In order to study the time-domain characteristics of DC corona current pulse, a high-frequency current measurement system is built to measure corona currents on HV terminal under different conditions. Based on the numerous experimental results, the characteristics of positive and negative corona current pulses, such as the amplitude, rise time, half-wave time and repetition frequency, are statistically analyzed. Statistical analysis of experimental results shows: the applied voltage has little effect on the pulse waveform of positive and negative corona, but it largely affects the pulse repetition rate； the rise time and half-peak time for negative pulse and positive pulse are respectively 20~30 ns, 90~110 ns and 40~50 ns, 140~160 ns； at the same applied voltage, the pulse repetition rate of negative corona pulse are 15~25 times larger than that of positive corona pulse.
The low frequency resonance of HVDC system may affect the safety and stability of the system operation. This paper studied the relationship between the loop impedance and frequency of ±1 100 kV UHVDC project, as well as its secondary frequency resonance. The characteristic curve of loop impedance and its resonance points were calculated by PSCAD/EMTDC simulation. The impact of transmission line length on the frequency of resonance points was proposed. The transient process of system under the fault that might cause secondary frequency resonance was studied, and the relationship between transmission line length and low frequency resonance phenomenon was obtained through the comparison with other UHVDC projects. The analysis results show that the length of transmission line is an important factor of loop impedance characteristics of DC system. The resonant frequency decreases with the increase of transmission line length； and 2000km is the sensitive length of low frequency resonance of HVDC system.
The improvement of DC transmission capacity proposes a higher request on the discharge current capacity, current-carrying reliability and heat resistance of valve hall fittings. In DC transmission project, valve hall fittings undertake the function of power equipments electrical connection, mechanical fixation and shielding. As the fittings operate with large current, high working voltage and poor heat dissipation, it is essential to study the reliability of existing fitting after capacity enhanced. Taking two typical valve hall fittings as research objects, equivalent cylinder was modeled as equivalent contact resistance, and thermal-electric coupled field was used for the finite element simulation. The temperature-rise test with high current was also carried out on the fittings, and the unbalanced current distribution of aluminium strip and stranded wire was surveyed and analyzed with using infrared thermal imager, as well as the local overheating phenomenon at the connection of welding and bolt. The test results and simulation analysis show that the local overheating and uneven current distribution of the fittings exist under 5620A excitation current. The simulation result agrees with temperature-rise test.
The economic transmission distance and capacity of DC transmission systems with different voltage levels serve as a guide in the selection of voltage level in actual DC transmission projects. The economic model of DC transmission system was built from four aspects of investment cost, maintenance cost, loss cost and transmission corridor depreciation cost. And the discounted cash flow model was adopted to calculate the comprehensive cost of DC transmission system. Taking a concrete transmission scenario with 1000 km transmission distance and 5 000 transmission power as an example, the comprehensive cost of DC project with different voltage levels was calculated, whose results were extended to other transmission scenarios to obtain the economic transmission applicable scope of DC transmission system with different voltage levels. Finally, the sensitivity analysis was carried out on parts of parameters that affected the economic transmission applicable scope. The results indicate that the DC transmission mode with high voltage is generally more economical along with the increase of transmission power and transmission capacity, and the electricity cost and equivalent annual loss time have significant influences on the economic transmission applicable scope of DC system.
The calculation of steady-state operation parameters plays an important role in the design process of back-to-back HVDC system. First of all, an explicit elaboration of the calculation model and control strategy used in the calculation of steady-state operation parameters was proposed, and then the calculation process was formulated based on these conditions. When HVDC system was operated in low power transmission, the operation parameters should be adjusted in order to engage converter in the balance control of reactive power between AC and DC systems. According to this problem, based on the discussion of the relationships among extinction angle and other steady-state operation parameters in back-to-back convertor station, an effective method of dichotomy iteration was put forward to solve the steady-state operation parameters that could meet the reactive power balance constraints during DC system low power operation. Finally, Gaoling back-to-back HVDC system was utilized to verify the correctness and effectiveness of this proposed algorithm. The result shows that this algorithm can be used in the packaged design of back-to-back HVDC system in the future.
The reactive power balance and control strategies in three typical HVDC transmission system (UHVDC, EHVDC, BTBHVDC) were comprehensively analyzed and researched. First of all, the electric equipment and influence factors related to reactive power in HVDC transmission system were introduced. Furthermore, the existing reactive power control strategies of HVDC projects were summarized and the concept of reactive power control factor was proposed. Moreover, based on the relationships between voltage fluctuation and reactive power in AC system, the simplified scheme was presented, in which AC voltage was converted into reactive power, and the intersection of transformed reactive power and AC system reactive power was taken as control factor. Finally, the computation results of the proposed method for reactive power were compared with the operating results of actual projects. The experimental results verify the effectiveness of the reactive power model in the proposed scheme.
±800 kV Jiuquan-Hunan UHVDC project sends large scale wind power to Hunan power system which is depending on the outside power source. Thus its reactive power compensation configurations not only need to consider the reactive power requirements of the DC system, but also should be optimized through equipped with reasonable dynamic reactive power compensation devices, according to weak reactive voltage supporting ability of AC grid in transmitter/receiver system. Because the AC grid connected to Jiuquan had large-scale wind power, the reactive power configuration scheme was proposed, in which static var compensator (SVC) was applied in the low-voltage side of substation-used transformer in converter station, and the filter capacity of converter station with SVC was determined. Aiming at low voltage risk of AC system connected to Hunan converter station, static synchronous compensator (STATCOM) was applied in the low-voltage side of substation-used transformer in Hunan converter station, the corresponding filter capacity was optimized and configured. Based on the calculation and analysis, it can be concluded that the dynamic reactive power compensation device applied in converter station can improve the voltage stability of the connecting AC system nearby converter station, reduce the voltage fluctuation of the converter bus caused by filter switching, and decrease the device investment.
The parameters design of main circuit and the structure study of main wiring were carried out aimed at DC system with 2 converters connected in parallel. Firstly, the main operation and connection schemes of DC system with two 12-pulse converters in parallel were introduced. Then, based on the basic theory of DC transmission and the characteristics of parallel converters, the DC main circuit was designed with considering the change of control mode, and the parameters of main devices such as converter transformers and converter valves, were calculated in detail. Finally, according to several typical parallel structures, the comparison of feasibility, floor space and difficulties in construction were carried out, and the recommended main electrical wiring scheme was proposed for converter station, combined with the simulation research under commutation failure condition.
The AC-side transient overvoltage level of converter station directly affects the insulation coordination and operation safety of the equipments in converter station. The mechanism and influence factors of the AC-side transient overvoltage of converter station were analyzed. The EMTDC electromagnetic transient simulation model was established based on Ximeng-Taizhou ± 800 kV UHVDC transmission project, and the AC-side transient overvoltage level of converter stations in transmitter/receiver system of this project were calculated. The results show that the switching impulse protective level can be set to 761 kV if the 500 kV AC bus in transmitter/receiver system uses the arrester with 396 kV rated voltage； the switching impulse protective level can be set to 858 kV if the rated voltage of arrester is 420 kV； and the switching impulse protective level can be set to 1460 kV if the 1 000 kV AC bus in receiver system uses the arrester with 828 kV rated voltage. The calculation results can provide technical reference for subsequent research and construction.
To meet the requirements of the intercontinental energy interconnection application, it is needed to develop the ±1 100 kV and 6 250 A ultra-large capacity UHVDC converter valve technology. Based on the existing test capability of China State Grid Corp Key Laboratory (Power System Power Electronics Laboratory), this paper studied the upgrading technologies of test capability, according to the research and development needs of the ultra-large capacity UHVDC converter valve. Through constructing the electromagnetic analysis models of DC and impacting equipment, this paper studied the electric field distribution characteristics and field intensity variation law on the equipment surface； proposed suppression measures for field strength； and completed the promotion of insulation test capability through the methods of optimizing laboratory shielding system, improving the curvature radius of shielding case, etc. According to the running test capability of converter valve, through the works of optimizing rectifier transformer, upgrading water-cooling systems parameters and the topology design of new fault current, the DC current test capability of operation test reached 7500 A, and the fault current test capability reached 60 kA, which completed the promotion of operation test capability. Finally, the test proved the correctness and effectiveness of ±1 100 kV and 6 250 A converter valve test capability.
The peak value of continuous operating voltage (PCOV) of HV transformer valve side voltage is an important data for the design of DC arrester parameters and the evaluation of insulation coordination scheme in UHVDC converter stations. Utilizing non real time operation functions of RTDS (real time digital simulator), in respect of the principle of the actual engineering facts, as well as to highlighting the key points, retain the related points and get equivalent of no significant points, this paper established a digital simulation high-frequency model which could simulate commutation overshoot transient process and calculate the PCOV of HV transformer valve side voltage with using multiple modeling techniques such as user-define valve component, adjustable DC voltage source equivalent for LV converter, the most simplification for control system according to the operating characteristics and so on. Taking Xiangjiaba-Shanghai ±800 kV UHVDC transmission project as an example, the accuracy and availability of the model were analyzed and verified from aspects of different operating conditions and equipment parameters.
The circuit breaker for switching AC filter is required the capacity of withstanding high voltage while the UHVDC direct accesses to 750 kV AC grid, So it is necessary to research the capacitive recovery voltage (CRV) of circuit breaker. This paper analyzed various AC fault scenarios of AC filter sub-bank circuit breaker, constructed the simulation calculation model of CRV by PSCAD software, and calculated the maximum value of CRV of 800 kV AC filter sub-bank breaker in the Lingshao UHVDC project. The simulation results show that the CRV of AC filter sub-bank breaker is determined by the short-circuit level of AC system and the AC-side insulator level, and the CRV exceeds the requirement of existing standards, so it is needed to develop new type of circuit breaker to meet the needs of engineering.
Converter valve equipment is the core element in the secondary system of DC transmission, the interface device between converter valve and DC control system, as well as the important part in the control and protection of converter valve. Based on Hazheng project, the overall structures of UHVDC control system and converter valve control system were introduced in detail, as well as the interface mode and signal between DC control system and converter valve control system with different technical route. A perfect interface test program was designed in view of the diversity and complexity of valve control system in Hazheng project. Based on the interface test and problems encountered during the test, some optimized suggestions were proposed for the interface between DC control system and converter valve control system, which could reduce the complexity and facilitate the standardization of operation and maintenance of converter valve system in the follow-up projects .
To save the land occupation of line corridor, parts of DC grounding electrode lines may be set up with AC external power lines on the same tower or corridor. These two layouts both have the mutual influence problems of AC lines and DC grounding electrode lines. By using electromagnetic transient program (EMTP), the mutual influences of AC transmission lines and DC grounding electrode lines were studied on the condition that grounding electrode lines were set up with 35 kV AC transmission lines on same tower or with 330 kV AC transmission lines on same corridor. The results show that: when DC grounding electrode lines are set up with 35 kV AC transmission lines on same tower, the induced electromotive force caused by 35 kV AC lines on DC grounding electrode lines is little, its maximum value is less than 2 kV, which will not affect the DC system； the induced electromotive force caused by DC grounding electrode lines on 35 kV AC system is far less than the insulation level of 35 kV AC equipments, which will not cause harm to the safe operation of 35 kV AC system； when DC grounding electrode lines are set up with 330 kV AC transmission lines on same corridor, the maximum induced electromotive force caused by 330 kV AC line on DC grounding electrode lines is not more than 18 kV, which will not cause harm to the safe operation of DC grounding electrode lines. The research result has great significance for ensuring the safe and stable operation of AC lines and DC grounding electrode lines.
The converter valve is a key component of Ultra High Voltage Direct Current system. The power loss calculation has attached great importance to the system design and radiator selection. The power loss of valve is coupled with the modulation strategies and categories of power electronic devices. It is difficult to calculate the power loss of valve accurately. In order to solve this problem, a power loss calculation method based on equivalent thermal model and junction temperature iteration method is proposed in this paper. Based on the analytical analysis of power loss, improved software is developed to calculate the power loss of valve. The commercial loss calculation software proposed by ABB is also introduced. And it is used to verify the effectiveness of power loss calculation method. At last, the relationship between power loss and operating parameters is obtained, which provides guidance for the designation of valves and radiator.
Vigorously developing and utilizing renewable energy is an important part of the sustainable development of energy and the sustainable economic development strategy in China. ±800 kV UHVDC transmission project from Jiuquan to Hunan with rated transmission capacity of 8 million kW is under construction, which transports thermal power, wind power and solar energy power of Northwest Power Grid. How to achieve the optimization of power transmission, consumption and economic benefit combined with the power delivery characteristics of Jiuquan and the load consumption characteristics of Hunan, it is needed to study the power running curve of UHVDC transmission power. The possible DC power transmission modes of Jiuquan and the consumptive situation and economic benefits of Hunan were analyzed. The ideas and methods of researches on these similar issues were proposed, which could provide technical reference for the operation of the project and important reference for other projects.
±800 kV Fulong-Fengxian UHVDC transmission system (XSH800 project) has successfully operated for 5 years. Its operation situation in last 5 years was analyzed, as well as the availability index and influence factors. In last 5 years, the cumulative transmission power of XSH800 was up to 895.53 billion kWh； bipolar and unipolar forced outage did not occur； single-valve group forced outage only happened five times； the unavailability of forced energy was only 0.0368%, which reached the international leading level. The practice results prove the safety and economy of UHVDC technology. The equipment problems exposed during operation, which may affect the availability, have been timely and fully corrected, and applied in the subsequent UHVDC transmission project. The successful operation of XSH800 in last 5 years has played a role in the demonstration and promotion for the large-scale construction and rapid development of UHVDC transmission project.
The recovery characteristics of DC system after commutation failure with different inverter control strategies were studied. Firstly, the operation principle of controller with commutation failure under constant extinguish angle and constant voltage control mode was analyzed. Then based on real-time digital simulation (RTDS) and the record data of actual project, the influences of these two control modes on the recovery characteristics of commutation failure were comprehensively compared. The results indicate that commutation failure is bound up with DC operation power, extinguish angle in steady state and failure moment, but has no relationship with inverter control mode. Finally, a novel optimization method of recovery characteristic was proposed, based on modifying current instruction parameter in VDCOL (voltage dependent current order limiter) module, and the simulation was carried out to verify its effect.
UHVDC project has been widely applied in power system because of its high-capacity and long transmission distance. However, the forced outage of monopole/bipole in DC system seriously affects its safe operation. According to the potential risk of the DC system, this paper studied the optimization of main wiring scheme, the restart logic of DC line, the automatic transformation scheme of metallic return, the differential protection action sequence of bipolar neutral bus and the protection configuration of DC filter in UHVDC transmission projects, from aspects of main circuit wiring mode, control & protection strategy, etc. Then, the key measure was proposed to prevent monopole/bipole blocking in UHVDC transmission projects. Finally, the effectiveness of the proposed measure was verified by real-time digital simulation analysis and engineering application. The research results have the practical guidance significance to improve the reliability of DC system and the design level of DC project.
The arrester fault of neutral bus in Nuozhadu-Guangdong UHVDC project during debugging process was analyzed, the energy stress of the arrester was simulated and calculated, whose results were compared with the field measured results. The research results show that the maximum absorption energy of neutral buss arrester during system test is the 11.9% of its nominal designed value and has a large margin, so the electrical energetic overload is not the cause of the arrester fault, but are quality defects of individual valve plates.
In EHV/UHVDC transmission systems, several AC filter circuit breakers (CB) were found broken due to internal insulation breakdown fault or external insulation flashover fault in several projects. The CBs supplied by domestic manufacturers were put into operation in China Southern Power Grid for the first time, and a CB fault occurred in a switching process because of porcelain bushing bursting caused by interrupter internal breakdown. This paper introduced the fault process, on-site disintegration inspection and failure analysis of this fault, and proposed the anti-accident measures for this batch of the circuit breakers, as well as the subsequent selection proposals of the filter CB.
