Inverter-based Distributed Generation Research Articles

Assessment and Enhancement of Static Voltage Stability With Inverter-Based Generators

In this letter, a systematic static voltage stability index for evaluating system loading status in distribution networks is deduced from complex power flow models. The proposed index facilitates impact analysis on the integration of inverter-based distributed generators under different control modes, which may affect the static voltage stability. The case study, including various scenarios, is performed to validate the accuracy of the theoretical results.

Protection of active distribution networks with conventional and inverter-based distributed generators

Fault current limiter (FCL) is one of the best choices for solving protection problems in active distribution networks comprising dispersed generation (DG). Due to the nonlinear nature of protection problems and characteristics of protective devices (PDs), the selection of FCL type and specification is one of the main challenges of this approach. In this paper, a simple and effective analytical method for FCL impedance calculation is proposed. Knowing only the properties of the fastest fuse of the feeder and the protective device installed at the beginning of the feeder (in this case recloser) suffices for the implementation of the proposed method. The method addresses issues of miscoordination caused by increased fault current levels, reduction of reach of PDs, miscoordination due to reduction of reach, blinding of protection, false tripping of PDs, and fuse-fuse and overcurrent relay-fuse miscoordination. The suggested approach has also been simplified so that the selection of FCL for one feeder only relies on DGs connected to this feeder. In the proposed scheme, the impact of both conventional and inverter-based DGs is considered, and the transient behavior of FCL is elaborated. Effectiveness of the proposed method is demonstrated by various simulations on a test system in MATPOWER 5.1 toolbox.

Optimal Harmonic Mitigation in Distribution Systems with Inverter Based Distributed Generation

In recent years, with the widespread use of non-linear loads power electronic devices associated with the penetration of various renewable energy sources, the distribution system is highly affected by harmonic distortion caused by these sources. Moreover, the inverter-based distributed generation units (DGs) (e.g., photovoltaic (PV) and wind turbine) that are integrated into the distribution systems, are considered as significant harmonic sources of severe harmful effects on the system power quality. To solve these issues, this paper proposes a harmonic mitigation method for improving the power quality problems in distribution systems. Specifically, the proposed optimal planning of the single tuned harmonic filters (STFs) in the presence of inverter-based DGs is developed by the recent Water Cycle Algorithm (WCA). The objectives of this planning problem aim to minimize the total harmonic distortion (THD), power loss, filter investment cost, and improvement of voltage profile considering different constraints to meet the IEEE 519 standard. Further, the impact of the inverter-based DGs on the system harmonics is studied. Two cases are considered to find the effect of the DGs harmonic spectrum on the system distortion and filter planning. The proposed method is tested on the IEEE 69-bus distribution system. The effectiveness of the proposed planning model is demonstrated where significant reductions in the harmonic distortion are accomplished.

An adaptive fault-component-based current differential protection scheme for distribution networks with inverter-based distributed generators

The high penetration of inverter-based distributed generators (IBDGs) T-connected into distribution networks has changed their structure from single-source networks to multi-source multi-branch networks; therefore, conventional distribution network protection schemes are difficult to apply. To this end, a mathematical model of IBDG fault component current considering the control strategy of IBDG is established, and then, a method for estimating the IBDG fault component current using only electrical information at both terminals of the protected line is proposed. On this basis, an adaptive fault-component-based current differential protection scheme suitable for distribution networks with IBDGs is proposed. The proposed scheme only needs to exchange the electrical information at both terminals of the protected line; that is, it can adapt to multiple IBDGs that are T-connected into the distribution networks, reducing the requirements of multi-terminal protection for communication channels and thus lowering costs. Finally, the effectiveness of the proposed protection scheme is demonstrated in the PSCAD/EMTDC software. The results show that the proposed protection scheme is not affected by the location, capacity of the IBDGs, or the fault type and has a high sensitivity.

Waveform Difference Feature-Based Protection Scheme for Islanded Microgrids

Unlike conventional synchronous generators, the fault characteristics of inverter-based distributed generators (IBDGs) are mainly determined by the inverter control strategies. Their limited fault current contributions make the protection design of islanded microgrids (MGs) become a major challenge. This article focuses on the fault characteristics and protection scheme design of islanded MGs. First, the phase differences between the positive-sequence current fault components (PSC-FCs) at both terminals of fault line are analyzed under different network topologies and fault severities. It points out the phase characteristic differences between internal and external faults. Then, on basis of this to overcome the problems of the phase measurement error and protection setting, a waveform difference feature-based protection scheme is proposed. The protection criteria are formulated using the first peak sign and time information of PSC-FCs between both terminals of line, which are extracted by the mathematical morphology (MM) technology. Only relatively low bandwidth communication is required to achieve accurate fault identification. Finally, the simulation results on a 10-kV islanded MG model demonstrate that the proposed scheme can identify internal faults reliably in both looped and radial islanded MGs under different fault resistances and measurement noises.

Hierarchical Control of Microgrid Using IoT and Machine Learning Based Islanding Detection

Due to the increase in penetration of renewable energy sources, the control technique plays a vital role to determine the performance of Microgrid (MG). Recently, the Internet of Things (IoT) and cloud computing has gained significance in solving various industrial problems. Robust and scalable Information Communication Technology (ICT) infrastructure is critical for efficient control of MG. IoT Devices with efficient measurement and control capability can play a key role in the MG environment. In this paper three layers hierarchical control of inverter based MG was developed using cloud-based IoT infrastructure and machine learning (ML) based islanding detection scheme. MG was operated in both island and grid connected mode. In the Primary layer, a voltage frequency (V-F) droop control with virtual impedance control was applied to avoid the disturbances in island mode. Moreover, Active Reactive (P-Q) power control was used for grid connected mode. In the secondary layer voltage and frequency deviations were removed by using the decentralized averaging based method. Voltage and frequency from each distributed generator (DG) were communicated by using a lightweight IoT-based protocol through an edge device (ED). Context-aware policy (CAP) was adopted in ED to optimize traffic flow over a communication network (CN) by comparing the difference in the present and previous data values. In the tertiary layer, a cloud-based ML model was developed using an artificial neural network (ANN) for islanding detection. ANN model was trained by data produced by simulating islanding scenarios in Matlab. Phasor measurement unit (PMU) data was communicated to the cloud for island prediction. The Proposed scheme was implemented on a modified IEEE-13 bus system with four inverter-based distributed generators (DGs) in Matlab, and Microsoft cloud services were used. The successful implementation of MG hierarchical control using an IoT feedback network with less data traffic along with cloud-based islanding detection using machine learning are the main contributions in this work. The whole system achieves stability within 2 seconds of islanding according to IEEE 1547 standards.

Synthetic Harmonic Distance Relaying for Inverter-Based Islanded Microgrids

Faults in inverter-based islanded microgrids can be a formidable protection challenge due to (i) low fault current magnitudes, (ii) compromised fault current phase angles, and (iii) bidirectional flow of fault currents. This paper proposes a protection scheme that disregards the fault signals altogether. Instead, it relies on decoupled synthesized signals introduced only during fault conditions. This scheme is achieved by exploiting the existing inverter-based distributed generation (IBDG) controllers to inject synthetic harmonic voltages and currents. These synthetic signals are measured locally by digital relays in the microgrid to develop a novel synthetic harmonic distance relay (SHDR). Apart from the utilization of high-order harmonic signals that enhance SHDR reactance reach, further reach improvement is achieved via the introduction of line reactance magnifiers (LRMs). Transient studies in PSCAD/EMTDC verify the performance of the proposed scheme under various faults, contingencies, and different microgrid configurations.

Modified Q–f Droop Curve Method for Islanding Detection With Zero Non-Detection Zone

In this paper, a modified Q-f droop curve method for inverter-based distributed generator is proposed. The proposed method has merits such as detecting islanding with zero non-detection zone, negligible effects on power quality and simple implementation. A new parameter is embedded in the reactive power reference to accelerate the frequency deviation at the point of common coupling, until the islanding is detected. Therefore, a passive under/over frequency relay will be simply sufficient to detect islanding within the permissible limit. The proposed method is modelled with computer-aided design SIMULINK/MATLAB environment. The proposed method is analysed for operation with unity and non-unity power factors. The performance is validated for various loading quality factors with a negligible change in detection time. In addition, it is confirmed that the proposed system is stable under unbalanced loading and switching conditions. The proper operation of proposed islanding detection method under multi distributed generators connection is validated. The results of the hardware experimentations are presented to confirm the proposed method analysis and simulations.

An Islanding Detection Technique for Inverter-Based Distributed Generation in Microgrids

This article proposes a new passive islanding detection technique for inverter-based distributed generation (DG) in microgrids based on local synchrophasor measurements. The proposed method utilizes the voltage and current phasors measured at the DG connection point (point of connection, PoC). In this paper, the rate of change of voltages and the ratio of the voltage and current magnitudes (VoI index) at the PoC are monitored using micro-phasor measurement units. The developed local measurements based decentralized islanding detection technique is based on the VoI index in order to detect any kind of utility grid frequency fluctuations or oscillations and distinguishing them from islanding condition. The simulation studies confirm that the proposed scheme is accurate, robust, fast, and simple to implement for inverter-based DGs.

An inertial droop control based on comparisons between virtual synchronous generator and droop control in inverter-based distributed generators

The adoption of the droop control provides a feasible scheme for the distributed generation connected to the utility grid. Nevertheless, it does not contribute to the inertia and damping factor necessary for power systems. In order to achieve increased stability of microgrids, some scholars simulate the operating characteristics of traditional synchronous generators through control equations, thus virtual synchronous generator control (VSG) was developed. Firstly, the small signal models of inverter based on droop control and VSG control are built in grid-connected (GC) state and stand-alone (SA) state to compare the dynamic response when load transition and power reference change. And we can deduce that in terms of dynamic response neither of these two control strategies can simultaneously satisfy the stringent requirements of different modes of operation. Then, by integrating the advantages of the traditional droop control and VSG control, inertial droop control is introduced in this paper. The presented inertial droop control not only features virtual inertia but also adds compensators to the output power part of the active power control loop to ensure the dynamic response of the system. In the GC state, inertial droop control helps to restrain power oscillations, shorten power adjustment time, and minimize frequency fluctuations to maintain frequency stability. In the SA state, the inertial droop control enables to achieve high enough inertia and damping properties. Finally, this paper demonstrates the superiority of the inertial droop control by means of simulation results.

The Power System and Microgrid Protection—A Review

In recent years, power grid infrastructures have been changing from a centralized power generation model to a paradigm where the generation capability is spread over an increasing number of small power stations relying on renewable energy sources. A microgrid is a local network including renewable and non-renewable energy sources as well as distributed loads. Microgrids can be operated in both grid-connected and islanded modes to fill the gap between the significant increase in demand and storage of electricity and transmission issues. Power electronics play an important role in microgrids due to the penetration of renewable energy sources. While microgrids have many benefits for power systems, they cause many challenges, especially in protection systems. This paper presents a comprehensive review of protection systems with the penetration of microgrids in the distribution network. The expansion of a microgrid affects the coordination and protection by a change in the current direction in the distribution network. Various solutions have been suggested in the literature to resolve the microgrid protection issues. The conventional coordination of the protection system is based on the time delays between relays as the primary and backup protection. The system protection scheme has to be changed in the presence of a microgrid, so several protection schemes have been proposed to improve the protection system. Microgrids are classified into different types based on the DC/AC system, communication infrastructure, rotating synchronous machine or inverter-based distributed generation (DG), etc. Finally, we discuss the trend of future protection schemes and compare the conventional power systems.

Impact of Microgrid Operation on the Performance of Overcurrent Relay Coordination and Assessment of Differential Relay Coordination

Penetration of distributed generations (DGs) enable microgrid to operate in different mode such as grid-connected and islanded mode with radial and mesh topology. The fault current magnitude and direction varies according to the operating conditions which imposes serious protection challenges and may lead to protection system failure and nonselective relay operation. Thus, the impact of operating conditions on the existing overcurrent relay coordination has been extensively studied in this paper. The overcurrent relay coordination is implemented using genetic algorithm (GA) considering different topology and operating modes of the microgrid. Extensive test results indicate that the existing overcurrent relay coordination may fail during some critical operating conditions and thus, a differential relay coordination scheme is proposed which provides improved performance for microgrid operating in different modes and different network topologies with both synchronous and inverter-based DGs.

A calculation method for the short-circuit current contribution of current-control inverter-based distributed generation sources at balanced conditions

When a grid-connected inverter-based distributed generation (IBDG) source behaves as a current source that can limit its magnitude in current loop control, the contribution from the inverter to the short-circuit current (SCC) is not as significant as those from conventional synchronous generators. However, the increased IBDG sources can change the magnitude and phase angle of the SCC, so they should not be ignored in SCC calculation. The objective of this study is to present a method for calculating the SCC of an IBDG source of acting as an internally either limited or unlimited current source at balanced conditions. For this purpose, negative- and zero-sequence networks are transferred to a positive-sequence network with balanced IBDG sources. Subsequently, SCCs and zero-, negative-, and positive-sequence induced voltage are determined by using zero- and negative-sequence current injection matrices. Then, new SCC calculation equations for a single line-to ground, line-to-line ground, line-to-line, or three-phase fault are also derived. As a result, the proposed method improves the conventional method in terms of taking the contribution from a balanced IBDG source to SCC into account.

Optimal Placement and Sizing of Inverter-Based Distributed Generation Units and Shunt Capacitors in Distorted Distribution Systems Using a Hybrid Phasor Particle Swarm Optimization and Gravitational Search Algorithm

In this paper, a novel hybrid population-based meta-heuristic algorithm, called the hybrid Phasor Particle Swarm Optimization and Gravitational Search Algorithm (PPSOGSA), is proposed to solve the problem of optimal placement and sizing of inverter-based distributed generation (DG) units and shunt capacitors in radial distribution systems with linear and non-linear loads. The objective of the problem is reduction of active power losses considering constraints of the fundamental frequency active and reactive power balance, RMS voltage, and total harmonic distortion of voltage (THDV) at each bus of the network, as well as the branch flow constraints. The performance of the PPSOGSA-based approach is evaluated on the standard IEEE 33- and 69-bus test systems under sinusoidal and non-sinusoidal operating conditions. Compared to the original PPSO and GSA and other algorithms commonly used in the optimal sitting and sizing problem of DG units and shunt capacitors, it is found that the proposed algorithm has yielded better results.

Optimal placement and sizing of distributed generation with small signal stability constraint

In recent decades, galloping growth of distributed generation (DG), and its vital role from technical, financial, and environmental point of view, has made it more prominent that DG and its utilization need to be analyzed meticulously. Since most DG units’ output power is DC or is not in accordance with network frequency, they need to be connected to the network via power electronic interface. Therefore, the behavior of power electronic interface and their low inertia can affect distribution system’s small signal stability and optimal size and location of DG unit consequently. In this paper, dynamic models for Inverter-Based DG unit (IBDG), network branches, and loads are utilized in order to scrutinize impacts of small signal stability constraint on optimal size and location of IBDG in radial distribution system (RDS). Similar to many previous studies, a genetic algorithm has been implemented to find optimal size and location of IBDG with the aim of total active power loss minimization. Simulation results on the IEEE 33-bus test system show that, DG placement studies without small signal stability consideration, can lead to inaccurate outcomes which have deleterious impacts on small signal stability.

Impact of inverter-based generation on islanding detection schemes in distribution networks

One of the most frequently used interface protections for Distributed Generators (DGs) is the Loss of Mains (LoM) protection. It detects the formation of an island at the connection point and disconnects the DG to protect the unit, the system and the personnel. The increased penetration of inverter-interfaced DG, in combination with the decommissioning of synchronous generators, reduces the system inertia and leads to faster changing and larger voltage and frequency deviations. Therefore, modern grid-codes require inverter-based DGs to provide support to the grid by modifying their active and reactive power injection based on local measurements. However, this leads to complex inverter-grid interactions and modifies the islanded system behavior, thus disturbing the operation of LoM protections that rely mainly on local voltage and frequency measurements. In this paper, we propose an improved analytical formulation for estimating the Non-Detection Zone (NDZ) of LoM protection devices in the presence of grid-feeding inverters, as well as novel NDZ approximations for grid-supporting and grid-forming inverter-based services. We verify the analytical results with detailed dynamic simulations and comment on the impact that the new inverter requirements have on the performance of LoM protections.

Performance of Droop Control Techniques Under Nonlinear Loading Conditions: Uniform and Nonuniform Configurations

Microgrids are seen as an emerging solution to locally provide clean energy. Power electronic-based distributed generators (DGs) operating in parallel are the main constituents of these systems, which extract, control, and convert electric power from renewable sources. Broadly, the approaches for controlling these systems are categorized under centralized, decentralized, and distributed strategies. Under the decentralized category, droop control and several variants of the same have been proposed in the literature over the years. A large portion of the work reported, considers linear loads. This article compares the performance of four commonly employed droop control techniques for inverter-based distributed generators, for their performance under nonlinear loading conditions and during plug and play functionality in the islanded mode of operation. The work attempts to investigate the performance of these strategies under both uniform (i.e., same control structure in the DGs) and nonuniform (DGs operating with different control structures) scenarios from the perspective of output power sharing and transients during the plug and play functionality. The evaluation is carried out through simulations of cases with constant impedance, constant current, and dynamic nonlinear loads on the modified 13-bus system using MATLAB-Simulink, and experiments on a laboratory scale microgrid.

Study on a Correlation-Based Anti-Islanding Method under Wider Frequency Trip Settings for Distributed Generation

Islanding phenomenon of distributed generation (DG), such as photovoltaic (PV) generation, is undesirable because it causes safety issues for utility service personnel and power system equipment. Many anti-islanding methods have been studied since DG appeared in electric power systems (EPSs). Most anti-islanding methods focus on disconnecting DG from the grid using functionality to detect islanding under narrow frequency trip settings, because safety issues have a higher priority. However, as DG plays a key part of an EPS, a significant loss of DG due to a short disturbance could result in a reliability issue for the EPS. Corresponding to this matter, new international standards, such as IEEE standard 1547–2018, require more sophisticated and complex functionalities for grid-connected DGs by adopting ride-through technologies and wider voltage/frequency trip settings. Since most anti-islanding functions of inverter-based DG have been based on the frequency of the inverter voltage, it is more difficult to detect islanding under wider frequency trip settings. This paper presents a correlation-based anti-islanding method (AIM) without depending on the frequency trip of inverter-based DGs. Simulation results are provided to verify the performance of the correlation-based anti-islanding method. As a result, the proposed method detects islanding at 0.116 s under wider frequency trip setting by the IEEE Std. 1547–2018 test condition, while the popular active frequency drift method with positive feedback does not detect islanding using the same current disturbance.

Protection of Inverter-Based Islanded Microgrids via Synthetic Harmonic Current Pattern Injection

Faults in inverter-based islanded microgrids can be a formidable protection challenge due to the limited fault current contribution of inverters. This paper designs a selective and sensitive protection scheme by injecting a pattern of up to three synthetic harmonics utilizing the flexibility of existing inverter-based distributed generation (IBDG) controllers (called injection pattern [IP]). The measured pattern (MP) by each relay is the result of the combination of IPs from IBDGs at different locations in a microgrid. To ensure that relays measure different MPs for forward and reverse faults, an optimization model is formulated to set and minimize the total number of IPs by IBDGs. A pattern-based directional element is proposed based on the cosine similarity measure between the MP and the predefined characteristic pattern (CP) of each relay. CPs are set offline as the normalized MPs of forward faults. Moreover, a new direction-comparison relaying scheme is devised, which comprises of permissive overreaching transfer trip (POTT) and direction zone-interlocking (DZI) for primary and remote backup protection, respectively. Transient studies in PSCAD/EMTDC verify the performance of the proposed scheme under various faults and different microgrid topologies.

A robust distributed secondary voltage control method for islanded microgrids

This paper proposes a robust distributed secondary control approach for voltage restoration of an islanded microgrid with inverter-based distributed generators (DGs). In the proposed approach, each DG only requires the knowledge of its own and immediate neighboring information and dispenses with the need of complex communication network. Unlike most of the existing methods relied on detailed and accurate mathematic models, the proposed approach adopts an active disturbance rejection control (ADRC) technique that only requires minimal model information and can sufficiently eliminate the model uncertainties as well as unknown disturbances. By means of dynamically linearizing compensation, the proposed robust distributed secondary voltage controller can be built on a quasi-linear model with proportional feedback control design. Another novel idea in this paper is that it can estimate the state of voltage derivative through an extended state observer (ESO), which obviates the requirement of a differentiator which is hard to be physically realized. The control law design and parameter selection of the proposed ADRC-based distributed secondary voltage controller are discussed considering the closed-loop stability constraint for the composite system. The robust performance of the proposed control approach is verified based on a 4 DGs-based islanded microgrid test system. Results show that the proposed control approach is entitled with high robustness against both constant disturbance and time-varying disturbance.