Electrical transmission or interconnection systems – With harmonic filter or neutralizer
Reexamination Certificate
2000-08-31
2002-11-05
Sircus, Brian (Department: 2836)
Electrical transmission or interconnection systems
With harmonic filter or neutralizer
C307S102000
Reexamination Certificate
active
06476521
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to devices, schemes and methods for network protection of electric power systems, and electric power systems comprising such devices and schemes. Network protection refers to measures in order to avoid or reduce a substantial disturbance in an electric power system. The invention relates in particular to such protection against poorly damped power oscillations in electric power systems.
BACKGROUND
Control and protections in electric power systems are of many different kinds. Single units are often provided with protection devices, which may detect any faults or if the unit is operated outside its limits. Such a protection device typically reduces the operation conditions or disconnects the unit, and is therefore only concerned about the local conditions.
In the present disclosure, “power system” and “power network” refers solely to electric power, even if not explicitly mentioned.
For system disturbances, where the whole or substantial parts of an electric power system are involved, system protection schemes are used, which detects the occurrence or an acute risk for occurrence of a major disturbance and provides measures to reduce the consequences. Such measures may e.g. be the disconnection of certain loads, the division of the power system network into smaller autonomously operating networks, etc. The situation, in which these system protection schemes are activated, are emergency or close to emergency situation, and the time for performing the necessary actions is very limited, typically in the order of a part of a second up to half a minute.
The present invention is aimed at indicating and protecting electric power systems against instability due to power system oscillations not sufficiently damped. An electric power system is in general terms considered stable if the oscillatory response of the power system during the transient period following a disturbance is damped and the system settles in a finite time to a new steady-state operating point. Power system stability depends on the existence of both the synchronizing and the damping torque. Lack of sufficient synchronizing torque results in instability through an aperiodic drift in rotor angle, while lack of sufficient damping torque results in oscillatory instability, i.e. oscillations of increasing amplitude. Rotor angle stability in electric power systems can be divided into transient (angle) and small-signal stability. Small-signal stability is the ability of an electric power system to maintain synchronism under small disturbances, where a disturbance is considered small if linearization of the system equations is permissible for the purpose of analysis. The present invention is concerned with damping of powers system oscillations by providing additional damping torque to maintain small-signal stability.
In many countries there is an on-going restructuring of the electric power industry. This restructuring includes deregulation, and in some cases privatization, of electric utilities. These changes in power markets around the world have led to substantially reduced investments in infrastructure, i.e. investments in hardware. A continuously increasing load level in combination with new power flow directions has led to that new operation conditions may appear for the system operators with respect to earlier operation conditions in well known electric power systems. New operation situations and fast production changes and power flow changes associated therewith increases the demands on the tools and facilities of the operator to have a continuous overview and control of the operation security and margins in the electric power system. The demands for models, measurement data and calculation programs will thereby increase.
One of the causes for the at present, in many places, increasing interest in stability issues is that a load growth without a corresponding increase in transmission capacity has resulted in that many power systems today are being operated closer to their limits. During the last decades, there has been an increase in generation capacity as well as in use of electricity in the industrial world. A problem is the power delivery infrastructure, which is becoming more stressed in the new high-traffic and more competitive electricity industry. The power grids have over the years also become more widely interconnected and cover larger geographical areas. The power grids built and extended during past decades were, in many cases, not planned for handling the large number of transactions taking place in today's deregulated power markets. As a consequence, power system damping has in some cases been reduced, which has led to an increased risk of poorly damped oscillations. As a result, there is likely a substantially increased risk for larger scale power system failures (black-outs).
The dependence in modern society on a reliable power supply must not be underestimated. Furthermore, more and more customers are today more and more sensitive to disturbances in the electric power systems. The increased focus on power quality (PQ) issues includes both unwanted variations in the power supply in form of e.g. voltage sags and dips as well as disruptions in the supply of power. For this reason, some of the existing defense plans have to evolve from systems designed in the 1960s or 1970s to meet the requirements of the actual power systems today. It is further not possible from a design viewpoint to build a power system that can withstand all contingencies that may occur.
In case of serious faults, combination of faults or extreme load or unexpected production changes in the power system, there are network protections at a number of locations over the world, which try to avoid extensive network breakdowns and instead limit the consequences and facilitate the recover of the network. The area of system protection schemes (SPS) comprises a number of different types of systems, where the information carrying signals may be control signals as well as information that certain measurement values have exceeded or fallen below their limits.
The defense plans of today against serious disturbances are mainly adapted for transient angle phenomena in the power network. These types of system protection schemes are mainly concerned with load disconnection or islanding of the network.
Small-signal stability can be enhanced by use of Power System Stabilizers (PSS), which is a device with a basic function of adding damping to the generator oscillations by modulation of the generator excitation control signal. Additional damping torque is achieved by modulating the generator excitation to develop a component of the electric torque in phase with rotor speed variations. The speed deviation is therefore a logical signal to use for controlling the excitation of the generator by using auxiliary stabilizing signals. However, in practice both the generator and the exciter exhibit frequency dependent gain and phase characteristics.
Conventional PSS devices are local, using exclusively local measurement for decisions on how to control generator excitation to damp power system oscillations. Commonly used input signals to a PSS for stabilization of poorly damped power oscillations via excitation control include speed deviation, frequency, electric power and accelerating power. These input signals are processed for to find indications of power system oscillations, which manifest themselves as variations in power, currents, voltages etc. The analysis is thus based on an indirectly obtained indication of power oscillations and therefore only gives an indirect picture for the rotor position of the generator. Additionally, supplemental stabilizing signals to enhance the damping of power system oscillations may be taken from modulation of generator input power control, control (switching) of active power loads, changes in power system operating conditions, control of SVC, HVDC converters and FACTS devices, etc.
The objectives of excitation control design is to maximize the damping of both local a
Karlsson Daniel
Löf Per-Anders
ABB AB
Dykema Gossett PLLC
Polk Sharon
Sircus Brian
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