Network Analysis - Introduction Transmission Planning Code Workshop 2 1 st 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?  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?