Electricity: motive power systems – Synchronous motor systems
Reexamination Certificate
2000-02-23
2003-02-25
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
C310S180000, C174SDIG001, C174SDIG001
Reexamination Certificate
active
06525504
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and device for controlling the magnetic flux in a rotating high voltage electric alternating current machine with at least one auxiliary winding in the stator.
The invention relates mainly to an electric high voltage rotating electric alternating current machine intended to be directly connected to a distribution or transmission network or power networks, operating at high, extra high and ultra high voltages, comprising a magnetic circuit with a magnetic core, a main winding and at least one auxiliary winding. Such electric machines are typically synchronous machines which mainly are used as generators for connection to distribution or transmission networks, generally referred to below as power networks. Such synchronous machines are also used as motors and synchronous compensators. The technical field also comprises double-fed machines, asynchronous machines, asynchronous converter cascades, outer pole machine and synchronous flux machines.
When building synchronous machines with cylindrical rotors and particularly synchronous machines with low power factor and long rotors the cooling of the rotor can be a problem.
A machine is usually designed in order to realize an economic yield from the electromagnetic circuit. This normally leads to generated harmonic electromotive forces in addition to the generated fundamental electromotive force. Third harmonic electromotive forces are generated mainly due to saturation effects in the machine. In the generated harmonics third harmonic usually is the largest.
When connecting a machine to a power network a delta/wye step-up transformer usually is used. This delta/wye connected step-up transformer effectively blocks third harmonics and multiples of third harmonics.
When a machine is directly connected to a directly grounded power network without a delta/wye connected step-up transformer third harmonics and multiples of third harmonics may start to flow in the current machine and in the power network. Such third harmonics may damage the machine and equipment in the power network. Other problems also may occur. However, the foregoing is an example of one significant problem.
It is well known that it is possible to manufacture such a machine with one or more extra windings in the stator. It is described for instance in SIX PHASE SYNCHRONOUS MACHINE WITH AC AND DC STATOR CONNECTIONS,
IEEE Transactions on Power Apparatus and Systems
, Vol. PAS-102, No. 8 August 1983.
When manufacturing a stator with conventional insulation techniques, it is difficult to manufacture such a machine with a rated voltage higher than approximately 10-25 kV. If the stator is manufactured with two separate windings the rated voltage of each winding is usually the same. This is due to the conventional insulation technique. Therefore manufacture of a machine with completely different voltage levels in the windings has not been of interest.
Furthermore, connecting electric equipment to such auxiliary winding is not particularly complicated, this is because the voltage seldom is higher than approximately 10-25 kV as noted.
When using directly connected high voltage machines, for instance above 36 kV, it is desirable to have windings designed for two or more voltage levels. For example, separation of voltage levels allows the main winding of the machine to be connected to a high voltage system and to use low voltage equipment with the auxiliary winding. With such an arrangement the equipment connected to the auxiliary winding may be simpler than if this equipment was to be connected to a high voltage winding (i.e., High voltage equipment is often more complicated than low voltage equipment).
The effectiveness of reactive power control on a power network may be of the utmost importance not only under normal conditions, but also during major system disturbances. It is often advantageous to operate the transmission parts of a power network with a fairly flat voltage profile, in order to avoid unnecessary reactive power flows; and reactive power capacity reserves available for use in connection with major disturbances and under generator, transformer or line outage conditions. The aim of the steady state voltage control is to keep the transmission bus voltages within fairly narrow limits, while the load transferred varies.
The basic voltage control of a power network is provided by the large synchronous generators, each having its own excitation system with an automatic voltage regulator. The generators are used for voltage control at the terminals to which they are connected. Reactive power is generated or absorbed, depending on the load conditions.
Transfer of reactive power from the generators to electrically remote points of the power network or vice versa is usually avoided under normal operating conditions. Generators are, however, very important as reserve sources of reactive power which may be needed also rather far from the generators. For example, if there is a sudden loss of a main generator or a major line section, short-time reactive overload capability of generators may be a valuable resource on such occasions.
Reactive power is present in all electric power networks that transfer alternating current. Many loads consume not only active power but also reactive power. Transmission and distribution of electric power itself results in reactive losses due to series inductance 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 ground potential.
Proper operation of an alternating current system requires agreement between active power production and consumption in order to obtain nominal frequency. An equally strong relationship exists between reactive power balance and voltage in the electric power network. If reactive power consumption and production are not balanced in a suitable manner, the result 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 which in worst case can lead to a power network collapse.
In a power network the synchronous machines are one of the most important producers of controllable reactive power. Production of reactive power by the synchronous generators is therefore vital for power network voltage control. When the loads in the power network are changing and the demands of active and reactive power changes the control equipment of the synchronous generators will change production of the active and reactive power from the synchronous generators.
When the power network requires more reactive power, e.g., when the bus voltage is decreasing, the control equipment of the synchronous generators stall to increase the production of reactive power and vice versa. At some point, see
FIG. 1
, the synchronous generator is not able to produce more reactive power, typically because the field winding reaches it's maximum allowable temperature. If the reactive power demand of the power network has not been fulfilled, the voltage in the power network may start to fall which can result in a power network collapse.
Generators supply active power, provide the primary voltage control of the power network; and bring about, or at least contribute to, the desired reactive power balance in the areas adjacent to the generating stations.
A generator absorbs reactive power when underexcited, and produces reactive power when overexcited. The reactive power output is continuously controllable through varying the excitation current.
The allowable reactive power absorption or production is dependent on the active power output as illustrated by the capability diagram of FIG.
1
. For short term operation the thermal limits can usually be overridden.
Synchronous generators are usually rated in terms of the maximum apparent power load at a specific voltage and power factor which they can carry continuously without overheating. The active power output
Berggren Bertil
Nygren Jan-Anders
ABB AB
Dykema Gossett PLLC
Ro Bentsu
LandOfFree
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