The development and test of China’s first ±525 kV cross-linked polyethylene （XLPE） DC cable are introduced. A type of heteromorphic copper conductor with a section of 3 000 mm² was designed， and a water-blocking tape was chosen as filler material. After calculation， the filling factor of the conductor reached 95.5%. Longitudinally permeability tests on the insulated conductor have proceeded under the condition of 2 MPa hydraulic pressure for 10 d and 1 m water column with thermal cycling for 10 times， the length of permeability was 7.2 m and 0.9 m respectively. Copper wires were twisted as a metallic shield of the ±525 kV XLPE DC cable， among the copper wires， two optical fibers were implanted in to achieve the requirements of communication and thermometry online. A composite waterproof sheath was compounded with aluminum-plastic tape and PE， hence， the outer diameter of cable has been minished 10% compared to corrugated aluminum-sheathed cable in the same voltage and section. To verify its electrical properties， voltage test and partial discharge test under AC 525 kV were carried out， then a whole performance type test was taken in National Center of Testing and inspection for Electric Cable and Wire （TICW） referred to CIGRE TB496—2012 and GB/T 31489.1—2015， including load cycle test， DC withstand voltage test， superimposed impulse test and so on， and the conductor passed all the tests. During the load cycle test， the maximum temperature of the conductor was over 70 ℃， and the temperature gradient in insulation exceeded 30 ℃.

The development and test of China’s first ±525 kV cross-linked polyethylene （XLPE） DC cable are introduced. A type of heteromorphic copper conductor with a section of 3 000 mm² was designed， and a water-blocking tape was chosen as filler material. After calculation， the filling factor of the conductor reached 95.5%. Longitudinally permeability tests on the insulated conductor have proceeded under the condition of 2 MPa hydraulic pressure for 10 d and 1 m water column with thermal cycling for 10 times， the length of permeability was 7.2 m and 0.9 m respectively. Copper wires were twisted as a metallic shield of the ±525 kV XLPE DC cable， among the copper wires， two optical fibers were implanted in to achieve the requirements of communication and thermometry online. A composite waterproof sheath was compounded with aluminum-plastic tape and PE， hence， the outer diameter of cable has been minished 10% compared to corrugated aluminum-sheathed cable in the same voltage and section. To verify its electrical properties， voltage test and partial discharge test under AC 525 kV were carried out， then a whole performance type test was taken in National Center of Testing and inspection for Electric Cable and Wire （TICW） referred to CIGRE TB496—2012 and GB/T 31489.1—2015， including load cycle test， DC withstand voltage test， superimposed impulse test and so on， and the conductor passed all the tests. During the load cycle test， the maximum temperature of the conductor was over 70 ℃， and the temperature gradient in insulation exceeded 30 ℃.

Aiming at proposing improvement to the protection strategy of sub-module overvoltage bypass according to the condition that sub-module capacitors are charged in modular multilevel converter （MMC） blocking state， the phenomena that the voltage magnitude of sound phase is raised are analyzed when a grounding fault occurs at valve side and MMC is blocked in an MMC-HVDC symmetrical monopolar bipolar system. Because there are distributed capacitances in DC lines， the voltage of sound phase arm capacitance is raised by charging. The conditions and factors to cause and influence the charging process are presented. Then a new protection strategy to prevent mistake bypass of sub-module because of overvoltage in blocking state is proposed. Finally， simulation analysis is performed on a PSCAD model of a three-terminal MMC-HVDC system and the boosting mechanism of submodule charging in blocking state is fully verified. The proposed new protection strategy can prevent mistaken bypass of sub-module in blocking state and reduce maintenance workload of MMC.

Aiming at proposing improvement to the protection strategy of sub-module overvoltage bypass according to the condition that sub-module capacitors are charged in modular multilevel converter （MMC） blocking state， the phenomena that the voltage magnitude of sound phase is raised are analyzed when a grounding fault occurs at valve side and MMC is blocked in an MMC-HVDC symmetrical monopolar bipolar system. Because there are distributed capacitances in DC lines， the voltage of sound phase arm capacitance is raised by charging. The conditions and factors to cause and influence the charging process are presented. Then a new protection strategy to prevent mistake bypass of sub-module because of overvoltage in blocking state is proposed. Finally， simulation analysis is performed on a PSCAD model of a three-terminal MMC-HVDC system and the boosting mechanism of submodule charging in blocking state is fully verified. The proposed new protection strategy can prevent mistaken bypass of sub-module in blocking state and reduce maintenance workload of MMC.

Seismic performance is meaningful for high voltage disconnect switch， which is vulnerable to the earthquake for its long span， great height， and flexibility. In this paper， a full-scale shake table test on 800 kV three-post disconnect switch is carried out to obtain its dynamical properties and seismic responses laws. The effect on disconnect switch’s posts caused by the main blade is studied in different direction via characteristics of acceleration amplification factor and the spectrum analysis method. The results indicate the space lattice tilted structure form can limit the adverse impacts on seismic responses induced by the height and flexibility of the equipment. Meanwhile， after the main blade closed， a certain constraint effect will occur in the along-span direction， while in the across-span direction constraint effect is weak. The constraint effect can amplify and reduce the seismic responses of the post with blade and the post with fixed contact respectively.

Seismic performance is meaningful for high voltage disconnect switch， which is vulnerable to the earthquake for its long span， great height， and flexibility. In this paper， a full-scale shake table test on 800 kV three-post disconnect switch is carried out to obtain its dynamical properties and seismic responses laws. The effect on disconnect switch’s posts caused by the main blade is studied in different direction via characteristics of acceleration amplification factor and the spectrum analysis method. The results indicate the space lattice tilted structure form can limit the adverse impacts on seismic responses induced by the height and flexibility of the equipment. Meanwhile， after the main blade closed， a certain constraint effect will occur in the along-span direction， while in the across-span direction constraint effect is weak. The constraint effect can amplify and reduce the seismic responses of the post with blade and the post with fixed contact respectively.

Carbon fiber composite core has a strong axial tensile capacity， while the properties of radial shear resistance， compressive capacity and splitting resistance are weak. In the process of line construction， such as wire crimping and conductor clamping by grip jaw， radial pressure failure is easy to occur， which may cause security risks in transmission line construction and safe operation. At present， there is little research on the radial pressure of carbon fiber composite core. The radial pressure tests of carbon fiber composite core of different diameters are carried out to get the failure pressure of carbon fiber composite core test samples. With ABAQUS software the radial pressure simulation model is established. According to the failure pressure data of carbon fiber composite core in the radial pressure test， the vertical radial force is applied to the pressure block. The radial pressure maximum value of carbon fiber composite core of different diameter is obtained by numerical calculation of finite element software. The results show that the situation of failure of carbon fiber composite core is the outermost glass fiber layer flattened causing the test sample to split integrally. The radial compressive allowable stress of carbon fiber composite core with different diameters increases with the increase of the diameter. The research results provide a reference for the design and application of construction machines and tools for carbon fiber composite core of transmission line.

Carbon fiber composite core has a strong axial tensile capacity， while the properties of radial shear resistance， compressive capacity and splitting resistance are weak. In the process of line construction， such as wire crimping and conductor clamping by grip jaw， radial pressure failure is easy to occur， which may cause security risks in transmission line construction and safe operation. At present， there is little research on the radial pressure of carbon fiber composite core. The radial pressure tests of carbon fiber composite core of different diameters are carried out to get the failure pressure of carbon fiber composite core test samples. With ABAQUS software the radial pressure simulation model is established. According to the failure pressure data of carbon fiber composite core in the radial pressure test， the vertical radial force is applied to the pressure block. The radial pressure maximum value of carbon fiber composite core of different diameter is obtained by numerical calculation of finite element software. The results show that the situation of failure of carbon fiber composite core is the outermost glass fiber layer flattened causing the test sample to split integrally. The radial compressive allowable stress of carbon fiber composite core with different diameters increases with the increase of the diameter. The research results provide a reference for the design and application of construction machines and tools for carbon fiber composite core of transmission line.

Three-phase un-synchronization time is an important parameter to reflect the mechanical characteristics of circuit breakers. For the shortage that the traditional detection method can only be carried out under power-off conditions， A new ultrahigh frequency （UHF） method for detecting non-synchronous characteristics of circuit breakers is proposed， the detection principle of which is introduced， the experimental model is established in the laboratory， and the feasibility is studied and measurement is conducted on the spot. The results show that the UHF method can realize the non-contact detection for un-synchronization time of circuit breakers. The research results have the value of further popularization and application.

Three-phase un-synchronization time is an important parameter to reflect the mechanical characteristics of circuit breakers. For the shortage that the traditional detection method can only be carried out under power-off conditions， A new ultrahigh frequency （UHF） method for detecting non-synchronous characteristics of circuit breakers is proposed， the detection principle of which is introduced， the experimental model is established in the laboratory， and the feasibility is studied and measurement is conducted on the spot. The results show that the UHF method can realize the non-contact detection for un-synchronization time of circuit breakers. The research results have the value of further popularization and application.

Icings on transmission lines occur frequently in China， which is a great threat to the safe and stable operation of transmission lines. Taking an actual ±500 kV strain tower as an example， a real-time warning method for the failure of strain tower under icing condition based on insulator string tension measurement system and finite element method （FEM） is proposed. An angle theoretical calculation method for insulator string is put forward， and the reliability of this method is verified by comparing the calculated results with measured data. The weak points of the tower under different icing condition are analyzed and the stress distribution of tower and critical failure curve is given. It can be seen from the curve that if the large side tension is too large or too small while the small side tension is certain， the tower may fail due to over-large unbalanced tension. Failure analysis of tower under the unbalanced de-icing condition is done at last. Combining with practical application effectively， this paper has certain guiding significance for the study on the failure warning of the strain tower under icing condition of small wind speed area.

Icings on transmission lines occur frequently in China， which is a great threat to the safe and stable operation of transmission lines. Taking an actual ±500 kV strain tower as an example， a real-time warning method for the failure of strain tower under icing condition based on insulator string tension measurement system and finite element method （FEM） is proposed. An angle theoretical calculation method for insulator string is put forward， and the reliability of this method is verified by comparing the calculated results with measured data. The weak points of the tower under different icing condition are analyzed and the stress distribution of tower and critical failure curve is given. It can be seen from the curve that if the large side tension is too large or too small while the small side tension is certain， the tower may fail due to over-large unbalanced tension. Failure analysis of tower under the unbalanced de-icing condition is done at last. Combining with practical application effectively， this paper has certain guiding significance for the study on the failure warning of the strain tower under icing condition of small wind speed area.

The load capacity of the operating cable is mainly affected by the thermal parameters of surrounding soil， such as thermal resistance coefficient， specific heat capacity， and ambient temperature. However， at present， little research has been done on the thermal characteristics of the soil around cables， and the influence of rainfall is seldom considered. Guangzhou Wushan area is taken as a typical representative， its soil temperature， thermal resistance coefficient and specific heat capacity are measured， and the effects of rainfall on soil thermal parameters are studied. Based on the dynamic load capacity forecasting model of transmission cable and the correlational studies of soil thermal parameters， the emergency load capacity of cables in different initial conditions is calculated. Finally， the influence of rainfall and its duration on the emergency load capacity of cables is analyzed. The research results can provide a reference to the studies of short-term emergency load capacity of cable and cable heating during short-circuit fault.

The load capacity of the operating cable is mainly affected by the thermal parameters of surrounding soil， such as thermal resistance coefficient， specific heat capacity， and ambient temperature. However， at present， little research has been done on the thermal characteristics of the soil around cables， and the influence of rainfall is seldom considered. Guangzhou Wushan area is taken as a typical representative， its soil temperature， thermal resistance coefficient and specific heat capacity are measured， and the effects of rainfall on soil thermal parameters are studied. Based on the dynamic load capacity forecasting model of transmission cable and the correlational studies of soil thermal parameters， the emergency load capacity of cables in different initial conditions is calculated. Finally， the influence of rainfall and its duration on the emergency load capacity of cables is analyzed. The research results can provide a reference to the studies of short-term emergency load capacity of cable and cable heating during short-circuit fault.

In recent years， the renewable energy in Northwest Power Grid has experienced fast development， and the HVDC has been sending increasing power to the rest of the country. Due to the fact that the renewable generation units do not have the ability to participate in the primary frequency regulation， the frequency control of large power grid has become an increasingly prominent predicament. It is therefore in desperate need to have renewable generation units participating in the frequency regulation. This paper introduces the typical schemes of the wind farms participating in the system frequency control as tested in the Northwest Power Grid. According to the field tests and the 2016 Northwest Power Grid frequency characteristics tests， this is the first time in our country to conduct a systematic actual measurement and analysis of the fast frequency response capacity of wind farms. The results show that wind farms are able to participate in the fast frequency response of power grid， and they have similar response capability compared to conventional generators. Therefore， it is very promising to implement the control scheme.

In recent years， the renewable energy in Northwest Power Grid has experienced fast development， and the HVDC has been sending increasing power to the rest of the country. Due to the fact that the renewable generation units do not have the ability to participate in the primary frequency regulation， the frequency control of large power grid has become an increasingly prominent predicament. It is therefore in desperate need to have renewable generation units participating in the frequency regulation. This paper introduces the typical schemes of the wind farms participating in the system frequency control as tested in the Northwest Power Grid. According to the field tests and the 2016 Northwest Power Grid frequency characteristics tests， this is the first time in our country to conduct a systematic actual measurement and analysis of the fast frequency response capacity of wind farms. The results show that wind farms are able to participate in the fast frequency response of power grid， and they have similar response capability compared to conventional generators. Therefore， it is very promising to implement the control scheme.

With the large-scale application of new power electronic devices in power grid， the risk of harmonic resonance is higher and higher. However， the design of trip criterion of security and stability control （SSC） device in the past seldom considers the new harmonic condition. Combined with the actual harmonic resonance happened in CSG， this paper carries out the study of harmonic impact on SSC device. According to defects found in studies that SSC device can’t effectively filter out specific frequency of high order harmonic， sampling algorithm of the device is analyzed theoretically， and harmonic impact of no-fault line trip criterion is also analyzed under existing conditions. Combined with the actual SSC devices in CSG， a temporary measure to deal with the risk of refusing action of SSC device is put forward. Also an ultimate optimization method of SSC software and hardware is proposed and tested on the SSC test platform. Test results verify the effectiveness of the proposed method， and meanwhile the method has important significance to the design and improvement of other SSC devices.

With the large-scale application of new power electronic devices in power grid， the risk of harmonic resonance is higher and higher. However， the design of trip criterion of security and stability control （SSC） device in the past seldom considers the new harmonic condition. Combined with the actual harmonic resonance happened in CSG， this paper carries out the study of harmonic impact on SSC device. According to defects found in studies that SSC device can’t effectively filter out specific frequency of high order harmonic， sampling algorithm of the device is analyzed theoretically， and harmonic impact of no-fault line trip criterion is also analyzed under existing conditions. Combined with the actual SSC devices in CSG， a temporary measure to deal with the risk of refusing action of SSC device is put forward. Also an ultimate optimization method of SSC software and hardware is proposed and tested on the SSC test platform. Test results verify the effectiveness of the proposed method， and meanwhile the method has important significance to the design and improvement of other SSC devices.

With the integration of a large number of the intermittent distributed generators such as wind turbine generators（WTG） into distributed system， the influences brought by random and correlated variables have to be taken into consideration in the reactive power optimization of distribution network. Firstly， rank correlation coefficient is used to represent the the correlations between wind speed and load demand， and the initial correlated samples are generated by means of Latin hypercube sampling（LHS） techniques. In addition， the simultaneous backward reduction is applied to reduce the scenarios to improve the computation efficiency. Secondly， a reactive power optimization model based on multiple scenarios is proposed， considering the volt/var control devices such as shunt capacitor（SC）， static var compensation （SVC）， on-load tap changer（OLTC）. And then， second order cone（SOC） relaxation is applied to transform the traditional reactive power optimization model to mixed integer second order cone programming（MISOCP） model， which can be tractably solved. Finally， the rationality of the proposed model is verified by the improved IEEE 33 node distribution network， and the effects of correlations on the reactive power optimization are emphatically studied.

With the integration of a large number of the intermittent distributed generators such as wind turbine generators（WTG） into distributed system， the influences brought by random and correlated variables have to be taken into consideration in the reactive power optimization of distribution network. Firstly， rank correlation coefficient is used to represent the the correlations between wind speed and load demand， and the initial correlated samples are generated by means of Latin hypercube sampling（LHS） techniques. In addition， the simultaneous backward reduction is applied to reduce the scenarios to improve the computation efficiency. Secondly， a reactive power optimization model based on multiple scenarios is proposed， considering the volt/var control devices such as shunt capacitor（SC）， static var compensation （SVC）， on-load tap changer（OLTC）. And then， second order cone（SOC） relaxation is applied to transform the traditional reactive power optimization model to mixed integer second order cone programming（MISOCP） model， which can be tractably solved. Finally， the rationality of the proposed model is verified by the improved IEEE 33 node distribution network， and the effects of correlations on the reactive power optimization are emphatically studied.

In order to improve the efficiency of voltage stability assessment （VSA） for large power grids， a three-stage progressive contingency screening and ranking method is proposed. In stage 1， the key contingencies are identified from an initial list of contingencies by defining the importance index of the branches. In stage 2， the critical contingencies are identified from the obtained key contingency set by combining sensitivity-based load margin estimation method and the minimum singular value method. In stage 3， the unstable contingencies are identified and ranked with contingency parameterization continuation power flow. Other critical contingencies are ranked according to their load margins obtained by using curve fitting method. The proposed method is successfully applied to an off-line VSA tool in CSG. The results show that it can be effectively applied in the practical large-scale power systems.

In order to improve the efficiency of voltage stability assessment （VSA） for large power grids， a three-stage progressive contingency screening and ranking method is proposed. In stage 1， the key contingencies are identified from an initial list of contingencies by defining the importance index of the branches. In stage 2， the critical contingencies are identified from the obtained key contingency set by combining sensitivity-based load margin estimation method and the minimum singular value method. In stage 3， the unstable contingencies are identified and ranked with contingency parameterization continuation power flow. Other critical contingencies are ranked according to their load margins obtained by using curve fitting method. The proposed method is successfully applied to an off-line VSA tool in CSG. The results show that it can be effectively applied in the practical large-scale power systems.

In order to improve the consumption level of intermittent new energy in active distribution network （ADN）， this paper presents an economic optimization model for ADN energy dispatching based on multi-agent dynamic topology reconfiguration. The model takes the minimum reconstruction power loss， the maximum utilization efficiency of distribution new energy sources and the maximum profit of distribution companies as the decision-making conditions， and the economic maximization is achieved through the energy scheduling of each link in ADN. ADN energy dispatching is implemented by three-layer time-division and space-distribution control strategies using multi-agent system （MAS）. The chaotic quantum particle swarm optimization algorithm is used to solve the ADN energy optimal dispatching equations to realize the benefit optimization from the individual to the whole. Finally， taking four typical scenes of IEEE 69 node distribution network as examples， ADN energy dispatching is simulated. The effectiveness of the model is verified while the economic optimal solution is obtained.

In order to improve the consumption level of intermittent new energy in active distribution network （ADN）， this paper presents an economic optimization model for ADN energy dispatching based on multi-agent dynamic topology reconfiguration. The model takes the minimum reconstruction power loss， the maximum utilization efficiency of distribution new energy sources and the maximum profit of distribution companies as the decision-making conditions， and the economic maximization is achieved through the energy scheduling of each link in ADN. ADN energy dispatching is implemented by three-layer time-division and space-distribution control strategies using multi-agent system （MAS）. The chaotic quantum particle swarm optimization algorithm is used to solve the ADN energy optimal dispatching equations to realize the benefit optimization from the individual to the whole. Finally， taking four typical scenes of IEEE 69 node distribution network as examples， ADN energy dispatching is simulated. The effectiveness of the model is verified while the economic optimal solution is obtained.

The sub-synchronous oscillation （SSO） incidents caused by the dynamic interactions between wind farms and the rest of power system threaten the safe and reliable operation of practical power grids. Firstly， this paper introduces several incidents occurred in practical power systems caused by wind farms， and several key factors of wind turbine types， mechanism， and the consequences are described. Secondly， the paper illustrates the commonly used analysis methods which are frequency-domain method （complex torque coefficient analysis and impedance-based method） and model analysis method for analyzing SSO caused by grid-connected wind farms in terms of principles， applicability， and characteristics. Thirdly， the paper summarizes the researches which have been carried out in the area of SSO caused by grid-connected wind farms on the basis of the analysis methods and the object system types adopted in the literature. In addition， the current research status of mechanism investigation on the issue of SSO caused by grid-connected wind farms has been demonstrated. Finally， the analysis methods and issues to be addressed urgently for mechanism study of SSO caused by grid-connected wind farms are discussed.

The sub-synchronous oscillation （SSO） incidents caused by the dynamic interactions between wind farms and the rest of power system threaten the safe and reliable operation of practical power grids. Firstly， this paper introduces several incidents occurred in practical power systems caused by wind farms， and several key factors of wind turbine types， mechanism， and the consequences are described. Secondly， the paper illustrates the commonly used analysis methods which are frequency-domain method （complex torque coefficient analysis and impedance-based method） and model analysis method for analyzing SSO caused by grid-connected wind farms in terms of principles， applicability， and characteristics. Thirdly， the paper summarizes the researches which have been carried out in the area of SSO caused by grid-connected wind farms on the basis of the analysis methods and the object system types adopted in the literature. In addition， the current research status of mechanism investigation on the issue of SSO caused by grid-connected wind farms has been demonstrated. Finally， the analysis methods and issues to be addressed urgently for mechanism study of SSO caused by grid-connected wind farms are discussed.