Tower structures of DC and AC overhead transmission lines are shown in Fig. 2.There are some environmental issues must be considered for the converter stations. These issues are focused in [43]-[45]. The use of ground or sea return paths in monopolar operation, electromagnetic compatibility, visual impact, and audible noise are explained in [46]-[48]. In [49], an overview of the voltage stability analysis of AC/DC systems. This index is used to classify the system into soft and non-soft modal systems. The latter is defmed as the system with constant dQt/eig_min for all the SCRs and vice versa for the former. This index also serves as a basis to decide the type of reactive power compensation and HVDC control strategy.
engineering methods, tools, and design solutions is introduced. Verification methods used in HVDC converter stations design considering acoustic requirements are also explained.
IV.
VOLTAGE STABILITY OF HVDC VERSUS HVAC
INTERCONNECTIONS
Long transmission lines are required to deliver the power to the major load centers or the nearest connection point of the
existing transmission network. For long transmission of bulk power several technical and economic issues have to be considered before an optimal decision can be made. Voltage stability in general is one of the main technical issues to be considered [50]-[51]. Several methods, used to obtain the stability margin of a HVDC system, are well presented in the literature [51]-[59]. The most common voltage stability indices used for HVDC systems are maximum available power (MAP), critical effective short circuit ratio (CESCR) and voltage stability factor (VSF).
The maximum power method, which determines MAP and the voltage sensitivity method to detennine VSF are best described in [52]. These two methods coincide, i.e. the MAP point is reached when VSF nears infinite, if the converters are operated in constant extinction angle and constant power control mode. The basic P-V stability equations are also derived taking into account load characteristics and system parameters. These methods are applied in [53] to determine the most unfavorable load characteristics with respect to degrading power/voltage stability margins. This is done by analyzing the impact of load characteristic on maximum power instability (dP/dI) and MAP of the HYDC system. The Short Circuit Ratio (SCR) or CESCR are also considered as stability factors for an HYDC system, but only appropriate to evaluate the impacts of AC system on the stability margin of HVDC [60].
Authors in [59] introduced a new index (dQt/eig_min) for DC- tower
Fig.2. Typical transmission line structures for approximately 1000 MW.
While the above mentioned indices can be used to compare voltage stability margins between HVDC systems, they are not applicable for HVAC and HVDC comparison. In [56], authors extend the conventional point of collapse (PoC) method developed for AC systems to detennination of saddle-node bifurcation in systems including HVDC links. In [57], a comparison of the performance of the PoC and continuation rights- of-way without degradation of reliability, and mitigate environmental concerns. In all of these applications, HVDC nicely complements the AC transmission system. The following points highlight different advantages and disadvantages of the HVDC systems [29].
A.
Advantages
methods for large AC/DC systems is presented. The proposed continuation method is applied in the two free software- packages for stability studies; (UWPflow) and (PSAT) [61]- [63].
A nonlinear programming approach for estimating the voltage stability in AC/DC systems based on the above mentioned algorithms is presented in [59] where PoCs are found by solving an optimization problem for several test systems. However, more in-depth analytical explanation is required, and control issues of HVDC systems need to be considered. Inappropriate control schemes of firing, extinction, and overlap angles results in commutation failure or singularity in the Jacobian matrix. Therefore, PoC based on this method is not reliable to be used
in the comparison of voltage stability of HVDC and HVAC systems. The dVac/dq factor at a particular bus is a commonly used voltage stability index in both AC and DC systems [51], [54]-[55]. However, it has never been used for comparison purposes between HVAC and HVDC systems.
V.
ADVANTAGES AND DISADVANTAGES OF
HVDC Although the rationale for selection of HVDC is often
economic, there may be other reasons for its selection. In many cases more AC lines are needed to deliver the same power over the same distance due to system stability limitations. Furthermore, the long distance AC lines usually require intermediate switching stations and reactive power compensation. This can increase the substation costs for AC transmission to the point where it is comparable to that for HVDC transmission [29].
HVDC may be the only feasible way to interconnect two asynchronous networks, reduce fault currents, utilize long cable circuits, bypass network congestion, share utility
1) Greater power per conductor.
2) Simpler line construction and smaller
transmission towers.
3) A bipolar HVDC line uses only two insulated sets of conductors, rather than three. 4) Narrower right-of-way.
5) Require only one-third the insulated sets of conductors as a
double circuit AC line.
6) Approximate savings of 30% in line construction. 7) Ground return can be used.
8) Each conductor can be operated as an
independent circuit.
9) No charging current at steady state.
10) No Skin effect. 11) Lower line losses.
12) Line power factor is always unity.
13) Line does not require reactive compensation. 14) Synchronous operation is not required. 15) Distances are not limited by stability.
16) May interconnect AC systems of different frequencies. 17) Low short-circuit current on D.C line.
often provide a more economical alternative to AC transmission, for exploiting the high electrical power generated at long-distances and bulk-power delivery from clean remote resources, such as; hydroelectric developments, mine-mouth power plants, solar, large-scale wind farms, or major hot-rock geothermal energy. This transmission is established using fewer lines with HVDC than with AC transmission.
18) Does not contribute to short-circuit current of an AC
system.
19) Controllability allows the HVDC to 'leap-frog' multiple
'choke-points'.
20) No physical restriction limiting the distance or
power level for HVDC underground or submarine cables 21) Can be used on shared ROW with other utilities 22) Considerable savings in installed cable and losses
costs for underground or submarine cable systems [29].
B. Disadvantages
1) Converters are expensive.
2) Converters require much reactive power. 3) Multi-terminal or network operation is not easy.
4) Converters generate harmonics and hence, require filters. 5) Break-even distance is influenced by the costs of right-of- way and line construction with a typical value of 500 km [38]-[40].
VI.
ApPLICATIONS OF HVDC TRANSMISSION SYSTEMS
HVDC has gradually become a mature technology for AC system interconnection since the commissioning of the first commercial project between Mainland Sweden to Gotland island in 1954 [30]. The applications of HVDC technology are justified by some special conditions where HVDC is the most feasible or may be the only solution. Such applications include bulk power transmission over long distances, sub-marine cable transmission, and asynchronous systems inter-connection [64].HVDC transmission applications can be broken down to the following different basic categories [29], [37] AND [64].
A. Long Distance Bulk Power Transmission
As shown above, HVDC transmission systems B. Cable Transmission
Unlike the case for AC cables, there is no physical restriction limiting the distance or power level for HVDC underground or submarine cables. Underground cables can be used on shared ROW with other utilities, without impacting reliability concerns over use of common corridors. Saving advantages of underground and submarine cable systems 'have been shown previously, knowing that depending on the power level to be transmitted; these savings can offset the higher converter station costs at distances of 40 km or more.
On the other hand, for AC transmission over a
remote wind generation arrays require a collector system,
distance, there is a drop-off in cable capacity due reactive power support, and outlet transmission. to its reactive component of charging current, since cables have higher capacitances and lower inductances than AC overhead lines. Although this can be compensated by intermediate shunt compensation for underground cables at increased expense, it is not practical to do so for submarine cables [65]-[66].
C. Asynchronous Ties
With HVDC transmission systems, interconnections can be made between asynchronous networks for more economic or reliable system operation. The asynchronous interconnection allows interconnections of mutual benefit while providing a buffer between the two systems. Often these interconnections use
back-to-back converters with
no
transmission line [67]. Asynchronous HVDC links effectively act against propagation of cascading outages in one network from passing to another network.
Higher power transfers can be achieved, with improved
voltage stability in weak system applications, using
capacitor commutated converters. The dynamic voltage support and improved voltage stability offered by voltage source converter (VSC) based converters permits even higher power transfers without as much need for AC system reinforcement. VSC converters do not suffer commutation failures, allowing fast recoveries from nearby AC faults. Economic power schedules which reverse power direction can be made without any restrictions since there is no minimum power or current restrictions [68].
D. Offihore Transmission
Self-commutation, dynamic voltage control, and black-start capability allow compact VSC HVDC transmission to serve isolated and orphaned loads on islands, or offshore drilling and production platforms over long distance submarine cables. This capability can eliminate the need for running uneconomic or expensive local generation or provide an outlet for offshore generation such as that from wind.
The VSC converters can operate at variable frequency to more efficiently drive large compressor or pumping loads using high voltage motors. Large Transmission for wind generation must often traverse scenic or environmentally sensitive areas or bodies of water. Many of the better wind sites with higher capacity factors are located offshore. VSC based HVDC transmission not only allows efficient use of long distance land or submarine cables but also provides reactive support to the wind generation complex and interconnection point [29].
E. Power Delivery to Large Urban Areas
Power supply for large cities depends on local generation and power import capability. Local generation is often older and less efficient than newer units located remotely. Air quality regulations may limit the availability of these older units. New transmission into large cities is difficult to site due to right-of-way limitations and land use constraints. Compact VSC-based underground transmission circuits can be placed on existing dual-use rights-of-way to bring in power, as well as to provide voltage support allowing a more economical power supply without compromising reliability. The receiving
terminal acts like a virtual generator delivering power and supplying voltage regulation and dynamic reactive power reserve. Stations are compact and housed mainly indoors making siting
in urban
areas
somewhat easier.
VII. DIFFERENT HVDC SCHEMES [69]-[73]
A. Back-To-Back Converters
The \that the rectifier and inverter are located in the same station. Back-to-back converters are mainly used for power transmission between adjacent AC grids which cannot be synchronized. They can also be used within a meshed grid in order to achieve a defined power flow.
Furthermore, the dynamic voltage support offered by the VSC can often increase the capability of the adjacent AC transmission [29]. These applications can be summarized as follows:
I) Power transmission of bulk energy through long distance overhead lines.
2) Power transmission of bulk energy through sea cables. 3) Fast and precise control of energy flow over back-to-back HVDC links, creating a positive damping of electro- mechanical oscillations, and enhancing the network stability, by modulating the transmitted power.
4) Linking two AC systems with different frequencies usingasynchronous back-to-back HVDC links, which have no constraints with respect to systems' frequencies or phase angles.
5) Multi-terminal HVDC links are used to offer necessary strategically and political connections in the traversed areas of the potential partners, when power is to be transmitted from remote generation locations, across different countries, or different areas within one country. 6) Link renewable energy sources, such as hydroelectric, mine-mouth, solar, wind farms, or hot-rock geothermal power, when are located far away from the consumers.
7) Pulse-Width Modulation (PWM) can be used for the VSC based HVDC technology as opposed to the thyristor based
conventional HVDe. This technology is well suited for wind power connection to the grid.
8) Connecting two AC systems without increasing the shortcircuit power, that the reactive power does not get transmitted over a DC links. This technique is useful in generator connections, various applications of an HVDC system shown in Fig. 3.
B. Monopolar Long-Distance Transmissions
For very long distances and in particular for very long sea cable transmissions, a return path with ground/sea electrodes will be the most feasible solution. In many cases, existing infrastructure or environmental constraints prevent the use of electrodes. In such cases, a metallic return path is used
in spite of increased cost and losses.
C. Bipolar Long-Distance Transmissions
A bipolar is a combination of two independent poles in such a way that a common low voltage return path, if available, will only carry a small unbalance current during normal operation. This configuration is used if the required transmission capacity exceeds that of a single pole. It is also used if requirement to higher energy availability or lower load rejection power makes it necessary to split the capacity on two poles. During maintenance or outages of one pole, it is still possible to transmit part of the power. More than 50% of the transmission capacity can be utilized, limited by the actual overload capacity of the remaining pole, while require only one-third the insulated sets of conductors compared to a double-circuit AC line.
Other advantages of a bipolar solution over a solution with two monopoles are reduced cost, due to one common or no return path, and lower losses. In [74]-[76] the bipolar HVDC system configuration has been modeled. The reliability models in these three papers are similar to each other but the objectives in the papers differ.
高压直流输电系统的英文翻译
