Page 257 - Fluid Power Engineering

224 Chapter Eleven

must match demand. Because there is no storage on most networks,
excess supply or excess demand lead to imbalances. On a second-by-
second basis, supply and demand are kept in balance with load—
following spinning reserves generators. On a day-to-day basis, a grid
operator plans grid operations by seeking day-ahead generation plans
from each generation entity on the grid. The day-ahead plan, or fore-
cast, is in the form of an hour-by-hour generation output that will be
made available to the grid. This is called generator’s schedule. In the
United States, if a generation entity is unable to supply to the hour-by-
hour schedule (of the day-ahead plan), then there are penalties. These
penalties are called imbalance charges; the intent is to encourage ac-
curate forecasting such that reliability of the system can be enhanced.
This mechanism works well for fossil fuel-based generators (base-load
or spinning reserves) because there is no variability in availability of
raw material. The mechanism does not work well for renewable en-
ergy sources like solar and wind.
An alternate mechanism that favors renewable energy has been
developed by California Independent System Operator (CAISO),
which removes penalties based on hour-by-hour schedule and allows
for netting of schedules at the end of the month. In order to avail of
this netting, the wind plant operator must participate in the central-
ized wind output forecasting program through which CAISO pro-
vides forecasting and scheduling. Other approaches that have been
floated, not necessarily implemented, are to allow wind plant opera-
3
tors to provide 4-h ahead forecasts for wind resource and for system
operators to balance supply and demand at this granularity.

Single-Line Diagram
A wind project of a reasonable size requires a diagram to represent the
various electrical components of the wind project and interfaces with
the grid. A simple method to achieve this is with a single-line diagram.
The basic components in a circuit for wind project are seen in Fig. 11-1.
Most turbines produce AC power at 480 V (smaller turbines) or
690 V (utility-scale turbines). In order to minimize losses, this power
is transformed into distribution-level voltage of 11 to 34.5 kV using a
transformer at the base of the tower. The circuit on the higher voltage
side of the transformer is called the medium voltage (MV) circuit. The
MV circuit consists of switches to disconnect power and underground
cables to carry the power to a substation. At the substation, the power
is transformed again and the voltage is lifted to the grid voltage. A
circuit breaker in the substation monitors the grid parameters and
compares to the power parameters that are on the output side of the
substation. The circuit breaker trips when it detects a fault and discon-
nects the entire wind farm from the grid. Switchgear is a combination
of fuses, circuit breakers, and others. In this section, the two terms
circuit breaker and switchgear will be used interchangeably.