Transient Damping of Virtual Synchronous Generator for Enhancing Synchronization Stability during Voltage Dips

Virtual synchronous generators(VSGs)are widely introduced to the renewable power generation,the variablespeed pumped storage units,and so on,as a promising gridforming solution.It is noted that VSGs can provide virtual inertia for frequency support,but the larger inertia would worsen the synchronization stability,referring to keeping synchronization with the grid during voltage dips.Thus,this paper presents a transient damping method of VSGs for enhancing the synchronization stability during voltage dips.It is revealed that the loss of synchronization(LOS)of VSGs always accompanies with the positive frequency deviation and the damping is the key factor to remove LOS when the equilibrium point exists.In order to enhance synchronization stability during voltage dips,the transient damping is proposed,which is generated by the frequency deviation in active power loop.Additionally,the proposed method can realize seamless switching between normal state and grid fault.Moreover,detailed control design for transient damping gain is given to ensure the synchronization stability under different inertia requirements during voltage dips.Finally,the experimental results are presented to validate the analysis and the effectiveness of the improved transient damping method.

Cost Estimation of Voltage Dips in Small Industries Based on Equipment Sensitivity Analysis

Voltage dip is one of the detrimental power quality problems that can lead to huge financial losses in industries. Its economic impact is not only associated with the quality of the supply but also with sensitivity of electronic controls and equipment of the industry which are susceptible to voltage dips. Mitigating solutions are available but the choice depends on the careful assessment of the economic impact of voltage dips and economic gains of solutions. This paper presents an approach for estimating the economic cost of voltage dips based on sensitivity analysis. The voltage-tolerance curves of the sensitive equipment are obtained from experimental tests under different conditions. From the behavior and interaction of process equipment, different failure modes and economic sensitivity density are determined for different types of voltage dips. Voltage events monitored in the MV-network for several years are assessed to determine the frequency and severity of voltage dips at the customer terminal. The economic values of equipment and processes are assessed to get insight into alternative solutions with more rewarding measures. Then, cost-benefit analysis is performed to compare the economic gains of solutions protecting equipment or processes showing more rewarding economic values.

A direct power control of a DFIG based-WECS during symmetrical voltage dips

The Wind Energy Conversion System(WECS)based Doubly Fed Induction Generator(DFIG)has experienced a rapid development in the world,which leads to an increasing insertion of this source of energy in the electrical grids.The sudden and temporary drop of voltage at the network can affect the operation of the DFIG;the voltage dips produce high peak currents on the stator and rotor circuits,without protection,the rotor side converter(RSC)will suffer also from over-current limit,consequently,the RSC may even be destroyed and the generator be damaged.In this paper a new Direct Power Control(DPC)method was developed,in order to control the stator powers and help the operation of the aero-generator during the faults grid;by injecting the reactive power into the network to contribute to the return of voltage,and set the active power to the optimum value to suppress the high peak currents.The DPC method was designed using the nonlinear Backstepping(BS)controller associated with the Lyapunov function to ensure the stability and robustness of the system.A comparison study was undertaken to verify the robustness and effectiveness of the DPC-BS to that of the classical vector control(VC)using Proportional-Integral(PI)correctors.All were simulated under the Simulink®software.

Realization of Unified Power Quality Conditioner for Mitigating All Voltage Collapse Issues

This paper proposes about a powerful control mechanism of UPQC (Unified Power Quality Conditioner) work on voltage source inverter which can effectively compensate source current harmonics and also mitigate all voltage collapse such as dip, swell, voltage unbalances and harmonics. The consolidation of series and parallel active power filters sharing mutual DC bus capacitor forms UPQC. PI (Proportional Integral) controller is mainly used in order to maintain continual DC voltage along with the hysteresis current controller. The parallel and series power filters were designed using 3-phase voltage source inverter. The reference signals for shunt and series active power filters were obtained by Synchronous Reference Frame (SRF) theory and Power Reactive (PQ) theory respectively. By using these theories, reference signals were obtained which was fed to the controllers for generating switching pulses for parallel and series active filters. The UPQC dynamic performance is obtained through testing terms like the compensation of voltage, current harmonics and all voltage distortion associated with 3-phase 3-wire power system which is simulated using MATLAB-Simulink software.

Influence of the Inverse-Time Protection Relays on the Voltage Dip Index

The probability-assessment analyses on the characteristic value of voltage dip by using Monte Carlo stochastic modeling method to stimulate the randomness of the short circuit fault are introduced. Using Matlab and Power Systems CAD(PSCAD), we design control interface which combines the electromagnetic transient simulation with the Monte Carlo method. Specifically, the designing of interface which is meant to employ the method of Matlab programming to control the electromagnetic transients including direct current(EMTDC)simulation is introduced. Furthermore, the influences of the protection devices on the voltage dip to ensure the authenticity and the referential reliability are simulated. A system with the inverse-time protection devices equipped on each line which can coordinate together is designed to cut off the short-circuit fault. The voltage dip of the designed system is assessed by the pre- and post-system average root mean square(RMS) variation frequency index, and the voltage dip index is compared with the Information Technology Industry Council(ITIC)curves. The simulation results demonstrate that the inverse-time relay protection equipments are well-coordinated,and the severity and the range of the voltage dip are influenced by the cooperation of the equipped inverse-time protection devices.