Boots – shoes – and leggings
Patent
1995-03-08
1997-10-14
Trammell, James P.
Boots, shoes, and leggings
364483, G01R 1900
Patent
active
056778520
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for detecting and damping transient oscillations in a power transmission system, and an arrangement for carrying out the method.
BACKGROUND OF THE INVENTION
In a plant for transmission of high voltage direct current, an HVDC-plant, transient oscillations can arise due to various reasons. They can occur, for example, due to earth faults, short circuits and misfiring valves, as well as during certain control operations during normal operation, such as ramps during starting, stopping, restarting after faults or stabilizing operations. The natural resonance of transient oscillations at certain frequencies is particularly large, whereby the natural resonant frequencies are well defined depending on the configuration of the system and the frequency of the alternating current side of the circuit. At resonance, the amplitude of the oscillations can quickly grow to dangerous levels which considerably exceed the rated quantity of the plant. This causes the energy transmission to be interrupted by safety systems. In addition, lasting oscillations can affect other regulating systems in the alternating current parts of the total network. Furthermore, larger alternating current components with fundamental frequency in the direct current transmission can result in the connected alternating current system obtaining an undesired direct current component due to saturation effects in transformers.
It can be very difficult to design normal regulators in power transmission systems so that they fulfil both the requirements which are made during normal operating conditions, whilst at the same time offering effective damping of critical resonant frequencies which exist in the network.
Similar resonance problems are present in alternating current networks for transmission when turbine generators are used. Because of the construction of such turbines, with their rotating masses affixed to necessarily torsion-elastic shafts, mechanical natural oscillations at subsyncronous frequencies (so-called negative damping) and similar oscillations can thus be fed to the network via the generators and also transmitted to other turbine generators in the network so that connected oscillations arise there-between. Similarly, interaction can occur with the network itself which often has large series condensers which, in cooperation with other reactances, can cause the network to oscillate. The subsyncronous oscillations (SSR) can, in certain cases, cause the turbine generators to break down whilst, in other cases, they are eventually damaged due to fatigue.
From U.S. Pat. No. 4,434,376 it is known to detect subsyncronous oscillations in alternating current systems, whereby the time separation between zero crossings of the voltage is detected and it is determined whether this differs from that which the system frequency should give. Such detected oscillations are thus damped by means of a regulating action which connects in one or more damping resistors. It is also known to dampen SSR by modulation of current/active power in a connected HVDC-station, whereby narrow band conventional filter circuits are used for diagnosis. In the cases where the thyristor circuits of the HVDC-system are sufficiently quick, the regulation itself can be performed without particular problems.
In order to avoid interruption, it is desirable to detect and dampen the transient oscillations at an early stage before the amplitude grows to critical values, or if the duration becomes too long. In order that this may be performed in a satisfactory manner, it is important to be able to differentiate between the operationally stipulated transient building-up processes and other processes which can result in resonant oscillations with subsequent disconnection or damage to other parts of the system.
In certain underwater cables for HVDC, and also in other cases, combinations of cable capacitance and smoothing inductances in the system can give rise to natural resonant frequencies which can create oscillations which are not self dampenin
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Asea Brown Boveri AB
Trammell James P.
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