Ireland is committed to supplying more than 40% of electrical energy from renewable sources — primarily wind generation, with a 10% penetration of electric vehicles and national smart metering — by 2020. The sole distribution system operator for the Republic of Ireland, ESB Networks (ESBN) will play a key role in achieving these targets.
Over the last decade, ESBN, which serves more than 2 million customers with its transmission system and distribution network that extends 180,000 km (111,847 miles), has invested strategically in infrastructure. As a result, Ireland has a modern, well-maintained electricity system in excellent condition.
Smart grid technology is fundamental to ESBN's strategy in delivering the national targets. In 2009, ESBN joined the Electric Power Research Institute's smart grid demonstration project to develop this expertise, sharing its experiences with the other project members, and accessing the pool of knowledge and technological breakthroughs of other project members. This ambitious demonstration includes the management and maximization of wind generation, preparing for electric vehicles, proving the potential to influence customer behavior through smart metering, and further improving the efficiency and security of the distribution network.
The intermittent nature of wind generation necessitates other technologies be available to compensate for this variable output in a coordinated and optimized manner. In addition, at night, when the load on the network is low, the output of the installed wind generation capacity will exceed the load on the network. Already, more than 50% wind penetration has been experienced at times of low load; in order to deliver the 2020 targets, three times the current level of wind energy is required.
High-Penetration Wind Generation
At ESBN, more than 50% of wind generation capacity is to be connected to the grid through the distribution network, a characteristic different than that of other countries. The 40% renewable energy target will require more than 5,000 MW of installed wind generation, which is a four-fold increase of the current 1,700-MW installed capacity. With a current system peak demand of 4,640 MW and limited interconnection, both the connection and the management of this scale of potential wind energy create significant challenges, demanding innovative solutions.
ESBN is undertaking several studies and field demonstrations to address these challenges:
Develop a coordinated local volt/volt-ampere-reactive (VAR) control strategy for a cluster of wind farms connected to the distribution network
Manage network voltage variations under contingency supply conditions through the novel deployment of a voltage regulator
Design substations optimized for the distributed generation market in Ireland, currently planned for two sites.
Coordinated Volt/VAR Control
Through the innovative use of built-in voltage-control systems in modern wind generation technology and coordinating the requirements of clusters of wind farms on distribution networks, it is possible to employ smart, cost-effective strategies to manage voltage with minimal reactive demand on the associated transmission system.
To explore the potential for volt/VAR control of wind generators and establish whether its practical implementation meets theoretical specification, a demonstration site with two large wind farms connected to a dedicated 110/38-kV transformer was selected. The cooperation of the two wind farm operators — Scottish and Southern Electricity (SSE) and Bord Gái — was secured to carry out the demonstration.
Baseline data was collected for several operational conditions and used by University College Dublin in simulation studies to examine the potential operational parameters and to test further operational control modes. The key conditions addressed were to assess voltage-control operation on a live network and to determine the optimum setpoints for managing volt/VAR control on clustered networks.
For the initial test periods, the wind farms operated in fixed voltage-control mode and behaved as required, providing the reactive power support to keep the network operating as normal. Additionally, experience and understanding of the full operational range of the wind farm control systems were developed and demonstrated.
The sensitivity of systems that result in control actions can be varied through the droop setting. Exploring droop settings has allowed ESBN to determine the setpoints required to ensure satisfactory regulation of network voltages without an abnormally high level of control activity that can result in hunting and an unwarranted and excessive number of transformer tap-change operations.
Initially, a relatively high droop setting of 4% was applied, which means there was a slightly reduced sensitivity of the feedback to the control system, causing some voltage variation. For subsequent trials, tighter droops of 1% and 2% were applied. A slightly less-sensitive reaction setting was initially implemented with both wind farms acting in voltage-control mode to establish a baseline for the system before testing the extremities of its operation.
The output of one of the wind farms when operating in fixed voltage-control mode confirmed that, where there is 1 MW or more, the reactive power-voltage (Q-V) characteristic behaves in a strict, linear manner along the predefined droop setting. This level of control is valuable and qualifying its performance in a real deployment is a key step in planning for such systems being more widely deployed in the future.
Following the field tests with both wind farms in active voltage-control mode, it is important to analyze the results of the test period. To date, the trial has been run on a network that does not supply demand customers, mitigating the risk for the demonstration period. However, the on-load tap changer at the 100/38-kV Trien Substation was maintained on automatic tap in normal operational mode as when supplying customers. Thus, the trials to date were carried out in such a way the network was not tested strictly for demand connections.
A key result of the trials was the networks operated within standard voltage levels. Moreover, there was sufficient additional voltage headroom to afford the provision of reactive power to higher network levels. This is an important consideration in Ireland, where the geographical location of wind generation that is distant from major conventional generation means the delivery of the required reactive power can lead to network congestion.
Innovation at Work
High levels of wind generation on a distribution network can cause a voltage rise because of the bidirectional power flow that can restrict the wind energy in-feed. This is particularly so in rural areas that have insufficient load to offset the level of generation. Where there are high levels of wind generation, it is unlikely back-feed network configurations could maintain the voltage within standard limits. As restricted or non-firm access is currently not an option, considerable investment is required to reinforce the network or, alternatively, the wind generation must be connected to an alternative distribution network or substation.
An example of this problem is on the 38-kV network in Rathmore, County Kerry, in southwest Ireland. The 38-kV circuit feeding this substation supplies three substations at Cloonbanin, Rathmore and Lacka. In case of a single fault on the 38-kV network feeding the Lacka Substation, the voltage at the Rathmore Substation would be in excess of distribution code limits because of the 17 MW of wind generation connected through the two wind farm in-feeds at Lacka (7.5 MW) and Gneeves (9.5 MW).
A novel approach is being demonstrated on this network to keep the voltage within statutory limits at load points while allowing the voltage to deviate from these limits on the connecting networks. By installing at Rathmore a voltage regulator usually deployed by ESBN to increase rather than decrease voltage, the power-quality criteria for load customers can be met without the requirement to invest heavily in largely redundant network reinforcements.
The network, as configured for a fault situation but allowing the wind farms to continue to feed as under normal circumstances, was initially put into operation in winter 2010 /2011. During this period, there were no departures from the statutory voltage limits. However, the most onerous conditions for this configuration occur when there is high-wind generation and low load, conditions that prevail over the summer period.
A key feature of this demonstration is that it was conducted on a network supplying existing customers. Part of the final trial of this project will be to test a new supervisory control and data acquisition (SCADA) system that will eliminate any possibility of operator error in the remote control of the voltage regulator. The SCADA design was completed mid-summer 2011 and implemented in late 2011.
The results of the network configuration from late July 2011 indicate the wind generation increases as the night-period load at Glenlara decreases, leading to a level of reverse power flows that causes a voltage rise at Rathmore. However, while the voltage on the 110-kV Rathmore-Cloonbanin line increases because of the low load, generation ratio (the voltage regulator installed at Rathmore ensures the busbar voltage from which customers are supplied) remains within network supply limits.
While the final review of the 30-second data is ongoing, the results of this demonstration appear to be positive as the system shows the potential to benefit the wind generators, network operators and interested parties planning to optimize the wind penetration on the Irish networks. By mitigating the voltage rise, wind generation offers support and valuable backup supply to networks already under pressure because of fault conditions. Furthermore, the demonstration that voltage can be maintained within statutory limits for supply customers could enable the maximum wind generation capacity to be installed on the network.
These trials offer a range of solutions that increase the capacity of existing networks for wind farm connections while reducing the potential cost of connection for the wind farm developer, and maintaining high power quality and security of supply for all customers.
In addition to the projects already in progress, ESBN has designed single transformer substations for the optimal, efficient and economic connection of clusters of wind farms. The new substation design was implemented at one site in 2011. These projects serve to illustrate ESBN's approach to ensuring Ireland's national targets on electrical energy from renewable resources can be met by directly addressing the practical, technical and economic challenges with innovative, creative and effective solutions.
Teresa Fallon ([email protected]) graduated with a degree in industrial engineering and information technology from the National University of Ireland, Galway in 1992. She has worked for ESB since graduation and has undertaken many roles in the organization. Fallon's most recent position was network investment manager for distribution capital work, but since June 2010, she has served as manager of smart networks.
Ellen Diskin ([email protected]) graduated with a bachelor's degree in engineering (electronic) with honors from Dublin City University in 2009, followed by research experience on telecommunications. In 2009, she joined ESB in network planning, and since 2010, she has been smart networks. In conjunction with work in ESB smart networks, she is performing research toward a Ph.D., supported by ESB Networks, on the integration of distributed energy resources, initially focusing on high penetration, non-firm wind integration.
John J. Simmins ([email protected]) has a bachelor's degree and Ph.D. in ceramic science from Alfred University, and his current position is senior project manager for the Electric Power Research Institute (EPRI). He develops robust system architecture for EPRI's smart grid projects and brings thought leadership in the area of integrating diverse applications including advanced meter infrastructure, meter data management systems, distribution management systems, customer information systems, geospatial information systems and outage management systems.
Summary of Site Test Periods
Bord Gáis | www.bordgais.ie
Electric Power Research Institute | www.epri.com
ESB Networks Ltd. | www.esb.ie/esbnetworks
Scottish and Southern Electricity | www.sse.com