Power System Studies

Power system studies, or power system analysis is a critical part in the design process of any HV power system. If the system has not been designed correctly, formal computer analysis will identify any problems before construction starts, allowing redesign to be affected at minimal cost and any potential safety risks to be eliminated.

Power system analysis, is the process of formally (mathematically) modelling a power system, and analyzing it to understand how the system operates for a variety of given scenarios. The starting point for any power system analysis, is a set of reasonably complete single line diagrams, load lists and layouts diagrams; which are used as a basis for the studies engineer to model the system in the chosen software package. Once the model is complete, a full mathematical equivalent of the system will have been created, which can then be easily evaluated for a variety of different loadflow, short circuit, harmonic and dynamic/transient conditions. These are generically known as ‘system studies’.

Traditionally power system analysis used to be done by lengthy hand calculations, which were very complex and time consuming to undertake, but modern software and computer advances have simplified the whole process considerably. As it helps to eliminate errors, reduce analysis time, allow different scenarios to be examined easily, and can be quickly and easily reproduced. It is worth noting however, that many experienced studies engineers will still perform hand calculations, to validate the computer results.

Power system studies are a key part of any system design, and its importance should not be underestimated. Fundamentally the power system studies are undertaken to demonstrate that the electrical system is safe, performs as expected and that all items of electrical equipment have been correctly sized. The goal of the study is to validate the design, identify any problem areas, and where appropriate suggest appropriate solutions to design issues. The computer model simulates a theoretical network and can easily identify and problems, allowing the design to be modified before any equipment is purchased. In practice this means that the studies should show that all the equipment is correctly sized for the required duty, can be operated satisfactorily and poses no risk to operations staff, or the public. It is also worth noting that some clients require a 3^rd party quality inspectorate (such as Lloyds or DNV) to verify the design, and review of the power system study will be a key a factor in achieving satisfactory approvals.

Load Flow Studies

Loadflow studies is the generic name given to a variety of studies that evaluate how a power system operates in a steady state, normal (non-faulted) condition. When undertaking these studies it is usual to set the electrical power system into different configurations. These configurations, or study cases, let the study engineer identify how the plant behaves in different conditions. Typical areas that are examined during loadflow analysis are:

• Power flow through the system
• Equipment sizing
• Total power demand calculation
• Volt drop calculations
• Voltage profile & transformer tap settings
• Power factor identification & correction

Short Circuit Calculations

Short circuit calculations are undertaken to identify the current that flows in the system during fault conditions. From basic three phase theory, it is reminded that a whole variety of faults can occur, such as three phase faults, two phase faults, phase-earth faults and cross-country faults. In a short circuit study we are only usually interested in a 3-phase fault and a phase earth fault, as these represent the most onerous conditions, and all other faults are variations of these. Short circuit studies are undertaken for two main reasons: The first is to determine the maximum short circuit level, and is used primarily for working out equipment specifications and ratings. The second type is the minimum short circuit levels and is used for motor starting studies and developing protection settings.

It is also briefly worth noting that for most studies the fault is considered to be ‘bolted’ (i.e zero impedance in the fault path), this means that the maximum fault current will occur. It is sometimes useful to consider an impedance in the path of the fault (i.e. reducing the fault current), but this is primarily used to consider the range of protection settings necessary.

Arc Flash Analysis

Arc Flash analysis is really a subset of short circuit calculations, but with the specific aims of identifying the amount of energy in a certain type of fault, known as an arcing fault. Arcing faults are generally short-lived faults with a lot less energy than a bolted 3-phase fault, but they are much more likely to occur and present a serious risk to operators and equipment.

The studies are relatively new and interestingly the dominant standard is an American one known as IEEE 1584, as arc faults were not really considered in much detail until after 2000. There is an effort underway to produce an IEC equivalent, but this is still some way off yet. Studies are developed to help determine the level of arc-proof containment required for the electrical switchgear, and to determine the level of PPE required by operational staff. The results of the studies normally result in an amount of energy per unit area, which will indicate the kind of PPE operators need to wear during switching operations.

Under most electrical studies it is best to always take the worst-case scenario for determining ratings, but during arc-flash studies, it is better to be practical and realistic, otherwise it may result in specifying large and cumbersome PPE that is difficult for the operators to use.

Harmonic Analysis

Harmonic analysis studies are only usually required when there is a large proportion of harmonic sources on the electrical system and are usually caused by power electronic switching devices such as variable speed drives, AC UPS, DC UPS, HVDC power systems, solar PV farms, certain types of wind turbines etc. They are commonly encountered in:

• Large industrial plants with lots of VSDs
• Data centres
• Office blocks with lots of PCs
• DC traction systems (for rail / trams)

Typically, there is a little point undertaking any harmonic studies in the conceptual or front-end design stages, as actual equipment manufacturer information is required so that the ‘harmonic fingerprint’, of the relevant equipment can be identified.

Studies for harmonic distortion fall into two categories, determination of overall Total Harmonic Distortion (THD) and identification of any resonance points; these two studies are different and approached in a different manner.

Calculation of the THD is based on a harmonic load flow calculation that sums of all the integer harmonics on the electrical system, expressed as a ratio of the main power supply. A high value of THD implies that the main system AC waveform will suffer significant distortion, which may lead to poor performance of equipment and excess heating. It is usually accepted that the THD should be 5% of less, if the THD is above this then it is necessary to consider installation and specification of harmonic filters at key points on the system.

Harmonic studies are carried out to ENA G5/4 in the UK, which is derived from the IEC 61000 standard on harmonics. The IEC standard is a little hard to understand, so it is not unusual to find many companies prefer to work to the American IEEE 519 standard instead.

Protection Study

Protection studies are some of the most complex studies to be carried out on a project and require that the study engineer has an in-depth knowledge of the system and the theory of protection. The aim of a protection study is to develop the protection settings necessary, so that only the correct relays closest to a fault trip, and upstream relays do not trip first – this is done by choosing setting for the relay ‘pickup’, curve type and time delay setting. This statement sounds simple, but it covers a very complex area.

To ensure that this requirement is met, the study must be able to identify what are normal operating conditions, overload conditions, and fault conditions are, and then ensure that each relay above the other has a sufficient grading margin (usually 200ms to 400ms), to ensure that the upstream breaker does not trip first or at the same time.

One of the interesting and challenging things about protection studies, is that they are very much an art of compromise. It is virtually impossible to get all but the most basic system to coordinate fully, and the protection engineer, must try and identify the most important parts of the system and find the optimal compromises.

Once a protection study has been completed, the protection engineer should be able to provide a settings summary to allow the site commissioning engineers to load into the various protection relays and trip units.

Motor Starting

Motor starting studies are undertaken to prove that a motor can be started satisfactorily against the electrical system and the driven load. This is done by examining the voltage profile of the network during the motor starting, and by confirming that the motor accelerates and can generate enough pull-up torque and break-out torque to prevent the motor from stalling.

There are two main types of studies undertaken with power system analysis software: The first type is known as a static study and is based on a simple impedance calculation to compare the motors starting power against the network strength in terms of fault level (strong networks are better for starting motors). These studies are usually done when only a quick verification is needed and does not check the motors capability in regard to the load and will typically show the level of voltage depression at the motors terminals and related switchboards.

A dynamic motor starting study is more complex, and takes into account the motors dynamic characteristics, excitation system (for synchronous motors) the load type and rating and any generators governor/exciter response. The studies are much more involved and need a lot of detailed information to produce meaningful results, for this reason they are generally only done in the later stages of a project, or when a conceptual study identifies a particularly problematic motor.

Dynamic & Transient Stability

These power system studies are often just called transient stability studies, which can somewhat misleading, as they usually refer to both transient events (<1s) and dynamic events (>1s to <20s. Fundamentally these studies are used to show how the electrical power system responds to temporary disturbances on the system. Depending on the type of disturbance a temporary voltage depression of frequency depression may be experienced and quickly recover, or it is potential that the voltage may collapse, the frequency deviation exceeds the defined limits, or even for the system to pull out of synchronism. Typical events that are examined in a transient stability study are:

• Loss of main item of plant i.e. generator, transformer, grid supply, overhead line
• Islanding of a power system i.e. trip of a generator switchboard bus-section
• System Response to a severe 3-phase fault

In all transient stability studies, it is particularly important to make sure that any turbine’s / engine governor is modelled correctly, as many larger GTGs have limited load pick-up capability (typically 1MW/sec), whilst STGs accept load at an even slower rate of around 10% of rating per minute. If an incorrect governor has been specified, then this can give misleading results. Similarly, the exciter for any generator or synchronous motor must be correctly modelled.

Earthing / Grounding Study

Grounding studies are a little unusual, as although they are a key part of electrical system design they are not always done as part of a power system analysis exercise. It should be emphasised that grounding is an exceptionally complicated area, and a good deal of knowledge is required of approach a grounding study, as it requires a strong theoretical underpinning as well as a good understanding of the practicalities and realities of system design. Fundamentally the aim of a grounding system study is to determine the maximum allowable touch and step potentials that persons can be exposed to, and then supplement the earthing design to ensure that these cannot be exceeded.

Summary

We hope that this post has given a general overview of the most common types of power system studies that are carried out. We will be preparing some more blog posts on each of the specific study types over the next few weeks, to try and explain the detail of each study type. Please check back when you can!

At Aurora we recognise that we have a wide customer base and therefore own and operate a number of different packages. Aurora currently owns license from ETAP, Digsilent Powerfactory, EMTP-RV and EMTP-ATP. This lets us model virtually any power system problem and choose the most appropriate tool for the project.

Lastly, please remember to carry out these studies early on in the plant design, as leaving the study to the last minute can create many problems when the results indicate that the system does not perform as expected.

Please contact Aurora if you would like to know more, or to discuss your project requirements.