Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1998-03-25
2004-12-14
Mullins, Burton (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S179000, C310S180000, C310S195000, C174SDIG001, C174SDIG001, C174SDIG002
Reexamination Certificate
active
06831388
ABSTRACT:
TECHNICAL FIELD
The present invention relates to electric machines intended for connection to distribution or transmission networks, hereinafter termed power networks. More specifically the invention relates to synchronous compensator plants for the above purpose.
BACKGROUND ART
Reactive power is present in all electric power systems that transfer alternating current. Many loads consume not only active power but also reactive power. Transmission and distribution of electric power per se entails reactive losses as a result of series inductances in transformers, overhead lines and cables. Overhead lines and cables also produce reactive power as a result of capacitive connections between phases and between phases and earth potential.
At stationary operation of an alternating current system, active power production and consumption must be in agreement in order to obtain nominal frequency. An equally strong coupling exists between reactive power balance and voltages in the electric power network. If reactive power consumption and production are not balanced in a suitable manner, the consequence may be unacceptable voltage levels in parts of the electric power network. An excess of reactive power in one area leads to high voltages, whereas a deficiency leads to low voltages.
Contrary to active power balance at a nominal frequencies, which is controlled solely with the aid of the active power starter of the generator, a suitable reactive power balance is obtained with the aid of both controllable excitation of synchronous generators and of other components spread out in the system. Examples of such (phase compensation) components are shunt reactors, shunt capacitors, synchronous compensators and SVCs (Static Var. Compensators).
The location of these phase compensation components in the electric power network affects not only the voltage in various parts of the electric power network, but also the losses in the electric power network since the transfer of reactive power, like the transfer of active power, gives rise to losses and thus heating. It is consequently desirable to place phase compensation components so that losses are minimized and the voltage in all parts of the electric power network is acceptable.
The shunt reactor and shunt capacitor are usually permanently connected or connected via a mechanical breaker mechanism to the electric power network. In other words, the reactive power consumed/produced by these components is not continuously controllable. The reactive power produced/consumed by the synchronous compensator and the SVC, on the other hand, is continuously controllable. These two components are consequently used if there is a demand for high-performance voltage control.
The following is a brief description of the technology for phase compensation with the aid of synchronous compensator and SVC.
A synchronous compensator is in principle a synchronous motor running at no load, i.e. it takes active power from the electric power network equivalent to the machine losses.
The rotor shaft of a synchronous compensator is usually horizontal and the rotor generally has six or eight salient poles. The rotor is usually dimensioned thermally so that the synchronous compensator, in over-excited state, can producr approximately 100% of the apparent power the stator is thermally dimensioned for (rated output) in the form of reactive power. In under-excited state, when the synchronous compensator consumes reactive power, it consumes approximately 60% of the rated output (standard value, depending on how the machine is dimensioned). This gives a control area of approximately 160% of rated output over which the reactive power consumption/production can be continuously controlled. If the machine has salient poles with relatively little reactance in transverse direction, and is provided with excitation equipment enabling both positive and negative excitation, more reactive power can be consumed than the 60% of rated output stated above, without the machine exceeding the stability limit. Modern synchronous compensators are normally equipped with fast excitation systems, preferably a thyristor-controlled static exciter where the direct current is supplied to the rotor via slip rings. This solution enables both positive and negative supply as above.
The magnetic circuits in a synchronous compensator usually comprise a laminated core, e.g. of sheet steel with a welded construction. To provide ventilation and cooling the core is often divided into stacks with radial and/or axial ventilation ducts. For large machines the laminations are punched out in segments which are attached to the frame of the machine, the laminated core being held together by pressure fingers and pressure rings. The winding of the magnetic circuit is disposed in slots in the core, the slots generally having a cross section in the shape of a rectangle or trapezium.
In multi-phase electric machines the windings are made as either single or double layer windings. With single layer windings there is only one coil side per slot, whereas with double layer windings there are two coil sides per slot. By coil side is meant one or more conductors combined vertically or horizontally and provided with a common coil insulation, i.e. an insulation designed to withstand the rated voltage of the machine to earth.
Double-layer windings are generally made as diamond windings whereas single layer windings in the present context can be made as diamond or flat windings. Only one (possibly two) coil width exists in diamond windings whereas flat windings are made as concentric windings, i.e. with widely varying coil width. By coil width is meant the distance in arc dimension between two coil sides pertaining to the same coil.
Normally all large machines are made with double-layer winding and coils of the same size. Each coil is placed with one side in one layer and the other side in the other layer. This means that all coils cross each other in the coil end. If there are more than two layers these crossings complicate the winding work and the coil end is less satisfactory.
It is considered that coils for rotating machines can be manufactured with good results up to a voltage range of 10-20 kV.
A synchronous compensator has considerable short-duration overload capacity. In situations when electromechanical oscillations occur in the power system the synchronous compensator can briefly supply reactive power up to twice the rated output. The synchronous compensator also has a more long-lasting overload capacity and is often able to supply 10 to 20% more than rated output for up to 30 minutes.
Synchronous compensators exist in sizes from a few MVA to hundreds of MVA. The losses for a synchronous compensator cooled by hydrogen gas amount to approximately 10 W/kvar, whereas the corresponding figure for air-cooled synchronous compensators is approximately 20 W/kvar.
Synchronous compensators were preferably installed in the receiving end of long racial transmission lines and in important nodes in masked electric power networks With long transmission lines, particularly in areas with little local generation. The synchronous compensator is also used to increase the short-circuit power in the vicinity of HVDC inverter stations.
The synchronous compensator is most often connected to points in the electric power network where the voltage is substantially higher than the synchronous compensator is designed for. This means that, besides the synchronous compensator, the synchronous compensator plant generally includes a step-up transformer, a busbar system between synchronous compensator and transformer, a generator breaker between synchronous compensator and transformer, and a line breaker between transformer and electric power network, see the single-line diagram in FIG.
1
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In recent years SVCs have to a great extent replaced synchronous compensators in new installations because of their advantages particularly with regard to cost, but also in certain applications because of technical advantages.
The SVC concept (Static Var. Compensator) is today the
Berggren Bertil
Leijon Mats
ABB AB
Dykema Gossett PLLC
Mullins Burton
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