Energy storages in high-power STATCOM applications

Renewable energy sources are connected to the electric grid at an increasing pace. Previously synchronous generators have been the basis of stable grid operation, but nowadays and in the future, synchronous generators are being replaced by renewable energy sources that are grid connected using power electronics converters. Replacement of synchronous machines can cause for example decrease of inertia which can make the power grid more vulnerable to stability issues. To avoid such problems, new control methods have been developed to maintain stable grid operation in a situation where synchronous machines are not present. Grid-forming control method is being applied to a static synchronous compensator (STATCOM) compensation device that is traditionally used just for reactive power compensation, but for inertial response also active power injection and absorption are required. STATCOM can provide active power response if the device has sufficiently large energy storage for active power injection and absorption. Previously capacitors have been used as the capacitors have a long lifetime of well over 10 years, fast response time and can tolerate high voltages. In reactive power compensation the capacitors have been used to provide voltage for the modular multilevel converter in order to be able to inject or absorb reactive power. Nowadays transmission system operators have showed interest in STATCOM devices that are capable of active power injection and absorption. However, the energy density of capacitors is quite low, so more cost and size effective solutions need to be investigated which presents the main objective of this thesis. The new energy storage should be safe, capable of producing high power output and have larger stored energy than previously. In addition, the size, complexity and cost should be minimal. Supercapacitors, Li-ion batteries, superconducting magnetic energy storages, flywheels and Li-ion capacitors were identified as possible energy storage options. Based on characteristics of each energy storage option, supercapacitors and Li-ion batteries are seen as the best options. Closer analysis of the supercapacitors and Li-ion batteries has been made using commonly used simulation models. Especially different supercapacitor models can have significant differences that will affect the sizing and ratings. Battery sizing was done using LTO cell parameters while supercapacitor sizing was done based on 3200 F cell. Simulating the behavior of the energy storage as a part of STATCOM model showed that the voltage change during the start of discharge and charge can be significantly large. Based on the decreasing voltage characteristics of batteries and supercapacitors during discharge, it is seen that a DC-DC converter should be used as it will also have a significant effect on the overall costs. Major difference between batteries and supercapacitors is that the equivalent series resistance of batteries is much larger. Charging and discharging rate of batteries is lower than with supercapacitors which means that many more parallel battery strings are needed to limit the current experienced by one cell in common DC-link implementation. Benefit of using batteries is that the voltage during discharge and charge is very linear with a small slope unlike with supercapacitors. Voltage change for batteries during short charging and discharging is small because the batteries are oversized in terms of energy for the given power requirement. Based on simple calculations, the initial investment to buy the energy storage is smaller with batteries than with supercapacitors