Subtransmission System Research Articles

A novel scheme for current-only directional overcurrent relay

Overcurrent relays are widely used as main protection in sub-transmission and distribution systems. In mesh and multi-source networks, application of directional relay is unavoidable. Traditional directional overcurrent relays use the reference voltage phasor as the polarizing quantity to estimate the direction of the fault. Traditional direction distinguishing scheme is unreliable in the case of close-in faults. In this paper, a novel algorithm for directional overcurrent relay is proposed. The new algorithm uses only current signals for determining the fault direction. It uses superimposed component of the current signal and does not require phasor estimation. This new algorithm uses pre-fault current signal as the polarizing quantity. The proposed method is tested on simple power system in different situations. The results show it leads to fast and reliable directional protection.

送電線合成インピーダンスを用いた分散形電源を含む2次系統の縮約手法

This paper proposes a reduction method of subtransmission system including various distributed generators (DG) mainly for the efficient transient stability studies. The proposed method is based on the concept of expressing the original branch impedances consisting of the reduced system as equivalent branch impedance. The reduced system consists of two nodes and two branches with two equivalent impedance, those are obtained respectively for loads and DG's of the original system. Each node in the reduced system represents either load or DG, and both the value of two reduced branch impedance can be kept in the reasonable positive range. The reduction system is configured so that the operating condition at boundary node after reduction is matched to that of the original network, also the DG terminal voltage magnitude at the reduced system is maintained as the average DG voltage magnitude in the original system. Demonstrative result to the application in the transient stability simulation is presented in order to verify the practicability of the proposed method.

Implementation of Directional Over-current Relay Coordination Approaches in Electrical Networks

As the smart grid concept is employed in the electric power system, network load flow and topology changes intensively to meet the best generation-demand balancing point. These changes must account for protection devices to enhance their performance. The coordination of directional over-current relays is most commonly studied based on fixed network operation and topology within a mesh power system. But sub-transmission and distribution systems constantly operate differently to satisfy the variety of load demand levels throughout the day and year. Hence, if the setting of directional over-current relays changes according to the different operations of the system, then relay operation time and sensitivity can both be enhanced. This can potentially improve the protection performance in a smart grid system. Therefore, this study is carried out based on the comparison among three coordination approaches: conventional (fixed settings), discrete (groups of settings), and continuous or real time (dynamic settings).

Online coordination of directional overcurrent relays using binary integer programming

This work presents a binary programming model for online coordination of directional overcurrent relay problems in interconnected power distribution and subtransmission systems. The proposed model considers discrete time dial and pickup current settings as in microprocessor-based relays. The coordination problem is solved for every topological change in the network and whose optimized settings are remotely adjusted on each relay through a SCADA system, enhancing the speed and sensitivity of the protection system. A pre-processing step is performed to ensure that the operation time of each relay be below the melting curve of the protected cable. Computational results for the binary programming model solved by CPLEX optimization package are compared to heuristic-based techniques. The simulation results indicates that our approach is suitable for online applications, since an optimal solution can be found in reduced times.

A Fast Outage Planning Method Focusing on Circuit Breakers Near Outage Facilities

SUMMARYJapanese electric power utilities installed many electric power facilities to respond to increasing power demand in the era of high economic growth. However, these facilities have aged about 40 years and should be replaced in the near future. The utilities will now have to replace larger numbers of facilities at the same time because of the increase in the number of power facilities to be replaced. Therefore, outage planning should be made more efficient. However, creating an outage plan requires time and labor, because the plan must specify (1) the outage schedule, (2) the power system configuration during outage operations, and (3) the system switching of the power system in order to conduct the outage operations. To solve these problems, a fast outage planning method focusing on circuit breakers near the outage facilities is proposed in this paper. Simulations run on IEEJ subtransmission system model I confirm the validity of the proposed method.

A Review of Distributed Generation Resource Types and Their Mathematical Models for Power Flow Analysis

The emergence of Distributed Generation (DG) in distribution network has changed the configuration of this century’s power system in terms of power flow. The reason for this is that DG affects the power flow and voltage conditions in the distribution system; contrary to its traditional unidirectional nature in radial configuration. It is worth mentioning that the change in the direction of power flow is not limited to the distribution network, but can as well extend to the transmission or sub-transmission systems, especially when DG penetration is high. This paper gives an overview of DG types and modeling techniques of the DG for power flow analysis during planning and operations. Various DG technologies are highlighted, different models of DGs are presented and some key challenges ahead with current drive towards smart grid networks is also discussed.

Coordinated Voltage Control of Weak Sub-transmission Networks Considering Wind Power Variability

In this paper, a novel coordinated control method is proposed to handle the negative voltage impacts on weak sub-transmission systems, which are caused by wind power fluctuations. Firstly, wind variations are divided into large and small fluctuations. Then, according to the different control characteristics of on-load tap changers (OLTCs) and static compensators (STATCOMs), the OLTCs are used to minimize the impacts caused by large fluctuations, while the STATCOMs are used to eliminate the impacts introduced by small fluctuations. The model predictive control (MPC) and a consensus based distributed control techniques are adopted to design the control schedules of OLTCs and STATCOMs, respectively. It is shown that by the coordination of OLTCs and STATCOMs, voltage violations of points of interconnection (POIs) are eliminated, and voltage fluctuations at unregulated load buses are mitigated. The effectiveness of our proposed control scheme is presented through a case study on the modified IEEE 14-bus test system with a real wind power profile. According to the simulation results, it turns out that the proposed approach can reduce the operation number of OLTCs significantly, and decrease the required capacity of STATCOMs dramatically by comparison with the existing control scheme.

Implementation of Dynamic Line Rating in a Sub-Transmission System for Wind Power Integration

Based on conventional static line rating method, the actual current carrying capability of overhead conductors cannot be judged. Due to continuous increment in electricity demand and the difficulties associated with new line constructions, the overhead lines are therefore required to be rated based on a method that should establish their real-time capability in terms of electricity transmission. The method used to determine the real-time ampacity of overhead conductors not only can enhance their transmission capacity but can also help in allowing excessive renewable generation in the electricity network. In this research work, the issues related to analyzing an impact of wind power on periodical loading of overhead line as well as finding its static and dynamic ampacities with line current are investigated in detail. Moreover, the investigation related to finding a suitable location for the construction of a 60 MW wind farm is taken on board. Thereafter, the wind park is integrated with a regional grid, owned by Fortum Distribution AB. In addition to that, the electricity generated from the wind park is also calculated in this project. Later on, the work is devoted to finding the static and dynamic line ratings for “VL3” overhead conductor by using IEEE-738-2006 standard. Furthermore, the project also deals with finding the line current and making its comparison with maximum capacity of overhead conductor (VL3) for loading it in such a way that no any violation of safe ground clearance requirements is observed at all. Besides, the line current, knowing the conductor temperature when it transmits the required electricity in the presence of wind power generation is also an important factor to be taken into consideration. Therefore, based on real-time ambient conditions with actual line loading and with the help of IEEE-738-2006 standard, the conductor temperature is also calculated in this project. At the end, an economic analysis is performed to evaluate the financial advantages related to applying the dynamic line ratings approach in place of traditional static line ratings technique across an overhead conductor (VL3) and to know how much beneficial it is to temporarily postpone the rebuilding and/or construction of a new transmission line. Furthermore, an economic analysis related to wind power system is taken into consideration as well to get familiar with the costs related to building and connecting a 60 MW wind farm with the regional grid.

Selecting the Best Compatible SVC Type in Power Systems for transient Stability

Considering the increased trend to use renewable energy to meet electrical energy demands, a reliable performance also should be adopted in transmitting the generated power through the stable transmission and subtransmission systems for which nowadays Flexible AC Transmission System (FACTS) controllers are used extensively. Transient stability is one of the most important issues in power system stability analysis. FACTS devices have confirmed their capabilities in various aspects in power networks. In this paper, to improve transient stability of power systems, a shunt connected FACTS devices have been introduced, i.e., Static VAR compensators (SVCs). The SVC has the most diversified branches among FACTS devices. The main aim of this work is to study and analyze the impact of eight branches of SVC on improvement of transient stability and to choose the best of them. Simulations are performed in MATLAB/SIMULINK environment.

Circuit Breaker Transient Recovery Voltage Requirements for Medium-Voltage Systems With NRG

The industrial distribution power systems in Northern Alberta supplies electrical energy to satellite locations and mining areas. In some aspects, these systems differ from their counterparts in regular utility distribution cases. Accordingly, they require special attention when performing transient recovery voltage (TRV) studies and identifying ratings for new breakers to be added to the system. Meanwhile, North American (IEEE) and European (IEC) Standards are embarking on significant efforts to harmonize breaker specifications and testing requirements, including TRV tests. An electromagnetic transient program (EMTP) study has been performed to verify system TRV requirements under different conditions against Standard requirements and supplier's provided test data. Concerns, lessons learned, and some other findings associated with the study are documented in this paper for future references and for advancing robust usage of EMTP (alternative transient program) for the performance of such studies for subtransmission and distribution systems in different electrical systems.

Dynamic expansion planning of sub-transmission substations and defining the associated service area

In this paper, a new methodology is presented for dynamic expansion planning of sub-transmission substations (DEPSS). The proposed method deals with the expansion schemes of the facilities which should be installed and/or reinforced in order to make the sub-transmission system capable of supplying the forecasted demand at the lowest cost while all technical constraints are satisfied. DEPSS is inherently a mixed integer nonlinear programming (MINLP) problem due to the prevalent electrical and expansion constraints, cost indices in objective function and decision variables. This nonlinear problem is simplified to a linear problem without any neglecting by the proposed method. The location, capacity and construction time of substations and MV feeders, as well as the optimal service area of substations are determined through a dynamic approach for the planning years. Meanwhile, the optimal operation capacities of substations are determined at each load level in every planning year. The effectiveness of the proposed optimization method is discussed in the first case study which is related to just placement and defining the associated service area. Also, the proposed dynamic method is tested on a realistic case study and compared with the static and multistage approaches.

Detection of Islanding Feasibility in Subtransmission Systems

The penetration of Renewable Energy Sources (RES) is increasing in modern power systems. The integration of large amounts of RES is a challenge but the technology provides many tools to exploit them for uses and functions considered, so far, a prerogative of more traditional power plants. For example, since a large amount of RES is installed in HV subtransmission grids, it is possible to exploit them in wanted and unwanted islanding conditions. In this paper, a procedure for evaluating off-line the feasibility of islanded operation and planning it is presented. This procedure defines which generators and loads could remain connected after islanding to balance the real power of the island. For each possible set of islanding events, the set of discrete control actions (opening switches) is determined and communicated to local Substation Automation Systems, to be put in place in case of actual islanding. Results, including dynamic simulation responses, are shown with reference to real power systems

A multi-stage MINLP-based model for sub-transmission system expansion planning considering the placement of DG units

This paper presents a new multi-stage model, based on the mixed integer nonlinear programming (MINLP) approach, to determine the optimal sub-transmission system expansion planning (SSEP). This model considers the placement of distributed generation (DG) units in distribution networks over the planning periods. The SSEP determines the facilities which should be installed and/or reinforced so that the system serves the forecasted demand at the lowest cost while satisfying the operational constraints. These constraints include the maximum allowable voltage drop, and the operation capacity of substations, MV feeders, and DG units. Meanwhile, the proposed model presents the optimal planning of existing and new sub-transmission substations, as well as the routes of medium voltage (MV) feeders. Moreover, the allocation and operation planning of DG units over the planning period is considered, as well. On the other hand, the expanding limitations of existing and candidate sub-transmission substations and DG units are considered as the expansion constraints in the SSEP problem. The effectiveness of proposed MINLP-based model is demonstrated using several case studies.

Research on dynamical characteristics of wind turbine gearboxes with flexible pins

Wind energy is believed as one of the most efficient clean renewable energies and is explored more often currently. The dynamic behaviors of wind turbine gearboxes are concerned by engineers since they affect the whole working performances and the service lives of wind turbines. In this work, a coupled dynamic model is developed for a wind turbine gearbox with flexible pins. For the planetary gear stage, the sun gear is floated and the planetary gears are flexibly supported by flexible pins. The sub-transmission system of the gearbox, which consists of a planetary gear stage and a parallel gear stage, and the body sub-system are coupled through the bearings which are simulated as springs. The dynamic behaviors of the system are studied using a lumped parameter model, in which the stiffnesses of the gearbox body is predicted using the finite element method, the results is also verified by experiments conducted on a test rig.

On-line coordination of directional overcurrent relays: Performance evaluation among optimization algorithms

The coordination of directional overcurrent relays is most commonly studied based on a fixed network topology in a mesh electrical system. However, in real life, the network topology and the operation of elements of sub-transmission and distribution systems are constantly changing. In addition, modern electrical systems tend to operate near boundary conditions to satisfy the clients. As a result, the directional overcurrent relays (DOCRs) lose speed and sensitivity. Therefore, the principal objective of this paper is to perform on-line coordination of the directional overcurrent relays in order to enhance speed and sensitivity. The secondary objective is to formulate the differential evolution algorithm and compare it with the genetic algorithm and ant colony algorithm. The objectives were accomplished by developing an on-line algorithm that worked with the optimization algorithms.

Hybrid procedure including subtransmission systems and substations for reliability assessment

A new procedure focused on reliability analysis of subtransmission systems supported by the state enumeration technique is presented. This new methodology is conducted in three stages. First, a classical state enumeration reliability assessment is performed for the branch-node model of a subtransmission system, assuming that substations are perfectly reliable. Second, a detailed model of the subtransmission system is considered and the reliability of each substation is assessed by considering them in a ‘one-by-one’ process, supposing perfect operation for the branch-node model. Finally, the reliability indices calculated in the first and second stages are analytically combined to obtain the reliability indices for the subtransmission system (system reliability indices) and for the load nodes of the distribution system (load-node reliability indices). Test results show that the proposed methodology is suitable for both planning studies and 24 h-ahead security assessment.

A Study of Eliminating the Protective Coordination Curve Intersection of Electromechanical Overcurrent Relays in Subtransmission Systems

This paper presents a practical new method to quickly and effectively eliminate coordination curve intersection problems in subtransmission systems resulting from improper protective coordination settings in electromechanical overcurrent relays. Firstly, a database representing the relay operating characteristics was constructed based on the electromechanical overcurrent relay characteristic curves published by the manufacturer. Then, combining with this database, a fixed-point coordination curve adjustment procedure dedicated to electromechanical overcurrent relays was proposed to tackle the curve intersection situation of the coordinated upstream and downstream relay pair appearing in the original design. In addition to correcting manmade errors that could cause unexpected widespread power outages when fault occurs, the proposed technique can also enhance the effectiveness and accuracy of coordination. This study also simulated an actual case in Taiwan Power Company to verify the feasibility and practicality of the proposed method.

Dynamic sub-transmission system expansion planning incorporating distributed generation using hybrid DCGA and LP technique

Since the emerging of distributed generation (DG) technologies, their penetration into power systems has provided new options in the design and operation of electric networks. In this paper, DG units are considered as a novel alternative for supplying the load of sub-transmission system. Thus, the mathematical model of considering DG on the expansion planning of sub-transmission system is developed. Fix and variable costs of the plan and the related constraints are formulated in the proposed model. The proposed objective function and its constraint are converted to an optimization problem where the hybrid decimal codification genetic algorithm (DCGA) and linear programming (LP) technique are employed to solve it. Solution of the proposed method gives the optimal capacity of substations; optimal location and capacity of DGs as well as optimal configuration of the sub-transmission lines. To demonstrate the effectiveness of the proposed approach, it is applied on a realistic sub-transmission system of Zanjan Regional Electrical Company, Iran, and the results are evaluated.

A Fast Outage Planning Method Focusing on Circuit Breakers Near Outage Facilities

SUMMARY Japanese electric power utilities installed many electric power facilities to respond to increasing power demand in the era of high economic growth. However, these facilities have aged about 40 years and should be replaced in the near future. The utilities will now have to replace larger numbers of facilities at the same time because of the increase in the number of power facilities to be replaced. Therefore, outage planning should be made more efficient. However, creating an outage plan requires time and labor, because the plan must specify (1) the outage schedule, (2) the power system configuration during outage operations, and (3) the system switching of the power system in order to conduct the outage operations. To solve these problems, a fast outage planning method focusing on circuit breakers near the outage facilities is proposed in this paper. Simulations run on IEEJ subtransmission system model I confirm the validity of the proposed method.

Impact of smart subtransmission level fault current mitigation solutions on service reliability

Superconductive fault current limiters and substation automation systems are two major smart grid technologies that can be directly or indirectly used for mitigation of high fault current levels at subtransmission systems. In comparison to the other conventional fault current mitigation solutions, it is expected that these solutions have much less negative impacts on the service reliability. This paper aims to quantitatively evaluate how these smart fault current mitigation solutions may affect the major attributes of the service reliability. Sustained interruptions, momentary interruptions and voltage sags are considered as major events that might appear in the electricity service of the customers due to a typical short circuit fault on the subtransmission systems. A typical Finnish subtransmission system is utilized in this paper for directing the quantitative reliability case studies. The study results show that in addition to the fault current mitigation aspects of the targeted solutions, their possible impacts on the service reliability should also be taken into account.