الفهرس 
Towards HVDC Transmission Projects in EgyptOnly 14 pages are availabe for public view
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Abstract
The HVDC (high voltage direct current) transmission systems have become a significant competitor in various electrical power transmission and integration projects. Given the conventional HVDC systems that depend on thyrisors, many drawbacks have been successfully avoided with the VSC (voltage source converter) schemes. With the
VSC-HVDC systems, there is no need for reactive power compensation, no
commutation failure problems, it is feasible to integrate with static or weak AC system, and it is applicable to operate in multiterminal configuration. This promotes the wide applicability of the VSC-HVDC systems to be adopted in many projects and applications as with integrating renewable wind energy resources. The major challenging concern with the VSC-HVDC systems is the protection perspectives. The VSC systems are restricted by its limited handling capability with disturbances particularly with the DC fault conditions. With the occurrence of DC fault condition, a significant voltage reduction is attained beside very fast high transient currents. In a very short transient duration, the converter units are exposed to severe
stresses which may lead to damage if there is no fast and reliable protection scheme
employed. Furthermore, with the multiterminal system operation, a more challenging point arises. Successfully identifying the faulty section only with a reasonable speed in detection still represents an interesting aspect. Given the permanent fault cases, after
successfully detecting and isolating the faulty line/cable, it is then favored to determine where the fault is located. This helps in the maintenance phase of the faulted transmission section. The fault location is a well-established approach with AC systems; however, it is not straightforward enough to find a reliable and accurate fault location scheme with the DC systems. The HVDC transmission systems do not face stresses with DC faults only however, it may encounter overvoltage problem. This could happen with the occurrence of significant imbalance between transferred powers through terminals due to AC fault conditions particularly at the inverter side. This is a common problem with the HVDC links employed for integrating wind farms with the occurrence of AC grid fault conditions. These highlighted concerns are considered within the proposed study in this thesis. Two different fast, reliable, and secure DC fault protection schemes are proposed. For more selective performance, the introduced schemes are based on incorporating the measured signals at both ends of the transmission system. The provided DC protection schemes are mainly based on Bergeron transmission system model. The proposed investigation avoids the most drawback of two-terminal data algorithms, which is the delay problem due to communication system. The proposed schemes succeeded in handling the communication time delay problem where this is a considerable advantage. Extensive tests are performed for validating the proposed schemes with
considering different fault types, worst fault scenarios, and healthy disturbances.
Determining the location of the DC fault condition is considered as well in the
proposed study. A single-end data based strategy is proposed and this is featured by avoiding the need to collect data from different locations or check the synchronization condition between captured signals. The proposed DC fault location scheme is proposed such that both ground and pole faults could be successfully located. In addition, the metallic and non-metallic fault conditions are considered. The HVDC overvoltage problem is investigated. A renewable wind energy resource is included in analysis and is integrated via VSC-HVDC link with the grid. A proactive detection scheme for the DC overvoltage condition is proposed by depending on the AC grid side measurements; this is where the problem primarily originates as a consequence after AC grid fault condition. Under such condition, it is not recommended
to isolate the HVDC link in order to keep the system in operation and support the grid. This is according to the grid code fault ride through requirements (FRT). A nonconventional overvoltage protection scheme is proposed by alleviating the voltage increase in the DC link by balancing the power transferred at both ends of the HVDC link. Faster control modification scheme is proposed to help in fast response and keep the HVDC voltage level within its acceptable limits.
The proposed investigations in the thesis are validated with the help of PSCAD and
MATLAB software platforms. In addition, the proposed studies are tested in real time for more verification by the help of a real time digital simulator (RTDS) system at Birmingham University, UK.