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

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In general, the requirement for this reserve is the larger of 7 % of total capacity or the size of the
largest operating generator. It must accommodate the largest single contingency on the system
and provide power within a period measured in cycles or seconds depending on the requirement.
Underfrequency Load Shedding
Loss of transmission lines and generators may lead to a decrease in system frequency as the
available generation attempts to supply the load. This condition will continue until a balance
between generation, transmission capability and load are reached. If this balance does not occur
rapidly (e.g., by the overall system generation increasing to accommodate the existing load), load
shedding (dropping customer load) may be required to avoid loss of synchronism and system
blackout.
Circuit Breaker Reclosing
Power line faults cause circuit breakers to open, thereby isolating the fault and eliminating or
reducing any damage. Once the fault is cleared, the circuit breakers are reclosed and the isolated
section is returned to service. If the separated sections of the power system are not otherwise
tightly connected, they will drift in phase while the circuit breaker is open. If the power angle
difference across the breaker is too large, protective relays will prevent it from closing. The
injection of real power while the circuit breaker is open can reduce the time for reclosure.
Power Quality and Backup Power
A variety of loads--ranging from modest industrial installations to substations of significant
capacity--require energy to provide power quality and backup power. This energy is used for a
variety of conditions such as when momentary disturbances require real power injection to avoid
power interruptions. In the case of industrial customers, a local source of power may be required
when there is an interruption of power from the utility. This power source may function until the
power feed from the utility is restored, until a reserve generator is started, or until critical loads
are shut down in a safe manner. In the case of a substation, a variety of momentary disturbances
such as lightning strikes or transmission flashovers cause power trips or low voltages. The total
energy storage requirement is greater and there may be a need power flow separation to insure
continuous power to important customers.
Sub synchronous Resonance
Long-distance, high-power transmission lines typically have high levels of series capacitors that
provide VAR compensation. Generators directly connected to the transmission lines can become
a part of what is called Sub-Synchronous Resonance SSR when their feed back or control
systems cause them to form a resonant circuit with the transmission lines and associated
capacitors. The impact of SSR on the grid is to reduce the transfer capability of the transmission
lines.
SMES Activities in Japan
There are differences between US and Japanese efforts in SMES development. The US has been
generally project focused, i.e., a specific application and site are chosen and one or more teams
develop plans and perhaps devices to meet the project goal. In Japan, the developments are
generally part of a program that has a variety of players and is coordinated by a national
committee.
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