Overview

Grid development based on underground cables due to renewable generation integration at a weak part of a transmission system necessitated studies to check whether possible resonance issues will result in excessive harmonic amplifications and risk to exciting temporary overvoltages. Studies identified that the possible mitigation of technical issues while employing cables will be extremely difficult and hence the alternative of using overhead lines with some partial cabling was investigated with a possible mitigation solution for its implementation.

Challenge

PSC was asked to investigate possible technical effects of a proposed cable circuit with two alternative routes on the transmission system. The need for the cable circuits arose due to increasing renewable generation capacity in the region of interest. The proposed cable was planned to be installed in a weak part of the system and likely to create issues with harmonic distortion amplification and possible excitation of resonances causing temporary overvoltages (TOV). PSC was further tasked to check what the maximum cable length that could be implemented without the need for excessive mitigation measures.

Solution

PSC initially modelled the cables circuits in PSS/E in order to establish any requirement for shunt compensation. Once reactive power compensation needs to keep the system within planning levels were established, frequency sweep calculations using DIgSILENT PowerFactory were performed to obtain system frequency response and calculate harmonic gain values in terms of possible voltage amplification.

Results indicated significant amplifications with the use of cables and so the challenge moved into checking what would be the situation if one of the routes were to be overhead line instead of underground cable. The harmonic amplification effect even with a full overhead line showed harmonic gains that would still result in voltage amplifications, but these were of the level that can be mitigated. Automated scripting was then instigated in order to find an optimum mitigation measure in the form of shunt passive harmonic filters. Once a filter solution was identified, cable sections were introduced into the overhead line route to establish the level that can be accommodated without breaching harmonic distortion limits. This was identified to be a low ratio if complex mitigation measures were to be avoided.

Time domain electromagnetic transient analysis using ATP/EMTP were then conducted for the proposed overhead line and filter solution in order to identify the possibility of TOV risk. In a few cases, values marginally exceeding the company allowable level were established but that the risk was very low.

Results

PSC conducted specialist analysis using three different software platforms in order to utilise the most sophisticated components of each platform applicable to the technical issue at hand. Technical issues were identified, and appropriate solutions for the project to advance to the next stage was put forward. Following agreement, different alternatives were explored and once issues identified with the alternatives also, mitigation measures were established and checked for their effectiveness. Recommendations were given at every stage of the analysis with significant effect on the way to advance the project to the next analytical stage.

The methodology established in the analysis as well as the developed scripting to check multiple scenarios both at establishing effect and solution were made generic so that they can be applied for other similar development on this transmission system.