Civil Engineering Reference
In-Depth Information
In the variable-speed induction generator system using the inverter at the
interface, the inverter gate signal is derived from the grid voltage to assure
synchronism. The inverter stability depends a great deal on the design. For
example, with line commutated inverter, there is no stability limit. The power
limit in this case is the steady state load limit of the inverter with any short-
term overload limit.
13.2.3
Load Transient
During steady state operation, if the renewable power system output is fully
or partially lost, the grid will pick up the area load. The effect of this will
be felt in two ways:
•
the grid generators slow down slightly to increase their power
angle needed to make up for the lost power. This will result in a
momentary drop in frequency.
•
small voltage drop results throughout the system, as the grid con-
ductors carry more load.
The same effects are felt if a large load is suddenly switched in at the green
power site, starting the wind turbine as the induction motor draws a large
current. This will result in the above effect. Such load transients are mini-
mized by soft-starting large generators. In wind farms consisting of many
generators, individual generators are started in sequence, one after another.
13.2.4
Safety
Safety is a concern when renewable power is connected to the utility grid
lines. The interconnection may endanger the utility repair crew working on
the lines by continuing to feed power into the grid even when the grid itself
went down. This issue has been addressed by including an internal circuit
that takes the inverter off line immediately if the system detects grid outage.
Since this circuit is critical for human safety, it has a built-in redundancy.
The site-grid interface breaker can get suddenly disconnected, accidentally
or to meet an emergency situation. The high wind speed cut out is a usual
condition when the power is cut off to protect the generator from overload-
ing. In systems where large capacitors are connected at the wind site for
power factor improvement, the site generator would still be in the self-
excitation mode, drawing excitation power from the capacitors and gener-
ating terminal voltage. In absence of such capacitors, one would assume that
the voltage at the generator terminals would come down to zero. The line
capacitance, however, can keep the generator self excited. The protection
circuit is designed to avoid both of these situations, which are potential safety
hazards to unsuspecting site crew.
