22-05-2013, 11:44 AM
Wind Power in Areas with Limited Transmission Capacity
Introduction
The amounts of installed wind power continue to increase; however, the transmission
system is usually not dimensioned for a new generation of considerable size. Some
measures are necessary to make installation of large-scale wind power possible: grid
reinforcement, optimal planning and control of wind power sources and existing
conventional sources etc.
The first part of this chapter gives an overview of the congestion types and measures to
overcome limitations. The second part will describe the Swedish power system as an
example of the system with congestion problems. And, finally, the third part will give an
insight into optimisation and probabilistic methods to overcome the transmission limits.
Transmission Limits
Power transmission in a system can be subjected to the following limits: thermal limits of
the overhead lines, voltage stability limits, and transient stability limits. The following
subsections provide an overview of these congestion types and suggested reinforcement
measures is given.
Thermal limits
To define the thermal limits for the transmission line the ratings of the corresponding
equipment (such as breakers, voltage and current transformers, power transformers) should
also be considered. The limit is given by the lowest rating. The current-carrying capacity of
the conductors depends on the ambient temperature, wind velocity, solar radiation, surface
conditions of the conductor, and altitude above the sea level.
To allow operation of the transmission line at the higher temperature the usual measure is to
increase the height of the poles. This measure will permit sags for overhead lines at higher
power transmission.
Voltage stability limits
Voltage stability is the ability of the system to maintain steady acceptable voltages at all
buses in the system under normal conditions and after disturbance. The main factor of
voltage instability is the inability of the system to meet the demand for reactive power.
For any bus in the power system so-called nose curve or P-U curve can be drawn for the
analysis. From this curve it becomes obvious that voltage stability strongly depends on load
power factor and length of the transmission lines. Series compensation and shunt
compensation can be applied to solve voltage stability problems. With series compensation
the transmission capacity of the line can be increased 15-30%
Transient stability limits
Transient stability is the ability of the power system to maintain synchronism after being
subjected to severe transient disturbance. The effect of different wind turbines on transient
stability depends on turbine concept and connection point.
The system stability can be improved by changing protection settings and adding control
equipment.
Situation in Sweden
In Sweden the main power production (hydropower) is concentrated in the north. 8 long
400 kV transmission lines connect northern part of the transmission system with central and
southern parts, where the main load is concentrated. During cold days when the system is
highly loaded or during the spring flood when a lot of power is transmitted from north the
power transmission is almost at the limit.
Swedish TSO runs load flow simulations for normal operation and for the most severe
faults such as faults on the busbars. The limit for the voltage stability is defined by the most
severe fault. The calculated limits are compared with actual load flow and if the operation
is close to the limit the transmission has to be decreased.
Thermal limits are not decisive for the Swedish transmission system but those are important
between neighbouring countries.
The question under discussion is installation of large-scale wind power in mountainous area
in the north of the country. Good wind conditions make the area attractive for this purpose,
but the question is how much wind power that can be installed without violating the
transmission limits.
Other Methods to overcome the Congestions
In the previous sections the methods to overcome the transmission limits by means of grid
reinforcement were suggested. In this section optimisation and probabilistic methods are
introduced as a helpful tool to estimate the amount of wind power that is possible to install
in areas with congestions.
One of the measures to install more wind power in areas with transmission congestions is to
compensate wind power variations with hydropower production. This can be achieved most
effectively by common planning of wind power production and hydropower production.
It is also possible to assume that each time the transmission is at the limit some amount of
wind energy should be spilled as water in hydropower plants. Using probabilistic methods
it is possible to make a trade-off between wind power installed and wind energy spilled.