Baojin Liu

Load power estimation based secondary control for microgrids

The well-known active power-frequency and reactive power-voltage amplitude droop control is widely used in the coordinative control of parallel inverters in microgrids. However, this conventional droop method may cause frequency and voltage deviation, which affects the accuracy of power supply. This paper proposes a novel secondary control strategy to compensate this deviation. This strategy mimics the Master-Slave control but requires no communication lines among the parallel inverters. The master inverter adopts conventional droop method (using power to control frequency and voltage amplitude) and is controlled as a voltage source while the slave inverters adopt reversed droop method (using frequency and voltage amplitude to control power) and are controlled as current sources. The droop characteristic bias of slave inverters is designed online based on the estimation of load power demand. Through this method, frequency and voltage deviation can be eliminated and power sharing can be realized among all the slave inverters. Simulation and experimental results are provided to prove the effectiveness of the proposed control strategy.

A new master-slave based secondary control method for virtual synchronous generator

VSG (Virtual Synchronous Generator) is a control method for inverters that mimics both the steady-state droop characteristic and transient characteristic of Synchronous Generators (SGs). Because the steady states of VSG and droop controlled inverters are similar, the deviation of frequency and voltage amplitude also exists in the VSG. This paper proposes a new secondary control method mimicking the master-slave control but requiring no communications among VSGs. In the proposed method, the master VSG is controlled as a voltage source while the slave VSGs are controlled as current sources. The droop characteristic biases of slave VSGs are regulated online based on the load power estimation. Through this method, frequency and voltage deviation can be compensated and the majority of load power can be shared equally among slave VSGs. Simulation and experimental results are provided to verify the effectiveness of the proposed control strategy.

Modeling and analysis of droop based hybrid control strategy for parallel inverters in islanded microgrids

The well-known active power-frequency and reactive power-voltage amplitude droop scheme is widely used in islanded microgrids to automatically share load power and regulate output voltage of parallel voltage-controlled inverters (VCIs) in microgrids. However, droop controlled VCIs tend to lose stability as droop slopes increasing. Meanwhile, parameter discrepancies extend synchronization time between VCIs which degrade system dynamic performance. In order to compensating above limitations of traditional method, this paper proposed a droop based hybrid control strategy by exploiting advantages from both voltage-controlled and current-controlled inverters. Capturing the detail of inner control loops, a small-signal state-space model is derived to analyze characteristics of the overall parallel system. Comparing to traditional method, eigenvalues of the hybrid control strategy indicate better stability and dynamic performance. In agreement with theoretical analysis, both simulation and experimental results are presented to validate the advantages of this proposed strategy.

Efficiency-based optimization of steady-state operating points for parallel source converters in stand-alone power system

The operation efficiency is always considered to be a significant issue for power electronics system, such as parallel source converters in stand-alone power system. Quite a few solutions have been developed to increase power conversion efficiency of a single converter. However, existing work concerning efficiency issue on system level is scarce. On the basis of hierarchical multilevel control theory, this paper proposed a coordinative control strategy on tertiary level to optimize steady-state operating points for each source converter in the system, reducing losses and improving the overall efficiency. Based on the quadratic loss model, the optimal power distribution ratio between two converters is firstly analyzed. By introducing the concept of equivalent converter loss model, a forward-backward sweep calculation method is generalized to N-parallel-converter system. Furthermore, the successive switching points of N converters are precisely designed to maintain minimum system losses under full load range. According to datasheets of typical industrial products, calculation examples are presented to demonstrate the validity of this proposed strategy. The maximum and average efficiency improvement is 1.55 and 0.56 in percentage respectively. Considering the rising development of renewable energy generation and microgrids, this efficiency increase will contribute considerable energy and economic savings in the long run.

Comparison between virtual synchronous generator and droop controlled inverter

With the growing interest in microgrids to support renewable-energy sources, such as wind power and photovoltaic systems, control of microgrid components, such as inverters, is of increasing importance. Droop controls are methods that without communication lines can stabilize power sharing and grid formation for power generators. A virtual synchronous generator (VSG) is a control method for inverters that mimics both steady-state droop characteristics and dynamic characteristics of synchronous generators (SGs) to enhance the inertia of microgrids. In this paper, some representative VSG control methods are presented and compared. Some characteristics that are not described clearly in the literature are analyzed, such as the different forms of damping effects in VSGs and the necessity of an inner voltage loop. A comparison of VSGs and droop controls leads to the conclusion that a VSG can be regarded as a special type of droop control that mimics the rotor inertia and damping effect of SGs. This is done by changing the compensator parameters of a droop control. Furthermore, in the droop control, if the compensator is designed properly, the dynamic performance of the system can be even better than SGs. Simulations and experimental results are provided to support the conclusions.

Small AC signal droop based secondary control for Microgrids

The well-known active power-frequency and reactive power-voltage amplitude droop control is widely used in the coordinative control of parallel inverters in Microgrids. However, this conventional droop method may cause deviation on the frequency and voltage amplitude, which will affect the accuracy of power supply. This paper proposes a novel secondary control method to compensate this deviation. This method is based on the control of a small AC voltage signal, which serves as a communication link among the parallel inverters. The output power of the inverter can be automatically regulated by establishing a droop relationship between the frequency of the small AC signal and the bias of the inverters droop characteristics. Through this method, frequency and voltage amplitude deviation can be eliminated and power sharing can be realized among the parallel inverters. Simulation and experimental results are provided to prove the effectiveness of the proposed control method.

A decoupled current droop control method for parallel inverters in microgrids

The well-known active power-frequency and reactive power-voltage amplitude droop control is widely used in the coordinated control of parallel inverters in microgrids. However, the dynamics of the conventional droop method is slow due to the low-pass filters used for power calculation purposes. Stability concerns also exist in the conventional droop method. This paper proposes a novel droop control scheme, which is based on droop relations between the output voltages and currents in a synchronous reference frame (SRF). The SRF voltage and current are decoupled by introducing a virtual current vector, which is aligned with the voltage vector. The proposed current droop relation forms automatically a virtual impedance loop, which can compensate the line impedance mismatch. In addition, a proportional regulator-based phase-locked loop is used to synchronize the parallel inverters in an equalized manner. The proposed method provides accurate current sharing, faster dynamics and higher system stability in diverse line impedance conditions. The effectiveness of the proposed method is verified by both simulations and experiments.

A seamless transfer strategy based on indirect current control and droop control

The distributed generation (DG) should supply continuous power to local critical load, even in the condition of utility outage. This paper proposes a novel seamless transfer control strategy, which combines indirect current control and droop control. A microgrid structure of multi-masters and multi-slaves is also proposed, which based on the master-slave control and droop control. The master DGs are controlled by the novel control strategy and the slave DGs are controlled as conventional current sources based on PQ control. The advantages of this proposed strategy and microgrid structure are as follows. Firstly, the quality of critical load voltage can be maintained when utility outage occurs, Secondly, the reliability of microgrid in islanding mode can be improved because of the existence of multiple master DGs. The simulation results are provided to verify the effectiveness of this proposed control method.

A seamless transfer strategy based on special master and slave DGs

In the case of sensitive and critical load, the distributed generation (DG) should supply continuous power, even in the condition of utility outage. This paper proposes a novel seamless transfer control strategy, which based on special master and slave DGs. The master DG unit is controlled by a novel control strategy, which combines indirect current control and droop control. A part of slave DGs are controlled as conventional current sources based on PQ control, which are named as Slave-Type1. Another part of slave DGs are controlled based on PQ control and droop control, which are named as Slave-Type2. When the islanding is confirmed, the control block of Slave-Type2 DGs will transfer from PQ control to droop control. The advantages of this proposed strategy and microgrid structure are as follows. Firstly, the quality of critical load voltage can be maintained when utility outage occurs, Secondly, the reliability of microgrid in islanding mode can be improved because of the existence of multiple master DGs. The simulation results are provided to verify the effectiveness of this proposed control method.

Analysis and design of cutoff frequency for power calculation low-pass filters in droop control

Droop control method has been widely applied to achieve equal power sharing among distributed generations in microgrids. In practice, low-pass filters are usually required to mitigate the harmonics and noises in the calculated instantaneous power and obtain the average power used for the generation of frequency and voltage references. Unfortunately, the design of low-pass filters has not been studied systematically in the existing research. This paper, by proposing a small-signal model of the droop controlled system, analyzes the effects of the low-pass filters on system stability and provides a design method for an optimal cutoff frequency. The effectiveness of the proposed design method is verified by simulations.