A new method for integrating and controlling synchronous generators in power systems (GB2012267.7)

Synchronous generators (SGs) are the most popular type of generators that are used worldwide to generate electricity. There are two main types of SG: round-rotor (also known as turbo-generator) and salient-rotor. The principle of the operation of both types are the same: the rotor is connected to a mechanical source of energy, known as the prime mover, and rotates with it. Therefore, the rotating magnetic field (using either electro- or permanent magnet) of the rotor induce a voltage in the stator windings, which is connected to the load/grid. The prime movers for fossil/bio/nuclear fuelled power stations are either steam or gas turbines. SGs also are used as distributed generation units e.g. in CHP plants, where they are called microgenerators. The SGs are even used for renewable systems (e.g. wind), which are not the concern of this patent. The ever-increasing penetration of Power Electronic Converter (PEC)-based generation units (e.g. renewable energy), has been causing numerous challenges for the network operators and has put the power networks on the verge of instability. Some of these challenges, which are due to the intermittent nature of renewable energy, do not depend on the power system structure. For example, as the penetration of renewable energy increases, more energy storage (ES) facilities will be needed to balance the generation with the demand (regardless of the system structure). However, there are some challenges (such as reduction in the short circuit level, phase angle movement, and rate-of-change-of-frequency) that do depend on the system structure. To alleviate these issues the popular approach (in both industry and academia) is to make the PECs behave like SGs. However, this approach is not optimized. For example, a SG can inject between 5-7 pu (per unit) fault current, known as short circuit level (SCL), while that of PEC-based unit is about 1.1-1.2 pu. Therefore, in order to be able to supply similar SCLs by the PEC-based units we must either “de-load” during normal operation or use over-sized power electronics switches. Neither de-loading nor using over-rated switches is an optimized solution and increase the energy price. This patent proposes a “steer into the skid” strategy involving the decoupling of SGs from the network using AC/DC/AC PECs and controlling them such that the PECs impose the power on the SGs according to the local voltage and frequency (virtual AVR and virtual Governor)

A low-voltage ride-through strategy using mixed potential function for three-phase grid-connected PV systems

This paper presents a new control strategy for low-voltage ride-through for 3-phase grid-connected photovoltaic systems. The proposed fault ride through control algorithm, which is designed based on mixed potential function, can protect the inverter from overcurrent failure under both symmetric and asymmetric faults, reduce the double frequency oscillation and provides reactive power support by applying a voltage compensation unit. With the proposed method, the inverter can also inject sinusoidal current during asymmetric faults. The method does not require a hard switch to switch from the Maximum Power Point Tracking (MPPT) to a non-MPPT algorithm, which ensures a smooth transition

Universal and Seamless Control of Distributed Resources-Energy Storage for all Operational Scenarios of Microgrids

This paper proposes one control paradigm that can operate in both grid-connected and islanded modes, hence, does not need any sort of islanding detection method. The proposed method automatically and seamlessly rides-through a fault on the grid side, and controls the microgrid’s voltage and frequency during islanded operation. During islanded operation it utilises the combination of distributed generation-energy storage similar to the prime-mover of a synchronous generator to control the frequency. A comprehensive active and reactive power control is proposed that minimises the usage of a local fossil-fuelled auxiliary generator. The method is based on expanding the so called non-detection zone to all operational scenarios including islanded mode, hence, having small, “undetectable” voltage and frequency deviation. As soon as the grid is reconnected the distributed generator is automatically and seamlessly synchronised with the grid. This is achieved through keeping PLL as part of the operation in islanded mode without altering its phase angle. The proposed method is validated using PSCAD/ EMTDC simulation

Stability analysis of a PMSG based Virtual Synchronous Machine

This paper proposes a Virtual Synchronous Machine (VSM) strategy for Permanent Magnet Synchronous Generator based wind turbines which enables seamless operation in all operating modes. It guarantees Maximum Power Point Tracking in grid-connected operation, Load Following Power Generation in islanded operation and Low Voltage Ride Through capability during faults. To achieve optimal performance in all operating modes, the stability of the VSM is investigated in the event of small and large perturbations. The small-signal stability analysis of the VSM is conducted using a linearized state space model and the impact of the controllers on the dominant modes are derived using participation factor analysis. The transient stability and dynamic performance of the VSM are analyzed using a non-linear model. Based on this analysis, design guidelines and operational limits of the VSM are established. The results of this analysis are validated using time-domain simulations in MATLAB/SIMULINK

Impact of Virtual Synchronous Machines on Low-Frequency Oscillations in Power Systems

The low-frequency oscillations (LFOs) inherent inpower systems will be impacted by the increasing penetrationof renewable energy sources (RESs). This paper investigates theimpact of virtual synchronous machine (VSM) based RESs on theLFOs in power systems. A detailed two-machine test-bed has beendeveloped to analyze the LFOs which exists when VSMs replacesynchronous generators. The characteristics of the LFO modesand the dominant states have been comprehensively analyzed.Furthermore, this study analyzes the LFO modes which existsin an all-VSM grid. The role of the power system stabilizers inthe all-VSM grid has been comprehensively evaluated. The IEEEbenchmark two-area four-machine system has been employed tocorroborate the results of the small-signal analysis and observethe transient performance. The analysis in this paper have beenperformed in MATLAB/SIMULINK environment

Buffered-microgrid Structure for Future Power Networks; a Seamless Microgrid Control

This paper proposes a new structure and control scheme for future microgrid-based power system, which is designed to achieve a seamless operation in both islanded and grid-connected modes, while the load is appropriately shared by all units (i.e. renewable sources, energy storage systems and the grid). The proposed method, which involves physical separation of the microgrid from the grid by using AC/DC/AC converters, ensures safe, secure and seamless operation of both modes. Such a “buffered” structure enables reduction in the transmission losses by reducing the exchanged energy with the grid through using a dead-zone in the control of the buffering AC/DC/AC converter. An inverse-droop control technique has been implemented to control the voltage magnitude and frequency, using current control in the dq-frame. PSCAD/EMTDC software has been used to validate the proposed method through simulating different scenarios. The solution provides a simple, smooth, and communication-free decentralized control for multi-sources microgrids. Moreover, the proposed buffered structure separates the dynamics of the microgrid and the grid, which enables a faster microgrid voltage and frequency control and protects the grid and the microgrid from faults on the other side

A new control strategy for low-voltage ride-through of three-phase grid-connected PV systems

Power quality and current limitation are the most important aspects of the grid-connected power converters under fault. Since the distributed energy resources are widely used, fault management strategy is important for micro-grids applications. This paper presents a new control strategy for low-voltage ride-through for 3-phase grid-connected photovoltaic systems. The proposed method, which is designed in a synchronous frame using positive and negative sequence components, can protect the inverter from overcurrent failure under both symmetrical and unsymmetrical faults and provides reactive power support. The method does not require a hard switch to switch from MPPT to a non-MPPT algorithm, which ensures a smooth transition

Wind generator-energy storage control schemes for autonomous grid

Conventionally the power network operators were obliged to buy all the wind energy generated by wind farms. However, as the penetration of wind energy (or generally any other sort of renewable source) in a power system is increased, the ability of other generators to balance the demand becomes limited. This will necessitate the control of wind turbines in order to generate a given demand power rather than extracting the maximum wind power. This control approach is termed “Power Demand Control” in this thesis. In contrast to Power Demand Control, “Power Smoothing Control” utilizes energy storage systems in order to absorb high frequency wind fluctuations, hence, delivering a smoother version of wind power into the grid/load. The drawback of the Power Smoothing approach is that the average power into the grid/load is still determined by the available wind power rather than the system operator. The Power Demand Control approach, which has received little attention in literatures, is the main focus of this thesis. This research proposes control schemes with and without external energy storage for the Power Demand Control strategy. This thesis studies different possible methods of applying Power Demand Control, in particular the droop control method. It is shown that a droop-controlled wind farm does not need a central “Supervisory wind Farm Control” unit to determine the power demanded from each DFIG. Moreover, a droop-controlled wind farm has the advantage of controlling the local grid voltage and frequency. This means that no external voltage and frequency source is required which makes a droop-controlled wind farm a more suitable option for integration of wind energy at distribution level. The classical droop control is modified in order to make the DFIGs share the demand power not only according to their ratings but also to their associated available wind power. The applications of the control paradigm are discussed, including: integration into microgrids, AC grids and HVDC connection feeders. This work mainly concentrates on microgrid applications. An Energy Management System is proposed in order to keep the energy level of the energy storage (or the DFIG’s shaft speed) within its limits using an Auxiliary Generator and a Dispatchable Load. Different possible system configurations are introduced and their advantages and drawbacks are discussed. It is illustrated through simulation that the proposed control scheme can inherently ride-through a grid fault with no need for communication. Furthermore, it is shown that the control scheme can operate if the wind speed drops to zero. The simulations are carried out using the PSCAD/EMTDC software

Wind generator-energy storage control schemes for autonomous grid

Conventionally the power network operators were obliged to buy all the wind energy generated by wind farms. However, as the penetration of wind energy (or generally any other sort of renewable source) in a power system is increased, the ability of other generators to balance the demand becomes limited. This will necessitate the control of wind turbines in order to generate a given demand power rather than extracting the maximum wind power. This control approach is termed “Power Demand Control” in this thesis. In contrast to Power Demand Control, “Power Smoothing Control” utilizes energy storage systems in order to absorb high frequency wind fluctuations, hence, delivering a smoother version of wind power into the grid/load. The drawback of the Power Smoothing approach is that the average power into the grid/load is still determined by the available wind power rather than the system operator. The Power Demand Control approach, which has received little attention in literatures, is the main focus of this thesis. This research proposes control schemes with and without external energy storage for the Power Demand Control strategy. This thesis studies different possible methods of applying Power Demand Control, in particular the droop control method. It is shown that a droop-controlled wind farm does not need a central “Supervisory wind Farm Control” unit to determine the power demanded from each DFIG. Moreover, a droop-controlled wind farm has the advantage of controlling the local grid voltage and frequency. This means that no external voltage and frequency source is required which makes a droop-controlled wind farm a more suitable option for integration of wind energy at distribution level. The classical droop control is modified in order to make the DFIGs share the demand power not only according to their ratings but also to their associated available wind power. The applications of the control paradigm are discussed, including: integration into microgrids, AC grids and HVDC connection feeders. This work mainly concentrates on microgrid applications. An Energy Management System is proposed in order to keep the energy level of the energy storage (or the DFIG’s shaft speed) within its limits using an Auxiliary Generator and a Dispatchable Load. Different possible system configurations are introduced and their advantages and drawbacks are discussed. It is illustrated through simulation that the proposed control scheme can inherently ride-through a grid fault with no need for communication. Furthermore, it is shown that the control scheme can operate if the wind speed drops to zero. The simulations are carried out using the PSCAD/EMTDC software