A Less Intrusive Solution To Stablize VSC Transmission Against Highly Variable Grid Strength

A less-intrusive solution to stabilize a Voltage Source Converter (VSC) over an unknown grid strength is presented in this paper. The existence of equilibrium point is investigated as a prerequisite to stabilization. By partially imposing grid forming control, a simple auxiliary outer loop is proposed to exhaust the physical limit of power delivery in steady state and provide support to fault-ride-through operations over a wide range of grid strength. The proposed control can be used to upgrade a commissioned VSC with inner current loop intact; it also offers a non-intrusive solution to stabilize VSCs externally. The effectiveness of the proposed approach and schemes are verified by analysis in frequency domain and case studies in time domain including change of grid strength and fault-ride-through

Improving the Transient Stability of the Virtual Synchronous Generator

The majority of the Distributed Energy Resources (DERs), i.e., Energy Storage Systems (ESSs) and Renewable Energy Systems (RESs), utilize inverters to convert the Direct Current (DC) power to the Alternating Current (AC) power needed by the majority of the consumers. Proliferation of the inverter-based DERs has caused significant changes in the operation of the modern electric power systems. Inverters lack the mechanical inertia that is inherent in the traditional power generators, i.e., rotating electrical machines. As a result, the emerging inverter-dominated power systems suffer from lower stability margins, excessive frequency deviations, and poor dynamic response to disturbances. This issue has adversely affected the integration of the highly advantageous inverter-based renewable energy systems in microgrids and active distribution systems. Appropriate inverter control can be used to emulate virtual inertia by imitating the behavior of traditional generation units. Based on this idea, the concept of virtual synchronous generator (VSG) has been proposed. VSGs suffer from the transient stability issues that affect the operation of the Synchronous Generators (SGs). They can become unstable due to prolonged faults. Unlike the SGs that can handle significant over-current stress, VSGs have limited overcurrent capacity. The studies conducted in this research indicate that the current limiting strategy of the VSG significantly impacts its transient stability. The impacts of different inverter current limiting strategies on the performance of the VSG are investigated and the one that leads to the largest transient stability margin is identified

Low voltage ride-through strategies for a 3-phase grid-connected PV system

Grid codes is a technical specification which defines the parameters a power system that are connected to the national power systems has to ensure safe, secure and eco-nomic proper functioning of the electric system. One of these requirements is to stay connected to the grid during faults. In such scenarios, the generating unit should remain connected to the grid for a certain period and provide reactive power to support the grid. This is called low voltage ride-through capability. At the early stage, low voltage ride-through requirements were imposed for large scale generators connected to the trans-mission network. However, with the increased penetration of distributed generation, such as PV panels implemented in the distribution network, the low voltage ride-through requirements are also required for distributed generation. With the maturity of PV technology, the cost of PV generation has decreased. Therefore, the total installed capacity of grid-connected PV generation has increased; this has cre-ated new challenges to the low voltage ride-through. Power quality and transient per-formance are the most critical aspects of the grid-connected PV systems under grid faults. PV generation is permitted to switch off from the grid during a fault; however, with the high penetration of the installed PV system, it will degrade the power quality if the same method applied. It is necessary to make sure that the inverter currents remain sinusoidal and within the acceptable limits at the instant of the fault, during and after the fault clearance for different types of faults. Accordingly, this thesis proposes two low voltage ride-through strategies for a 3-phase grid-connected PV system in different reference frames. The presented low voltage ride-through control algorithm in the synchronous reference frame, which fulfils a voltage compensation unit and the reactive power injection block is designed to protect the inverter from overcurrent failure under both symmetrical and asymmetrical faults, reduce the double grid frequency oscillations and provides reac-tive power support by applying a voltage compensation unit. The inverter can also inject sinusoidal current during asymmetrical faults. The method does not require a hard switch from the Maximum Power Point Tracking to a non-Maximum Power Point Tracking algorithm, which ensures a smooth transition. The proposed method in the stationary reference frame provides a fast post-fault recov-ery, which is essential to minimize the fault impacts on the loads and the converter. The method, which consists of a new reference currents calculation block and the voltage compensation unit, maintains the converter current within acceptable limits, produces sinusoidal current even during asymmetrical faults, improves the post-fault recovery performance, and provides independent control for active and reactive powers