The analysis of the small-signal stability of conventional power systems is well established, but for inverter based microgrids there is a need to establish how circuit and control features give rise ...to particular oscillatory modes and which of these have poor damping. This paper develops the modeling and analysis of autonomous operation of inverter-based microgrids. Each sub-module is modeled in state-space form and all are combined together on a common reference frame. The model captures the detail of the control loops of the inverter but not the switching action. Some inverter modes are found at relatively high frequency and so a full dynamic model of the network (rather than an algebraic impedance model) is used. The complete model is linearized around an operating point and the resulting system matrix is used to derive the eigenvalues. The eigenvalues (termed "modes") indicate the frequency and damping of oscillatory components in the transient response. A sensitivity analysis is also presented which helps identifying the origin of each of the modes and identify possible feedback signals for design of controllers to improve the system stability. With experience it is possible to simplify the model (reduce the order) if particular modes are not of interest as is the case with synchronous machine models. Experimental results from a microgrid of three 10-kW inverters are used to verify the results obtained from the model
Reliability assessment tools are of crucial importance for planning and modernisation of distribution power systems. In recent years there has been an increased deployment of renewable energy, ...distributed generation, energy storage, electric vehicle, protection device automation and demand response schemes in the distribution networks. All these technologies contribute in their own way to network reliability. The objective of this paper is to provide a critical survey of the reliability assessment techniques used for the evaluation of distribution networks, emphasising the importance of an increased penetration of distributed energy resources and a more widespread application of control, protection and communication technologies. A detailed analysis and a comparison between different techniques and models used for the reliability assessment will be provided for each technology along with the guidelines for their application. A case study will be used to demonstrate the properties and the modelling procedure of the reliability assessment tools for modern distribution networks.
The emulation of synchronous machines is becoming an attractive solution to provide virtual inertia and voltage support to weak grids and microgrids, representing an alternative to conventional ...current-injection control strategies. However, synchronverters operate as voltage sources and this makes it difficult to limit the current when an event like voltage sag takes place. Here, a method to limit the output current of synchronverters in parallel connection is proposed. First of all, a fictitious reference current is generated, and its value is then followed by an internal current controller. A method to avoid the loss of synchronism when the current saturates is proposed and studied in detail. Also, the implementation of the controller is addressed carefully. All the control system improvements are tested on a prototype consisting of two 15 kVA synchronverters connected in parallel to a configurable weak grid.
Inverters are a necessary interface for several forms of distributed generation (DG) and where they form a microgrid they have the potential to offer high power quality. The challenge is to ...coordinate the actions of a group of inverters so that they offer the level of power quality known to be possible from fast local control of a single inverter. The case examined here is a power park of several inverter-based DG in relatively close proximity. A basic requirement is that the inverters regulate the grid voltage and share the real and reactive power demands according to their ratings. In small girds with high proportions of nonlinear and unbalanced loads it is also important to actively control the waveform quality in terms of harmonics, transient disturbances, and balance. Further, it is important that these duties are shared equally between the units rather than having one master unit taking the lead in the voltage control function. A constraint faced in designing a sharing system is the limited bandwidth of signal communication even over distances of a few meters. A control method is proposed that separates the control tasks in the frequency domain. Power sharing and voltage regulation are controlled centrally and commands are distributed through a low-bandwidth communication link. Waveform quality functions are controlled in high bandwidth controllers distributed to each local inverter. Experimental tests on a grid of three 10-kVA inverters are used to show that the method fully exploits the inherent fast response of the inverters while also ensuring voltage balance even with extreme load imbalance. It is shown that circulating currents are avoided during steady state and transients
Rectifiers and voltage regulators acting as constant power loads form an important part of a microgrid's total load. In simplified form, they present a negative incremental resistance and beyond ...that, they have control loop dynamics in a similar frequency range to the inverters that may supply a microgrid. Either of these features may lead to a degradation of small-signal damping. It is known that droop control constants need to be chosen with regard to damping, even with simple impedance loads. Actively controlled rectifiers have been modeled in nonlinear state-space form, linearized around an operating point, and joined to network and inverter models. Participation analysis of the eigenvalues of the combined system identified that the low-frequency modes are associated with the voltage controller of the active rectifier and the droop controllers of the inverters. The analysis also reveals that when the active load dc voltage controller is designed with large gains, the voltage controller of the inverter becomes unstable. This dependence has been verified by observing the response of an experimental microgrid to step changes in power demand. Achieving a well-damped response with a conservative stability margin does not compromise normal active rectifier design, but notice should be taken of the inverter-rectifier interaction identified.
The grid synchronisation block is a key component of power electronics converters and their control systems. In some cases, synchronisation algorithms are based on non-linear oscillators and, ...therefore, their stability boundaries should be analysed by using non-linear techniques. However, as this topic is relatively novel, it has been seldomly addressed in the literature. This article investigates the transient stability of limit cycle oscillators for grid-connected voltage source converters. The analysis is based on the Krasovskii theorem, which is derived from the Lyapunov theory. Numerical results are used to calculate the regions where stability is guaranteed for specific states of the synchronisation system such as the frequency, phase and voltage error. Experimental results obtained from the Power-Hardware-In-the-Loop laboratory are used to validate the theoretical developments.
•Application of the Krasovskii theorem for analysing a synchronisation algorithm.•Derivation of the stability regions of a LCO-FLL used to control power converters.•Presentation of analytical and simulation results, for specific cases.•Validation of the proposed method in an experimental platform.
The term Virtual Synchronous Machine (VSM) commonly refers to the emulation of synchronous machines operation by using power electronics converters. However, a wider application of this control ...technique is still pending on further refinement of its design aspects and digital implementation. In this paper, a systematic approach to design the VSM parameters is introduced. First, a simplified third-order small-signal model of a VSM based on the conventional model of synchronous generators is developed. Unlike the existing works published in literature, the proposed model takes into account the effects of inductive and resistive weak grids. The model is then used to design the controller parameters by appropriately placing the poles of the closed-loop system. VSM robustness is studied for weak grids dominated by either inductance or resistance, and the aspects of digital control implementation are addressed. All the control system features and improvements were tested on a 15 kW prototype of a battery-supported VSM connected to a weak grid.
•Reliability of distribution networks with active network management is evaluated.•A novel reliability assessment methodology is proposed for such evaluation.•Reliability improvement provided by ...soft-open points is calculated and analysed.•Active network management and soft open-points significantly improved the reliability.
Active Network Management (ANM) technologies represent an effective tool for increasing the penetration of Distributed Generators (DGs) in electrical distribution systems. Under normal operating conditions, these technologies allow a massive integration of renewable generation without exceeding rated values of electrical equipment. In addition to that, these technologies can be used to restore the supply when there is a network outage, yet this additional use has been commonly neglected in reliability studies. In this paper a novel methodology to assess the contribution of ANM technologies to reliability of distribution networks is proposed. Technologies like renewable DGs with controllable output power, on-load tap-changers (OLTC) transformers, demand control solutions and soft open points (SOPs) are all included in the reliability study. Coordinated operation of these technologies during outages and performance of multi-terminal SOPs are also modelled and evaluated. The proposed methodology was tested on two distribution networks. The contribution of different ANM technologies to reliability was assessed and their benefits confirmed.
Within electrical distribution networks, substation constraint management requires that aggregated power demand from residential users is kept within suitable bounds. Efficiency of substation ...constraint management can be measured as the reduction of constraint violations w.r.t. unmanaged demand. Home batteries hold the promise of enabling efficient and user-oblivious substation constraint management. Centralized control of home batteries would achieve optimal efficiency. However, it is hardly acceptable by users, since service providers (e.g., utilities or aggregators) would directly control batteries at user premises. Unfortunately, devising efficient hierarchical control strategies, thus overcoming the above problem, is far from easy. In this article, we present a novel two-layer control strategy for home batteries that avoids direct control of home devices by the service provider and at the same time yields near-optimal substation constraint management efficiency. Our simulation results on field data from 62 households in Denmark show that the substation constraint management efficiency achieved with our approach is at least 82% of the one obtained with a theoretical optimal centralized strategy.
This paper presents a novel approach to demand side management (DSM), using an "individualized" price policy, where each end user receives a separate electricity pricing scheme designed to ...incentivize demand management in order to optimally manage flexible demands. These pricing schemes have the objective of reducing the peaks in overall system demand in such a way that the average electricity price each individual user receives is non-discriminatory. It is shown in this paper that this approach has a number of advantages and benefits compared to traditional DSM approaches. The "demand aware price policy" approach outlined in this paper exploits the knowledge, or demand-awareness, obtained from advanced metering infrastructure. The presented analysis includes a detailed case study of an existing European distribution network where DSM trial data was available from the residential end-users.
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