Synchronous Distributed Generation Research Articles

Analysis the securable operation and protection coordination indices on multi‐objective sitting and sizing of synchronous distributed generations on distribution networks

AbstractDespite improving various technical indices such as operation and voltage profile, the integration of synchronous distributed generations (SDGs) may lead to the loss of protection coordination in the distribution network (DN). To tackle the problem, this paper presents a multi‐objective siting and sizing of SDGs on DN considering operation, economic and protection coordination indices simultaneously. The proposed scheme is structured in the framework of a four‐objective optimization problem, which aims to minimize the energy losses, the worst voltage security index (VSI), planning cost of SDGs and the protection index (PI) (the deviation of coordination time interval). Therefore, it is constrained to power flow equations, VSI, and protection coordination of overcurrent relays (OCRs). Then, the Pareto optimization based on the method of the weighted functions summation obtains an integrated single‐objective problem for the proposed scheme. Next, a hybrid evolutionary algorithm formed by merging particle swarm optimization (PSO) and crow search algorithm (CSA) is incorporated to achieve the optimal solution with unique response approximate conditions. Eventually, the suggested scheme is applied on distribution portion of IEEE 30‐bus ring and IEEE 69‐bus radial DN and numerical results confirm the efficient performance of SDG planning in terms of technical indicators and protective devices coordination.

A Coordination Scheme for a Combined Protection System Considering Dynamic Behavior and Wind DGs Fault Ride-Through Constraints

• A proposed design of a hierarchical protection scheme is presented. • Protective devices are optimally coordinated based on fault current transients. • Hybrid superconducting fault current limiters are optimally allocated to keep coordination under synchronous and inverter-based wind DGs. • Ride-through capability of wind DGs is integrated into the fault current limiter sizing problem. • Changes of network topology and variation of detected fault current are considered. A combined protection system has multiple types of devices such as directional overcurrent relays, reclosers, and fuses to protect distinct areas in a DG-incorporated network. An approach for setting all protective devices to realize complete coordination of adjacent ones taking transient responses into account is presented. The participation of synchronous generator-based and/or inverter-based wind DGs can degrade protection coordination. So, a scheme for sustaining coordination by optimal hybrid superconducting fault current limiters is proposed. The scheme can cope with variation of network topology; and handles the change of fault current level perceived by the protection device under a given topology. Moreover, instead of the conventional steady-state models, more realistic dynamic models of relays, reclosers, DGs, and fault current limiter are used. As a serious operation problem, undesired tripping of wind DGs under fault condition can occur due to the sag of terminal voltage below certain limits defined by national and international standards. Therefore, the proposed scheme prevents this undesired tripping of wind DGs by integrating its ride-through capability constraints into the fault current limiter allocation problem. The proposed approach is applied to the IEEE 9-bus and 30-bus systems. Results confirm the efficacy of the proposed method.

Innovative Optimal Nonstandard Tripping Protection Scheme for Radial and Meshed Microgrid Systems

The coordination of optimal overcurrent relays (OCRs) for modern power networks is nowadays one of the critical concerns due to the increase in the use of renewable energy sources. Modern grids connected to inverter-based distributed generations (IDGs) and synchronous distributed generations (SDGs) have a direct impact on fault currents and locations and then on the protection system. In this paper, a new optimal OCR coordination scheme has been developed based on the nonstandard time–current characteristics (NSTCC) approach. The proposed scheme can effectively minimize the impact of distributed generations (DGs) on OCR coordination by using two optimization techniques: genetic algorithm (GA) and hybrid gravitational search algorithm–sequential quadratic programming (GSA–SQP) algorithm. In addition, the proposed optimal OCR coordination scheme has successfully employed a new constraint reduction method for eliminating the considerable number of constraints in the coordination and tripping time formula by using only one variable dynamic coefficient. The proposed protection scheme has been applied in IEEE 9-bus and IEC MG systems as benchmark radial networks as well as IEEE 30-bus systems as meshed structures. The results of the proposed optimal OCR coordination scheme have been compared to standard and nonstandard characteristics reported in the literature. The results showed a significant improvement in terms of the protection system sensitivity and reliability by minimizing the operating time (OT) of OCRs and demonstrating the effectiveness of the proposed method throughout minimum and maximum fault modes.

Two‐stage planning of synchronous distributed generations in distribution network considering protection coordination index and optimal operation situation

This paper presents economic planning of synchronous distributed generations (SDGs) constrained to protection and operation indices in the distribution network. The proposed scheme has a two-stage formulation. In the first stage, optimal protection coordination (OPC) is modelled in the first phase in a network without SDGs. This phase considers the total operating time of all overvoltage relays in primary and backup protective modes for different fault locations constrained to the coordination interval and setting parameters of the relays. Then, candidate buses and maximum installable size of SDGs are extracted in the second phase in proportion to preserving the OPC based on calculating the protection coordination index (PCI). In the second stage, bi-objective planning of SDGs is modelled based on the results extracted in the first stage. In this stage, minimization of the weighted sum of planning cost of SDGs is presented by taking into account optimal AC power flow constraints and operation-planning model of SDGs. By implementing the scheme on standard distribution networks, numerical results verify the validity of the suggested scheme in maintaining the protection coordination when placing SDGs in distribution network and enhancing economic and operation indices in comparison to power flow studies.

A New Algorithm for Harmonic Impacts with Renewable DG and Non-linear Loads in Smart Distribution Networks

With the development of renewable energy sources (RECs), distributed generation (DG), micro-grid, electric vehicles, etc., in traditional distribution network which was earlier supply-side driven, single power supply, one-way service and basic reliance on manual management of business model turns into the direction of user interaction, the trend of two-way flow and highly automated these changes gradually formed the smart distribution networks (SDN). The rapid advancement in power electronic devices increases the fast growth of RECs into SDN and also increases the non-linear portion of loads. Due to this, harmonic currents will be injected into SDN. This paper proposed a new power flow algorithms (PFAs) for fundamental and harmonic impacts in SDN with integration of renewable energy based DG and presence of non-linear loads. The proposed methodology completely exploits the radial feature in distribution networks and developed the matrix of bus numbers (BUS_NUM) and matrix of branch numbers (BRANCH_NUM). These matrices make the implementation of PFAs simple. The accuracy of fundamental and harmonic PFAs is tested on IEEE-13 bus test feeder and they are found to be accurate with the literature. Different case studies are carried on IEEE-13 bus test feeder for analyzing the impacts of integration of synchronous based-renewable DG and power electronic based-renewable DG.

Distributed Coordinated Voltage Control for Distribution Networks With DG and OLTC Based on MPC and Gradient Projection

This paper proposes a distributed coordinated voltage control scheme for distribution networks with distributed generation (DG) and on-load tap changer (OLTC). In this scheme, static synchronous compensators (STATCOMs), DG units and OLTC are coordinated to regulate voltages of all buses to be close to the nominal value in the distribution network, mitigate voltage fluctuations, and minimize the number of operations of OLTC while considering different temporal characteristics of voltage regulation devices. The optimization problem of coordinating DG units and STATCOMs is decomposed by the gradient projection (GP) method. The local controller optimizes the reactive power outputs of DGs and STATCOMs according to local voltage and reactive power measurements, and still achieves the optimal coordination of DG units and STATCOMS in a decentralized manner without a central controller or communication between local controllers. The OLTC control scheme is designed to correct the long-term voltage deviations based on model predictive control (MPC) while minimizing the number of operations. The local controllers send the calculated reactive power references of DG and STATCOMs to the OLTC controller, which achieves distributed coordinated voltage control and mitigates the computation burden. A distribution network with two 20 kV feeders and 8 DG units was used to validate the control performance of the proposed coordinated voltage control scheme.

Microgrid Protection through Adaptive Overcurrent Relay Coordination

The inclusion of distributed energy resources (DER) in Microgrids (MGs) comes at the expense of increased changes in current direction and magnitude. In the autonomous mode of MG operation, the penetration of synchronous distributed generators (DGs) induces lower short circuit current than when the MG operates in the grid-connected mode. Such behavior impacts the overcurrent relays and makes the protection coordination difficult. This paper introduces a novel adaptive protection system that includes two phases to handle the influence of fault current variations and enable the MG to sustain its operation. The first phase optimizes the power flow by minimizing the generators’ active power loss while considering tolerable disturbances. For intolerable cases, the second phase opts to contain the effect of disturbance within a specific area, whose boundary is determined through correlation between primary/backup relay pairs. A directional overcurrent relay (DOCR) coordination optimization is formulated as a nonlinear program for minimizing the operating time of the relays within the contained area. Validation is carried out through the simulation of the IEEE 9, IEEE 14, and IEEE 15 bus systems as an autonomous MG. The simulation results demonstrate the effectiveness of our proposed protection system and its superiority to a competing approach in the literature.

An offline three-level protection coordination scheme for distribution systems considering transient stability of synchronous distributed generation

The growing penetration of synchronous distributed generation (SDG) affects the protection and stability of power distribution systems. The connection of SDG to distribution network may lead to change in the amplitude and direction of the current of feeders during the fault conditions and thus disturb the coordination of protection devices (PDs). On the other hand, the transient stability of SDGs as low-inertia machines after fault occurrence strictly depends on the fault clearing time which relies on the performance of the protection system. Therefore, an effective protection scheme should simultaneously consider both aspects of protection coordination of PDs and transient stability of SDGs. In this paper, an efficient and comprehensive offline three-level protection scheme is proposed to consider both protection coordination and transient stability indices. In the first level, the transient stability of SDGs is assessed in terms of critical clearing time (CCT) using offline simulations. In the second level, PDs are coordinated considering clearing time interval (CTI) of the main and backup PDs and CCT of SDGs. Finally, in the third level, the characteristic curves of PDs are modified if required by the transient stability of SDGs. The effectivity of the proposed method is demonstrated using the simulations on an active IEEE 33-bus distribution network in the ETAP software.

A Control Strategy to Mitigate the Sensitivity Deterioration of Overcurrent Protection in Distribution Networks With the Higher Concentration of the Synchronous and Inverter-Based DG Units

This article focuses on the indeterminate protection challenge of the sensitivity deterioration (SD) (also known as protection blindness) of the overcurrent protection (OCP) devices due to the large-scale integration of distributed energy resources (DERs) in the unbalanced distribution networks (DNs). The protection blindness of the OCP has been demonstrated through novel mathematical formulations to quantify the percentage of the SD for different types of distributed generation (DG) units and for the varying DG penetration levels. Furthermore, this issue has been realistically analyzed for the different fault impedances and DG locations to examine its impact on the existing OCP system. A mitigation strategy has been proposed to overcome the SD issue of the OCP protective devices (PDs) for a steadfast operation of the OCP devices without requiring costly upgrades of the protection system. This protection blindness issue has been explored and the mitigation strategies have been tested for both synchronous and inverter-based DGs using the IEEE standardized test feeders. The proposed mitigation strategy has been validated in the IEEE 34-bus test system. The simulation results prove the effectiveness of the proposed mitigation strategy to restore the protection coordination and suitability to improve grid resilience.

A Novel Synchronous DGs Islanding Detection Method based on Online Dynamic Features Extraction

Reliable and fast state discrimination are among the most important requirements in islanding detection methods (IDMs) to avoid islanded operation problems of distributed generation (DG) systems. This paper proposes a novel, IDM based on the dynamic response characteristics of synchronous DGs. The proposed method is based on four features of the dynamic behavior of synchronous DGs, namely, participation matrix; zero mode; oscillation frequency; and damping factor. This allows to distinguish between islanding conditions and grid-connected disturbances, even when demand and generation are closely matched. These features are extracted in practice from local measurements using Hilbert-Huang transform (HHT) technique. Moreover, a new method for online observation of the participation matrix based on online power system measurements is proposed. The impacts of synchronous DG controllers and motor loads are considered in the proposed method. Different scenarios such as load shedding, load restoration, capacitor switching, and short circuit faults have been simulated to assess the performance of the proposed method. Simulations are performed on a typical distribution system with real data for different scenarios to verify the high speed performance, reliability, robustness, and applicability of the proposed islanding detection algorithm. Comparison with alternative islanding detection methods prove the superiority of the proposed approach.

A first swing stability improvement approach in microgrids with synchronous distributed generators

A first swing stability improvement approach in microgrids with synchronous distributed generators

Leveraging a Genetic Algorithm for the Optimal Placement of Distributed Generation and the Need for Energy Management Strategies Using a Fuzzy Inference System

With the rising load demand and power losses, the equipment in the utility network often operates close to its marginal limits, creating a dire need for the installation of new Distributed Generators (DGs). Their proper placement is one of the prerequisites for fully achieving the benefits; otherwise, this may result in the worsening of their performance. This could even lead to further deterioration if an effective Energy Management System (EMS) is not installed. Firstly, addressing these issues, this research exploits a Genetic Algorithm (GA) for the proper placement of new DGs in a distribution system. This approach is based on the system losses, voltage profiles, and phase angle jump variations. Secondly, the energy management models are designed using a fuzzy inference system. The models are then analyzed under heavy loading and fault conditions. This research is conducted on a six bus radial test system in a simulated environment together with a real-time Power Hardware-In-the-Loop (PHIL) setup. It is concluded that the optimal placement of a 3.33 MVA synchronous DG is near the load center, and the robustness of the proposed EMS is proven by mitigating the distinct contingencies within the approximately 2.5 cycles of the operating period.

A Novel Passive Islanding Detection Scheme for Synchronous-type DG using Load Angle and Mechanical Power Parameters

This paper offers a novel passive islanding detection scheme for synchronous distributed generations (SDGs) based on the combination of (dδ/dt) and (dPm/dt). By the occurrence of islanding and grid-connected events, the load angle (δ) and mechanical power (Pm) of SDGs will change. These signals will oscillate during non-islanding events, while they will reach a stable point without any oscillation during islanding events. However, during some grid-connected disturbances such as high impedance faults and small capacitor switching, the oscillations of δ and Pm may not be significant. Therefore, the swing feature cannot be a reliable index and can lead to an inappropriate operation. Hence, to select the best parameters, a combination of four parameters, including dPm/dt, dδ/dt, dPm/dδ, and dδ/dPm is employed. The sensitivity analysis to discriminate islanding from non-islanding events has been reported in the presence of SDGs with various inertia constants, voltage levels, and loads. Based on the results, the combination of dδ/dt and dPm/dt has high accuracy and speed in clustering the occurred events. The efficiency of the proposed approach is tested on the IEEE 33-bus system. It is proved that the proposed approach decreases the non-detection zone to 0.01 MW and 0.01 MVAr of the powers nonconformity.

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.

Stochastic Assessment of the Impact of Distributed Synchronous Generators on Voltage Sags due to System-Wide Faults

In this paper, the authors use a stochastic approach to investigate the impact of distributed synchronous generation (DSG) on the severity of voltage sags due to system-wide occurring faults. Based on the method of fault positions, faults of various types are applied at pre-determined locations along all lines in the system and remaining voltages on the faulted phase at a monitoring point are recorded. This is done for a base case and a case with DSG. The results show with DSG, there is a general improvement in magnitudes of remaining voltage. Comparison of the populations of voltages for the two cases, using The Wilcoxon Signed Rank Sum Test, showed that these improvement are statistically significant. Further analysis showed that DSG either reduces severity of or eliminates some voltage sags. In addition, the number of expected trips due voltage sags is reduced with DSG, for any voltage sensitivity level assumed. The study concluded that incorporation of distributed synchronous generation is beneficial to the improvement of voltage sags and this should be a further incentive for their integration, especially into weak networks.

Determining Optimal Size of Superconducting Fault Current Limiters to Achieve Protection Coordination of Fuse-Recloser in Radial Distribution Networks with Synchronous DGs

In this paper, Superconducting Fault Current Limiter has been used to restore the missing protection coordination between fuse and recloser in the distribution network in presence of distributed generation resources. In order to evaluate different SFCL models, various types of this equipment including resistive, inductive and hybrid are modelled and investigated for achieving protection coordination. The purpose of this paper is to limit the current injected by DGs connected to the radial distribution network with fuse and recloser protection in presence of DGs protection coordinate between fuses and reclosers confirmed. Since increasing the size of SFCL increases its price, using the Genetic and the PSO Algorithms to calculate the minimum size of different types of SFCL in order to maintain network protection coordination. Results of simulation performed on an IEEE-34 bus network in DIgSILENT® software show that using this optimization method, not only missing protection coordination of the fuse and recloser caused by presence of DG can be solved but also impedance of SFCL can be minimized.

The Differential Protection Scheme for Microgrid Using S-Transform

The micro-grids with distributed generators enhance the aggregate power supply to the increasing load demand. However, the conventional over current relay protection may fail to operate when magnitude and direction of current differs significantly in grid connected mode and islanded mode depending upon DGs types and generating capacity. This paper presents a differential protection scheme using S-transform for detection and phase identification of faults in distribution systems integrated with different types of DGs. A radial distribution system with photovoltaic and synchronous distributed generators is simulated for data generation in PSCAD/EMTDC software. The proposed technique is tested on various fault cases and confirms accurate detection of faults and correct identification of faulty phases within a cycle for a 50 Hz grid nominal frequency.

Assessment of optimal allocation of renewable distributed generator sources in distribution network

AbstractThe integration of distributed generators (DGs) into distribution networks in optimal allocation is one of the main issues facing power system engineers to ensure improved stability and economic operation. This article presents a detailed analysis of the impacts of the optimal allocation and the number of DGs on both system steady‐state and transient performances of distribution networks. An oscillatory particle swarm optimization (OPSO) algorithm was used to find the optimal allocations of DGs via minimizing various objective functions that deal with Total Transmission Losses, Voltage Regulation, and Power Performance Index. The OPSO is used to optimize these functions as a single and as a multiobjective optimization problem. The effectiveness of the method is demonstrated with the IEEE‐14 bus as an example of distribution networks with a 50% increase in system loading. Two penetration scenarios have been considered, the optimal sizes and locations of DGs are obtained, and the results are presented. In addition, the impact of the penetration level of Photovoltaic and Wind Energy sources on transient performance is obtained using detailed nonlinear models of both synchronous machines and DGs sources. The system response is then obtained when the system is subjected to a three‐phase short circuit fault for six cycles and the results are presented in a comparative form for different penetration levels. The techniques and results presented in the article form a useful base for power system engineers in planning and operating distribution systems with high penetration levels of RDG sources.

Mesh grid power quality enhancement with synchronous distributed generation: optimal allocation planning using breeder genetic algorithm

Mesh grid power quality enhancement with synchronous distributed generation: optimal allocation planning using breeder genetic algorithm

Fast and systematic approach for adjusting ROCOF relay used in islanding detection of SDG

Frequency-based relays, commonly used for islanding detection of synchronous distributed generation (SDG), may either have large non-detection zones (NDZs) or wrongly operate for non-islanding disturbances unless they are efficiently adjusted. Trial and error simulation-based methods, employing application region (AR) and/or power imbalance AR (PIAR), are presented in the literature to adjust these relays for quick islanding detection without false trips. However, the associated settings are not optimal and finding them is time-consuming. In this study, an approach utilising particle swarm optimisation algorithm is proposed to adjust a rate of change of frequency (ROCOF) relay, one of the most effective relays in SDG islanding detection. The proposed approach needs a set of analytical formulas for determining the ROCOF relay performance curve and NDZ as well as AR and PIAR. These formulas are developed to be employed in the relay adjustment. By applying the developed analytical formulas to the optimisation-based approach, the ROCOF relay is adjusted fast and systematically. It is shown that while the performance of the relay adjusted by the proposed approach is close to the one with setting obtained by corresponding trial and error-based method, the latter is much more time-consuming than the former.