Electrical Power Systems Quality, Second Edition phần 9

Distributed Generation and Power Quality 422 Chapter Nine Time delay for low currents Time Adjustable delay time Adjustable instantaneous trip threshold Instantaneous trip for higher currents 100 Current (% of transformer rating) Figure 9.32 Adjustable reverse-power characteristic. capability to electromechanical network relays. In the past, these sup-plemental relays had minimum time delays of 1 s or more since their mission was to wait for the elevator to descend. However, not all util-ities endorse this low-current, time-delay technique. Some feel that any time delay in opening the network protectors degrades the high service quality that the network system is intended to provide. The load-generation control and DG tripping schemes mentioned above are intended to ensure that the network protectors are never opened by exported power. As long as the schemes work properly, the network protectors are never exposed to the out-of-phase voltage con-ditions that may exceed the switch capability. However, because of the potentially catastrophic consequences of causing a network protector failure, it is prudent to provide a backup. An interlocking scheme that trips the DG instantaneously when a certain number of network pro-tectors have opened ensures that the network protectors will not be exposed to out-of-phase voltages for more than a few cycles. The deci-sion as to how many protectors must open before the DG is tripped (one, two, or all) is a tradeoff between security of the protectors and nuisance tripping of the DG. Note that this scheme does not relieve the DG installer from the responsibility of providing stuck-breaker backup protection for the DG’s switching device. An even more secure approach to avoiding overstressing the network protectors is to replace existing protectors with new designs that are capable of interrupting fault currents from sources with higher X/R Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Distributed Generation and Power Quality Distributed Generation and Power Quality 423 ratios and of withstanding out-of-phase voltages across the open switch. One major U.S. manufacturer of network protector units has recently introduced such high-capacity protectors in 800- to 2250-Arat-ings and plans to introduce them in ratings up to 6000 A. These pro-tectors are designed to be retrofitted in many existing types of network units. Apossible DG interconnection problem exists that would involve net-work protectors without a network bus interconnection. If a DG is interconnected on a feeder that also supplies a network unit, then if its feeder breaker is tripped and the DG is not rapidly isolated, it may impact one or more of the network units as if it were isolated on the net-work bus. For this type of event to occur, the DG output does not have to be matched to the feeder load. For the excess generation case, it only has to be momentarily greater than the load on the network bus. Under this condition the power continues to flow to the network bus from the feeder with the interconnected DG, which keeps that protector closed. However, the excess power flows through the network back to the other feeders, resulting in the opening of the protectors connected to those feeders. Once open, these protectors will be separating two indepen-dent systems. For the case of less generation than load, the protector connecting to the feeder with the generation may trip. Again, such a condition would have a protector separating two independent systems. Therefore, such DG applications should be avoided unless the DG breaker is interlocked with the feeder breaker with a direct transfer trip scheme. 9.7 Siting DG The value of DG to the power delivery system is very much dependent on time and location. It must be available when needed and must be where it is needed. This is an often neglected or misunderstood concept in discussions about DG. Many publications on DG assume that if 1 MW of DG is added to the system, 1 MW of additional load can be served. This is not always true. Utility distribution engineers generally feel more comfortable with DG installed on facilities they maintain and control. The obvious choice for a location is a substation where there is sufficient space and com-munications to control centers. This is an appropriate location if the needs are capacity relief on the transmission system or the substation transformer. It is also adequate for basic power supply issues, and one will find many peaking units in substations. However, to provide sup-port for distribution feeders, the DG must be sited out on the feeder away from the substation. Such generation will also relieve capacity constraints on transmission and power supply. In fact, it is more effec- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Distributed Generation and Power Quality 424 Chapter Nine tive than the same amount of DG installed in the substation. Unfortunately, this generation is usually customer-owned and distrib-ution planners are reluctant to rely on it for capacity. The application of DG to relieve feeder capacity constraints is illus-trated in Fig. 9.33. The feeder load has grown to where it exceeds a limit on the feeder. This limit could be imposed by either current rat-ings on lines or switchgear. It could also be imposed by bus voltage lim-its. There is DG on the feeder at a location where it can actually relieve the constraint and is dispatched near the daily peak to help serve the load. The straightforward message of the figure is that the load that would otherwise have to be curtailed can now be served. Therefore, the reliability has been improved. This application is becoming more common as a means to defer expansion of the wire-based power delivery infrastructure. The gener-ation might be leased for a peak load period. However, it is more com-mon to offer capacity credits to customers located in appropriate areas to use their backup generation for the benefit of the utility system. If there are no customers with DG in the area, utilities may lease space to connect generation or, depending on regulatory rules, may provide some incentives for customers to add backup generation. There is by no means universal agreement that this is a permanent solution to the reliability problem. When utility planners are shown Fig. 9.33, most will concede the obvious, but not necessarily agree that this situation represents an improvement in reliability. Three of the stronger arguments are 1. If the feeder goes out, only the customer with the DG sees an improvement in reliability. There is no noticeable change in the ser-vice reliability indices. DG Sited to Provide Feeder Relief Feeder Limit DG Dispatched ON Daily Load Profile Figure 9.33 DG sited to relieve feeder overload constraint. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Distributed Generation and Power Quality Distributed Generation and Power Quality 425 2. Customer generation cannot be relied upon to start when needed. Thus, the reliability cannot be expected to improve. 3. Using customer-owned generation in this fashion masks the true load growth. Investment in wire facilities lags behind demand, increasing the risk that the distribution system will eventually not be able to serve the load. It should also be noted that the capacity relief benefit is nullified when the distribution system is upgraded and no longer has a con-straint. Thus, capacity credits offered for this application generally have a short term ranging from 6 months to 1 year. If one had to choose a location on the distribution feeder, where should the DG be located? The optimal DG siting problem is similar to the optimal siting problem for shunt capacitor banks. Many of the same algorithms can be used with the chief difference being that the object being added produces watts in addition to vars. Some of the same rules of thumb also apply. For example, if the load is uniformly distributed along the feeder, the optimal point for loss reduction and capacity relief is approximately two-thirds of the way down the main feeder. When there are more generators to consider, the problem requires computer programs for analysis. The utility does not generally have a choice in the location of feeder-connected DG. The location is given for customer-owned generation, and the problem is to determine if the location has any capacity-related value to the power delivery system. Optimal siting algorithms can be employed to evaluate the relative value of alternative sites. One measure of the value of DG in a location is the additional amount of load that can be served relative to the size of the DG. Transmission networks are very complex systems that are sometimes constrained by one small area that affects a large geographical area. A relatively small amount of load reduction in the constrained area allows several times that amount of load to be served by the system. This effect can also be seen on distribution feeders. Because of the simple, radial structure of most feeders, there is generally not a con-straint so severe that DG application will allow the serving of addi-tional load several times greater than the size of the generator. However, there can be a multiplying effect as illustrated in Fig. 9.34. This example assumes that the constraint is on the feeder rather than on the substation. If 1 MW of generation were placed in the sub-station, no additional load could be served on the feeder because no feeder relief has been achieved. However, if there is a good site on the feeder, the total feeder load often can grow by as much as 1.4 MW. This is a typical maximum value for this measure of DG benefit on radial distribution feeders. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Distributed Generation and Power Quality 426 Chapter Nine Pload Pgen Pload = 1.4 gen Figure 9.34 Ability of DG to increase the capacity of a distribution feeder is dependent on DG location. Another application that is becoming common is the use of DG to cover contingencies. Traditionally, utilities have built sufficient wire-based delivery capacity to serve the peak load assuming one major fail-ure (the so-called N-1 contingency design criterion). At the distribution feeder level, this involves adding sufficient ties to other feeders so that the load can be conveniently switched to an alternate feeder when a failure occurs. There must also be sufficient substation capacity to serve the normal load and the additional load expected to be switched over during a failure. This results in substantial overcapacity when the system is in its normal state with no failures. One potentially good economic application of DG is to provide sup-port for feeders when it is necessary to switch them to an alternate source while repairs are made. Figure 9.35 depicts the use of DG located on the feeder for this purpose. This will be substantially less costly than building a new feeder or upgrading a substation to cover this contingency. The DG in this case is located near the tie-point between two feeders. It is not necessarily used for feeder support during normal conditions although there would often be some benefits to be gained by operating the DG at peak load. When a failure occurs on either side of the tie, the open tie switch is closed to pick up load from the opposite side. The DG is dispatched on and connected to help support the backup feeder. Locating the DG in this manner gives the utility additional flexibil-ity and more reconfiguration options. Currently, the most common DG technology used for this application is currently diesel gensets. The gensets may be mounted on portable trailers and leased only for the peak load season when a particular contingency leaves the system vul-nerable. One or more units may be interconnected through a pad- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ... - tailieumienphi.vn