Grid-Forming Converter Control Method to Improve DC-Link Stability in Inverter-Based AC Grids

As renewable energy sources with power-electronic interfaces become functionally and economically viable alternatives to bulk synchronous generators, it becomes vital to understand the behavior of these inverter-interfaced sources in ac grids devoid of any synchronous generation, i.e. inverter-based grids. In these types of grids, the inverters need to operate in parallel in grid-forming mode to regulate and synchronize their output voltage while also delivering the power required by the loads. It is common practice, therefore, to mimic the parallel operation control of the very synchronous generators that these inverter-based sources are meant to replace. This practice, however, is based on impractical assumptions and completely disregards the key differences between synchronous machines and power electronic inverters, as well as the dynamics of the dc source connected to the inverter. This dissertation aims to highlight the shortcomings of conventional controllers and derive an improved grid-forming inverter controller that is effective in parallel ac operation without sacrificing dc-link stability. This dissertation begins with a basis for understanding the control concepts used by grid-forming inverters in ac grids and exploring where existing ideas and methods are lacking in terms of efficient and stable inverter control. The knowledge gained from the literature survey is used to derive the requirements for a grid-forming control method that is appropriate for inverter-based ac grids. This is followed by a review and comparative analysis of the performance of five commonly used control techniques for grid-forming inverters, which reveal that nested loop controllers can have a destabilizing effect under changing grid conditions. This observation is further explored through an impedance-based stability analysis of single-loop and nested-loop controllers in grid-forming inverters, followed by a review of impedance-based analysis methods that can be used to assess the control design for grid-forming inverters. An improved grid-forming inverter controller is proposed with a demonstrated ability to achieve both dc-link and ac output stability with proportional power-sharing. This dissertation ends with a summary of the efforts and contributions as well as ideas for future applications of the proposed controller

Development of Multiport Single Stage Bidirectional Converter for Photovoltaic and Energy Storage Integration

The energy market is on the verge of a paradigm shift as the emergence of renewable energy sources over traditional fossil fuel based energy supply has started to become cost competitive and viable. Unfortunately, most of the attractive renewable sources come with inherent challenges such as: intermittency and unreliability. This is problematic for today\u27s stable, day ahead market based power system. Fortunately, it is well established that energy storage devices can compensate for renewable sources shortcomings. This makes the integration of energy storage with the renewable energy sources, one of the biggest challenges of modern distributed generation solution. This work discusses, the current state of the art of power conversion systems that integrate photovoltaic and battery energy storage systems. It is established that the control of bidirectional power flow to the energy storage device can be improved by optimizing its modulation and control. Traditional multistage conversion systems offers the required power delivery options, but suffers from a rigid power management system, reduced efficiency and increased cost. To solve this problem, a novel three port converter was developed which allows bidirectional power flow between the battery and the load, and unidirectional power flow from the photovoltaic port. The individual two-port portions of the three port converter were optimized in terms of modulation scheme. This leads to optimization of the proposed converter, for all possible power flow modes. In the second stage of the project, the three port converter was improved both in terms of cost and efficiency by proposing an improved topology. The improved three port converter has reduced functionality but is a perfect fit for the targeted microinverter application. The overall control system was designed to achieve improved reference tracking for power management and output AC voltage control. The bidirectional converter and both the proposed three port converters were analyzed theoretically. Finally, experimental prototypes were built to verify their performance

Smart PV Inverter Control for Distribution Systems

PV solar systems employ inverters to transform dc power from solar panels into ac power for injecting into the power grids. Inverters that perform multiple functions in addition to real power production are known as “smart inverters”. This thesis presents a novel control of PV inverter as a dynamic reactive power compensator – STATCOM. This “smart PV inverter” control enables a PV solar inverter to operate in three modes – i) Full PV, ii) Partial STATCOM, and iii) Full STATCOM, depending upon system needs. The novel control is developed and demonstrated for the objectives of a) symmetrical voltage regulation, b) temporary overvoltage reduction, c) power factor correction, and d) reactive power control. In Full PV mode, the inverter performs only real power production based on solar radiation. In Partial STATCOM mode, the controller uses the remaining capacity of the inverter for voltage control, power factor correction and reactive power control. The Full STATCOM mode is invoked in emergency scenarios, such as faults, or severe voltage fluctuations. In this mode, the real power production is shut down temporarily and the entire inverter capacity is utilized for voltage regulation or TOV curtailment for providing critical support to the power system. This thesis presents a comprehensive design of the proposed smart inverter controller with all its associated system components. The performance of the smart inverter is simulated using the electromagnetic transients software PSCAD/EMTDC. It is further validated through Real Time Digital Simulation and Control Hardware in the Loop (CHIL) simulation. Finally the successful performance of the smart inverter controller is demonstrated on a 10 kW inverter in the laboratory on a simulated feeder of Bluewater Power, Sarnia, where this smart inverter is proposed to be installed. The smart PV inverter control is further shown to enhance the connectivity of PV solar farms in a realistic 44 kV Hydro One distribution feeder. It is demonstrated that if such a novel control is implemented on a 10 MW solar farm, the need for the actually installed STATCOM for voltage regulation and TOV control can be either minimized or altogether eliminated, bringing a significant savings for the utility PV solar systems employ inverters to transform dc power from solar panels into ac power for injecting into the power grids. Inverters that perform multiple functions in addition to real power production are known as “smart inverters”. This thesis presents a novel control of PV inverter as a dynamic reactive power compensator – STATCOM. This “smart PV inverter” control enables a PV solar inverter to operate in three modes – i) Full PV, ii) Partial STATCOM, and iii) Full STATCOM, depending upon system needs. The novel control is developed and demonstrated for the objectives of a) symmetrical voltage regulation, b) temporary overvoltage reduction, c) power factor correction, and d) reactive power control. In Full PV mode, the inverter performs only real power production based on solar radiation. In Partial STATCOM mode, the controller uses the remaining capacity of the inverter for voltage control, power factor correction and reactive power control. The Full STATCOM mode is invoked in emergency scenarios, such as faults, or severe voltage fluctuations. In this mode, the real power production is shut down temporarily and the entire inverter capacity is utilized for voltage regulation or TOV curtailment for providing critical support to the power system. This thesis presents a comprehensive design of the proposed smart inverter controller with all its associated system components. The performance of the smart inverter is simulated using the electromagnetic transients software PSCAD/EMTDC. It is further validated through Real Time Digital Simulation and Control Hardware in the Loop (CHIL) simulation. Finally the successful performance of the smart inverter controller is demonstrated on a 10 kW inverter in the laboratory on a simulated feeder of Bluewater Power, Sarnia, where this smart inverter is proposed to be installed. The smart PV inverter control is further shown to enhance the connectivity of PV solar farms in a realistic 44 kV Hydro One distribution feeder. It is demonstrated that if such a novel control is implemented on a 10 MW solar farm, the need for the actually installed STATCOM for voltage regulation and TOV control can be either minimized or altogether eliminated, bringing a significant savings for the utilit

A DC-DC Multiport Converter Based Solid State Transformer Integrating Distributed Generation and Storage

abstract: The development of a Solid State Transformer (SST) that incorporates a DC-DC multiport converter to integrate both photovoltaic (PV) power generation and battery energy storage is presented in this dissertation. The DC-DC stage is based on a quad-active-bridge (QAB) converter which not only provides isolation for the load, but also for the PV and storage. The AC-DC stage is implemented with a pulse-width-modulated (PWM) single phase rectifier. A unified gyrator-based average model is developed for a general multi-active-bridge (MAB) converter controlled through phase-shift modulation (PSM). Expressions to determine the power rating of the MAB ports are also derived. The developed gyrator-based average model is applied to the QAB converter for faster simulations of the proposed SST during the control design process as well for deriving the state-space representation of the plant. Both linear quadratic regulator (LQR) and single-input-single-output (SISO) types of controllers are designed for the DC-DC stage. A novel technique that complements the SISO controller by taking into account the cross-coupling characteristics of the QAB converter is also presented herein. Cascaded SISO controllers are designed for the AC-DC stage. The QAB demanded power is calculated at the QAB controls and then fed into the rectifier controls in order to minimize the effect of the interaction between the two SST stages. The dynamic performance of the designed control loops based on the proposed control strategies are verified through extensive simulation of the SST average and switching models. The experimental results presented herein show that the transient responses for each control strategy match those from the simulations results thus validating them.Dissertation/ThesisPh.D. Electrical Engineering 201

Bidirectional Electric Vehicles Service Integration in Smart Power Grid with Renewable Energy Resources

As electric vehicles (EVs) become more popular, the utility companies are forced to increase power generations in the grid. However, these EVs are capable of providing power to the grid to deliver different grid ancillary services in a concept known as vehicle-to-grid (V2G) and grid-to-vehicle (G2V), in which the EV can serve as a load or source at the same time. These services can provide more benefits when they are integrated with Photovoltaic (PV) generation. The proper modeling, design and control for the power conversion systems that provide the optimum integration among the EVs, PV generations and grid are investigated in this thesis. The coupling between the PV generation and integration bus is accomplished through a unidirectional converter. Precise dynamic and small-signal models for the grid-connected PV power system are developed and utilized to predict the system’s performance during the different operating conditions. An advanced intelligent maximum power point tracker based on fuzzy logic control is developed and designed using a mix between the analytical model and genetic algorithm optimization. The EV is connected to the integration bus through a bidirectional inductive wireless power transfer system (BIWPTS), which allows the EV to be charged and discharged wirelessly during the long-term parking, transient stops and movement. Accurate analytical and physics-based models for the BIWPTS are developed and utilized to forecast its performance, and novel practical limitations for the active and reactive power-flow during G2V and V2G operations are stated. A comparative and assessment analysis for the different compensation topologies in the symmetrical BIWPTS was performed based on analytical, simulation and experimental data. Also, a magnetic design optimization for the double-D power pad based on finite-element analysis is achieved. The nonlinearities in the BIWPTS due to the magnetic material and the high-frequency components are investigated rely on a physics-based co-simulation platform. Also, a novel two-layer predictive power-flow controller that manages the bidirectional power-flow between the EV and grid is developed, implemented and tested. In addition, the feasibility of deploying the quasi-dynamic wireless power transfer technology on the road to charge the EV during the transient stops at the traffic signals is proven

Inter-Microgrid Operation: Power Sharing, Frequency Restoration, Seamless Reconnection and Stability Analysis

Electrification in the rural areas sometimes become very challenging due to area accessibility and economic concern. Standalone Microgrids (MGs) play a very crucial role in these kinds of a rural area where a large power grid is not available. The intermittent nature of distributed energy sources and the load uncertainties can create a power mismatch and can lead to frequency and voltage drop in rural isolated community MG. In order to avoid this, various intelligent load shedding techniques, installation of micro storage systems and coupling of neighbouring MGs can be adopted. Among these, the coupling of neighbouring MGs is the most feasible in the rural area where large grid power is not available. The interconnection of neighbouring MGs has raised concerns about the safety of operation, protection of critical infrastructure, the efficiency of power-sharing and most importantly, stable mode of operation. Many advanced control techniques have been proposed to enhance the load sharing and stability of the microgrid. Droop control is the most commonly used control technique for parallel operation of converters in order to share the load among the MGs. But most of them are in the presence of large grid power, where system voltage and frequency are controlled by the stiff grid. In a rural area, where grid power is not available, the frequency and voltage control become a fundamental issue to be addressed. Moreover, for accurate load sharing a high value of droop gain should be chosen as the R/X ratio of the rural network is very high, which makes the system unstable. Therefore, the choice of droop gains is often a trade-off between power-sharing and stability. In the context, the main focus of this PhD thesis is the fundamental investigations into control techniques of inverter-based standalone neighbouring microgrids for available power sharing. It aims to develop new and improved control techniques to enhance performance and power-sharing reliability of remote standalone Microgrids. In this thesis, a power management-based droop control is proposed for accurate power sharing according to the power availability in a particular MG. Inverters can have different power setpoints during the grid-connected mode, but in the standalone mode, they all need their power setpoints to be adjusted according to their power ratings. On the basis of this, a power management-based droop control strategy is developed to achieve the power-sharing among the neighbouring microgrids. The proposed method helps the MG inverters to share the power according to its ratings and availability, which does not restrict the inverters for equal power-sharing. The paralleled inverters in coupled MGs need to work in both interconnected mode and standalone mode and should be able to transfer between modes seamlessly. An enhanced droop control is proposed to maintain the frequency and voltage of the MGs to their nominal value, which also helps the neighbouring MGs for seamless (de)coupling. This thesis also presents a mathematical model of the interconnected neighbouring microgrid for stability and robustness analysis. Finally, a laboratory prototype model of two MGs is developed to test the effectiveness of the proposed control strategies

Power Quality Improvement of Distributed Generation Integrated Network with Unified Power Quality Conditioner.

With the increased penetration of small scale renewable energy sources in the electrical distribution network, maintenance or improvement of power quality has become more critical than ever where the level of voltage and current harmonics or disturbances can vary widely. For this reason, Custom Power Devices (CPDs) such as the Unified Power Quality Conditioner (UPQC) can be the most appropriate solution for enhancing the dynamic performance of the distribution network, where accurate prior knowledge may not be available. Therefore, the main objective of the present research is to investigate the (i) placement (ii) integration (iii) capacity enhancement and (iv) real time control of the Unified Power Quality Conditioner (UPQC) to improve the power quality (PQ) of a distributed generation (DG) network connected to the grid or microgrid