Quantifying Voltage Support Capability of LCC-HVDC System

Wind generation is required to provide dynamic voltage support (DVS) during fault ride-through (FRT) to enhance system voltage stability. However, DVS causes the FRT behavior of wind generation to be segmented, and its impact on voltage stability in the wind farm (WF) integrated linecommutated converter-based high-voltage direct current (LCCHVDC) sending-end system remains unclear. To achieve a quantitative analysis to this gap, a novel quasi-static piecewise model for the sending-end system is developed, in which the segmented FRT behavior of WF is fully incorporated. Beyond the well-known saddle-node bifurcation (SNB), a new FRT-induced nonsmooth bifurcation (FRTNB), is identified. The influence of WF and the LCC-HVDC system on both SNB-related and FRTNBrelated voltage stability is thoroughly analyzed. Based on the acquired insights, a control strategy is proposed for the LCCHVDC system to proactively utilize its DVS capability, which is adaptive to the segmented FRT behavior of WF and can effectively enhance system voltage stability without requiring knowledge of grid parameters. Dynamic simulations validate the theoretical analysis and the effectiveness of the proposed method under both voltage sag and swell conditions

With increasing focus on economical as well as environmentally friendly means for mass transit, rail transport is gaining renewed momentum in many parts of the world. This means investing in novel rail infrastructure as well as upgrading and electrifying existing facilities. The TEN (Trans-European Network) program within the European Union is but one current example of the importance attributed to more and better mass transit all over the world. The feeding of heavy rolling stock from AC grids will in many cases cause unacceptable voltage drops as well as unsymmetry between phases of the feeding grid. A traditional way to improve the situation would be to build additional power transmission lines and/or step up the feeding voltage to a higher level. This, however, may be undesirable or even impossible, for cost reasons as well as for environmental reasons. FACTS devices offer means of remedy within the existing feeding grid, restoring grid symmetry as well as voltage stability, and also mitigating harmonic distortion emanating from rolling stock. FACTS devices such as SVC (Static Var Compensator) and STATCOM (Static Compensator) are well proven and commercially available today. The paper treats dynamic load balancing of grids supplying power to rail transport, as well as dynamic voltage support and harmonic mitigation, by means of SVC as well as STATCOM.

Conventional high-voltage direct current (HVDC) transmission systems are widely employed in large-capacity, long-distance power delivery scenarios. However, due to the reliance on thyristor-based line-commutated converter (LCC) lacking self-turn-off capability, commutation failures are likely to occur when the alternating current system is weak or subjected to disturbances. The static synchronous compensator (STATCOM), a shunt-connected reactive power compensation device, can inject or absorb reactive power to support the LCC-HVDC inverter side, thereby mitigating commutation failures. Nevertheless, traditional grid-following STATCOMs exhibit stability limitations under weak grid conditions, as their performance is inherently tied to grid strength. With the rapid integration of renewable energy sources, modern power systems increasingly exhibit "dual-high" characteristics, heightening instability risks for conventional STATCOM. In this study, grid-forming control technology is applied to STATCOM devices. Grid-forming control enhances stability in weak grids by emulating the rotor dynamics of synchronous generators to autonomously regulate voltage and frequency. A simulation platform integrating grid-forming STATCOM and LCC-HVDC systems is established in PSCAD/EMTDC. Case studies demonstrate the superior capability of grid-forming STATCOMs in suppressing commutation failures under weak grid conditions compared to conventional grid-following counterparts.

Issues for reactive power and voltage control pricing in a deregulated environment

This paper addresses issues related to reactive power, voltage support and transmission losses as dictated from a certain class of electric loads. Specifically, it addresses the impact of predominantly induction motor loads on voltage support, reactive power requirements, and transmission losses. These issues are examined with a model which explicitly models the induction motor mechanical load. Simulation results on a simplified electric power system are presented. The results are utilized to propose a pricing structure for voltage and reactive power support. The basic assumption of the paper is that, in a deregulated environment, the expense of the incremental requirements for voltage control should be charged to the member causing the additional requirements. Alternatively, the results can be used to justify long term pricing agreements between suppliers and customers.

In power grid modernisation, optimal network use is essential to preserving acceptable voltage profiles, boosting voltage stability, reducing power losses, and strengthening system security and dependability. This can be accomplished by strategically placing reactive power compensation devices within transmission and distribution networks, such as capacitor banks, synchronous condensers, flexible alternating current transmission system (FACTS) devices and custom power devices. The optimal location and size of these devices are essential for effective investment, but previous research has mostly concentrated on a variety of methods for this goal, using different indices to evaluate power loss, voltage stability, voltage profile and line loadability. Despite these initiatives, there is still a lack of a thorough analysis of how current indices and methodologies are applied to all varieties of reactive power compensation devices. This paper offers a detailed literature review on the ideal placement and sizing of these devices, encompassing analytical, conventional and hybrid-based techniques. It discusses key objectives such as power loss reduction, voltage deviation (VD) mitigation, voltage stability enhancement and improvements in system reliability and security. Additionally, the paper examines the relevance of reactive power for stakeholders, including transmission system operators (TSOs), distribution system operators (DSOs) and power generating companies, and explores the mathematical modelling of optimal reactive power dispatch (ORPD), considering the impact of renewable energy sources (RESs) and the role of FACTS devices.

Comprehensive review of the requirements of AC transmission grid codes and the role of FACTS devices in integrating renewable power plants

Reliable and efficient power systems operation requires voltage control as well as minimal reactive power flow between a load and the grid. To achieve this objective, reactive power compensating devices are typically utilized. In modern power systems, the synchronous static compensator (STATCOM) is primarily used for reactive power control, both in distribution and transmission lines. This study presents the modeling and simulation of a STATCOM based on a voltage source converter. Simulations are performed for both single phase and three phase power systems. Furthermore, two control techniques, utilizing Proportional Integral (PI) as well as Proportional Resonant (PR) controllers are explored. The PR controller presents several advantages compared to the PI controller. Simulation results show that in both cases, the STATCOM, connected in shunt with the load either provides reactive power to the load, or absorbs reactive power from the load. Thus, net reactive power transfer from the grid to the load is zero. The grid provides only active power, thus keeping the power factor to almost unity.

Dynamic voltage support is a type of critical ancillary service in electric power networks. With the increasing penetration of inverter-based renewable energy resources, utilizing their idle capacity to provide dynamic voltage support is becoming essential. However, the performance of traditional control methods is ofter limited due to the frequently changing operating point and network parameters, which are tuned only in a specific condition and suffer performance deterioration in others. This article presents a model-free dynamic voltage control method for inverter-based energy resources to supply dynamic voltage support. This method can dynamically estimate the system with pseudo gradients, in which a continuous parameter function is proposed to adaptively approximate the optimal intermediate parameters. It guarantees the performance when the host system operating states vary without tuning the controller's parameters, while in comparison the PID controllers require the parameters being tuned repeatedly. Both numerical simulation and field tests demonstrate that the proposed method significantly outperforms the classic PI and adaptive PI controllers, especially in the case when the host system varies over time.

The increasing penetration of inverter-based resources (IBRs) is changing grid dynamics and challenging grid planning, operation, and protection. Particularly, the increasing integration of IBRs may drive the power grid towards weak grid conditions, where potential dynamic stability issues may become significant. Recently, it was reported that the unintended loss of solar generation occurred in Southern California over a large geographic area. One of the major reasons for this generation loss is the tripping of solar generators due to the overvoltage in a less than one cycle time frame (i.e., sub-cycle overvoltage) experienced by solar photovoltaic (PV) inverters, especially when solar PV inverters enter the momentary cessation operation mode in response to abnormal grid disturbances. In this paper, the impact of grid strength on sub-cycle dynamics resulting from momentary cessation is investigated in a power system with distributed solar PV integration. In this investigation, distributed solar PVs are modeled with detailed grid-following inverter models considering inverter switching dynamics and momentary cessation function. It is found that undesired sub-cycle overvoltage has a positive relation to grid strength at points of integration (POIs) of solar PVs. At the weak POIs, server sub-cycle overvoltage occurs not only when momentary cessation starts to cease energy injection but also when momentary cessation restarts to inject energy into the system during the restoration process. Furhtermore, the interaction between solar PVs making POIs weak exacerbates the severity of the sub-cycle overvoltage at the POIs. Thus, in the power system with high penetration of solar PVs, it is important to improve grid strength in power system planning and quickly recover grid strength in power system operation following disturbances.

Cybersecurity is crucial for modern power systems due to their high digitization. The open information and communication technology, that is (ICT) being used for the operation of such systems, is highly vulnerable to cyber threats. The adopted smart grid concept around the globe enables the utilization of demand-side for providing ancillary services based on well-known demand response (DR) programs. These programs aggregate smart appliances in homes and electric vehicles (EVs) for providing vital services such as frequency regulation and voltage support. Since the aggregation is based on cyber layer, any cyber threat could affect the ancillary services that are being delivered from the aggregators, which might lead to stability and security issues resulting in brownout or massive blackouts. This chapter discusses the cybersecurity in DR program and shows its importance for modern and future smart power systems due to their stability and security margins. Furthermore, the cyberattack case study is implemented in a power system with demand side program responsible for providing primary frequency support ancillary service, where the results confirm the high vulnerability of modern power systems to cyber threats on DR-active power reserve providers. Moreover, technical suggestions are provided for enhancing the cybersecurity in DR programs in power systems with high power share from renewable energy sources.

The synchronization stability of grid-connected inverters is crucial for the reliable operation of renewable energy system. It is found in this article that under weak grid conditions, as the increase of active power, the inverter is prone to collapse due to phase locking failure when the output reactive power is insufficient. The reason lies in the positive feedback loop in the phase-locked loop. An increase in the active power and a decrease in reactive power can both lead to the expansion of the positive feedback effect and even result in the collapse of the phase-locked loop. A frequency deviation-based feedforward control strategy is proposed in this paper, which can weaken the response of the positive feedback loop and enhance the synchronization stability. Finally, the simulation results verify the theoretical analysis.

Optimal Reactive Power Dispatch (ORPD) is a critical optimization problem in power system operations, aiming to minimize Active Power Loss (APL) and Voltage Deviation (VD), and maintain voltage levels within acceptable limits. This study employs the Artificial Bee Colony (ABC) algorithm to solve the ORPD problem in the IEEE 30-bus system. The ABC algorithm, inspired by the foraging behavior of honeybee colonies, optimizes key control variables such as generator bus voltages, transformer tap settings, and reactive power compensations. The performance of the proposed method is evaluated by comparing it with other metaheuristic approaches, demonstrating its superior ability to reduce power losses and improve voltage profiles. Results indicate that the ABC algorithm significantly decreases APL and enhances voltage stability, outperforming conventional optimization methods. The study confirms the effectiveness and robustness of the ABC algorithm in addressing complex ORPD challenges, making it a viable tool for enhancing power system efficiency and stability.

The reactive power is the background power without which the active power cannot be transmitted in the power systems. In the modern power system, the reactive power pricing is considered as essential in order to maintain the voltage in the transmission line. The modern power system is the grid that functions with smart innovative technological system that provides flexibility, efficiency and availability for the users. The smart grid uses the Internet of Things (IoT) technology to identify and provides the requirement of system reactive power for reactive power pricing. The IoT based wireless communication platform significantly reduces the latency and provides better accuracy. The system requirements for reactive power are optimally dispatched by the generator and other reactive power provider using optimization algorithm like Self Balanced Differential Evolution. Here the analytics and opportunities of smart grid for reactive power service are discussed using 62 bus Indian Utility System (IUS).

Chapter 8 - Reactive power balance control based on flexibility theory