Menu
A new requirement introduced by NESO in GC 0141 requires that all new Grid Code compliant projects, provide an EMT model of their system to NESO – currently section PC.A.9.4. This model has a series of very specific performance requirements and must be provided with a user guide and validation report. Special techniques often need to be used for model aggregation, and may require development of custom control modules and analysis tools. EMT modelling is a challenge at the best of times, and developing a new model to meet NESO requirements is difficult, however failure to meet these requirements will lead to a delay in achieving ION.
GC0168 is an upcoming NESO Grid Code modification that is required for all existing power stations connected to the NESO network, to provide an EMT model. This may sound innocuous but places some significant burdens and constraints on an asset owner. The OEMs have typically moved on, and will be reluctant to engage in historical plants, data can be missing, and actual site models can be out of step with latest models. In many cases it could be necessary to develop a bespoke generator governor model.
For requirements and discussion of your project please get in touch ASAP. PSCAD modelling skills are in high demand and failure to provide a working model is one of the key causes of project delay.
Insulation coordination studies, is a generic name that is used to describe the system voltage performance and response for a wide range of transient events in accordance with IEC 60071. These can include temporary overvoltage (TOV), switching overvoltages (SOV), lighting transient impulses and very fast frequency impulses from GIS type switchgear. These types of studies are carried out using EMT software such as PSCAD, EMTP-ATP or DIgSILENT. For more details of the specific study type see below.
These two study groups are usually carried out together and combined into a single report. The primary aim of these studies is to identify which events could cause large transient voltage spikes and the need and specification for any surge arresters on the network.
Temporary Overvoltages (TOV) are generally the slowest type of transient events and typically last for a few cycles. They can include events such as transformer energisation, single phase to earth faults, load / generator rejection and parallel resonance. Switching Overvoltages (SOV) are usually faster transients associated with line energisation, reactor and capacitor bank switching and behaviour during 3-phase faults and associated reclosing.
TGN 288E is a specific standard used by NGET for the England Wales network, to assess TOV / SOV events on the system. The TGN 288E method is a little bit of an unusual approach, and needs some interpretation to use correctly.
These studies should not be confused with lighting risk assessments (see here), and are related to the system response should a lightning impulse enter the electrical system, and propagate through the network. They are intended to identify if the equipment can withstand a lighting impulse without a flashover occurring, and (usually) the need and specification for surge arresters on the system.
These studies are generally not required on private network unless, they contain significant sections of overhead lines, or very long cable networks. This is because most lightning impulses are collected via overhead lines associated with the DNO / TSO network, which are already provided with a high level of lightning protection systems.
Selection of the correct type and rating of surge arresters, and their location on the system is a critical part of insulation coordination protection that is often overlooked. Surge arresters are specified in accordance with the expected Maximum Continuous Operating Voltage (MCOV) for the network configuration and earthing type and appropriate Earthing Factor. As well as selection of the MCOV, the surge arrester energy capability must be specified to prevent thermal damage during operation.
Contact: Steve Sommerville
Phone: 01305 300 210
Aurora Power Consulting are a leading independent power systems consultancy. We provide specialist power systems analysis services using software such as DIgSILENT, ETAP, PSCAD, EMTP-ATP, CDEGS and XGSLab.
