Milan Prodanovic

Energy Management in Autonomous Microgrid Using Stability-Constrained Droop Control of Inverters

This paper presents an energy management system (EMS) for a stand-alone droop-controlled microgrid, which adjusts generators output power to minimize fuel consumption and also ensures stable operation. It has previously been shown that frequency-droop gains have a significant effect on stability in such microgrids. Relationship between these parameters and stability margins are therefore identified, using qualitative analysis and small-signal techniques. This allows them to be selected to ensure stability. Optimized generator outputs are then implemented in real-time by the EMS, through adjustments to droop characteristics within this constraint. Experimental results from a laboratory-sized microgrid confirm the EMS function.

High-Quality Power Generation Through Distributed Control of a Power Park Microgrid

Inverters are a necessary interface for several forms of distributed generation (DG) and where they form a microgrid they have the potential to offer high power quality. The challenge is to coordinate the actions of a group of inverters so that they offer the level of power quality known to be possible from fast local control of a single inverter. The case examined here is a power park of several inverter-based DG in relatively close proximity. A basic requirement is that the inverters regulate the grid voltage and share the real and reactive power demands according to their ratings. In small girds with high proportions of nonlinear and unbalanced loads it is also important to actively control the waveform quality in terms of harmonics, transient disturbances, and balance. Further, it is important that these duties are shared equally between the units rather than having one master unit taking the lead in the voltage control function. A constraint faced in designing a sharing system is the limited bandwidth of signal communication even over distances of a few meters. A control method is proposed that separates the control tasks in the frequency domain. Power sharing and voltage regulation are controlled centrally and commands are distributed through a low-bandwidth communication link. Waveform quality functions are controlled in high bandwidth controllers distributed to each local inverter. Experimental tests on a grid of three 10-kVA inverters are used to show that the method fully exploits the inherent fast response of the inverters while also ensuring voltage balance even with extreme load imbalance. It is shown that circulating currents are avoided during steady state and transients

Control and filter design of three-phase inverters for high power quality grid connection

The trend toward using inverters in distributed generation systems and micro-grids has raised the importance of achieving low-distortion, high-quality power export from inverters. Both switching frequency effects and pre-existing grid voltage distortion can contribute to poor power quality. A well designed filter can attenuate switching frequency components but has an impact on the control bandwidth and the impedance presented to grid distortion. This paper describes a filter designed to incorporate an isolating transformer and the design of a complementary controller that rejects grid disturbance, maintains good waveform quality and achieves real and reactive power control. A realistic discrete time implementation is discussed and validated with experimental results.

Dynamic Stability of a Microgrid With an Active Load

Rectifiers and voltage regulators acting as constant power loads form an important part of a microgrid’s total load. In simplified form, they present a negative incremental resistance and beyond that, they have control loop dynamics in a similar frequency range to the inverters that may supply a microgrid. Either of these features may lead to a degradation of small-signal damping. It is known that droop control constants need to be chosen with regard to damping, even with simple impedance loads. Actively controlled rectifiers have been modeled in nonlinear state-space form, linearized around an operating point, and joined to network and inverter models. Participation analysis of the eigenvalues of the combined system identified that the low-frequency modes are associated with the voltage controller of the active rectifier and the droop controllers of the inverters. The analysis also reveals that when the active load dc voltage controller is designed with large gains, the voltage controller of the inverter becomes unstable. This dependence has been verified by observing the response of an experimental microgrid to step changes in power demand. Achieving a well-damped response with a conservative stability margin does not compromise normal active rectifier design, but notice should be taken of the inverter–rectifier interaction identified.

Energy Management System with Stability Constraints for Stand-alone Autonomous Microgrid

This paper presents an energy management system (EMS) for a stand-alone droop-controlled microgrid, which adjusts generator outputs to minimize fuel consumption and also ensures stable operation. It has previously been shown that droop gains have a significant effect on stability in such microgrids. Approximate relationships between these parameters and stability margins are therefore identified, using qualitative analysis and small-signal techniques. This allows them to be selected to ensure stability. Optimized generator outputs are then implemented in real-time by the EMS, through adjustments to droop characteristics within this constraint. Experimental results from a laboratory-sized microgrid confirm the EMS function.

Control of power quality in inverter-based distributed generation

Power quality is an important additional service of inverter-based interfaces for distributed generators. In grid connected applications the power quality depends on the harmonic content of the current injected at the point of common coupling. By careful design of the power converter and its output filter the switching frequency components in the output current spectrum can be reduced to low levels. The effect of the harmonic distortion of the grid voltage on the output current can be minimised by using an appropriate inverter control strategy. Conventional control methods (manipulation of inverter voltage magnitude and phase) offer active and reactive power control, but not the control of the output current quality. This paper describes a new choice of control structure and explains the interaction between the applied control loops. The inverter is used to control the current in the first element of an LCL filter. A further controller is wrapped around this loop to control power export to the grid. The usefulness of this arrangement in providing big power quality is emphasised. Experimental results from a 10 kVA prototype are used to evaluate the distortion rejection properties and the regulation of active and reactive power control. The results show high quality of generated power and excellent transient and steady state-response to both active and reactive power demands.

Distributed voltage control in AuRA-NMS

This paper presents real time test results arising from the application of a case based reasoning technique for voltage control of a section of existing UK 11kV network. The test network includes two distributed generation schemes. The objective of the case based reasoning technique is to maintain voltages within statutory limits while also maximizing the DG access to the network. The control algorithm is embedded on a commercially available hardware platform designed for installation in power system substations. The case based reasoning technique employs the following control actions to achieve its objectives: DG real power control, DG reactive power control and transformer tap change control. The voltage control technique is tested during over-voltage conditions and for situations where only partial sensor data is available.