability. The steady-state stability limit of induction generators can also limit their application on very weak distribution networks because a very high source impedance, or low network short-circuit level, can reduce their peak torque to such an extent that they cannot operate at the rated output.
Introduction 17
1.5.6 Network operation
Embedded generation also has important consequences for the operation of the distribution network in that circuits can now be energised from a number of points. This has implications for policies of isolation and earth-ing for safety before work is undertaken. There may also be more difficulty in obtaining outages for planned maintenance and so reduced flexibility for work on a network with embedded generation connected to it.
1.6 Economic impact of embedded generation on the distribution