Overview
Due to a significant upsurge in the number of applications for large scale demand (data centers) and renewable generation (solar farms) a need arose to conduct harmonic distortion analysis in order to establish emission limits. The main driver was the type of connections employing power electronic converters that have the potential to introduce harmful harmonic distortion onto the transmission system that could impair the power supply to other customers. Comprehensive system modeling and analysis resulted in emission limits being issued to several customers connecting onto the transmission system.
Challenge
PSC was asked to support a transmission system operator in issuing harmonic emission limits to a large number of new distorting installations that include solar farms and data centers with connections taking place in an area with a high degree of uncertainty in terms of system development. Transmission grid capacity is limited, and any new installations may require new developments. The possible closure of conventional generators, which is known to have a significant impact on power quality, provided further uncertainty. PSC was tasked to develop “generic” emission limits that are fair, practical and achievable and that can be issued to the customers connecting onto the transmission system.
Solution
PSC initially conducted a complete review of the transmission system model for its suitability to perform frequency-domain studies and included updates for all the proposed system developments and the new customer connections.
A methodology was then developed to take into account the background harmonic distortion that already exists on the system, how this will be affected under various system developments and operational scenarios and the effect of the existing customers that are contracted to connect onto the system with harmonic emission limits but have not yet connected. A major part of the methodology had to account for the effect each node will have on other nodes in the system within that area. This includes utilizing a portion of available headroom not just for a customer connecting at that particular node but also of other customers connecting in the vicinity.
Automated scripting was heavily used in order to check various sensitivities and system contingencies. Considering that the system operator’s approach to harmonic limits allocation is based on apportioning the available headroom, no active injections were considered. However, the effect of all changes on the existing background distortion was considered based on a voltage source behind system impedance approach. IEC 61000-3-6 approach was used in the overall analysis which proved to have a number of limitations in terms of the effect of concurrent connections along with the background distortion consideration. Issues were established as a result of system development, i.e. change in system topology and these were remedied before moving into a wider system study to determine customer connection emission limits.
A number of individual connections were checked simultaneously in determining harmonic emission limit allocation. Limits were developed for each of the connecting customers and individual emission limits reports were prepared for each of them which included system impedance characteristics required for any mitigation measure design.
Results
PSC conducted specialist power quality analysis and determined separate harmonic emission limits for all new customers (including solar farms and data centers) connecting onto the transmission and distribution systems. All limits were issued on a timely basis taking into account several uncertain system developments, change in generation merit order, uncertainty on some connections and limits that have been already issued but connections not yet materialized. These considerations result in potentially rather conservative limits if some of the projected system developments and/or customer connections do not materialize. To avoid this it was recommended that careful consideration is taken to implementing proactive solutions which may lead to over-investment.
The methodology established in the analysis as well as the developed scripting both to establish limits and determine system impedance characteristics were made generic so that they can be applied for other similar developments on this transmission system.