Protection Devices and Solutions for Evolving Distribution Smart Grid

摘要

This paper presents novel solutions for the ever-evolving Smart Grid. The presented solutionsuse Rogowski coils as current sensors, voltage dividers as voltage sensors, and multifunctiondigital relay platforms designed for enhanced digital communication technology. Even thoughthis paper presents entire solutions for Smart Grid applications, the main focus is onRogowski coil design and application aspects since they have a major impact on the paradigmchange regarding protection, control, and metering solutions.Rogowski coils are well-suited devices for Smart Grid applications because they accuratelymeasure currents under all operating conditions, they have linear performance characteristic(cannot saturate), they are compact in size, and they weigh many times less than conventionaliron-core current transformers. The same Rogowski coils can be used to perform metering,control, and protection functions. They can be designed as split-core style devices to allow forinstallation without requirements to open primary conductors.Rogowski coils presented here were uniquely designed using printed-circuit boards, achievingperformance characteristics which enable advanced solutions for evolving distributionsystems. Their performance characteristics can be summarized as follows:1. Accuracy class meets requirements for both metering and protection applications;2. Coils have linear voltage-current characteristics (do not saturate even at extreme faultcurrents);3. Possess high immunity to external electro-magnetic fields and nearby conductorsconducting high currents;4. The same coil can be applied for a wide operating current range (1A to over 100 kA);5. Can be easily designed with dimensions to fit in limited compartments such as insidecubicles around power cables;6. Have rigid and durable designs;7. Cannot produce hazardous secondary voltages (output voltages during faults may be onlyseveral volts; open secondary circuit does not create hazard);8. Easy to install; and9. Maintenance free.A series of tests were performed to verify their performance characteristics. Tests at roomtemperature confirmed 0.1 accuracy class for the tested coils. Accuracy class tests performedat temperatures -20°C to +70°C verified that the maximum change of the coil Scale Factorwas 0.1%. Linearity tests were performed up to 60 kA. Test results verified that the coilspreserve linearity in the whole test current range. Rejection of external electromagnetic fieldswas confirmed as being minimal. The impact from nearby conductors that carry high currentsis one of the most important performance characteristics that classify the Rogowski coilaccuracy. Maximum impact is when the coil is located next to a nearby conductor. However,even under these conditions the impact of a conductor carrying 60 kA was approximately 1%of the measured current. When conductors are at a distance from the coil — like in actualapplications, the impact is negligible. Accuracy class tests were also performed with coilspositioned 70° and 110° angled to the primary conductor. Maximum error as compared to thecoil’s reference position of 90° perpendicular to the primary conductor was 0.3%. Thisverifies that the position has minimal impact on the coil accuracy for the tested coils.The entire protection system was tested. The test results confirmed that digital protectionrelays interfaced to Rogowski coils possess adequate accuracy and resolution to reliablyperform in all types of distribution systems.