With the increase of power electronic equipment in the power grid, harmonics and their hazards gradually appear, and have drawn more and more attention. As converters are the main harmonic sources in DC transmission system, it seems particularly important to further analyze them on harmonics. Taking the line commutated converter high voltage direct current (LCC-HVDC) system as the objective, the paper uses an harmonic calculation model based on the switching function to analyze characteristic harmonics. Firs-tly, based on the modulation theory, AC-DC harmonic calculation equations considering the switching function are established, while the switching functions of voltage and current are solved based on the working principle of the converter. Then the equivalent harmonic impedance model of DC system is obtained by structure analysis of DC system. Based on the above analysis and flow analysis on the AC side, the harmonic equations are solved to obtain the LCC characteristic harmonics of AC and DC sides. Finally, by comparing with the PSCAD simulation model, the feasible and accurate of harmonic calculation method are verified. The paper can provide theoretical basis and method reference for power system harmonic analysis, harmonic suppression and so on.

Integrated gate commutated thyristor (IGCT) has the advantages of low on-state loss and high current-through capacity, and still has the potential for further development. It is suitable for the application of modular multilevel converter (MMC). This paper describes the principle and calculation of the mean equivalent method applied to loss assessment of the IGCT based MMC sub-module. Especially, losses of IGCT device, buffer circuit and energy extraction power supply are analyzed in detail. An example is given and is compared with that of simulated calculation. The accuracy of mean equivalent method is verified.

Facing the increasingly severe problems like short circuit current exceeding limit, serious AC-DC interaction and difficulty in setting up the third defensive line, system partition in multi-DC infeed receiving power grids like Guangdong power system is urgently required. In order to solve the contradiction between system partition and system stability and reliability, and support regional power consumption growth and large-scale clean energy access, an idea of building the outer ring of the Guangdong-Hong Kong-Macao Greater Bay Area is proposed. Key issues like coverage of the outer ring of the Bay, partition scheme of Guangdong power grid and capacity selection of the outer ring are specifically discussed. Then the flexible DC interconnection network scheme based on the outer ring of the Guangdong-Hong Kong-Macao Greater Bay Area is formed. Simulation analysis indicate that the scheme could meet the relevant safety standards, the capacity of Guangdong power grid could be increased to the expected level and problems such as short circuit current exceeding standards, serious AC-DC interaction and difficulty in setting up the third defensive line are effectively dealt with.

The occurrence of the 8·9 blackout in the United Kingdom makes us think that there are certain technical bottlenecks in the high proportion of distributed generation systems (DGs). Based on this, this paper proposes a current-limiting control strategy adapted to AC microgrid ground faults, which can limit the fault current to within twice the rated current, so that DGs will not be instantaneously disconnected in the event of a grid fault. Meanwhile, the method does not need to switch the reference current in the normal and faulty states of the power grid, and can achieve smaller current harmonics and lower current amplitude asymmetry in the two states. Firstly, the reference value of the voltage and phase-locked loop are obtained by the improved droop control strategy with washout filter, and the static error-free tracking of the outer-loop voltage is realized by the quasi-PR controller; Secondly, the difference between the filter inductor reference current output by the outer-loop controller and the actual inductor current is passed through an improved passivity-based control controller with anti-parameter disturbance capability to realize the tracking of the reference current. Fianlly, an AC microgrid model with four distributed generation built by simulation verifies the effectiveness of the method proposed in this paper.

Renewable energy grid connection brings new challenges to the stability of the power system, and it is a feasible solution to adopt motor-generator pair (MGP) for grid-connection method. This paper first introduces the structure of the renewable energy via MGP grid-connected system and its control method. Then the mathematical model of the system is established, and the power transmission model and electromagnetic-mechanical coupling characteristics of the system are analyzed to reveal influence of the control parameters, MGP inertia and damping parameters on system frequency and active power response. The simulation model of renewable energy via MGP grid connection is built in PLECS. In order to analyze the effect of different parameters on the system, by setting voltage faults in the simulation model, and comparing the changes of system frequency and active power. The results show that the optimization of the parameters is beneficial to suppressing and attenuating the oscillations of system frequency and active output. Finally, the frequency and power response are compared at different voltage rising levels in the experiment platform of renewable energy via MGP grid connection. The results show that MGP is helpful to improve the stability of renewable energy grid connection.

Under the current renewable energy consumption guarantee mechanism, this paper constructs the system dynamics model of renewable energy consumption market and renewable energy power market, and analyzes the impact of different consumption weight formulation schemes on consumption market, non-aqueous renewable energy installation and market electricity price under given conditions. The results show that non-aqueou renewable energy consumption market and electricity market respond differently to different policy scenarios, and a stable clearing price of the renewable energy consumption and energy power can be fromed in the long term under the mediation of market mechanism after a period of fluctuation, which ensures the stabilization of growth rate of renewable energy, and the promotion of the installed capacity of renewable energy.

In order to improve the dynamic performance of double-fed wind generator (DFIG) grid-side converter (GSC), a nonlinear compound control strategy is proposed based on GSC affine nonlinear model. In this control, the feedback linearization decoupling of the current is performed under stationary coordinate system and no need of grid voltage orientation instead of conventional feedback linearization. Simultaneously, sliding mode control based on a saturation function is applied to the voltage loop, which achieves fast stabilization of the DC-bus voltage and the current tracking. The stability of the GSC and the control of power factor on the grid side are achieved through simulation, which verify the correctness of the theory. Compared with the traditional control method, the control strategy proposed in the paper can reduce the variables of calculation and ensure the rapid convergence of the system.

In view of the lack of quantitative evaluation method for energy efficiency of the existing AC and DC distribution network, the complexity of the AC and DC distribution network and the wide distribution of energy efficiency impact factors lead to the lack of comprehensive and objective energy efficiency evaluation. This paper establishes a set of energy efficiency evaluation index system suitable for AC-DC hybrid distribution network from three aspects of source load according to relevant national standards and references. On this basis, the combined weighting method based on G1 anti entropy weight is established to determine the comprehensive evaluation model of index comprehensive weight from subjective and objective perspectives, and the approach ideal solution sorting method is used to comprehensively evaluate the energy efficiency level, fully reflecting the gap between various schemes. Finally, four simulation examples are used to verify the correctness of the index and the feasibility of the evaluation model, which has a certain reference value for the further scientific and reasonable planning, management and operation of the AC and DC distribution network.

To improve the transformer fault diagnosis effect and weaken the adverse effect of unbalanced training samples, a transformer fault diagnosis method based on variational auto-encoders (VAE) preprocessing deep learning and DGA for unbalanced samples is proposed. The DGA characteristics of each sample and the probability distribution of each fault state are used as the input and output of the diagnosis model respectively. Firstly, the minority training samples are preprocessed by VAE, and the training samples are generated automatically based on the learning to determine the distribution characteristics of the minority training samples, so as to improve the balance of the training samples. Secondly, the transformer fault diagnosis model is constructed based on the stack sparse auto-encoder (SSAE) deep learning network with three-hidden layers, and the parameters of the diagnosis model are updated and optimized with the balanced training samples preprocessed by VAE. The effectiveness of the proposed method is verified based on examples. The experimental results show that the proposed method can improve the adverse effects of unbalanced training samples. Under each training set, the accuracies of diagnosis results using the proposed method maintain above 91%, and the false negative rate are relatively low.

In the test of the maximum 900 kV DC superimposed 1 800 kV operating or 2 400 kV lightning impulse voltage, a special resistor element needs to be designed to protect the DC voltage generator from being damaged by the impulse voltage, and the appropriate shape is designed of the resistor to prevent flashover. Based on the simulation calculations, the resistance value is selected in the DC superimposed impact test circuit. And based on the external insulation flashover occurred during the test, the resistance wire winding method and the electrode shape are optimized. The results show that the optimum protection resistor is 500 kΩ which can meet the role of protecting the DC voltage generator in the test circuit; The externally insulated length is selected to be 10 m, the resistance wire winding method uses two resistance wires for double-wire winding, The high-voltage terminal electrode is equipped with ring diameter 2 700 mm, hood depth 500 mm and pipe diameter 250 mm of a grading ring on the high-voltage end, which can significantly improve voltage and electric field distribution on the resistor, and meet the requirement of no external insulation flashover and prevent flashover along the surface.

Actual operation experiments show that invalid protection of parallel gaps often occurs when parallel gaps recommended by relative standard are used. New type of 500 kV parallel gap used in normal region has been developed, while in high altitude region, the size of new type 500 kV parallel gap cannot be used directly. In this paper, U_50% lightning impulse test, volt-time characteristic test, discharge visible corona and radio interference test of parallel gaps with different size are carried out in National Engineering Laboratory for UHV Engineering Technology (Kunming), the altitude of which is about 2 100 m, and the effectiveness of protection of parallel gap is analyzed; then the lightning performances with and without parallel gap are acquired by simulation. The result shows that, when the numbers of insulator of insulator string are 33 and 37, the crosswise lengths of electrode are respectively recommended as 800/800 mm and 850/850 mm, and after installing parallel gaps, the lightning performance of single circuit is reduced compared with no parallel gap, while the lightning performance of double circuit simultaneously flashover is improved, and the increased percentage is respectively 11.3% and 11.6%.

The tower structure and voltage level of 35 kV line are between distribution line and transmission line, which makes it difficult for live operators to choose appropriate electric field protection measures. In order to ensure the safety of live-line working on 35 kV lines, the spatial electric field distribution of 35 kV single circuit and double circuit towers and the electric field intensity of live line operators at different working positions are simulated and calculated by finite element method, and the electric field intensity on the surface of live-line operators at different working positions is analyzed. The distribution characteristics and change rules of electric field on the human body surface of the operators, and the corresponding personnel protection requirements are put forward. The calculation results show that the electric field intensity on the human body surface at the working position is within the limit value when the workers are working on the 35 kV single circuit and double circuit line poles under four typical working conditions, which meets the standard requirements and does not need to wear the live working shielding clothing. The research results provide a technical basis for guiding the safe development of live line operation of 35 kV lines.

With the rapid development of demand-side field and the improvement of informatization level, more and more demand response (DR) terminals will be connected to the power system. As the demand response device is quite close to end user, the existing information processing techniques of the demand response terminal also has great potential for improvement. By investigating the existing DR business, this paper proposes a modified multi-streaming space time coding (STC) precoding scheme (preSTC-MC) based on the existing STC coding schema. By introducing the zero-forcing equalization compensation coefficient, the transporting performance of DR business can be improved. The BER and computing overhead performance are also investigated. The results show that the preSTC-MC approach performs better under the same SNR condition compared with other precoding schemes, without introducing too much computational overhead.

After the transformation of AC cable into DC operation, it is very important to conduct temperature and electric field simulation on the power cable joint and study its temperature distribution and electric field intensity to accurately evaluate the power supply capacity. At present, researches on the DC transformation of AC cable almost focuse on the cable body, and there are few researches on the cable joint. In this paper, the 3D simulation model of 10 kV three-core cable joint which is commonly used in the urban distribution network is established, first, the ANSYS heat-electricity coupling module is used to simulate temperature field of previous statement 3D model in which the contact resistance coefficient is four and the temperature distribution is analyzed. Then, the influences of contact resistance, and air convective heat transfer coefficients on cable joint temperature are studied. At last, the DC electric simulation of cable joint is conducted, and choose the DC current capacity and suitable DC voltage according to the temperature and electric simulation, the maximum transmission power is compared with AC cable joint. The results show that the power transmission capacity of AC cable joint is improved after the DC transformation.

According to the large-scale use of electric vehicles in the commercial field of China, this paper proposes an orderly charging and discharging scheduling strategy suitable for commercial electric vehicle groups to increase the income of commercial electric vehicle group owners. This strategy uses the aggregator to act as a bridge between electric vehicles and the grid, rationally schedules the charging time of electric vehicle groups and the number of charging vehicles, and participates in real-time transactions in the electricity market. First of all, in the case of satisfying the safety of the power system, a model that takes into account the income of electricity trading and vehicle operation is established, and is optimized through the multi-attribute utility theory. Then the genetic algorithm is used to minimize the multi-attribute utility function and daily scheduling of electric vehicles. Finally, based on a small-scale commercial electric vehicle group, a simulation example is carried out, and the load comparison analysis of orderly charging and disorderly charging are carried out in a medium-scale commercial electric vehicle group. The results show that this strategy has certain economic benefits under the premise of ensuring the safety of the power grid, and can play a role in peak shaving and valley filling.