Modelling and Analysis - Loadflow & Fault Levels

Data Collection

The vast majority of information required for any of the modelling and analysis exercisescan be derived from the site inventory of electrical assets. Where this is either incomplete or otherwise lacking information we can usually gather the necessary data through inspections at site whilst the equipment is energised. Occasionally it is necessary to inspect some items whilst de-energised, particularly where items such as current transformers are encased in the switchgear and there is no external rating plate.

Loadflow Analysis
Loadflow analysis typically falls into two categories - reviewing existing system configurations & loading levels such as when reviewing the impact of running with new loads or designing new installations where predicted loadflow and voltage deviations need to be quantified such as when reactive compensation is to be dimensioned for grid code compliance.

Existing system loadflow analysis is usually requested when considering the impact of new equipment or re-configuration of existing equipment in order to facilitate other works. The loadflow analysis will often be completed alongside a protection review as the equipment both needs to be able to carry the predicted load current within it's rating and also without inadvertant operation of any protective devices.

New installations are often analysed for loadflow to ensure that the effect of the new equipment on the existing system is within acceptable limits. Windfarms, for example, require to be able to control their reactive power profile as the Grid Code and their connection agreement identify. This usually entails (although this is highly dependant on the size, location and connection type of the windfarm) the ability to control the overal power factor to +-0.95 and run with a nett reactive profile of zero (i.e. neither importing nor exporting reactive power). This involves modelling the wind turbine generator profile and turbine step up transformer in order to create a model of each turbine installation and then integrate this into the cable network proposed (usually based on predicted full load currents & simplicity of cable selection) through to the main grid connection transformer or terminal point. The model can then be simulated at the various load points on the windfarm reflecting the variable yield from the wind and the effect on the voltage at the terminal point and reactive power flows quantified. This hence leads to a specification of the reactive power compensation.

Fault Level Analysis
The fault level analysis on a system will quantify the maximum current that will flow for faults between phases and to earth in the electrical system. Given that different supply configurations result in different supply fault levels, these permutations need to be taken into account and then any other configuration changes within the system need to be modelled such as closing bus switches, paralleling transformers etc.

There are additional variations to take into account with regard to nominal system voltage, transformer taps and reactive compensation in order to ascertain the maximum fault currents. However it is also very important to determine the minimum fault currents that will flow as the protection devices need to be confirmed as having enough fault current to cause operation and hence prevent a long-term overload condition. Typically, fault level analysis in the UK will be completed to BS EN 60909 although the fault levels utilised for Arc Flash Analysis are derived from the American standards for fault current calculation. Although similar in output the methods of calculation are different and hence are not exactly comparable.






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Director: Stuart Shaw


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Last Modified 8th October 2018