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A dual-source powered mixed effect AC (absorption chiller) is applied in a CCHP (combined cooling heating and power) system. This CCHP system integrates both fossil fuel energy and solar energy together. Refrigeration performance of this mixed effect AC under both waste heat mode and solar mode are tested and compared. In waste heat mode, the AC is simultaneously powered by exhaust gas waste heat and hot water waste heat from an ICE (internal combustion engine). And five working conditions under different electric output of ICE are tested to study the refrigeration performance. In solar mode, the AC is powered by hot water from a solar thermal collector matrix, and its refrigeration performance is tested in a selected day to study the operation characteristics. Test results show that, the mixed effect AC can reach a COP (coefficient of performance) of 0.91 in waste heat mode. But its COP is only about 0.6 in solar mode for there is only hot water heating source. Comparison results show that, waste heat mode is not necessarily better than solar mode and vice versa. In terms of primary energy consumption, waste heat mode is worse than solar mode when the part load ratio of ICE is below 0.62.

Combined cooling, heating and power (CCHP) systems are thought to be highly efficient in energy utilization. But there are still potentials to further improve system performance. This work proposed a CCHP system based on internal combustion engine (ICE) for power generation, refrigeration and domestic hot water production. Thermoelectric generator (TEG) and condensing heat exchanger are applied to efficiently recover the exhaust gas waste heat of ICE. All the energy flows are designed based on energy cascading utilization principle. Basing on the test results of a 16 kW ICE, CCHP system characteristics are investigated by simulation from idling condition to full load condition. Especially, the part load performance of TEG and absorption chiller are simulated and discussed. The feasible operating zone of ICE and feed water flow rate are figured out to keep the domestic hot water temperature within a certain range. Results show that the primary energy efficiency of system can reach 0.944, thanks to the condensing heat recovery from exhaust gas. The primary energy saving ratio and cost saving ratio can reach 0.304 and 0.417, respectively. Considering some more equipment is incorporated, the total investment increment is about 11.1%.

Tri-generation system is efficient in supplying thermal energy and electricity. But it is very difficult to satisfy multiple load demands simultaneously in real applications. Therefore, the benefit of tri-generation system is not so easy to realize. If the electric load is separated from the thermal load, difficulty of control would be avoided. In this work, combined cooling and heating (CCH) mode of tri-generation system is proposed. The generated electricity is only used to power the HVAC device, which converts all the electricity into thermal energy. Namely, there is only thermal output in this kind of mode. As for the rest electric load of user, it can be ensured by the public grid. Considering 20% of total energy consumption and 50% of building energy consumption may come from the HVAC system in developed countries. The CCH mode of tri-generation system would be potential in energy saving. The influences of system capacity, HVAC performance, public grid efficiency, and energy prices on system performances are analyzed. Results show that the CCH mode of tri-generation system can be much better than an independent HVAC system. For a system with generator capacity over 100 kW, the payback period could be less than 4 years.

A micro-CCHP (micro combined cooling, heating and power) system is proposed and investigated. It mainly consists of an internal combustion engine (ICE), an adsorption chiller (AD), a thermal management controller (TMC) and some other devices. The waste heat of ICE is collected and managed by the TMC. So the waste heat can be automatically assigned to different water cycles for heating, cooling, or to be released. Considering the complexity of micro-CCHP system, both experiment and simulation are conducted. Firstly, the experimental system and methodologies are described in detail. Then, simulation models of ICE and AD are proposed in order to simulate the system performance. Based on results of experiment and simulation, system characteristics for heating and cooling are investigated. Some influence factors are taken into account, such as the part load ratio of ICE, buffer tank set point temperature, hot water tank set point temperature, chilled water temperature and cooling water temperature of AD. Results show that the system can realize 17.7 kW heating output, 6.5 kW cooling output and 16 kW electric output simultaneously. The experimental primary energy ratio reaches 0.765 in heating mode and 0.56 in cooling mode.

In this paper, a sensorless pitch angle control method for a wind generation system is suggested. One-step-ahead prediction control law is adopted to control the pitch angle of a wind turbine in order for electric output power to track target values. And it is shown that this control scheme using the inverse dynamics of the controlled system enables us to predict current wind speed without an anemometer, to a considerable precision. The inverse input-output of the controlled system is realized by use of an artificial neural network. The proposed control and wind speed prediction method is applied to a Double-Feed Induction Generation system connected to a simple power system through computer simulation to show its effectiveness. The simulation results demonstrate that the suggested method shows better control performances with less control efforts than a conventional Proportional-Integral controller.

As the practical load conditions vary with time, the performance of micro combined cooling heating and power (micro-CCHP) system depends a lot on the operation strategy. So, it is significant to study the system performance and find the optimal operation strategies for various load conditions. There are many devices can be considered in micro-CCHP system. In this study, a comprehensive micro-CCHP system is built, basing on gas engine and adsorption chiller. Auxiliary devices, such as gas boiler, heat pump and electric chiller, are also considered in the study of operational optimization. In order to find the optimal operation strategies and discuss why they are the optimal ones, a mixed-integer non-linear programming model is developed. Energy saving ratio and cost saving ratio are chosen as the objectives and they are calculated hierarchically. Operation strategies under various load conditions are analyzed in detail and two dimensional distributions of system performance are presented. Results show that, optimal operation strategy changes with load conditions for energy saving optimization while it also changes with energy prices for cost saving optimization. For energy saving optimization, micro-CCHP system is always superior to conventional separated system when the heating load is over 12 kW in CHP (combined heating and power) mode or over 21 kW in CCHP mode. For cost saving optimization, micro-CCHP system can be superior to conventional separated system when the dimensionless energy price ratio is less than 0.45.

Converter valve possesses the function of AC-DC voltage conversion and power transmission. It is an important device in the valve hall of UHVDC converter station. Since the corona discharge has a serious impact on the safe and stable operation of converter valve, it is necessary to analyze its surface electric field (E-field) distribution. In this paper, the structure characteristics of three typical double valve designs have been analyzed, which were respectively used in three high voltage (400-800kV) valve halls of one ±800kV UHVDC transmission project in China. Based on the project drawings, the equivalent model of each type of double valve with shielding structure was built with three-dimensional (3D) finite-element method (FEM) software ANSYS. Considering the interaction of double valves in actual valve hall, three double valve FEM models of same type in one hall, which formed the upper 6-pulse bridge in the 12-pulse converter unit, were built for the analysis of E-field distribution. Due to the hybrid AC-DC operating voltage, the transient E-field analysis with discrete loading voltage waveforms was adopted in these models. After calculation, the E-field distribution characteristic of different double valve types were summarized and compared under the normal operating condition. The results showed that the surface E-field strength on the shielding structure of all the double valves was lower than the critical value of corona inception and could meet the project requirements. This research could provide valuable references for shielding structure design and optimization of converter valve in the UHVDC transmission project.

The converter valve is one of the key equipments in the ultra-high voltage direct current (UHVDC) transmission systems. Shield case design is an important part in the process of converter valve design, appropriate shield case configuration can improve the electric field distribution of converter valve, extend the service life and enhance the long-term operation reliability of the UHVDC systems. In this paper, an electric field distribution equivalent calculation model of the double converter valve was discussed, and a 3-D transient calculation model of ±800 kV double converter valve was established by using a finite element method (FEM) analysis software ANSYS. The calculation results of potential and electric field distribution of the double converter valve were presented, the law of electric field distribution of the shield cases which run on different working conditions and locate in different positions of the double converter valve was revealed, the effect of electric field and potential distribution of different shield cases configuration schemes was discussed, and an optimal scheme was put forward. Verified by the computer simulation, the optimal scheme satisfied the external insulation and electromagnetic environment requirements of the double converter valve. The results provide an important basis on the design of the UHVDC double converter valve.

For three-legged Y-YG core type transformers, due to the high reluctance of zero-sequence magnetic return path, the zero-sequence magnetizing impedance is low. The low zero-sequence magnetizing impedance will allow fault current to flow for a line-ground fault in the YG side of a three-legged core type transformer. When calculating line-ground fault current of a three-legged core type transformer, the zero sequence equivalent circuit needs to include the zero-sequence magnetizing impedance. The zero-sequence equivalent circuit from most classic reference books does not take magnetizing impedance into account and needs to be modified. This paper gives the modified zero-sequence equivalent circuit for a three-legged core-type transformer. Theoretical calculation, computer simulation, and actual staged tests verify that fault current based on the modified zero sequence circuit matches the result from the simulation and actual test.

It is recommended that differential protection be used for the protection of transformers of 10 MVA (self-cooled) and higher. Differential protection generally is considered the best protection for transformers. However, inrush current due to transformer energization exists mostly only in one winding of the transformer; therefore, the relay sees the energization condition as a fault. To improve security while maintaining the required levels of dependability, many restraint methods have been proposed to block the operation of the differential element due to the inrush current. Methods using harmonic restraint, wave-shape recognition, and artificial neural networks (ANN) have been proposed to discriminate magnetizing inrush and internal faults. The second harmonic restraint method is the most common one used by various relay manufacturers and application engineers. There are a few variations of harmonic restrained differential protection. This paper will analyze factors affecting the second harmonic ratio in inrush current, and describe various harmonic restraint methods and compare their performance.