At a glanceRapidly increasing load growth is a ma-jor challenge and driver for large-scalesystem expansion in Ghana. In the lastfive years the transmission system op-erator, GRIDCo, increased its trans-former capacity by over 50 per cent.Even so, the transmission system is of-ten forced to operate close to its limitsdue to a lack of generation capacity. Inthe past, mal-operation of the protec-tion system and cascading events wereobserved. Considering low critical faultclearing times, the situation poses ahigh risk of total system blackout incase of failure of the primary protec-tion system.

In order to reduce the risk of systemcollapse, GRIDCo initiated a system-wide protection system review. Theobjective was to ensure system securi-ty and to maximize the utilization ofthe system through improvement ofthe performance of the protection sys-tem.

Initial situation and challengeIn general, protection settings werecalculated by different substation ven-dors or consultants. The protectionsystem model and settings were not

available in a central database orpower system simulation software. Nosystem-wide coordination and check ofprotection settings was possible.

The following tasks and workflow weredefined to review and improve the ex-isting systems’ performance:

· review of existing protection systemand generation system stability toachieve performance improvements

· development of enhanced protec-tion schemes and relay settings cal-culation guidelines

· calculation of new improved set-tings and verification of their per-formance through simulation

· definition of short-term actions, anda medium- and long-term strategicprotection system development plan

The solutionPower and Protection SystemModeling:A detailed power and protection sys-tem model was built and benchmarkedwith GRIDCo‘s existing PSS®E model.The geographic system model that wasdeveloped includes a detailed internalsubstation model with all circuit break-ers (CBs), current transformers (CTs),voltage transformers (VTs) and protec-tion relays.

Figure 1: Power and protection system model

Increased securityand maximizedutilizationProtection and system securityimprovement in Ghana

siemens.com/power-technologies

GRIDCo is the national interconnected transmission system operator ofGhana. Its mission is to provide reliable, secure, and efficient electricitytransmission services and wholesale market operations to meet stakehold-er expectations within Ghana and the West African Sub-region, in an envi-ronmentally sustainable and commercially viable manner.

All protection setting parameters werecollected in the field and imported intothe system model. The zero-sequenceand mutual coupling was amended tosimulate single-phase faults with thehighest possible accuracy. The modelbuilding process was complementedwith multi-level data plausibility checksfor highest data quality.

Scenario Building: To systematicallyassess protection system performance,a multitude of fault scenarios with var-ying fault location, type and arc re-sistance were defined.

Simulation and Performance Assess-ment: Siemens PTI’s SIGUARD PSA (Pro-tection Security Assessment) solutionwas used to simulate selectivity, sensi-tivity and speed of the protection sys-tem. In each scenario, all stages of theentire fault clearance sequence weresimulated in detail. This provides deepinsight into the protection system re-sponse for a given fault condition. Allcurrents, voltages and impedancesmeasured by protection devices areavailable.

Generation system stability was as-sessed for the existing and newly pro-posed and improved under-frequencyload shedding schemes, taking intoaccount different system configura-tions. In total, hundreds of thousandsof fault scenarios were simulated. Theresults were automatically assessedand visualized in a concise way withSIGUARD PSA.

Figure 2: SIGUARD PSA protection performancevisualization (color code: red - fault notcleared, orange/yellow – over-/underfunction)

In this state-of-the-art approach, theprotection system performance is as-sessed by simulation, and not only us-ing the classical approach of solely de-picting setting values. The new meth-odology sets itself apart from the con-ventional, and individual, evaluationsof distance, differential and overcur-rent time relay characteristics by as-sessing the combined ‘system’ re-sponse with all main and back-up pro-tection relays in the power system.

The quality of the protection systemperformance is depicted by color-coding that is meaningful for bothtechnical and non-technical staff. Re-sults can be aggregated for completenetwork regions or detailed for indi-vidual power system components orprotection relays. This allows the iden-tification of incorrect settings, weak-nesses and limitations of protectionsystem.

The high degree of automation ena-bles the efficient assessment of a verylarge number of fault scenarios andcomplex system conditions, and mini-mizes the individual treatment of spe-cial cases.

Protection System Improvement:False or improvable protection settingswere corrected using a rule-based al-gorithm. The underlying protectiongrading rules were improved by iterat-ing simulation, assessment and settingimprovement until optimum resultswere reached. In this way, system-widecoordination and optimization of theprotection settings was achieved. Inaddition, the quality of the new protec-tion settings is ensured before approv-al.

Key achievementsThe benefits of the systematic protec-tion system review and improvementproject were:

· complete data survey, documenta-tion and modeling of the power andprotection system with validatedhigh quality

· assessment of the performance ofthe existing protection system,showing its specific characteristics,behavior, weak points and limita-tions

· development of newly improvedprotection settings, grading rules,scheme recommendations and anunder-frequency load sheddingscheme based on the review find-ings. Validation of new settings bysimulation before approval

· recommendations for the efficientfuture strategic development of theprotection system

· knowledge transfer for staffthrough regular workshops and cus-tomized training

It was demonstrated that the adapta-tion of protection settings can help in-crease system security and maximizeutilization of the existing system,which in turn avoids CAPEX invest-ment.

Published bySiemens AG 2017

Energy Management DivisionFreyeslebenstrasse 191058 Erlangen, Germany

For more information, please contact:power-technologies.energy@siemens.com

AL=N, ECCN=EAR99Subject to changes and errors. The infor-mation given in this document only con-tains general descriptions and/or perfor-mance features which may not alwaysspecifically reflect those described, orwhich may undergo modification in thecourse of further development of theproducts. The requested performance fea-tures are binding only when they are ex-pressly agreed upon in the concluded con-tract.

“The project results covered all required aspects of the Terms of

Reference and contain important information for an improved net-

work protection system. We are looking forward to continue our

business relationship with Siemens PTI.”

Dr. Thomas Ansah, CEO at GRIDCo