This chapter develops a robust decentralized voltage tracker for islanded MGs. The proposed controller is robust against the plug and play operation of the MG, loads, and line parameter uncertainties. The problem is solved in the framework of linear matrix inequality (LMI). The proposed robust control represents the load changes and the parameter variations of lines connecting the DGs as a norm-bounded uncertainty. The proposed controller utilizes local measurements from DGs (i.e., it is totally decentralized). Control decentralization is accomplished by decomposing the global system into subsystems. The effect of the rest of the system on a specific subsystem is considered as a disturbance to minimize (disturbance rejection control). The controller is designed by the invariant-sets (approximated by the invariant ellipsoids). Different time-domain simulations are carried out as connecting and disconnected one or more DGs, connecting and disconnecting local loads DGs and transmission line parameters variation.
TopIntroduction
A microgrid (MG) is a small-scale power grid composed of various interconnected local loads, distributed generation (DGs), storage devices, master controller, power electronics converters, protective devices, along with communication, and automation systems. The growth of MGs permit extensive penetration of renewable energy sources (RESs) into the power systems. Consequently, MGs can diminish transmission, distribution system losses, global warming gas emissions. It also enhance reliable and secure service of power to customers (Bayoumi E.H.E., et al. 2020; Olivares D. E., et al. 2014).
MGs have two modes of operations: grid-connected and islanded modes. In the grid-connected mode, MGs transfer power to the utility grid at point of common coupling (PCC). In the islanded mode, the MG works autonomously without connection to the utility grid. The characteristics of the microgrid are: bidirectional power transfer, and power electronics devices installed with DGs. The control of MG in each operation mode along with the switching between the modes are the challenge that require to be resolved in order to use MGs effectively and successfully (Sadabadi M. S., et al. 2017).
The islanded mode of operation is able to be scheduled or unscheduled and it arises when MGs are separated from the utility grid (Yazdanian M., et al. 2014). The unscheduled islanded mode of operation is started when the utility grid faces a continual abnormal condition. When the MG is disconnected, real and reactive power mismatch between the load and generation causes deviation in voltage and frequency. The main function of the control system in islanded mode of operation is to reestablish the balance to preserve the MG stability (Olivares D. E., et al. 2014). To guarantee stability during the islanded mode of operation, new control techniques has to be implemented to ensure both voltage and frequency stability through suitable real and reactive power sharing arrangements. One of the well-known control hicrarchy used for the MGs is the Energy Management System (EMS). This control technique specifies the DGs power and voltage set points (Bidram A., et al. 2017; Xuand Y., 2020).
Sustaining the stability of MGs operation in both grid-connected and islanded modes along with the switching between the two modes introduces excessive challenge as a result of the increasing penetration of renewable energy-based DGs (Shafiee G., et al. 2014). This challenge is due to low- inertia inverter-interfaced DGs compared to large rotational inertia of the conventional rotating synchronous generators (Schiffer J., et al. 2014). The DGs with low-inertia inverters do not improve grid stability. One of the main important challeges in MGs is plug -and-play (PnP) operation of DGs due to the inherently discontinuous nature of renewable energy sources.
Plug-and-play (PnP) characteristic must be impeded in any ideal MG system. It should permit normal integration of any new loads or sources without the necessity to redesign the MG control system. There are some efforts in achieving this target. Generally, most of these efforts can be classified into two main categories: stability, and Communications. MG system stability must be guaranteed when it is operating Plug-and-play (a new DG unit is added/removed due to any abnormal condition or the intermittency of renewable energy resources) (Xuand Y., et al. 2020). Under this performance, the frequency and voltage stability of MGs have to be preserved even when communication links are lost without the need to retune the MG control system. To attain this performance, a decentralized control strategy is required. The droop control is normally applied to MGs control. This strategy is based on the power balance as of synchronous generators (Olivares D. E., et al. 2014; Sadabadi M. S., et al. 2017). The frequency or the rotor speed of a synchronous generator decreases linearly with its output power (Kundur P., et al. 1994). Although the primary droop control strategy delivers a proportional power sharing between DGs, its implementation is limited because it has only one tunable parameter. Extra challenges related to this control technique involve the coupling between frequency and voltage deviations taking into consideration the real and reactive power dynamics. Furthermore, the droop control technique has a poor behaviour in the case of parameters’ changes of lines connecting the DGs (Etemadi A. H., et al. 2014).