Abstract

Traditionally, remote communities worldwide consist of autonomous power systems (mini-grids) supplied almost exclusively by diesel-engine generator sets (gensets) at relatively high costs. Integration of renewable energy sources (RESs), such as, photovoltaic (PV) and wind, can substantially reduce the cost of electricity generation and emissions in these remote communities. However, the highly variable load profile typical of mini-grids and the fluctuating characteristics of the RESs, cause frequent operation of the diesel genset at low loading condition, at low efficiency points and subject to carbon build up, which can significantly affect the maintenance costs and even the life time of the genset. Another important issue that is frequently overlooked in small (< 100 kVA) mini-grids, which usually present a low number of loads thus reducing the averaging effect, is load unbalance. Diesel gensets supplying unbalanced loads experience overheating in the synchronous generator and vibration in the shaft. For efficient operation, the genset should be operated near its full capacity and also in balanced mode.
In order to address the above mentioned issues, a fast and reliable multi-mode battery energy storage system (BESS) employing voltage source inverter (VSI), is proposed in this thesis. In the genset support mode, as a basic feature, it can provide minimum loading for the genset and supplement it under peak load conditions. In addition, it can also provide load balancing and reactive power compensation for the mini-grid system. Therefore, the genset operates in balanced condition and within its ideal power range. In cases when the power demand from the genset is low, due to high supply of RESs and/or low load consumption, the genset can be shut-down and the BESS forms the grid, regulating voltage and frequency of the mini-grid system in the grid forming mode. Besides, the logic for defining the operating mode of the BESS and for achieving smooth transitions between modes are also presented in this thesis.
The conventional approach for the control of three-phase VSI with unbalanced loads uses three-phase vector (dq) control and symmetrical components calculator which usually results in slow dynamic responses. Besides, the common power (P) vs. frequency (f) droop characteristic of the genset results in the mini-grid operating with variable frequency what further complicates the design of the controller for the VSI. Therefore, a new frequency adaptive per-phase dq-control scheme for three-wire and four-wire three-phase VSI based on the concept of fictive axis emulation is presented. It enables the control of current/voltage of each phase separately to achieve better dynamic performance in the variable frequency diesel hybrid mini-grid system. The effectiveness of the proposed techniques is demonstrated by means of simulation and experimental results.