Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2003-01-16
2004-10-12
Berhane, Adolf (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C323S207000
Reexamination Certificate
active
06804128
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to current modulation of direct current transmission lines.
BACKGROUND OF THE INVENTION
In utility, industrial, commercial, marine, and military power systems Alternating Current (AC) supply lines have intrinsic limits to the amount of power they can transfer from source to load. That limit is proportional to both voltage and current, voltage being limited by the insulation provided, the current by the thermal limit of conductors. It is not unusual for power demand to outgrow either the capacity or the reliability limits of the transmission system already in place, both circumstances being addressed by the invention. While not limited to high voltage electric power transmission lines, the invention will be illustrated in that context.
Line loading on large AC power systems varies according to demand but seldom reaches the thermal maximum of the conductors. Actual transfer limits are more often set by characteristics of the electrical network of which a line is a part than by the thermal limitations of its conductors. Beyond a certain level of transfer; (1) synchronism of the AC system may be jeopardized (2) voltages may become depressed or unstable or (3) the inadvertent loss of the line in question could not be accommodated by other lines on the system. Furthermore the flow on an AC line cannot easily be controlled independent of flow on other lines in the system. For example when the thermal limit is reached on the weaker of two parallel AC lines, the stronger cannot be independently controlled to carry its maximum.
There are a number of AC equipment options, well described in the technical literature, that can overcome some or all of the system limitations to flow on a particular line. They include series compensation, phase shifting transformers, and other options, sometimes enhanced by thyrister controls. Such recourses, to the extent they are effective, are less expensive than construction of additional lines where that is even a practical option.
Present trends in many countries exacerbate the system-based limitations on AC line transfers, particularly those which interconnect major system segments. Growth in the ratio of intra-regional generating capacity to inter-regional transmission capacity may eventually threaten stable operation. Furthermore power brokering, now a common practice in the new generation market place is made difficult on AC systems since contracts must specify or infer a path over which the power is to flow even though it is difficult on an AC system to control the actual route of flow.
High Voltage Direct Current (HVDC) transmission overcomes many of the above limitations. It is possible, for example, to insert an AC/DC/AC (Back-to-Back) converter at one end of an AC line, thus operating the line at AC but gaining the power control advantages of DC. That option can usually bring the line's maximum loading up to the thermal limit of conductors, thus making full use of the line investment. However by leaving the line itself as an AC medium of transfer, back-to-back schemes forfeit the advantage HVDC has in its ability to make more efficient use of overhead or cable insulation. Insulation on AC systems is dimensioned to accommodate the peak of a sinusoidal voltage wave, even though the voltage is lower during other parts of the cycle. HVDC uses the insulation constantly and does not require as high an allowance for surges. The product of these two factors alone can boost transfer capacity by a factor from 2 to as much as 2, depending on insulation conditions.
In addition to improved transfer ability due to physical attributes of the line or cable, transfers on an HVDC lines have far fewer system-imposed limitations. Thus where there is a demand for more transfer, it is easier for an HVDC line to operate up to the thermal limits of the conductors. Being controllable, HVDC is also much more compatible with modern power brokering practices.
HVDC is inherently more reliable than AC, primarily because the loss of one conductor on an AC line forces the whole line out of service. Operation with two of three phases in service is not possible. Bipolar HVDC lines, equipped with metallic ground return, can loose one conductor and still operate at half power.
It is for the above reason that the industry contemplates conversion of selected existing AC lines to HVDC. The primary barrier to that recourse is economic. To convert an AC line to DC, all of the transmitted power must pass through an AC/DC inverter at one terminal and a DC/AC converter at the other. Thus even if the conversion achieves no increase in power level, the full cost of the conversion equipment must be justified by operational advantages of HVDC. On the other hand if system-imposed limitations allow the AC line to operate at only half its thermal limit and HVDC conversion allows loading to be brought to that limit, the conversion, while sized for full capacity, can be written off over the incremental one half of full capacity achieved.
In the case of some underground or underwater cables there are technical limits to HVDC conversion as well. Paper insulated cables are judged to be suitable for either AC or DC, thus delivering the advantages cited above. But cables with solid insulation, e.g. cross-linked polyethylene have two problems when used for DC. Over a period of time a static electrical charge builds up in the solid insulation which, upon reversal of power flow, achieved by most systems through a sudden reversal of polarity, can almost double the local electrical gradient in the insulation causing a risk of failure. Additionally, continuous DC systems applied to solid cable there is tendency for impurities to migrate in the insulation causing a weakened insulation path. Both problems are ameliorated this invention.
A double-circuit AC line, having two three-phase circuits on one structure (six conductor positions), is easily adapted to bipolar DC since it can be converted into three independent 2-pole circuits or into a single 2-pole circuit with three parallel conductors for each pole, thus making full use of the conductivity available.
The odd number of phase positions (3) on a single circuit AC line means that only two phase positions can be fully used, the third serving as an emergency ground should one normal pole be out of service. Thus whatever per-conductor advantages are achievable in the double circuit case, are penalized by a factor of ⅔ in the single circuit case Normally this would render the thermal limit of single circuit AC lines, converted to DC, about the same as the prior AC limit.
The same arguments apply to underground cables where, in the case of a single three-phase cable, one third of the inherent conductivity would be left unused under normal conditions.
SUMMARY OF THE INVENTION
Conductor “thermal limit” in the ensuing explanation is simplified as though it corresponded to a fixed ampere limit. Actual temperature rise of bare conductors depends not only on conductor characteristics but a number of environmental factors including, ambient temperature, sun exposure, and the magnitude and direction of wind. Technical literature provides details of these effects for specific conductors and current levels. The effects of high conductor temperature which depend on the magnitude of current, its duration and the cumulative exposure time, include mechanical weakening and embrittlement. To simplify application, most transmission owners use very conservative assumptions and assign a single “thermal limit,” in amperes, which is applied irrespective of actual conditions. Some assign two or more such values, each depending on conditions. Some actively monitor conditions and set the limits to correspond more closely to actual conditions. It will be apparent that any of the foregoing approaches would adapt equally well to implementation of the invention as they do to operation of the AC line.
Heating of a conductor is proportional to the square of current flowing through it and directly proportional to its electrical resi
Berhane Adolf
Dingman, Esq. Brian M.
Mirick O'Connell DeMallie & Lougee LLP
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