From cases 2 and 3, it is evident that the optimal QRA solution should include mitigation of most voltage dips as well as interruptions. This case is built upon case 3, except that distributed energy resources (DER) (in this case, on-site generation) is considered for protecting against long interruptions. The QRA analysis for dips is the same as in Case 3. DER information is given in Table A.9.
Table A.9 DER Information DER failure rate (λ) 2/year DER repair time 6 hours
Redundancy 1-out-of-2 Initial cost of DER $1,000,000
Installation cost $100,000 Fuel cost/yr $95,278 O&M cost/yr $50,000 DER life span 10 years
The facility is assumed to have two DER units with N-1 redundancy. FMEA analysis for this case is shown in Table A.10. NPV for long interruptions is shown in Figure A.4. The block of “Local DGs”
represents the equivalent parallel combination of two DER units in parallel at the facility.
Table A.10 FEMA Analysis: Case 4 Failure Mode for Interruption Effect
The utility feeders fail. Backup generator should come online. Critical load won’t be affected.
Utility feeders fail and the DERs
fail to start. Critical load will be interrupted.
Utility feeder
Local DGs
Local switchgear
Fig.A.4 Configuration of local distributed generation with utility feed
The results of the QRA analysis are shown in Table A.11. For the cost analysis, it is assumed that DER provides ancillary benefits such as CHP and peak shaving. The savings due to these are assumed to be 5%
of the total initial cost. Positive NPV indicates that DER can be an economical option for the
semiconductor-fabrication process. Note that DER will not provide any protection against voltage dips.
The NPV in Table A.11 does not include the losses due to twelve voltage dips per year. If losses due to voltage dips are considered, NPV will come out to be 0.88M$. Therefore, installing DER without any dip ride-through technology is not an effective QRA solution.
Table A.11 QRA Results: Case 4
Distributed generation is able to mitigate the duration of power outages; however, it’s lack of impact on voltage dips is a strong disadvantage in this particular analysis. At $44M, the overall improvement in NPV of this option is roughly comparable to that of a redundant power feed (-$51M – (-$95M) = $44M).
B Case Comparison and Sensitivity
The results of the Base Case and three alternative cases are illustrated in Figure B.1. Although a myriad of other options can be considered, among the three cases, Case 2: Battery UPS offers the best 10-year NPV and should be seriously considered, along with other solutions that mitigate the impact of voltage dips.
It is also important to note, however, that the other options may offer benefits not taken into account in this analysis, such as improved safety, environmental issues, or options for combined heat and power (CHP) or cogeneration. These and many other issues can be considered in the NPV analysis depending on the sophistication of the application and those designing it. The key is that all cases considered be treated equivalently.
Comparison of NPV Cases
-$100,000,000 -$80,000,000 -$60,000,000 -$40,000,000 -$20,000,000
$0
Base Case Case 1: Redundant Feeder
Case 2: Battery UPS
Case 3: On-Site Generation
Cases
NPV (10-yr)
Figure B.1 NPV Values for Mitigation Cases
It is also important to consider the sensitivity of such an analysis to variations in the input parameters used—in particular, the number of voltage dips assumed per year can have a profound impact on the results of the NPV calculations. For example, if the true cost of voltage dips to this facility were actually
$250,000 per event rather than $500,000, the economic analysis would be impacted as illustrated in Figure B.2 below. Although the relative rankings of the various solutions considered here are not changed, the disparity in their impact on 10-year NPV is considerably reduced. For PQ solutions that address only voltage dips (such as dynamic voltage restorers, etc.), reassessment of the cost of individual dips would likely have a profound impact on economic performance.
Comparison of NPV Cases
-$80,000,000 -$60,000,000 -$40,000,000 -$20,000,000
$0
Base Case Case 1: Redundant Feeder
Case 2: Battery UPS
Case 3: On-Site Generation
Cases
NPV (10-yr)
Figure B.2 NPV Values for Mitigation Cases with reduced impact of voltage dips
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