The objective of this research is to develop new dynamic sequence component models for doubly-fed induction wind generators in order to calculate the positive, negative and zero sequence currents from such large wind plants to a grid fault. The approach is to simulate these faults in the time domain using accurate machine and power electronic models, and develop their sequence component models in the frequency domain. Protection relays measure the sequence component currents, but until now utility protection engineers have not been able to accurately calculate relay settings at the nodes where wind plants are connected to the utility network.
When the wind generator circuit topology changes within the milli-second range, it is necessary to have time varying or dynamic sequence component models based and adapting this information to relay settings will be a fundamentally new area to power engineering. This method could also be applied to other three phase systems and machines, and even lead to completely new relay designs.
Reliable and secure electricity is the backbone of society. Incorrect relay settings based on incorrect sequence modeling, can lead to failure to interrupt faults, or to nuisance tripping of the power. Failure to interrupt could in turn lead to personal injury, or severe damage to equipment. The results will be disseminated through publications and various IEEE Working Groups, and included in a graduate level course on electrical machines taught by Dr. Harley. Two undergraduate minority students from the Opportunity Scholars Program (OSP) at Georgia Tech will be involved.
The project commenced on September 15, 2010. The objective was to develop sequence network models of wind turbine generators (WTGs) to provide power system protection engineers with an accurate method for calculating protective relays settings in wind power plants. Most protection relays sense the positive, negative or zero sequence component values of current. Power system protection engineers therefore rely on positive, negative, and zero sequence component models of every element in the power network, including the generators, in order to study the inherent imbalance caused by single-line to ground, double-line to ground, and line-to-line electrical faults. These sequence component model circuits estimate the values of the sequence currents and these are set on the relays by protection engineers. Sequence component models for traditional generators, transformers, and lines are well understood, but they do not apply to certain classes of WTG’s and until now utility protection engineers have not been able to accurately calculate relay settings at the nodes or buses where wind plants are connected to the utility network. Moreover, certain types of wind generators change their topology during the fault, which needs changing or dynamic sequence component models which currently do not exist. Mis-operation of protective relaying systems could be attributed to inadequate sequence modeling of wind generating plants during the simulation studies. Therefore, it is vital to provide protection engineers with the correct procedures to calculate accurately the fault current contribution of a newly installed wind power plant. Before this work, utility protection engineers did not have a fault current model of the various wind generators and had to estimate the wind generator fault currents based on fault current models of conventional coal fired thermal power plant generators. This typically led to setting the sensing relay of protection systems too sensitive or insensitive, thus leading to nuisance tripping of circuit breakers and disconnection of the wind farm which in turn cause disturbances to the utility grid and all its customers. Electric generators contribute large fault currents into network faults and if left undetected could lead to personal injury and severe damage to equipment. Faults are detected and identified by relays which then activate trip commends of circuit breakers to isolate the fault and minimize damage and service interruption. Having a reliable protection system of many relays is therefore essential for a safe and secure power network, but most of the modeling and simulation tools developed for wind plants prior to this project have not kept up with the needs of protection engineers, since such models have concentrated on planning and dynamic stability studies. Reliable and secure power generation systems are of national interest for sustainability. As wind energy penetration grows, the problem of incorrect protective relaying on certain types of wind generators will become even more urgent than it already is. The utility grid as a whole will benefit from this knowledge that will enable the correct calculation of fault currents and thereby protecting the generators and the grid. Several technical papers have been published and a section written in an IEEE Power and Energy Scoiety technical report that will be used as a guideline for protection engineers to calculate their relay settings. This will become even more urgent as the sizes of individual generators as well as the total amount of wind generation grow. Skilled manpower has been produced, both at PhD and at undergraduate levels to serve as champions in the quest for renewable green energy from wind generators. The PhD student has graduated and was hired by a leading manufacturer of wind generators.
