The Role of Distributed Generation in Power Quality and Reliability

Table 4.3 Power Quality Disruptions for System with UPS and Standby Generator
Power Quality Disruptions Occurrences
per Year
Duration per
Occurrence
Facility
Disruption per
Occurrence
Total Annual
Facility
Disruption
Voltage Sags 0.2 0.14 Seconds 50 Minutes 0.2 Hours
Momentary Interruptions 0.02475 5.3 Seconds 1.4 Hours 0.0 Hours
Long-Duration
0.00025 60 Minutes 5.0 Hours 0.0 Hours
Interruptions
Total Events 0.225 0.02 Minutes 0.2 Hours
Voltage Availability 100.00000%
Customer Availability 99.99769%
MTBF
1622.2 Days
Given the nature of the PQ disturbances at this site, the majority of incidents are short-term
fluctuations that are handled by the UPS. The standby generator only handles the single longterm
outage that occurs about once every other year. The facility availability can be evaluated
separately for the UPS and the standby generator with the incremental break-even costs to the
facility calculated. Table 4.4 summarizes this calculation. A UPS without back-up is
economically justified for customers whose avoided costs of facility disruption is $4.93 per kW
or higher. A standby generator is comparatively more expensive for the additional protection
provided with a break-even cost of $31.76/kW. The standby system, however, controls against
the possibility of a potentially devastating long-duration outage, so, in that sense, the system
provides an insurance value. A facility operator would prefer a known annual cost to one that is
highly variable and uncertain.
Table 4.4 Mitigation Effectiveness, Costs, and Break-even Value
Facility
Disruption
Annual Cost
of Mitigation
Break
Even PQ
Value
Hours/year $/year $/kW
No Mitigation Equipment 22.0 $0 NA.
UPS without Standby 2.7 $142,576 $4.93
Standby without UPS 20.3 $81,454 $31.76
Combined UPS/Standby System 0.25 $224,030 $6.90
Energy and Environmental Analysis / Pace Energy 20 The Role of DG in Power Reliability