Prime-mover dynamo plants – Electric control – Fluid-current motors
Reexamination Certificate
2001-07-10
2003-12-30
Ponomarenko, Nicholas (Department: 2834)
Prime-mover dynamo plants
Electric control
Fluid-current motors
C290S002000, C322S037000, C307S031000
Reexamination Certificate
active
06670721
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention is directed to systems, methods, rotating machines (as well as power electronic converters associated with the rotating machines) and computer program products that relate to electric power that is applied to an electric power grid after being generated from renewable power generation facilities (“renewables”). More specifically, the present invention is directed to systems, methods, rotating machines and computer program products for enhancing electric power produced by renewable facilities, such as wind turbine plants, solar electric power plants and the like, so as to make electric power generated thereby as commercially valuable and fungible as electric power produced by traditional sources, e.g., fossil fuel power plants, hydroelectric plants, nuclear plants and the like.
DISCUSSION OF THE BACKGROUND
Renewables are “natural” power production sources that instinctively should be regarded as optimal sources of energy for producing electric power. Renewables do not require burning of fossil fuels, do not result in nuclear waste by-products, do not require the channeling of water sources, and do not otherwise disturb the environment.
On the other hand, renewables are burdened by “weakness” and “variability” (each defined below), thus not offering AC power grid operators the (voltage) “stiffness” and type of planned control that the power grid operators and owners possess with conventional power generation facilities, especially during normal operation as well as during faults. The term “weakness” refers to a condition or quality of being weak. Moreover, in the context of the present document, weakness in reference to a power grid refers to (1) a risk of failure when subject to pressure, stress or strain, (2) having a lack of physical strength, energy or vigor, (3) having a lack of proper strength for resisting failure in a strained environment, (4) having a lack of ability to function normally or fully, and (5) having a lack of aptitude or skill. The term “variability” in reference to a power surge relates to (1) a quality, state or degree of being variable or changeable, (2) having characteristics that are subject to vary or have a tendency to variation, (3) have a characteristic (such as wind or currents) tending to change direction, and (4) being capricious (irregular and unpredictable).
“Stiffness,” (defined below) in the context of voltage in a power grid scenario relates to (1) corresponding to an infinite bus in a power grid, (2) having a quality, state or degree of being difficult to change or disturb, (3) being firm as in purpose, or resolute, and (4) being potent or strong.
As recognized by the present inventors, the AC power grid is not yet exposed to full stress regarding the weakness associated with renewable power generation facilities. Such facilities have not reached sufficient penetration in the grid, as compared to the existing methods and mechanisms known for supporting stable power grid operation and for proper fault handling. An increase in percentage of conventional renewable facilities would tend to limit the transmission availability and capability of existing power grids. Existing methods and mechanisms for stable power grid operation and fault handling are based on the fact that the existing power plants, converting energy from the traditional sources into electrical power, possess a transient built-in overload capability, especially regarding reactive power and fault currents. Consequently, modem AC power grids exhibit voltage stiffness sufficient for normal operation. This enable the grid to be operated within a few percent voltage variation around its nominal value.
By way of background, reactive power is used to stabilize the AC power grid's operation and its voltage stiffness and thus the grid's power quality. Fault currents are considerably larger than the nominal current levels, say 3 to 10 times, which ensure proper and rapid fault handling with existing methods and mechanisms.
Modern designers of electric conversion systems embedded in renewable facilities have tried to identify ways to use the same conversion systems to provide both active and reactive power from renewable facilities with a power throughput through only one type of power converter. Some designers thus mainly focus on the reactive power issue during normal operation and the associated perpetual AC voltage control in the operative AC power grid. However, they neglect the transient fault current and the transient voltage stiffness issues, thus leaving transient needs to be solved by others, such as AC power grid owners and operators and traditional AC power plant owners. Thus, traditional renewables may be environmentally friendly, but the nature of the power they produce and the strain they place on the system is viewed as a burden on the power production system, if deployed in great numbers. When attempting to address the power quality issue, renewable designers sometimes rely on a solution that includes expensively dimensioned power semiconductor hardware when the renewable facility has the duty to support, or even maintain, the voltage stiffness and/or the supply of short circuit power during faults.
DEFINITIONS AND TECHNICAL DESCRIPTIONS
Short-circuit Power, Voltage Stiffness
Short-circuit Capacity, SCC
A product of the pre-fault bus voltage and the post-fault current is referred to as short-circuit capacity (SCC), or fault level of a bus in question. By definition it is the value
SCC=EI=E*E/X
where E is bus voltage magnitude and X is reactance for the Thevenin circuit.
SCC and voltage stiffness
As E (pre-fault bus voltage, in the above equation) equals the pre-fault voltage of the shorted bus and this voltage normally has a magnitude of approximately 1 pu, (p.u.=per unit) the SCC becomes
SCC=(approximately)1/
X
(where X is the Thevenin reactance; same as X above)
The SCC has a direct bearing on the choice of circuit breakers, which must have an interrupt megavoltampere capacity equalling at least the fault level value for the bus in question. It should be noted that there is a direct relationship between the SCC and the ‘voltage stiffness’ of a system.
AC/DC System Interconnection—Short-circuit Ratio, SCR
Since the AC power grid's system strength has an important impact in the AC/DC system interconnection, it is useful to have a simple way of measuring and comparing relative strength of AC systems. The short-circuit ratio (SCR) has evolved as such a measure. It is defined as:
SCR=(short-circuit MVA of AC system)/(DC converter MW rating).
The operation of a DC system when connected to a weak (low short-circuit capacity) AC system is, as an example, associated with the following problems:
(1) high dynamic over-voltages, (2) voltage instability, (3) harmonic resonance, and (4) objectionable voltage flicker.
Stability Improvement
Practically, useful methods to improve power system (transient) stability include:
Increase of system voltage;
Reduction of transfer reactance; and
Use of high-speed circuit breakers and re-closing breakers.
Rotating Machine and Power Grid Interaction
To install a rotating machine and for a rotating machine drive system to function well, it is of importance to carefully study interconnection issues with the network. Voltage levels, short-circuit levels (capacity), type of network (distribution or industry), connected phase compensation equipment, disturbances (e.g. lightning), interruption frequency, etc. are to be studied.
An issue to address when installing new process equipment in industry is how large the installed rotating machine can be chosen (installed) without negatively effecting the voltage quality, especially if it is a motor that is started frequently and thereby behaving like a short-circuited rotating machine. Since a short-circuited rotating machine will result in a voltage drop at the point of connection, as well as in the area of connection, a maximum size of a rotating machine to be connected is determined
Gertmar Lars Gustaf Ingolf
Lof Per-Anders Kristian
ABB AB
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ponomarenko Nicholas
LandOfFree
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