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Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com Voltage Sags and Interruptions Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.co causing a voltage sag with duration of more than 1 cycle occurs within the area of vulnerability. However, faults outside this area will not cause the voltage to drop below 0.5 pu. The same discussion applies to the area of vulnerability for ASD loads. The less sensitive the equip-ment, the smaller the area of vulnerability will be (and the fewer times sags will cause the equipment to misoperate). 3.2.3 Transmission system sag performance evaluation The voltage sag performance for a given customer facility will depend on whether the customer is supplied from the transmission system or from the distribution system. For a customer supplied from the transmission system, the voltage sag performance will depend on only the transmission system fault performance. On the other hand, for a customer supplied from the distribution system, the voltage sag performance will depend on the fault performance on both the transmission and distribution systems. This section discusses procedures to estimate the transmission sys-tem contribution to the overall voltage sag performance at a facility. Section 3.2.4 focuses on the distribution system contribution to the overall voltage sag performance. Transmission line faults and the subsequent opening of the protec-tive devices rarely cause an interruption for any customer because of the interconnected nature of most modern-day transmission networks. These faults do, however, cause voltage sags. Depending on the equip-ment sensitivity, the unit may trip off, resulting in substantial mone-tary losses. The ability to estimate the expected voltage sags at an end-user location is therefore very important. Most utilities have detailed short-circuit models of the intercon-nected transmission system available for programs such as ASPEN* One Liner (Fig. 3.7). These programs can calculate the voltage through-out the system resulting from faults around the system. Many of them can also apply faults at locations along the transmission lines to help calculate the area of vulnerability at a specific location. The area of vulnerability describes all the fault locations that can cause equipment to misoperate. The type of fault must also be consid-ered in this analysis. Single-line-to-ground faults will not result in the same voltage sag at the customer equipment as a three-phase fault. The characteristics at the end-use equipment also depend on how the voltages are changed by transformer connections and how the equip-ment is connected, i.e., phase-to-ground or phase-to-phase. Table 3.1 summarizes voltages at the customer transformer secondary for a sin-gle-line-to-ground fault at the primary. *Advanced Systems for Power Engineering, Inc.; www.aspeninc.com. 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. Voltage Sags and Interruptions Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.co Figure 3.7 Example of modeling the transmission system in a short-circuit program for calculation of the area of vulnerability. The relationships in Table 3.1 illustrate the fact that a single-line-to-ground fault on the primary of a delta-wye grounded transformer does not result in zero voltage on any of the phase-to-ground or phase-to-phase voltages on the secondary of the transformer. The magnitude of the lowest secondary voltage depends on how the equipment is connected: Equipment connected line-to-line would experience a minimum volt-age of 33 percent. Equipment connected line-to-neutral would experience a minimum voltage of 58 percent. This illustrates the importance of both transformer connections and the equipment connections in determining the actual voltage that equipment will experience during a fault on the supply system. Math Bollen16 developed the concept of voltage sag “types” to describe the different voltage sag characteristics that can be experienced at the end-user level for different fault conditions and system configurations. The five types that can commonly be experienced are illustrated in Fig. 3.8. These fault types can be used to conveniently summarize the 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. Voltage Sags and Interruptions Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.co TABLE 3.1 Transformer Secondary Voltages with a Single-Line-to-Ground Fault on the Primary Transformer connection Phase-to-phase Phase-to-neutral Phasor (primary/secondary) Vab Vbc Vca Van Vbn Vcn diagram 0.58 1.00 0.58 0.58 1.00 0.58 0.00 1.00 1.00 0.33 0.88 0.88 0.33 0.88 0.88 — — — 0.88 0.88 0.33 0.58 1.00 0.58 Phase Number of Phases Shift 1 2 3 Angle None Sag Type D One-phase sag, phase shift Sag Type B One-phase sag, no phase shift Sag Type C Two-phase sag, phase shift Sag Type E Two-phase sag, no phase shift Note: Three-phase sags should lead to relatively balanced conditions; therefore, sag type A is a sufficient characterization for all three-phase sags. Sag Type A Three-phase sag Figure 3.8 Voltage sag types at end-use equipment that result from different types of faults and transformer connections. 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. Voltage Sags and Interruptions Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.co expected performance at a customer location for different types of faults on the supply system. Table 3.2 is an example of an area of vulnerability listing giving all the fault locations that can result in voltage sags below 80 percent at the cus-tomer equipment (in this case a customer with equipment connected line-to-line and supplied through one delta-wye transformer from the transmission system Tennessee 132-kV bus). The actual expected per-formance is then determined by combining the area of vulnerability with the expected number of faults within this area of vulnerability. The fault performance is usually described in terms of faults per 100 miles/year (mi/yr). Most utilities maintain statistics of fault perfor-mance at all the different transmission voltages. These systemwide statistics can be used along with the area of vulnerability to estimate the actual expected voltage sag performance. Figure 3.9 gives an exam-ple of this type of analysis. The figure shows the expected number of voltage sags per year at the customer equipment due to transmission system faults. The performance is broken down into the different sag types because the equipment sensitivity may be different for sags that affect all three phases versus sags that only affect one or two phases. 3.2.4 Utility distribution system sag performance evaluation Customers that are supplied at distribution voltage levels are impacted by faults on both the transmission system and the distribution system. The analysis at the distribution level must also include momentary interruptions caused by the operation of protective devices to clear the faults.7 These interruptions will most likely trip out sensitive equip-ment. The example presented in this section illustrates data require-ments and computation procedures for evaluating the expected voltage sag and momentary interruption performance. The overall voltage sag performance at an end-user facility is the total of the expected voltage sag performance from the transmission and distribution systems. Figure 3.10 shows a typical distribution system with multiple feed-ers and fused branches, and protective devices. The utility protection scheme plays an important role in the voltage sag and momentary interruption performance. The critical information needed to compute voltage sag performance can be summarized as follows: Number of feeders supplied from the substation. Average feeder length. Average feeder reactance. Short-circuit equivalent reactance at the substation. 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
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