05-03-2012, 01:20 PM
Power System Technology Navigator
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Asynchronous connection
The interconnected AC networks that tie the power generation plants to the consumers are in most cases large. The map below shows the European situation.
There is one grid in Western Europe, one in Eastern Europe, one in the Nordic countries. Islands like Great Britain, Ireland, Iceland, Sardinia, Corsica, Crete, Gotland, etc. also have their own grid with no AC connection to the continent. The other continents on the globe have a similar situation.
Even if the networks in Europe have the same nominal frequency, 50 cycles per second or Hertz (Hz), there is always some variation, normally less than ± 0.1 Hz, and in certain cases it may prove difficult or impossible to connect them with AC because of stability concerns. An AC tie between two asynchronous systems needs to be very strong to not get overloaded. If a stable AC tie would be too large for the economical power exchange needs or if the networks wish to retain their independence, than a HVDC page link is the solution.
Bottlenecks
Constrained transmission paths or interfaces in an interconnected electrical system
The term Bottlenecks is often interchangeable to congested transmission paths or interfaces. A transmission path or interface refers to a specific set of transmission elements between two neighboring control areas or utility systems in an interconnected electrical system. A transmission path or interface becomes congested when the allowed power transfer capability is reached under normal operating conditions or as a result of equipment failures and system disturbance conditions. The key impacts of Bottlenecks are reduction of system reliability, inefficient utilization of transmission capacity and generation resources, and restriction of healthy market competition.The ability of the transmission systems to deliver the energy is dependent on several main factors that are constraining the system, including thermal constraints, voltage constraints, and stability constraints.
Interruptions
Occur when the supply voltage drops below 10% of the nominal value
An Interruption occurs whenever a supply’s voltage drops below 10% of the rated voltage for a period of time no longer than one minute. It is differentiated from a voltage sag in that the late is not a severe power quality problem. The term sag covers voltage drops down to 10% of nominal voltage whereas an interruption occurs at lower than 10%. A Sustained Interruption occurs when this voltage decrease remains for more than one minute.
Long lines
Long lines need special consideration in the planning of a power system.
This transmission carries more than 12,000 MW over 800 km. There is an HVDC system with two 600 kV bipoles of 3150 MW each is direct route to São Paulo while the three 800 kV shunt and series compensated AC lines has two intermediate substations that allow connection to the local grids.
For long AC lines one must consider i.e. the reactive power compensation, the transient stability and switching overvoltages and how many intermediate substations one needs. If the line length is longer than approx. 600 km one should also consider if an HVDC alternative brings lower investment costs and/or lower losses or if the inherent controllability of an HVDC system brings with some other benefits.
