Handbook of Energy Storage for Transmission or ... - W2agz.com

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• Duty Cycle – Infrequent, e.g. 6 times per month, 75 momentary events per year
• Roundtrip Efficiency – 70-80% (assumes less than 1% duty cycle)
• No load Losses – Less than 3%
• Plant Footprint – 50 kW/m 2
• Environmental Issues – EMI, Harmonics, Air Quality (if back up diesel is used)
Figure 24 Typical Interruption and Sag Rates as a Function of Voltage Magnitude
There are very few practical non-energy storage alternatives for providing uninterruptible
protection of critical loads. Other available power sources or spinning reserves will serve
this need. This is usually in the form or a second independent feeder that is accessed
within the required response time via an electronic switch. Available of such
independent feeders is rare. Also, in this case, the small scale of the application is not
likely to justify the cost of such sources.
Battery-less Bridge to Stand by Generator (Standby UPS)
The objective of the bridging power system is to carry the critical load away from an outof-spec
or failing power source, and to a stable alternate source. The system is
effectively a battery-less standby UPS. Several key functions are required to accomplish
this objective. These are rapid isolation from the failing source, recovery using local
storage, energy conversion, synchronization, paralleling and soft transfer switching
between the primary and alternative power source. Optional functions that may add
value to this application are: additional power conditioning and filtering, full-time
reactive and real power stabilization, harmonic cancellation, control and dispatch of
distributed generation, interconnection protection and load control. Figure 25 shows each
of the basic functions in a generic circuit configuration.
Primary Source
Transfer
Switch
Isolator
Circuit
Breaker
Alternate Source
converter
Storage
Capacitor
Controller
Substation
Loads with
Priority
Sheading
Figure 25 Generic Circuit Configuration for a Substation Bridging Power System
Typically, the bridging application transfers the facility load from the primary power
source to a stand-by engine generator set. In addition, the application includes the
transfer back to the primary source after power is restored, and these transitions must be
seamless without causing any disruption to the source, load or facility. Characteristics of
available bridging power systems are:
Electrochemical Capacitors 46