Coordinated Control of Distributed Energy Resources in Islanded Microgrids

As the penetration of the distributed energy resources (DERs) in the power grid increases,new challenges are revealed, including: stability issues, frequency fluctuations, voltage control, protection system coordination, etc. A systematic approach for dealing with those issues is to view the DERs and associated loads as a subsystem or a microgrid (MG). MGs can operate either in the grid connected or islanded modes. As opposed to the grid connected mode, the voltage and frequency regulation and load/generation balancing during islanded mode is solely dependent on the local generation units. Therefore, stable and reliable operation of islanded MGs requires a real time coordinated control scheme. Conventionally, such coordination is achieved by means of the active power-frequency and reactive powervoltage droop control schemes. The conventional droop method, which is based on P-f droop concept in power systems, lacks compatibility with the resistive nature of networks as well as the low inertia of electronically interfaced DER units in MGs. As a result, it features a slow dynamic response but also a low power quality due to frequency and voltage fluctuations. This PhD research proposes a novel droop concept based on the global positioning system (GPS) and voltage-current (V-I) droop characteristics for coordination of inverter-based DER units in islanded MGs. The concept of V-I droop control is introduced in Chapter 2. In this control approach, each DER is equipped with a GPS receiver, which produces a pulse at frequency of 1Hz (1PPS). Since all GPS receivers are locked to atomic clocks of the GPS satellites, the 1PPS signal can be utilized to synchronize the time reference of the DER units. Using the common time reference and fixing the frequency at the nominal value, all of the units can share a common synchronous rotating reference frame (SRRF). Furthermore, proportional load sharing is achieved by drooping the d and q axis components of the reference voltage with respect to the d and q axis components of current, respectively. The proposed scheme not only circumvents the issue of frequency fluctuations but also is in accordance with the fast dynamics of inverter-based DER units and resistive nature of the networks in islanded MGs. The V-I droop scheme, in its basic form, relies on availability of GPS signals at each of the DER units. With the intention of improving the MG robustness with respect to GPS signal failure, a new control strategy based on V-I droop concept is presented Chapter 3. In this method, an adaptive reactive power-frequency droop scheme is used as a backup for the V-I droop controller to ensure synchronization in case of a GPS signal failure. Droop control schemes in general, and the proposed V-I droop strategy in particular are characterized by non-ideal sharing of current among the DER units due to the variations of voltage along the MGs. In order to improve the sharing accuracy of the V-I droop scheme iv while regulating the average voltage at the nominal value, a new distributed secondary control method based on consensus protocol is proposed in Chapter 4. In this method, the daxis droop characteristics is altered so as to regulate the average microgrid voltage to the rated value but also guarantee proper sharing of active power among the DERs. Additionally, the q-axis component of voltage is adjusted to perform proper sharing of current. Generally, DERs might be supplied from different energy sources, including renewables and storage systems. The intermittency of renewable energy resources on one hand and the limited capacity of the energy storage systems on the other hand, necessitate modification of droop characteristics based on an energy management plan. In Chapter 5, a novel distributed secondary control strategy is introduced for power management of integrated photovoltaicbattery DER units in islanded MGs. The distributed secondary controllers are coordinated based on a leader-follower framework, where the leader restores the MG voltage to the rated value and the followers pursue energy management. Unbalanced and nonlinear loads, which are quite common in MGs, adversely affect the power quality and sharing accuracy. In order to mitigate those issues, two new solutions are proposed in this thesis. In the first approach (Chapter 6), a new supplementary droop control scheme is added to the V-I droop controller to reduce the voltage unbalance while preventing current and power overload under unbalanced loading conditions. In the second approach (Chapter 7), a hierarchical control scheme, consisting of primary (modified V-I droop) and distributed secondary control levels is introduced to mitigate harmonic distortions and prevent overcurrent stresses under nonlinear and unbalanced loading conditions. Finally, the conclusions and possible future work are addressed in Chapter 8

Control and Stability of Residential Microgrid with Grid-Forming Prosumers

The rise of the prosumers (producers-consumers), residential customers equipped with behind-the-meter distributed energy resources (DER), such as battery storage and rooftop solar PV, offers an opportunity to use prosumer-owned DER innovatively. The thesis rests on the premise that prosumers equipped with grid-forming inverters can not only provide inertia to improve the frequency performance of the bulk grid but also support islanded operation of residential microgrids (low-voltage distribution feeder operated in an islanded mode), which can improve distribution grids’ resilience and reliability without purposely designing low-voltage (LV) distribution feeders as microgrids. Today, grid-following control is predominantly used to control prosumer DER, by which the prosumers behave as controlled current sources. These grid-following prosumers deliver active and reactive power by staying synchronized with the existing grid. However, they cannot operate if disconnected from the main grid due to the lack of voltage reference. This gives rise to the increasing interest in the use of grid-forming power converters, by which the prosumers behave as voltage sources. Grid-forming converters regulate their output voltage according to the reference of their own and exhibit load sharing with other prosumers even in islanded operation. Making use of grid-forming prosumers opens up opportunities to improve distribution grids’ resilience and enhance the genuine inertia of highly renewable-penetrated power systems. Firstly, electricity networks in many regional communities are prone to frequent power outages. Instead of purposely designing the community as a microgrid with dedicated grid-forming equipment, the LV feeder can be turned into a residential microgrid with multiple paralleled grid-forming prosumers. In this case, the LV feeder can operate in both grid-connected and islanded modes. Secondly, gridforming prosumers in the residential microgrid behave as voltage sources that respond naturally to the varying loads in the system. This is much like synchronous machines extracting kinetic energy from rotating masses. “Genuine” system inertia is thus enhanced, which is fundamentally different from the “emulated” inertia by fast frequency response (FFR) from grid-following converters. Against this backdrop, this thesis mainly focuses on two aspects. The first is the small-signal stability of such residential microgrids. In particular, the impact of the increasing number of grid-forming prosumers is studied based on the linearised model. The impact of the various dynamic response of primary sources is also investigated. The second is the control of the grid-forming prosumers aiming to provide sufficient inertia for the system. The control is focused on both the inverters and the DC-stage converters. Specifically, the thesis proposes an advanced controller for the DC-stage converters based on active disturbance rejection control (ADRC), which observes and rejects the “total disturbance” of the system, thereby enhancing the inertial response provided by prosumer DER. In addition, to make better use of the energy from prosumer-owned DER, an adaptive droop controller based on a piecewise power function is proposed, which ensures that residential ESS provide little power in the steady state while supplying sufficient power to cater for the demand variation during the transient state. Proposed strategies are verified by time-domain simulations