Network Analysis - Introduction

Transmission Planning Code Workshop 2

1st May 2008

Introduction

Network Analysis is used extensively by National Grid within the planning process

Aim of this presentation to introduce the fundamental concepts behind the network analysis tool used by National Grid for planning

Network Analysis – why do it?Network Analysis – why do it?

NTS is a complex network of pipes, compressors, regulators (control valves), multi-junctions and many other plant items required to transport high pressure gas safely

Gas leaving the NTS must be at adequate pressure

NTS must be able to meet the 1 in 20 peak day demand security standard

Mathematical models required to understand gas flow and pressure behaviour under different supply/demand conditions

Natural gas behaviour described by complex thermodynamic and hydraulic equations

What models are required?

Models are derived from fundamental physical and thermodynamic laws

Conservation laws (mass, momentum, energy) describe motion of a fluid under pressure and temperature changes in a fluid

Equation of state describes relationship between pressure, temperature and volume for gases or mixtures of gases

Major assets are modelled – these induce the largest pressure drops, control the flow of gas and control pressure

Pipes Compressors Regulators (control valves)

Network models are built up by joining these key components together Represents the topology of the network

What input data is required?

Network model topology describes only the major physical assets of the system

Additional data required to describe e.g. Volumes of gas flowing into and out of network, Design limits of components and other connected facilities Operating behaviour, gas properties, altitudes...

Data may be time dependent and vary with ambient conditions e.g. air temperatures

Models are usually visualised and manipulated through a graphical interface due to the amount of data required, and the complexity of the network topology

National Grid currently use the FALCON network analysis tool for planning

What output data is produced?

The key outputs from the software are Pressures at each modelled node (e.g. supply nodes, demand

nodes, pipe ends, compressor inlets/outlets) Volumetric flows through components Gas properties and temperatures at each node

All other outputs are derived from these results e.g. Linepack in each pipe Compressor power used

Models are deterministic i.e. they are not statistical models which simulate random or uncertain events

How is capacity modelled?

Entry and exit capacity not modelled directly - commercial rather than physical concepts

Derived by analysing the network under different supply/demand conditions Transmission capability usually analysed under steady flow assumptions (for

planning needs) Flexibility analysis is analysed assuming offtake flows are profiled across a

gas day (transient analysis)

Fixed flow conditions input to the model and analysed All pipeline and plant must operate within design limits Analyst changes the operating setpoints and configuration of the network to

find a model solution that supports the flow patterns

Each fixed flow condition must currently be analysed separately Requires skilled analysts to find a solution quickly to a steady flow problem A transient analysis problem may take an analyst several hours to solve

How are capacity constraints identified?

Capacity constraints are identified for planning needs through

Breach of minimum pressures required at an offtake and/or Over-pressurisation at an entry point or within the network

Constrained volume at an entry point is determined by the maximum flow that can be achieved at that entry point without breaching pressure limits

Constrained volumes will change with supply and demand patterns

Assumption of full plant availability used for longer term view

Summary

Network analysis is needed to understand the behaviour of the NTS under different supply and demand conditions, due to the underlying complex dynamics of natural gas flowing through the system

Outputs are the hydraulic variables of the system (pressures, flows)

Nodal capacity values are derived from different studies using the network models however these depend heavily on...

Supply and demand patterns Minimum and maximum operating pressure requirements Plant availability and capability

Questions?