The large-scale application of wind power and photovoltaic power solves the energy crisis and alleviates the environmental problems caused by the use of conventional energy. However, they are at risk of being randomly tripped from the network when faced to voltage sag and severe fault events, which will lead to a sudden reduction of active power output and also complicates fault recovery process of the whole system. Moreover, it may also aggravate failures and lead to large-scale power outages, which stimulates a growing interest in analyzing the low-voltage ride-through (LVRT) capabilities of the renewable energy systems (RES) and improving the performance through developing various mathematical models and analysis tools. In this paper, a systematical overview of cause, classification of voltage sag phenomena and voltage sag emulating techniques is presented, and four voltage sag generators (VSGs) are discussed and compared, which include generator based-VSG, shunt impedance based-VSG, transformer based-VSG and full converter based-VSG. Furthermore, a closed-loop detection platform based on real-time digital simulator (RTDS) for the converter controller of a permanent magnet synchronous generator (PWSG) set is introduced, to investigate the LVRT performance of the WT system under grid voltage sag conditions. Finally, the application of VSG in RES are presented and the future research directions are also discussed.

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Cause, Classification of Voltage Sag, and Voltage Sag Emulators and Applications: A Comprehensive Overview

This paper presents an analytical approach to the analysis of the interaction between induction motors (IMs) and voltage sags. The presented methodology enables quick assessment of the influence of IM on sag characteristics and avoids time-consuming transient simulations. The accuracy of the derived analytical formulae is verified through classical transient simulations using the PSCAD/EMTDC package. This paper further proposes a simple, yet efficient transformation of resulting nonrectangular voltage sags, based on the loss of voltage index, into equivalent rectangular sags suitable for conventional voltage sag performance benchmarking.

The Influence of Induction Motors on Voltage Sag Propagation—Part I: Accounting for the Change in Sag Characteristics

This paper describes a transformer-based voltage sag generator (VSG) suitable to evaluate the susceptibility of electrical equipment to voltage sag. The built VSG utilized one auto-transformer and two solid state relays (SSRs) to provide nominal voltage and sag voltage to the load. The switch statuses of two SSRs are controlled by nominal voltage and sag voltage duration signal produced by electronic circuits. The VSG operating result shows that it enables effective control of sag magnitude, duration, beginning and ending points on output voltage wave. If needed, it can work as a voltage swell generator and a voltage interruption generator. By supplying high voltage (HV) transformer from the primary side, the VSG also can provide HV sag, swell, and interruption. The presented VSG is easier to set up in the lab, and the construction cost is much lower than buying VSG products at the current market.

A single-phase voltage sag generator for testing electrical equipments

The uncertainty introduced by intermittent renewable energy generation and prosumer energy imports makes operational planning of renewable energy-assisted prosumer microgrids challenging This is due to the difficulty in obtaining accurate forecasts of the energy expected from these renewable energy sources and prosumers Operators of such microgrids therefore require additional grid-balancing tools to maintain power supply and demand balance during grid operation In this paper, the impact of demand response aggregators (DRAs) in a prosumer microgrid is investigated This is achieved by developing and solving a deterministic mathematical formulation for the operational planning of the grid Also, taking a cue from CAISO's proposed tariff revision which allows the state-of-charge of non-generator resources (like storage units) to be submitted as a bid parameter in the day-ahead market and permits scheduling coordinators of these resources to self-manage their energy limits and state-of-charge, the proposed formulation permits prosumers to submit the battery energy content as a bid parameter and self-manage their battery energy limits Furthermore, a robust counterpart of the model is developed Both formulations are constrained mixed integer optimization problems which are solved using the CPLEX solver in the Advanced Interactive Multidimensional Modelling System environment The results obtained from tests carried out on a hypothetical prosumer microgrid show that the operating cost of the microgrid reduces in the presence of DRAs In addition, the storage facility owner may benefit from self-managing its energy limits, but this may cut the amount of grid-balancing resource available to the microgrid operator, thereby increasing the operating cost of the microgrid

A robust energy and reserve dispatch model for prosumer microgrids incorporating demand response aggregators

A voltage sag generator device (30) for use in an electrical generator machine (2) such as a wind turbine, connected to an electrical network (1) including one to three parallel transformers (3, 4, 5) connected on one side to the wind turbine (2) via a coupling switch (7) and circuit breakers (13, 11, 10), and earthed on the other side via circuit breakers (18, 17, 16) and a control switch (6); means for causing short-circuits when the coupling switch (7) and the control switch (6) are actuated in such a way that a voltage sag having the required duration and type is generated; and means for protecting the transformers (3, 4, 5) during voltage sag generation.

Voltage sag generator device

The paper describes computer simulations used as an initial phase in process of designing a three-phase voltage sag generator. The influence of various parameters, different circuit topologies and different control strategies is investigated and discussed in the paper. The main results and conclusions are illustrated numerically and graphically. The investigation resulted in selection of optimal design and control parameters for voltage sag generator. The PSCAD/EMTDC software environment was used in all computer simulations.

K. A. Charalambous

Papers

Computer simulation of voltage sag generator

3rd IEEE Mediterranean Conference on Power Generation, Transmission, Distribution and Energy Conversion MEDPOWER 2002