Operation and control of voltage source converters in transmission networks for AC system stability enhancement

The rapid expansion in power transmission for the integration of large-scale renewables is foreseen in the future. This will be complemented by infrastructure reinforcements in the form of series compensation and high-voltage direct current (HVDC) links. These changes will bring forth new operability challenges to grid operators. The stability issues pertained to such reinforcements: potential threat of subsynchronous resonance (SSR) and frequency regulation will be investigated in this thesis. Utilising the existing and future voltage source converters (VSC) based HVDC links to support the AC system by proving ancillary services will be of significant importance in the coming decades. The research work presented in this thesis is aimed to address these challenges, in particular, the technical barriers associated with AC/DC interaction and to propose measures to avoid any potential instability. The main contributions of this research work comprise of four parts, namely, (1) analysis of interactions in-terms of SSR in AC/DC grids, (2) design of SSR damping (SSRD) controllers, (3) experimental demonstration of SSRD schemes, and (4) assessment and improvement of frequency regulation in a wind-thermal bundled AC/DC grid. An VSC-HVDC connected series-compensated AC system resembling the Great Britain (GB) power system has been used as the test network to evaluate the operability challenges pertained to the reinforcements. A state-space representation has been formulated and an eigenvalue analysis has been performed to assess the impact of VSC-HVDC on the torsional modes of nearby connected thermal generation plants. This is followed by damping torque investigation for SSR screening with the results compared against time-domain simulations for testing the accuracy of the small-signal models for SSR studies. A series of SSRD schemes is presented which have been integrated with the VSC-HVDC to damp SSR in the series-compensated GB power system. In addition, this thesis proposes an adaptive SSRD method based on the real-time estimation of the subsynchronous frequency v Abstract component present in series-compensated transmission lines–key information for the optimal design of HVDC subsynchronous damping controllers. Furthermore, the combined AC/DC GB network has been implemented in a real-time digital simulator and connected to a VSCHVDC scaled-down test-rig to performhardware-in-the-loop tests. The efficacy and operational performance of the AC/DC network while providing SSR damping is tested through a series of experiments. In order to provide frequency support in a wind-thermal bundled AC/DC system a dualdroop controlmethod is presented. The scheme binds the system frequency with the DC voltage of an HVDC network. For completeness, the performance of the proposed method is compared to conventional frequency regulation schemes. Sensitivity studies and eigenvalue analyses are conducted to assess the impact that wind penetration and changes in the dual-droop coefficient have on grid stability. Experimental validation is performed using a real-time hardware-inthe- loop test-rig, with simulation and experimental results showing a good agreement and evidencing the superior performance of the proposed frequency support scheme

Asset management strategies for power electronic converters in transmission networks: Application to HVdc and FACTS devices

The urgency for an increased capacity boost bounded by enhanced reliability and sustainability through operating cost reduction has become the major objective of electric utilities worldwide. Power electronics have contributed to this goal for decades by providing additional flexibility and controllability to the power systems. Among power electronic based assets, high-voltage dc (HVdc) transmission systems and flexible ac transmission systems (FACTS) controllers have played a substantial role on sustainable grid infrastructure. Recent advancements in power semiconductor devices, in particular in voltage source converter based technology, have facilitated the widespread application of HVdc systems and FACTS devices in transmission networks. Converters with larger power ratings and higher number of switches have been increasingly deployed for bulk power transfer and large scale renewable integration—increasing the need of managing power converter assets optimally and in an efficient way. To this end, this paper reviews the state-of-the-art of asset management strategies in the power industry and indicates the research challenges associated with the management of high power converter assets. Emphasis is made on the following aspects: condition monitoring, maintenance policies, and ageing and failure mechanisms. Within this context, the use of a physics-of-failure based assessment for the life-cycle management of power converter assets is introduced and discussed

Auxiliary dead-band controller for the coordination of fast frequency support from multi-terminal HVDC grids and offshore wind farms

High-voltage direct-current (HVDC) grids may provide fast frequency support to ac grids with the aid of supplementary control algorithms and synthetic inertia contribution from offshore wind farms. However, when all converters within the HVDC grid are fitted with these supplementary controllers, undesirable power flows and reduced power transfers may occur during a power imbalance. This is due to simultaneous frequency oscillations on the different ac systems connected to the HVDC grid arising during the support operation. To prevent this adverse effect, an auxiliary dead-band controller (ADC) is proposed in this paper. The ADC modifies the dead-band set-point of the fast frequency controllers using measurements of rate of change of frequency and frequency deviation. A four-terminal HVDC integrated with an offshore wind farm is modelled to analyse and study the effectiveness of three different supplementary fast frequency control algorithms. Results show that the proposed ADC scheme improves the performance of fast frequency control algorithms. For completeness, a small-signal stability analysis is carried out to confirm that a stable system operation is maintained

Comparisons of MVAC and MVDC systems in dynamic operation, fault protection and post-fault restoration

One of the most significant obstacles preventing the large-scale application of direct-current (DC) technology in medium voltage (MV) distribution networks is their fault protection. The existing AC relay protection needs to be changed or redesigned to protect the future overlay MVAC and MVDC distribution networks. Therefore, a comprehensive understanding of the dynamic and fault behavior and post-fault restoration strategies of MVAC and MVDC systems are critically important. Moreover, a comparison of MVAC and MVDC systems during a fault will also contribute to designing the protection systems of hybrid MV AC/DC systems. In this paper, the challenges of protecting DC faults of MVDC systems and possible solutions are first introduced. Then, the fault characteristics and post-fault restoration of MVDC and MVAC distribution systems are compared and investigated through case studies. Time-domain simulations have been conducted in PSCAD/EMTDC. The work in this paper will be valuable for the protection design for future hybrid MV AC/DC systems

Real-time estimation and damping of SSR in a VSC-HVDC connected series-compensated system

Infrastructure reinforcement using high-voltage direct-current (HVDC) links and series compensation has been proposed to boost the power transmission capacity of existing ac grids. However, deployment of series capacitors may lead to subsynchronous resonance (SSR). Besides providing bulk power transfer, voltage source converter (VSC)-based HVDC links can be effectively used to damp SSR. To this end, this paper presents a method for the real-time estimation of the subsynchronous frequency component present in series-compensated transmission lines-key information required for the optimal design of damping controllers. A state-space representation has been formulated and an eigenvalue analysis has been performed to evaluate the impact of a VSC-HVDC link on the torsional modes of nearby connected thermal generation plants. Furthermore, the series-compensated system has been implemented in a real-time digital simulator and connected to a VSC-HVDC scaled-down test-rig to perform hardware-in-the-loop tests. The efficacy and operational performance of the ac/dc network while providing SSR damping is tested through a series of experiments. The proposed estimation and damping method shows a good performance both in time-domain simulations and laboratory experiments

Power System Loading Margin Enhancement by Optimal STATCOM Integration:a case study

Safe and secure network operation with acceptable voltage level has become a challenging task for utilities requiring corrective measures to be implemented. Network upgrades using Flexible Alternating Current Transmission System devices are being considered to serve this purpose. To this end, static loading margin enhancement by optimal static synchronous compensator (STATCOM) allocation to enhance the power transfer capability with minimal voltage variation is presented. Maximum loadability is formulated as an optimization problem, subjected to voltage and small-signal stability constraints. Stability indices are presented and incorporated with the optimization problem to ensure secure operation under maximum loading. The scheme is executed with the IEEE system and an Indian utility network. Improved voltage regulation with different loading condition was achieved for both test networks, with the service rendered by the optimally placed STATCOM. Moreover, it facilitates an additional 50% capacity release in both test systems for hosting the active power and loads

Influence of active power output and control parameters of full-converter wind farms on sub-synchronous oscillation characteristics in weak grids

Active power outputs of a wind farm connected to a weak power grid greatly affect the stability of grid-connected voltage source converter (VSC) systems. This paper studies the impact of active power outputs and control parameters on the subsynchronous oscillation characteristics of full-converter wind farms connected weak power grids. Eigenvalue and participation factor analysis was performed to identify the dominant oscillation modes of the system under consideration. The impact of active power output and control parameters on the damping characteristics of subsynchronous oscillation is analysed with the eigenvalue method. The analysis shows that when the phase-locked loop (PLL) proportional gain is high, the subsynchronous oscillation damping characteristics are worsened as the active power output increases. On the contrary, when the PLL proportional gain is small, the subsynchronous oscillation damping characteristics are improved as the active power output increases. By adjusting the control parameters in the PLL and DC link voltage controllers, system stability can be improved. Time-domain results verify the analysis and the finding

Studies of commutation failures in Hybrid LCC/MMC HVDC systems

A hybrid of line commutated converters(LCCs) and modular multi-level converters(MMCs) can provide the advantages of both the technologies. However, the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system. In this paper, the system behavior during a commutation failure is investigated. Both halfbridge and full-bridge MMCs are considered. Control strategies are examined through simulations conducted in PSCAD/EMTDC

Experimental validation of dual H-Bridge current flow controllers for meshed HVdc grids

The current carrying capability of dc lines is limited by their thermal and electric stress limits. Thus, the line current must be maintained within the permissible operational region to protect the lines from damages. In a dense dc grid, control over each line current cannot be achieved without including additional control devices. In this paper a dual H-bridge current flow controller (2B-CFC) is used to manage the dc grid line power flow by providing dc voltage compensation in series with dc lines. A centralized hierarchical control system is proposed to coordinate the operation between multiple CFCs. A novel voltage sharing control scheme is demonstrated. It is shown that such scheme reduces the workload on a single CFC by sharing the required control voltage between multiple CFCs, and, additionally, can be used to avoid control conflicts among active CFCs during communication failure. An experimental platform consisting on a 3-terminal dc grid and small scale 2B-CFC prototypes has been developed to validate the concepts. For completeness, the CFC performance has been analyzed for overload conditions and when no communication exists. Small-scale dc circuit breakers have been developed to study the CFC performance under a pole-to-pole fault

Feasibility and reliability analysis of LCC DC grids and LCC/VSC hybrid DC grids

Power system interconnections using high-voltage direct-current (HVDC) technologies between different areas can be an effective solution to enhance system efficiency and reliability. Particularly, the multi-terminal DC grids, that could balance and ensure resource adequacy, increase asset utilization and reduce costs. In this paper, the technical feasibility of building DC grids using the line commutated converter based (LCC) and voltage source converter based (VSC) HVDC technologies are discussed. Apart from presenting the technical challenges of building LCC DC grids and LCC/VSC hybrid DC grids, the reliability modeling and analysis of these DC grids are also presented. First, the detailed reliability model of the modular multi-level converters (MMCs) with series connected high-voltage and low-voltage bridges are developed. The active mode redundancy design is considered for the reliability model. To this end, a comprehensive whole system reliability model of the studied systems is developed. The reliability model of each subsystem is modeled in detail. Various reliability indices are calculated using this whole system reliability model. The impacts of the redundancy design of the MMCs on these indices are presented. The studies of this paper provide useful guidance for DC grid design and reliability analysis