1. INTRODUCTION

1.1. BACKGROUND

The safe and economic operation of a nuclear power plant (NPP) requires the plant to be connected to an electrical grid system that has adequate capacity for exporting the power from the NPP, and for providing a reliable electrical supply to the NPP for safe startup, operation and normal or emergency shutdown of the plant. Connection of any large new power plant to the electrical grid system in a country may require significant modification and strengthening of the grid system, but for NPPs there may be added requirements to the structure of the grid system and the way it is controlled and maintained to ensure adequate reliability. The organization responsible for the NPP and the organization responsible for the grid system will need to establish and agree the necessary characteristics of the grid and of the NPP, well before the NPP is built, so that they are compatible with each other. They will also need to agree the necessary modifications to the grid system, and how they are to be financed.

For a Member State that does not yet use nuclear power, the introduction and development of nuclear power is a major undertaking. It requires the country to build physical infrastructure and develop human resources so it can construct and operate a nuclear power plant (NPP) in a safe, secure and technically sound manner. The IAEA is preparing a series of guides to assist Member States in these endeavours. Reference [1] is one of these guides which identifies and provides guidance on nineteen infrastructure issues that need to be addressed in the introduction of a nuclear power programme. One of these infrastructure issues is the development of the electrical grid.

Whilst most Member States already have an electrical grid system, it may require significant development to be suitable for the connection of an NPP. For a country that does not yet have an NPP, the necessary modifications to the grid system may be extensive. The modifications will include the new physical connections from the existing grid system to the NPP site, but may also include other changes to the grid system and the way it is operated and maintained to ensure that it can provide a reliable electrical supply to the NPP, to ensure safe and secure operation of the NPP. The key issues are the particular requirements of the NPP with regard to nuclear safety and a reliable electrical supply, and the large unit size of standard NPP designs. Experience in Member States that have operating NPPs shows that careful attention must be paid to the design and operation of the electrical grid system and the interface between the NPPs and the grid, in order to avoid events that might challenge the safety of the nuclear plant. Connecting an NPP to the grid requires the consideration of issues in addition to those that would be considered to connect a large fossil fuel power station or hydroelectric power station.

Even for a country that already uses nuclear power, the addition of a new NPP may require significant investment to develop and strengthen the existing electric grid, depending on the size and location of the new NPP. In many Member States, the organization of the electricity industry now is very different from the organization at the time that the earlier NPPs were built, with separation now between the transmission system operator and the operators of power plants, and with different commercial arrangements for the trading of electricity. Hence, the arrangements for connecting a new NPP may require negotiations and new commercial agreements between separate organizations.

Recent IAEA publications [1–3] provide a brief indication of some of the issues related to the electrical grid and NPPs; this publication provides more detail, and also updates the advice that was contained in technical reports published by the IAEA in the 1980s [4–6], making use of the experience in Member States since those publications were written.

In March 2011, while this publication was in preparation, the Fukushima Daiichi NPP in Japan suffered severe damage from an exceptionally strong earthquake followed by flooding from a tsunami. The details of this event are still being investigated, and it will be some time before all the lessons from it are known and published. However, as the vulnerability of the grid connections to the plant were a contributory cause of the severe damage, this publication makes some initial comments on this event.

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1.2. OBJECTIVES

This publication is intended to provide an understanding of the characteristics of the electrical grid system from the point of view of an NPP and the special requirements of an NPP with regard to its grid connection; the quality and reliability of its electrical supply. It should be of particular assistance to Member States that are considering the introduction of nuclear power for the first time to ensure that they consider all the interactions between the electrical grid and the NPP.

This publication is also intended to be helpful to Member States that already have NPPs in operation and may be considering the installation of new NPPs after an interval of many years. For these Member States, this report provides a reminder of the issues related to the electrical grid system that should be considered when installing a new NPP. This is important because many Member States have changed the structure of the electricity supply industry so that the organization that is responsible for the electrical grid system is now separate from the organization that will be responsible for a new NPP.

1.3. SCOPE OF THE PUBLICATION

This publication provides information on the requirements of the electrical grid and the requirements of NPPs, the way in which they rely on each other, and the recommended interactions. It describes the necessary characteristics of the electrical grid system that are required for the connection and successful operation of an NPP, and the characteristics of an NPP that are significant for the design and operation of the electrical grid system. The publication explains the issues that should be considered when a NPP is being planned, and the information that must be exchanged between the developer of an NPP and the organization that is responsible for the electrical grid during the design and construction of an NPP. It also describes the requirements for the way that the grid system is operated when the NPP is in operation, and the relationship between the operator of an NPP and the organization that operates the grid system at that time.

This publication is not intended to be a detailed guide to the design, analysis and operation of grid systems, which are described in many electrical engineering textbooks. Nor does this publication describe the detailed design of electrical systems within NPPs, which is dealt with in Ref. [7].

1.4. INTENDED USERS OF THE PUBLICATION

The intended users of this publication are Member States considering the construction of a new nuclear power plant. This includes Member States introducing nuclear power for the first time, and those that already have NPPs in operation and are considering building a new NPP after many years..

For Member States that are considering the introduction of nuclear power for the first time, this publication is intended to be useful to the organization set up to carry out a feasibility study and advise the government before a decision has been made to proceed with a nuclear power programme [2], to allow that organization to understand the issues related to the electrical grid, so they can be properly included in the feasibility study.

When a decision has been made to build a new NPP, it is necessary to consider all the issues where the NPP and the electrical grid system can interact. This publication is intended to enable engineers and managers in the organization responsible for the NPP to understand the issues related to the grid, and to enable engineers and managers in the organization responsible for the transmission system to understand the special requirements of NPPs that affect the design and operation of the electrical grid.

1.5.STRUCTURE

This publication has been divided into sections including this introduction followed by three appendices.

Section 2 presents an overview of the main features of an electrical grid system and outlines the differences in organization of the electricity supply industry that may be present in different Member States. It also defines and

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