A double-thimble-type fiber Bragg grating (FBG) temperature sensor that isolates the stress strain is developed, and the three materials of air, grease, and copper thimble are employed for encapsulating. To investigate the effect of different encapsulation materials on the time constant of the sensors under dynamic conditions, the transient heat conduction mathematical model is built according to the lumped heat capacity (LHC) system and thermal equilibrium theory, and the time constant is solved by an analytical solution. Then, a proportional three-dimensional sensor simulation model is established and the transient heat transfer process is numerically solved by the finite element analysis method. To verify the models, an experimental system is established to test the response speed of the three-type sensor and the experimental data are compared with the analytical and numerical solution results. The results show that the dynamic response performance depends on the encapsulation material parameters; the response speed is faster than recovery speed; and the response speed of the air packaging sensor is more than 20% faster than that of the grease packaging sensor, and more than 30% faster than that of the copper packaging sensor. The smaller the heat storage capacity and the larger the heat transfer coefficient, the faster the sensor’s response speed.

The hot-spot temperature of a dry-type air-core reactor is a crucial factor that determines the service life of the reactor. Although the environment elevation has a significant influence on the reactor’s heat dissipation performance and hot-spot temperature, studies seldom focus on the temperature distribution for different altitude scenarios. To this end, this paper proposes a temperature field simulation model with multi-parameter coupling constraints based on the laminar–turbulent flow state. The calculation of the temperature field with the hot-spot temperature at different altitudes is finally achieved. The simulation results show that the hot-spot is located at 6.3% from the top of the sixth encapsulated-winding. It also shows that the hot-spot temperature of the reactor increases by 5 K–11 K with the altitude ascending by 1 km. Moreover, the hot-spot temperature of the reactor exceeds the temperature index at an altitude of 3.25 km, which will result in a shortened service life. Fiber Bragg grating temperature sensors are embedded in encapsulated-windings to detect the temperature for verifying the validity of the temperature field model, which could provide critical temperature rise evaluation rules for the operation safety of the dry-type air-core reactor.

This paper aims at resolving problems of harmonic, reactive power and current imbalance. In the process of photovoltaic grid connection, a multi-objective control strategy is proposed, in which the three-phase network controller simultaneously controls the harmonic and realizes the reactive power and current balance. The [Formula: see text], [Formula: see text] current detection method, which is based on the current reactive power characteristic theory, can detect the compensation current. In order to reduce the measurement error of PLL, a software PLL based on decoupled double synchronous reference coordinate transformation is introduced, and the principle of multi-objective rule of three-stage network inverter is also given. The simulation and test results show that the harmonic distortion rate decreases from 2.57% to 1.10% with the proposed strategy, and the unbalance between reactive power and current is compensated, which verifies the correctness and feasibility of the strategy.