Drivers for Change
Following the political drivers to mitigate the effects of climate change on a global scale and the international trend towards privatization of the electricity supply industry, power systems are under immense scrutiny to enable the use of renewable energy sources while reducing the effects of centralized fossil fuel generation on carbon dioxide and toxic emissions. In Europe mandatory targets could mean that an increase of 34% of renewable energy sources are connected to the power networks of Europe leading up to the 2020 deadline. The increase of two-way power flows at sub-transmission and distribution level, the impact of intermittency and the freedom to connect to less than suitable parts of networks could all contribute to major instability and operational challenges for electrical network operators around Europe.

The impact of these regulated changes on the power network in general and substation protection and automation specifically will be profound. This is compounded by end users requirements for dependable, reliable and high quality power. This scenario is driving such initiatives as the European Technology Platform SmartGrids which has developed a holistic vision and road map to deliver a distributed architecture that is highly automated and driven by customer choices.

Current Status of Distributed Generation
The level of penetration of Renewable Energy Sources (RES) and Distributed Generation1 (DG) is set to increase dramatically over the next 30 years. The effect on substation protection and automation has been profound, requiring new and innovative solutions to evolve in order to enable the connection of uncontrollable and predominantly intermittent sources of energy at various voltage levels.

Up until recently the existence of DG was very limited, and therefore, had little impact on the operation of the system as a whole. Consequently, the regulatory framework and the connection requirements have been restrictive to protect the integrity of passive radial distribution systems.

However, in the past few years a gradual change of attitudes has been observed and in many countries more supportive regulations have already been implemented or currently are under revision. Although the situation is generally improving, it is still far from being resolved as the majority of the proposed changes are not coordinated internationally.

The fast, uninhibited expansion of privately owned DG in both distribution and transmission networks is not happening without consequences. There is potentially a conflict between the requirements of the power utility, whose primary concern is that of the security of its network, and the requirements of the DG whose primary objective is profit via the commercial production of electrical energy. Additionally, under deregulation, new plants are being built on more short term, cost-based decisions. Long-term investments such as large-scale expensive power plants or advanced communication based protection and automation infrastructures are not favored in deregulated markets where customers cannot be secured. As it is often difficult to resolve such conflicting interests, apart from the utilities and DG owners, this report is also aimed at the policy makers and legislative bodies whose decisions can make a fundamental impact on the future of DG and active networks. The restructured electricity market is not projected to stimulate renewable energy technologies unless required by government policies as it is foreseen that RES will remain more costly than fossil fuel alternatives to at least 2015.

The progressive increase in penetration levels and generator sizes, as well as the fluctuating nature of power and its dispersed locations, pose a number of technical challenges for the DNO/TSO relating to: Stability; Voltage profile and voltage transients; Congestion; Balancing of active and reactive power (reserve requirements); Losses; Fault Ride Through (FRT) capability; Short circuit levels; Power Quality; Control and monitoring; Protection co-ordination; Islanding Detection; Synchronization. Specifically, those issues concerning protection, such as islanding detection, FRT, stability, etc. are dealt with in this report.

Additionally, special attention needs to be paid to the additional benefits which the presence of DG can potentially bring to the network operators. These benefits include enhanced load profile management, provision of reactive power to support voltage regulation, contribution to frequency control, improvement of the supply reliability to customers, to name but a few. Most of these valuable services and opportunities unfortunately come at high initial cost. However, the benefits can be substantial if the changes are introduced in a coordinated manner and according to the best practice. The increased presence of power electronic based DGs will provide a natural asset for better control and stability of such networks through advanced control capabilities, improve flexibility and response time to changing network conditions facilitating more real-time automation and adaptive protection to maintain system integrity.

The purpose of this report is to present guidelines on the requirements for protection, control and monitoring of DG which can address the primary concerns of both the utilities and the generators.

Aim, Objectives and Scope
This report primarily aims to identify the impact of renewable and distributed energy sources on the substation protection and automation. The best practice from around the globe is highlighted through the international review of DG connection solutions. It forms a basis for identifying the future trends and possible solutions which will need to be adopted if power networks are to remain robust and resilient to the increasing consumer and environmental demands. The report seeks to improve the DG connection practices by contributing to the process of international standardization, specifically in the area of power system protection and automation associated with DG.

As an initial basis, the best practice from around the globe has been collected to form an informative compendium of current methods and solutions for the connection of DG at transmission and distribution levels. Both protection and automation aspects are analyzed leading to the conclusions and recommendations put forward by the working group. The intended audience includes primarily utility companies and generators, but also the policy makers and regulatory bodies which can make a decisive contribution to the integration of DG.

The scope of the project covers particularly the connection of the large renewable generation (excluding nuclear) associated with the sub-transmission and transmission power networks, although producers connected to MV network (35 kV and below) are also considered in the report as reference because of the unclear limits in regulations and practices in some countries and due to the possible impact on the transmission system performance.

From the outset of this work it has become apparent that the vocabulary used to describe system voltage levels (EHV, HV, LV, etc.) is different in each country, and also differs from many of the defined standards including CIGRE. For instance in the UK distribution level is generally referred to as including what would be considered sub-transmission level in continental Europe.

This report focuses on medium and large power plants because of their greater influence on the network performance, which include: synchronous rotating machines from diverse sources (hydro, gas, steam, CHP, CCGT, reciprocating engines, etc.), asynchronous wind turbines and converter based generators, such as full converter wind turbines and large PV installations. Microgeneration and energy storage systems (fuel-cells, batteries, etc.) are not covered in this report.
Although, many other considerations associated with DG connection are also included, the report does not cover in detail the following aspects: control and stability; protection practices for small-scale DG connected to LV distribution system; power quality; voltage and frequency regulation; economic considerations.

Main Findings
Based on the review of the practices in different countries and the available literature the main findings of the working group can be summarized as follows:

  • In the recent years, a spectacular growth of DG/RES has been happening widely in all countries and this is expected to continue over the next few decades
  • The increase of the DG penetration in distribution networks with the emergence of large DG power plants connected to sub-transmission network has pushed some countries to develop new requirements or revise the existing codes to cope with the new scenario. However, this process is has not been coordinated internationally. This work highlights the pressing need for better international co-ordination of these requirements
  • In view of the different connection analyzed for each country it is commonly acceptable to establish different DG connection criteria for distribution networks generally radial up to 69kV networks, and for (sub)-transmission meshed 100kV and above networks. In transmission networks, the DG is normally connected to a dedicated or existing substation, directly or through a dedicated line. Also, tapped connection to an existing two ended line is acceptable in some countries
  • All countries use practically the same set of interface protections, which are under/overvoltage protection, under/over-frequency protection, phase and ground overcurrent protection with transformer differential protection. For more detailed recommendations and best practice in generator interface protection refer to chapter 5
  • The impact of reduced short circuit current contribution from new generation technologies on the performance of the current line protection needs to be assessed, especially for those connections where current detected by the relays is exclusively supplied by the DG, i.e. direct line connections, tapped connections or connections to a substation with a few lines
  • With growing DG penetration levels of all sizes and connection voltages there is an increased need for high performance anti-islanding protection. Taking into account current relay offerings the implementation of anti-islanding protection is always a question of compromise between sensitivity and stability, and performance and cost. Traditionally, the stability aspect was of lesser importance as the network operation did not depend on the presence of DG. This situation is gradually changing and reliable islanding detection is becoming more important. The use of active methods and alternative communication technologies should be considered
  • With low DG penetration levels or small units, system operation does not depend significantly on the generated power from DG s and in case of abnormal situations all DGs are required to trip. In these cases conventional protection and restoration philosophies are valid and few adaptations in the existing network protections are required
  • As the penetration increases the units are considered more important for the security of the power system. Also, the implications in the network protection and the need for coordination, becomes greater. System operators are demanding capabilities from generators to stay connected under certain voltage (defined by FRT curves) or frequency excursions, even contributing to support the system parameters during these conditions. Classical frequency settings can result in significant tripping of DG for disturbances in the transmission network, so settings should be adapted to those of conventional generation for proper co-ordination. There is also a noted lack of coordination between undervoltage interface relays and FRT capabilities
  • The FRT requirements for wind generation in some networks are unnecessarily demanding. The reactive consumption and time responses are not appropriate for system stability. Therefore, the harmonization in FRT requirements is highly recommended
  • New technologies, such as inverter based generators, can offer capabilities of voltage, power factor and frequency control. In the future, the technology will evolve to provide new capabilities that maximize the output of non-synchronous generators, allow increased integration of new generation technologies and provide better system security
  • Current automation, monitoring and control practices are not sufficient for the needs of DG integration. Regional control centers for DG/RES should be extended and integrated. Also, more control capabilities (voltage and frequency) and contribution to ancillary services should be added. Local automation functions and the development of communication networks to control the operation of DG by the transmission system dispatch together with more advanced forecast applications will permit the integration of DG into the network and exploit their control capabilities similar to conventional generation. New control technologies such as pitch control and new power management tools will allow more flexible operation of wind farms and a greater integration into the market mechanisms
  • The IEC 61850 communications standard will help promote more use of WAP/SPS and adaptive protection schemes in the future, required for DG protection applications. Cyber security is a growing issue in the power industry and more secure communication networks are required in the future to keep data confidentiality and integrity.
  • Streamlining, simplification and harmonization of existing permission procedures and standardization of the grid codes for the connection of DG is also required to encourage large scale DG integration. The standardization in the connection requirements of DG, particularly of the protection equipment and settings criteria, will be very positive for the development of the DG, especially in highly interconnected networks

With increased levels of DG penetration in the future and the tendency of connecting these generators at higher voltage levels, it is expected that the benefits of islanded operation will be more significant and the additional cost relatively lower.
Intelligent protection schemes which can adapt their protection settings will play an important role to enable stable operation and protection of these power islands.

Let?s start with organization in protection testing