Directional relays without voltage sensors for distribution networks with distributed generation: Use of symmetrical components

This paper presents two local algorithms using only currents measurements that could be used as a back-up protection (after the loss of the voltage sensors) in directional relays for distribution networks with distributed generation (DG), or as additional directional relay dispatched along the feeders. These algorithms are based on the symmetrical components (0-zero, 1-positive and 2-negative sequences) of the 3-phases currents. Due to the power flows generated by the DG the positive sequence current argument is unforeseeable, thus it is not possible to use only the positive sequence. Then, we first propose in this paper an algorithm using the I₂/I₀ ratio to locate a phase-to-ground fault upstream or downstream the detector. The second algorithm measures the zero and positive sequence components of the fifth harmonic of the current and calculates the I_{0_5}/I_{1_5} ratio. The performances of these algorithms are analysed for several DG power, fault resistance, capacitive current and neutral grounding (resistive and compensative grounding). The fluctuations of the phasors measurements is also taken into account in the range ±5% for the modulus and ±5° for the argument. The present paper shows that these algorithms can be reliable in the major part of the studied cases.     

A Petri net based protection monitoring system for distribution networks with distributed generation

Connection of distributed generation (DG) essentially changes distribution network operation and creates a range of well-documented effects varying voltage levels and short circuit currents. Among others, DG can alter protection system operations in distribution networks, leading to failure of reclosing, disconnection of healthy feeder or prevention of protection operation. This paper proposes a procedure, based on Petri nets and supported by a centralized monitoring architecture for monitoring failures of the protection systems in radial distribution networks. Some case studies applied to a real Italian distribution network proved the effectiveness of the proposed procedure that can therefore represent an effective solution to improve distribution systems reliability in presence of DG.     

Dyn11联结配电变压器过电流保护的特点

论述了Ｄｙｎ１１联结配电变压器低压侧发生两相或单相短路故障时，低压和高坟侧电流分布的情况。     

Decentralized congestion mitigation in HV distribution grids with large penetration of renewable generation

The integration of large amounts of Renewable Energy Sources (RES) is a great challenge for the operation of current power systems. In the Italian HV distribution system the main threat to security is caused by congestions that are likely to result in significant RES curtailment. This paper proposes an automatic procedure to be implemented in the Substations Automation Systems (SASs) to locally define control actions to mitigate congestions in such a way that RES curtailment is minimized. The algorithm assumes a reduced set of available data, simple computations and is especially designed to operate on-line as a decentralized automation function. Hence, the methodology intentionally avoids burdensome computations: it neither requires the knowledge of the complete network structure nor imposes any particularly strict requirement on the communication system. The effectiveness of the proposed methodology is shown with reference to a real model of a section of the Italian 150 kV grid.     

Real-time voltage control algorithm with switched capacitors in smart distribution system in presence of renewable generations

One of the most important responsibilities of Distribution System Operator (DSO) is to maintain the customer voltage within specified ranges. Capacitor banks have long been used to provide voltage support and to correct displacement power factor on distribution network. This paper presents a new approach for real time voltage control of distribution networks that has improvements over the conventional voltage control models. This approach will be active in emergency conditions where, in real time, the voltages in some nodes leave their permissible ranges. In the proposed model, it is assumed that renewable distributed generations are integrated in the distribution system, and the communication infrastructure of smart grid has already been implemented. Also, all the capacitors are fitted with Remote Terminal Unit (RTU) and are completely accessible and controllable. Unlike previous voltage control methods, the proposed approach does not need the load and renewable generation forecast data to regulate voltage. Moreover, the calculation time of the proposed approach is considerably reduced. The proposed voltage control algorithm is applied on two different models, and each presented model has a substantial improvement over previous models. DSO can choose one of them based on a trade-off between cost and power quality index. To verify the effectiveness and robustness of the proposed control scheme, the developed voltage control scheme is tested on a typical distribution network. The simulation results show that the proposed real time voltage control has the capability to maintain distribution voltage in specified ranges.     

Islanding protection of active distribution networks with renewable distributed generators: A comprehensive survey

Anti-islanding protection schemes currently enforce the renewable distributed generators (RDGs) to disconnect immediately and stop generation for grid faults through loss of grid (LOG) protection system. This greatly reduces the benefits of RDG deployment. For preventing disconnection of RDGs during LOG, several islanding operation, control and protection schemes are being developed. Their main objectives are to detect LOG and disconnect the RDGs from the utility. This allows the RDGs to operate as power islands suitable for maintaining uninterruptible power supply to critical loads. A major challenge for the islanding operation and control schemes is the protection coordination of distribution systems with bi-directional flows of fault current. This is unlike the conventional overcurrent protection for radial systems with unidirectional flow of fault current. This paper presents a comprehensive survey of various islanding protection schemes that are being developed, tested and validated through extensive research activities across the globe. The present trends of research in islanding operation of RDGs are also detailed in this paper.     

A generalized power flow analysis for distribution systems with high penetration of distributed generation

In this paper, the element incidence matrix has been extended to develop a comprehensive three-phase distribution system power flow program for radial topology. Three-phase overhead or underground primary feeders and double-phase or single-phase line sections near the end of the feeder laterals have been considered. Unbalanced loads with different types including constant power, constant current and constant impedance are modeled at the system buses. Substation voltage regulator (SVR) consisting of three single phase units connected in wye or two single-phase units connected in open delta are modeled to satisfy the desired voltage level along the feeder. The mathematical model of distributed generation (DG) connected as PQ and PV buses are integrated into the power flow program to simulate the penetration of DGs in the distribution systems. The proposed method has been tested and compared with different IEEE test feeders result. The developed algorithm has been used to study the impact of both SVR and high penetration of DG on voltage profile and system power losses.     

Analysis of distribution network protection issues in presence of dispersed generation

The presence of a significant dispersed generation (DG) capacity in existing distribution systems would cause in most cases some conflicts with correct network operation. This is mainly due to conceiving a distribution system as a “passive” radial network, with neither generators operating in parallel nor power flow control. Issues such as voltage regulation, system protection, and, generally speaking, power quality are still being investigated by researchers in order to maintain adequate service to customers in presence of DG.     

Procedures for fault location and isolation to solve protection selectivity problems in MV distribution networks with dispersed generation

Present planning and operation criteria used for MV passive radial distribution networks are, in general, not suitable to cope with the presence of a significant distributed generation (DG) capacity. In fact, many technical problems are still to be solved in order to increase DG penetration and to provide, at the same time, high service quality levels to customers.     

《可再生能源法》推动可再生能源电力发展

我国可再生能源发展受到各级政府的重视,近年来可再生能源电力开发和利用取得了很大成绩,未来将呈现快速发展趋势。阐述《可再生能源法》对我国未来可再生能源电力发展所产生的促进作用,主要包括在制定可再生能源电力发展规划、消除上网障碍、获得合理回报、解决差额费用以及提供资金保障等方面的促进作用。     

Strategic approach for reinforcement of intermittent renewable energy sources and capacitor bank for sustainable electric power distribution system

To meet ever increasing load demand in a sustainable way, reinforcement of photovoltaic (PV) array, wind turbine (WT) and capacitor bank in distribution network is proposed in this paper. A comprehensive planning model is presented to determine location and required installation capacity of multiple PV array, WT and capacitor units in an electric power distribution network under heavy load growth situation. Intermittent power generation of renewable energy sources (RESs) are quantified with suitable probability distribution functions and incorporated in the planning model. The planning approach considers several welfare areas in the distribution systems, viz., increment of profit margin, reduction of carbon-di-oxide emission, minimization of distribution power losses, enhancement of voltage stability level and improvement of the network security considering power flow, voltage limit, line capacity, RES penetration, capacitor penetration and utility economy constraint. Non-dominated sorting based multi-objective particle swarm optimization algorithm along with fuzzy decision making criteria is used to find the best allocation alternative for mix RES and capacitor planning problem. The effectiveness of the proposed model has been tested on a typical 28-bus Indian rural distribution network. The results show that more efficient techno-eco-environmental optimization can be obtained from combined RES and capacitor planning model.     

Active management of renewable energy sources for maximizing power production

The continuous increasing penetration of Distributed Generation systems (DGs) into Distribution Networks (DNs) puts in evidence the necessity to develop innovative control strategies capable to maximize DGs active power production. This paper focuses the attention upon this problem, developing an innovative decentralized voltage control approach aimed to allow DGs active power production maximization and to avoid DGs disconnection due to voltage limit infringements as much as possible. In particular, the work presents a local reactive/active power management control strategy based on Neural Networks (NNs), able to regulate voltage profiles at buses where DGs are connected, taking into account their capability curve constraints. The Neural Network controller is based on the Levenberg–Marquardt algorithm incorporated in the back-propagation learning algorithm used to train the NN. Simulations run on a real Medium Voltage (MV) Italian radial DN have been carried out to validate the proposed approach. The results prove the advantages that the flexibility of the proposed control strategy can have on voltage control performances, generation hosting capacity of the network and energy losses reduction.     

California's renewable generation and pumped hydro storage's profitability

An analysis of 860 daily observations finds that rising renewable generation does not significantly diminish a pumped hydro storage system's daily operating profits from energy sales at the day-head and real-time market prices. The system, however, faces severely inadequate investment incentive because its annual operating profit can hardly pay for its annual fixed cost.     

Distributed generation and the voltage profile on distribution feeders during voltage dips

The presence of distributed generators in the distribution network results in an increase of the voltage magnitude close to these generators, during a fault elsewhere in the distribution system or in the transmission system. This voltage dip mitigation capability of converter-connected distributed generation (DG) units is dependent on the control strategy of the converter. To compare the influence of different types of converter-connected distributed generators on the voltage profile along distribution feeders during a fault, the quantity,“voltage ratio” is used. This voltage ratio is obtained by division of the voltage during the voltage dip by the voltage just before the voltage dip. The different converter types are modelled, and the influence on the voltage ratio is analysed.     

Bi-level stackelberg game-based distribution system expansion planning model considering long-term renewable energy contracts

With the deregulation of electricity market in distribution systems, renewable distributed generations (RDG) are being invested in by third-party social capital, such as distributed generations operators (DGOs) and load aggregators (LAs). However, their arbitrary RDG investment and electricity trading behavior can bring great challenges to distribution system planning. In this paper, to reduce distribution system investment, a distribution system expansion planning model based on a bi-level Stackelberg game is proposed for the distribution system operator (DSO) to guide this social capital to make suitable RDG investment. In the proposed model, DSO is the leader, while DGOs and LAs are the followers. In the upper level, the DSO determines the expansion planning scheme including investments in substations and lines, and optimizes the variables provided for followers, such as RDG locations and contract prices. In the lower level, DGOs determine the RDG capacity and electricity trading strategy based on the RDG locations and contract prices, while LAs determine the RDG capacity, demand response and electricity trading strategy based on contract prices. The capacity information of the DRG is sent to the DSO for decision-making on expansion planning. To reduce the cost and risk of multiple agents, two long-term renewable energy contracts are introduced for the electricity trading. Conditional value-at-risk method is used to quantify the RDG investment risk of DGOs and LAs with diferent risk preferences. The efectiveness of the proposed model and method is verifed by studies using the Portugal 54-bus system.     

Adaptive overcurrent protection scheme coordination in presence of distributed generation using radial basis neural network

The operational performance of conventional overcurrent protection relay coordination connected to a distribution network is adversely afected by the penetration of distributed generators (DG) at diferent buses in the network. To address this problem, this paper proposes a novel adaptive protection coordination scheme using a radial basis function neural network (RBFNN), which automatically adjusts the overcurrent relay settings, i.e., time setting multiplier (TSM) and plug setting multiplier (PSM) based on the penetration of DGs. Short circuit currents and voltages measured at diferent buses are acquired using the remote terminal units (RTU) connected to diferent buses within the terminal network. Communication between the various remotes and local end station RTUs is through hybrid communication systems of fber optic and power line communication system modules. The new adaptive overcurrent protection scheme is applied to the IEEE 33-bus distribution network with and without DGs, for single and multi-DG penetration using both the ETAP and MATLAB software. The simulation results show the proposed scheme signifcantly improves the protection coordination.     

Cost-effectiveness of a modest expansion of renewable generation capacity in Texas

Using a large sample of 15-minute real-time market (RTM) price data from the Electric Reliability Council of Texas (ERCOT), this paper estimates the RTM price reduction caused by a modest 500-MW renewable generation (RG) capacity expansion in Texas, thereby determining whether RG investors face inadequate investment incentive. Its key findings are: (a) the expansion is highly cost-effective with sufficient investment incentive for RG investors; and (b) a gradual RG development likely benefits Texas’s retail end-users. Hence, Texas should pursue RG growth, lest it triggers substantial increase in costs for transmission expansion and operating reserve.     

UK scenario of islanded operation of active distribution networks with renewable distributed generators

This paper reports on the current UK scenario of islanded operation of active distribution networks with renewable distributed generators (RDGs). Different surveys indicate that the present scenario does not economically justify islanding operation of active distribution networks with RDGs. With rising DG penetration, much benefit would be lost if the DGs are not allowed to island only due to conventional operational requirement of utilities. Technical studies clearly indicate the need to review parts of the Electricity Safety, Quality and Continuity Regulations (ESQCR) for successful islanded operations. Commercial viability of islanding operation must be assessed in relation to enhancement of power quality, system reliability and supply of potential ancillary services through network support. Demonstration projects under Registered Power Zone and Technical Architecture Projects should be initiated to investigate the utility of DG islanding. However these efforts should be compounded with a realistic judgement of the associated technical and economic issues for the development of future power networks beyond 2010.     

英国可再生分布式发电市场进展和配电网应对方案

本文介绍了英国可再生分布式发电市场现状和英国配电网(132kV～11kV)所取得的相关进展.简述了英国分布式发电相关政府法规、市场补贴机制和分布式电源入网规程.分析了高比例分布式发电对配电网造成的困难,并用实际工程案例阐述了应对方案.进而,为解决分布式发电带来的故障电流增高、电压升高和反向潮流等技术问题,列举了英国各地区试点项目应用的故障电流限值器、FACTS装置、储能设备和中压直流输电等技术方案.介绍了英国实时网络分析软件和配电系统优化技术,应用该技术可进一步释放配电网容量.