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Introduction

Power system modelling is a broad field of activity in which computer simulation is used in order to design or analyse several aspects of a real-world electrical power system.

Fundamentally the value offered in electrical system modelling is in its ability to simulate a complex real-world system within a computerised (non real-world) environment. In doing so, the system’s steady state characteristics and dynamic behaviour can be assessed in response to various planned or unforeseen events that may occur on that system in the real-world, and engineers can plan for these. Events may include:

• Various fault conditions, e.g., open circuit or closed circuit faults at various locations on the power system
• The starting of large motors and other ‘slow transient disturbances’
• Lightning strikes, circuit breaker switching events and other ‘fast transient disturbances’
• A changing generation or demand profile
• A changing configuration of the network, e.g., switching a line out-of-service for maintenance
• The connection of new equipment / decommissioning of damaged or ageing equipment

Assessing these events can enable better planning and operation of the electrical system in the real-world by providing engineers with additional knowledge relating to what stresses might be put on the system and how the system might behave in response.

A Note on Granularity

Simulation by its very nature requires a degree of simplification within its software environment.

If a power system modeller was to perfectly model every aspect of the real-world electrical system in software, the resultant model would be too large and complex to simulate efficiently and therefore would not be useful for the production of any meaningful insights.

For simulation to be effective it is necessary that the characteristics of components in the real-world system are accurately captured within the model but also suitably simplified so that the modelling software is able to evaluate results in a reasonable amount of time.

The system should only be complex enough to satisfy the accuracy requirements of the system studies under question. Additional complexity increases the study run time and introduces an increased risk of modelling errors.

This degree of complexity is known as the ‘model granularity’ and the degree of granularity adopted imposes a degree of uncertainty to the knowledge that the real-world system will behave as predicted by the model.

To assess whether a model is suitably granular, validation and verification of a model is an essential aspect of power system modelling.

We shall discuss this subject in more detail later in this course.

Consideration of the Time Domain

Power system studies can be loosely categorised by the time-domain across which they act. Examples of various power system studies mapped against the time domain is illustrated in the following figure.

Once the power system modeller defines aims of the modelling task, they will hence derive the time-domain across which it should be conducted. This consideration may inform the planning aspect of the subsequent modelling task. For example:

• If the aim of the model is to simulate long-term infrastructure planning, the modeller may omit information from the model that relates to the behaviour of the system in response to fast transient disturbances. These disturbances occur in the micro-second range and are not of great a priority.
• Similarly, the modeller may also choose to create the model within a different software platform altogether. Several platforms are designed specifically for conducting analysis across the long or short time domain.

Within this course, we shall generally be looking at those studies categorised as ‘steady-state’, ‘real-time’ or ‘slow transient’ as illustrated in the following figure:

Figure 1:Time Domain Categorisation of Various Power System Analyses

Types of Power System Studies

The following provide a description of some of the typical power system studies that may be encountered in power system modelling:

Load Flow (a.k.a. power flow)

A load flow study presents the power flows in a modelled electrical system for a steady state of operation. Parameters under study typically include the magnitude and phase angle of the voltage at a given node (bus) alongside the real and reactive power flows flowing in each element connecting busses.

Fault Analysis

A fault level study presents the fault currents present on each bus of a network in response to various fault modes such as phase to phase or three-phase to ground faults. Fault analysis in an electrical system is necessary to ensure that power system components are rated appropriately for safe operation under fault conditions and to inform the correct selection of protective devices such as relays and circuit breakers.

Protection Study

A protection study assesses the selection of (and settings for) protective devices installed on a network to ensure that the devices operate appropriately to isolate plant on the occurrence of a fault in accordance with the principle of electrical protection, i.e., that a fault is isolated in a timely manner to ensure safety of personnel and minimise damage to plant. A protection study should be undertaken in accordance with the equipment owner’s local protection philosophy which may add further consideration to other aspects, e.g., system continuity post fault, economic factors or reliability of supply.

Slow Transient Analysis

Transient analysis allows for the assessment of the dynamic behaviour of an electrical system in response to a change from a primary steady-state condition to a secondary steady-state condition, commonly the loss or addition of generation/load. The transient analysis of a system is important as the electrical parameters of the system during this transient phase may exceed the limits imposed by equipment selection but which do not appear to be exceeded when operating in either the primary or secondary steady-state conditions. In the real world, this could cause mal-operation or failure of equipment.

Motor Starting

A motor starter study is a type of slow transient analysis of the network as discussed previously but also considers the method by which motors are connected (direct-on-line, soft-starter, etc.). This study may also be required to consider computerised sub-station level motor controllers that manage a staggered start protocol.

Fast Transient Analysis

Fast transient analysis (and system stability analysis) differs from slow transient analysis in that it seeks to assess the resilience of an electrical system to an impulse (very short-time frame) transient event, such as a lightning strike to a conductor or a circuit breaker switching action. A fast-transient analysis assesses the high frequency oscillations imposed on the electrical system following such an event to ascertain whether the system can safely return to a steady-state conditions after the event.

There are numerous other analyses that can be undertaken for an electrical system (such as contingency analysis, harmonic analysis, unbalanced fault analysis, etc), which you may come across in your career. This course seeks to introduce you to modelling practices that could be adapted and applied to various power system modelling tasks.

Modelling Lifecycle

All typical modelling tasks should roughly follow a similar lifecycle, introduced here.

Figure 2: Introducing the Modelling Lifecycle

As illustrated in the following figure, Validation and Verification (V&V) should be conducted throughout the typical modelling lifecycle:

Figure 3: V&V in the Modelling Lifecycle

Validation and Verification for Modelling

In model V&V, the end product is a predictive model based on fundamental physics of the problem being solved.

The expected outcome of the V&V process is an understanding of the degree in which the modelled behaviour is representative with real-world behaviour and experimental data (i.e.: the predictive accuracy of the model).

Validation (am I modelling the right system?)

Validation is the process of determining the degree to which a model is an accurate representation of the real world from the perspective of the intended uses of the model.

Validation is concerned with quantifying the accuracy of the model by comparing numerical solutions to:

• real-world data
• previously validated models, or
• expected results (from mathematical methods)

Verification (am I modelling the system right?)

Verification is the process of determining that a model implementation accurately represents the developer’s conceptual idea of the model and the solution to the model.

Verification is concerned with identifying and removing errors in the model by comparing numerical solutions to analytical or highly accurate benchmark solutions.

Figure 4: Verification and Validation in Modelling

Examples of V&V Activities

Overview of Electrical System Modelling in MATLAB Simulink

Starting Simulink

Exercises and Further Work

Before continuing in this module, students should be familiar with the MATLAB Simulink software environment.

Students should feel comfortable with:

• Starting a new Simulink project;
• Opening and navigating the Library Browser;
• Bringing blocks (representing various system components) into the workspace;
• Running basic simulations; and
• Saving projects to return to later