04-07-2012, 02:08 PM
POWER T and D SOLUTIONS
POWER T and D SOLUTIONS.ppt (Size: 1.56 MB / Downloads: 124)
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 link is the solution.
And in other parts of the world (South America and Japan) 50 and 60 Hz networks are bordering each other and it would be impossible to exchange power between them with an AC line or cable. HVDC is then the only 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. These transmission limitations are usually determined by performing detailed power flow and stability studies for a range of anticipated system operating conditions. Thermal limitations are the most common constraints, as warming and consequently sagging of the lines is caused by the current flowing in the wires of the lines and other equipment. In some situations, the effective transfer capability of transmission path or interface may have to be reduced from the calculated thermal limit to a level imposed by voltage constraints or 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.
An interruption is usually caused by downstream faults that are cleared by breakers or fuses. A sustained interruption is caused by upstream breaker or fuse operation. Upstream breakers may operate due to short-circuits, overloads, and loss of stability on the bulk power system. Loss of stability is usually characterized by out-of-tolerance voltage magnitude conditions and frequency variations which exceed electrical machine and transformer tolerances. This phenomenon is often associated with faults and deficiencies in a transmission system but can also be the result of lack of generation resources. The concerns created by interruptions are evident and include inconvenience, loss of production time, loss of product, and loss of service to critical facilities such as hospitals.
Reactive Power Factor
Effects of reactive power on the efficiency of transmission and distribution
Reactive power is defined as the product of the rms voltage, current, and the sine of the difference in phase angle between the two. It is used to describe the effects of a generator, a load, or other network equipment, which on the average neither supplies nor consumes power. Synchronous generators, overhead lines, underground cables, transformers, loads and compensating devices are the main sources and sinks of reactive power, which either produce or absorb reactive power in the systems. To maintain efficient transmission and distribution, it is necessary to improve the reactive power balance in a system by controlling the production, absorption, and flow of reactive power at all levels in the system. By contrast, inefficient reactive power management can result in high network losses, equipment overloading, unacceptable voltage levels, even voltage instability and outages resulting from voltage collapse. Local reactive power devices for voltage regulation and power factor correction are also important especially for balancing the reactive power demand of large and fluctuating industrial loads.