Grid side inverter control scheme for robust fault ride through with enhanced overload capabilities

In this paper a predictive control scheme for current injection into three phase grids with power converters is presented. The proposed control scheme shows an excellent current quality even under adverse grid conditions like unsymmetrical grid faults. By usage of iron powder inductors as filter elements for the grid current a very high overload capability of the inverter can be achieved without oversizing these filter elements. The performance of the proposed control is experimentally verified with a two-level three-phase active front end

Protection of Microgrids: A Scalable and Topology Agnostic Scheme With Self-Healing Dynamic Reconfiguration

Momentum towards realizing the smart grid will continue to result in high penetration of renewable fed Distributed Energy Resources (DERs) in the Electric Power System (EPS). These DERs will most likely be Inverter Based Resources(IBRs) and will be an integral part of the distribution system in the near future. The drive towards resiliency with these IBRs will enable a modular topology where several microgrids are tied together, operating synchronously to form the future EPS at the distribution level. Since the microgrids can evolve from existing distribution feeders, they will be unbalanced in load, phases, and feeder impedances. A typical control strategy of a conventional inverter that follows the grid voltage and frequency while injecting positive-sequence current can lead to undesirable performance for the unbalanced systems, especially in the islanded mode of operation. So, the dissertation will first focus on the control aspect of IBRs in an unbalanced system. Acceptable operating conditions with stability against disturbances and faults are the primary focus. For the proper functioning of these microgrids, there is a need for grid-forming inverters that can enable acceptable performance and coexist with conventional grid-following inverters that supply only positive-sequence currents. In addition to the control objectives, limiting inverter output during faulted or overload conditions with a current limiter is essential. These control objectives can be implemented in both the synchronous reference frame ($dq$ coordinates) and the natural reference frame ($abc$ coordinates). Hence a comparison study is performed to understand the merit of each implementation related to this specific topology. As 100\% IBR-based microgrid becomes an integral part of the distribution system, the issues and challenges arising from its implementation should be addressed for successful operation. Designing reliable protection is one of the significant challenges for microgrids. Most microgrid protection schemes found in published literature suffer from a lack of generality. They work well for the assumed topology, including the type and placement of sources. Other generic protection schemes tend to be too complicated, expensive, or both. To overcome these drawbacks, a topology-agnostic, scalable, and cost-aware protection based on fundamental principles is developed that works in the presence of high penetration of inverter-based resources (IBRs). The protection system includes primary and backup. It also implements stable automatic reconfiguration of the healthy sections of the system after clearance of fault, thus increasing resilience by self-healing. The scheme is validated in PSCAD for primary and backup protection and reconfiguration on the IEEE 123-node feeder in grid-connected and islanded modes with 15 IBRs connected to the system. As the designed protection scheme requires communication between protective devices and the microgrid controller, the method must be validated in real-time with cyber-physical co-simulation for a successful demonstration. In this regard, a Hardware-In-the-Loop (HIL) platform between a simulated power system model using RTDS and physical protective devices is built. In the HIL platform, the primary protection of the scheme is programmed in SEL 421-7 relay, and backup protection is programmed in MATLAB on a generic computer acting as a microgrid controller. The IEC 61850 models are used to communicate between the SEL-421-7 relay and RTDS, whereas TCP/IP communication connects the microgrid controller to RTDS. The focus of the work is to demonstrate the co-simulation platform with communication links established using both protocols and validate the proposed scheme in real-time on the IEEE 123 node distribution feeder. The IEC 61850 and TCP/IP communications configuration are discussed as the interface requires proper hardware and software setup. The real-time performance indicates the Hardware In the Loop (HIL) framework as a competent testing environment for the developed protection scheme for microgrids. In summary, a scalable and topology agnostic protection scheme with self-healing dynamic reconfiguration is developed for microgrids. Clear guidelines for implementation of the proposed scheme on any microgrid topology are also described

Advanced control methods on three-phase inverters in distributed energy resources

“This research is an endeavor to apply new and well-established control methodologies to improve transient response, stability and reliability of three-phase inverters in grid-connected and isolated mode of operation. In the course of studying the effect of these methodologies, model-based control is introduced and is extensively applied which is relatively a new approach. In addition, the application of this concept has been studied on developing “grid-forming” controls to allow wind and solar inverters to support voltage and frequency levels like traditional generators. This research encloses the details of three major works of this research and their possible contributions on improving the performance of three-phase inverters in gridconnected and isolated mode of operation. The first one employs the concept of adaptive control using multiple models and a hierarchical control approach to smoothly switch between isolated and grid-connected modes of operation. In the second work, the features of the first research work have been applied and more nourished to control a grid-forming unit. The interactions of this grid-supporting converter with a grid- forming unit is the main subject of discussion in this work. The last work applies the concept of internal-model control to introduce a new control methodology in power-synchronization method. This approach has tackled the non-minimum phase issue attributed to power-synchronization methodology and offers a robust solution. Furthermore, in this research, detailed stability analysis of all the proposed control structures have been presented. Along with all simulation verification, FPGA-Based Hardware-in-the-Loop (HIL) has been utilized to verify the performance of the discrete control structure. The details of plant modeling, controller design, HIL and experimental results are presented for all of the proposed schemes in each section”--Abstract, page iv

Requirements to Testing of Power System Services Provided by DER Units

The present report forms the Project Deliverable ‘D 2.2’ of the DERlab NoE project, supported by the EC under Contract No. SES6-CT-518299 NoE DERlab. The present document discuss the power system services that may be provided from DER units and the related methods to test the services actually provided, both at component level and at system level

Enhancement of fault current contribution from inverter-based resources

The reduction in levels of fault current infeed as inverter-based resources (IBR) displace synchronous machines undermines the ability of a conventional protection system to identify and isolate faults in an effective manner and is therefore a concern for system operators (SOs). This observation provided the motivation to investigate the limitations of IBRs when injecting fault current and to explore how these limitations might be overcome. This thesis investigates techniques aimed at significantly increasing Fault Current Contribution (FCC) from an IBR system so that renewable energy resources can continue to be deployed without compromising the protection system. The techniques for enhancing FCC are at three different levels of an IBR system: at semiconductor or device level, circuit level and system level. The first study uses phase change materials (PCM) to provide a short-term overload rating to insulated-gate bipolar transistors (IGBTs) and found them to have very limited potential to provide FCC. A Finite Element Analysis (FEA) of heat-flow concluded that, although the PCM was useful for dealing with short over-load currents, it was unsuitable for facilitating large fault currents of several times normal load current. The view was that if the fault current cannot be created at device level through better thermal management, then a circuit level innovation would be required. The second study investigates series/parallel switching of submodules in modular converters. This takes advantage of the fact that during a fault, the line voltage is reduced, and if it falls below 0.5 pu then half of the sub-modules (SMs) can be put into parallel with the other half to double the FCC (2 pu) at half the voltage (0.5 pu). Similarly, if the voltage drops below 0.25 pu, parallel connection of four groups of SMs would enable 4 pu current capability. A model of a static synchronous compensator (STATCOM) was developed, inspired by the alternate arm converter (AAC), with the director switch of the AAC used as part of the reconfiguration circuit. The conclusion of this study was that the penalty paid in power losses in the additional semiconductor devices used for reconfiguration is reasonable for the 2 pu FCC case but not at the 4 pu FCC case. The third study was based on circuit reconfiguration but beyond the converter itself and in this case the windings of the coupling transformer of a STATCOM. Sections of winding were switched using thyristors to tap-change the transformer by a large factor. Using the proposed thyristor-based electronic tap-changer (eTC), the number of turns of the grid-side winding was reduced during a voltage dip, so that larger current can be delivered to the network for the same converter current. The STATCOM was controlled in the natural frame (abc frame) and this control is used to actively drive the currents in the tap-changer thyristors to zero when needed so that they can be commuted rapidly. The transformer was configured to give a normal ratio of 1:4 and be able to tap-down to 1:2 and 1:1 to increase FCC to 2 pu or 4 pu. Theoretical analysis of, and operating principles for, the proposed eTC, together with their associated control schemes, are verified by time-domain simulation at full-scale. The case-study circuit demonstrates delivery of substantial fault current contribution (FCC) of up to 4 pu at the point of common coupling (PCC) in less than half a cycle (10 ms) after detection of three- and single-phase faults. The results demonstrate that the proposed eTC is a good candidate for the enhancement of fault current from IBR systems that employ coupling transformers, allowing them thereby to make a contribution to future electricity networks dominated by IBR.Open Acces

Grid-connected converter with grid-forming and grid-following modes presenting symmetrical and asymmetrical fault ride-throug capability

Grid-connected converters must remain coupled to the grid during a fault. They have to control the reactive currents injected to the grid because of its limited overload capacity. In particular, grid-forming (GFM) converters ride through the fault either preserving GFM operation or switching from GFM to grid-following (GFL) mode. In the GFM mode, the current cannot be controlled, and therefore, mechanisms as virtual impedances become necessary. This article proposes a converter with GFM and GFL capabilities that demonstrates effective voltage tracking in the GFM mode and current tracking in the GFL mode, good dynamic disturbance rejection response, and wide stability range and robustness in both modes. During a fault, the proposed converter switches from GFM to GFL mode in order to precisely control the injected currents. The current control is able to limit the current at the inception of severe faults. This characteristic is preserved for a wide range of grid impedances, thanks to its good robustness and wide stability margin. As a result, the controller can operate in a stand-alone mode, connected to a weak grid or connected to a stiff grid with a short-circuit ratio of up to 50. The fault ride-through capability of the converter is tested under severe symmetrical and asymmetrical faults.Xunta de Galicia | Ref. ED431B2023/12Agencia Estatal de Investigación | Ref. PID2022-136908OB-I0