Everything has a price!
by Alex Apostolov, Editor-in-Chief
For the last couple of centuries human civilization has gone through a period of industrialization. It has completely changed our lives bringing many significant benefits as a result. We now enjoy electricity and all the devices that it can power, as well as the cars and trucks that can take us around and the planes that can fly us across the country or to a different continent. But as with everything else in life this did not come for free.
Burning coal and oil for a few hundred years resulted in high levels of pollution and what we call today Global warming that is threatening our civilization. It took some time to realize this threat and make a decision to do something about it by switching from fossil fuels-based electricity generation and transportation to the use of renewable energy resources.
This is all a very positive development that we hope will help us reduce pollution and its effect on the environment. But this has a price as well – a completely different electric power system.
For more than 100 years we became used to a system with large synchronous generators delivering power to the load centers over the transmission system and then to the individual customers over radial distribution feeders.
The protection system was designed to operate based on the high levels of current produced by the synchronous machines when a short circuit fault occurs. Protection engineers developed distance and differential protection relays capable of detecting a fault in about a cycle to maintain the dynamic stability during and after the fault. At the same time phase, ground and negative sequence overcurrent relays were widely used to protect the radial distribution feeders.
However, as a result of the ongoing significant penetration of inverter based distributed renewable energy resources the behavior of the electric power system during fault conditions has become very different and the traditional methods for protecting the transmission and distribution systems do not work very well anymore.
This is due to the fact that the inverters do not produce high levels of fault currents during short circuits which makes it difficult for the protection system to operate.
The development of microgrids introduces another set of challenges especially in the cases when you have only inverter-based resources. In the conventional electric power system, the frequency is used as an indicator of the balance between the load and generation. When we are operating at the nominal frequency this is an indication that we have a balanced system. Any deviation from it is an indication that this balance is not maintained anymore, and we need to do load or generation shedding to restore it. When we have a microgrid with renewable generation only the frequency is not a valid criterion for the balance of the system anymore.
So, the question is “What should we do?” Some people say that we should make the inverter controllers behave like synchronous generators and produce fault currents that will make the conventional protection relays happy because they will be able to operate when a short circuit fault occurs.
To me this doesn’t make sense because electric power system protection was developed to protect the electric power system equipment from the impact of the high fault currents. Instead of wanting to make the new power grid behave like what we are used to we need to take advantage of all the computer and communications technologies that are available to us today, get out of the box and develop new algorithms and operating principles that will allow us to detect that a fault has occurred and isolate the affected circuit.
To address all these challenges, we have dedicated this issue of the magazine to the impact of microgrids and inverter based distributed energy resources on protection systems.
The articles that you can read demonstrate that many PAC specialists are working hard and using innovation in developing new methods to address the challenges of the evolving grid.
” Innovation is the ability to see change as an opportunity –
not a threat “