Lenos Hadjidemetriou

A New Hybrid PLL for Interconnecting Renewable Energy Systems to the Grid

There is a need to develop new control strategies for interconnecting Renewable Energy Sources (RES) to the power system due to the ever increasing penetration of RES and in particular for wind power systems (WPS). The control strategies are typically based on a fast and accurate detection of the phase angle of the grid voltage which may be estimated by using a Phase-Locked Loop (PLL) control circuit. The performance of the PLL under normal and abnormal operational conditions is a crucial aspect, since the RES is desired to operate to support the power system under grid fault conditions. This paper investigates the performance of three different PLLs: a synchronous reference frame PLL (dqPLL), a stationary reference frame PLL (αβPLL), and a decoupled double synchronous reference frame PLL (ddsrfPLL). The results of this investigation motivate to the development of a new hybrid PLL which is a combination of the abovementioned PLLs and uses the advantages of each PLL. The proposed decoupled stationary reference frame PLL (dαβPLL) may be an appropriate solution to use in an interconnected RES with Ride Through Fault (RTF) capability, since it prevails the other PLLs with regards to its accuracy under unbalanced faults. Further, it has a lower deviation of the estimated phase after the fault occurs. The performance of the new hybrid dαβPLL is verified through simulations and experiments. Further the new PLL is used in an interconnected RES through experiments under normal and RTF operation.

A Robust Synchronization to Enhance the Power Quality of Renewable Energy Systems

The increasing penetration of renewable energy sources (RESs) in the power grid requires high-quality power injection under various grid conditions. The synchronization method, usually a phase-locked loop (PLL) algorithm, is directly affecting the response of the grid-side converter of the RES. This paper proposes a new PLL algorithm that uses an advanced decoupling network implemented in the stationary reference frame with limited requirements for processing time to enable a fast and accurate synchronization even under harmonic distorted voltage and low-voltage grid faults. The robust response of the proposed PLL is validated, and the effect of the proposed synchronization on the performance of the grid-connected renewable energy system is investigated. This investigation proves that the robust, accurate, and dynamic response of the new PLL can enhance the quality of the injected power from the RES and can also enable an appropriate fault-ride-through operation under harmonic distorted voltage and grid faults.

A Synchronization Method for Single-Phase Grid-Tied Inverters

The controllers of single-phase grid-tied inverters require improvements to enable distribution generation systems to meet the grid codes/standards with respect to power quality and the fault ride through capability. In that case, the response of the selected synchronization technique is crucial for the performance of the entire grid-tied inverter. In this paper, a new synchronization method with good dynamics and high accuracy under a highly distorted voltage is proposed. This method uses a Multiharmonic Decoupling Cell (MHDC), which thus can cancel out the oscillations on the synchronization signals due to the harmonic voltage distortion while maintaining the dynamic response of the synchronization. Therefore, the accurate and dynamic response of the proposed MHDC-PLL can be beneficial for the performance of the whole single-phase grid-tied inverter.

An Adaptive Tuning Mechanism for Phase-Locked Loop Algorithms for Faster Time Performance of Interconnected Renewable Energy Sources

Interconnected renewable energy sources (RES) require fast and accurate fault ride through (FRT) operation, in order to support the power grid, when faults occur. This paper proposes an adaptive phase-locked loop (adaptive dαβPLL) algorithm, which can be used for a faster and more accurate response of the grid-side converter (GSC) control of a RES, particularly under FRT operation. The adaptive dαβPLL is based on modifying the tuning parameters of the dαβPLL, according to the type and voltage characteristics of the grid fault, with the purpose of accelerating the performance of the PLL algorithm. The proposed adaptive tuning mechanism adjusts the PLL parameters in real time, according to the proposed fault classification unit, in order to accelerate the synchronization performance. The beneficial effect of the proposed adaptive tuning mechanism on the performance of dαβPLL is verified through simulation and experimental results. Furthermore, the benefits of using a faster synchronization method on the control of the GSC of RES are also demonstrated in this paper. Additionally, a new synchronization technique named frequency-phase decoupling (FPD)-dαβPLL is presented, which applies an FPD technique from the literature in the structure of dαβPLL, with the purpose of improving the performance of dαβPLL. Finally, the adaptive tuning mechanism proposed in this paper is combined with the FPD-dαβPLL in order to introduce the adaptive FPD-dαβPLL, which presents an even faster time performance and is an ideal solution for the synchronization of RES under grid faults.

A grid side converter current controller for accurate current injection under normal and fault ride through operation

Modern grid codes for interconnection of distributed energy resources have become more restrictive due to the massive integration of renewable energy sources. The interconnected renewable energy sources must operate accurately under normal and abnormal grid conditions in order to meet the grid requirements. In addition, the renewable energy systems require fault ride through capability in order to support the power grid under balanced or unbalanced low voltage grid faults. Therefore, the development of advanced current controller techniques is essential for the grid side power electronic converters in order to increase the renewable energy systems penetration and satisfy the grid codes. This paper proposes a current controller technique, which enables the appropriate operation of the grid side converter under unbalanced grid faults and harmonic distorted grid conditions. The proposed current controller is designed using multiple synchronous reference frames and can inject full positive or full negative sequence balanced sinusoidal currents under abnormal grid conditions. The accurate performance with robustness against unbalanced and harmonic distorted grid voltage indicates that the proposed current controller can be a useful tool in the fault ride through control of renewable energy systems.

Benchmarking of phase locked loop based synchronization techniques for grid-connected inverter systems

Grid-connected renewables are increasingly developed in recent years, e.g. wind turbine systems and photovoltaic systems. Synchronization of the injected current with the grid is mandatory. However, grid disturbances like voltage sags, harmonics, and frequency deviations may occur during operation, becoming inevitable challenges to the synchronization of the grid-connected renewable energy systems. In order to ensure the quality of the power generation from the renewables, robust and reliable synchronization methods are in demand. Among the prior-art solutions, Phase Locked Loop (PLL) based synchronization methods have gained much popularity in grid-connected applications. However, an appropriate selection and thus a proper design of the selected PLL synchronization remain of interest in practice, especially for single-phase systems. Therefore, in this paper, a benchmarking of the main PLL synchronization methods for single-phase grid-connected inverter systems in terms of accuracy, dynamic response, harmonic immunity, etc., has been conducted. Experiments on a 1-kW single-phase grid-connected system, suffering from different grid disturbances, are performed for the benchmarking. The experimental results have verified the discussions.

Synchronization of grid-connected renewable energy sources under highly distorted voltages and unbalanced grid faults

Renewable energy sources require accurate and appropriate performance not only under normal grid operation but also under abnormal and faulty grid conditions according to the modern grid codes. This paper proposes a novel phase-locked loop algorithm (MSHDC-PLL), which can enable the fast and dynamic synchronization of the interconnected renewable energy system under unbalanced grid faults and under highly harmonic distorted voltage. The outstanding performance of the suggested PLL is achieved by implementing an innovative multi-sequence/harmonic decoupling cell in order to dynamically cancel out the oscillation of the positive sequence voltage vector, produced by the existence of unbalanced and harmonic distorted voltage. The fast and accurate response of the proposed PLL under abnormal grid conditions is very important for the appropriate synchronization and control of the interconnected renewable energy systems. Therefore, the performance of the new PLL can increase the quality of the injected power under abnormal conditions and in addition enable the renewable energy systems to provide the appropriate support to the grid under balanced and unbalanced grid faults.

Investigation of different Fault Ride Through strategies for renewable energy sources

The Fault Ride Through (FRT) capability of renewable energy sources (RES) for providing support to the power grid under several disturbances allows us to consider them not only as passive elements in the power system network, but also as ancillary services for the mitigation of evolving contingencies. In this paper, an investigation of the FRT operation of RES according to current grid regulations highlights some impacts on the system response that were not considered previously and require to be addressed. Slight modifications of the current grid codes are therefore necessary to solve these problems. A critical dead band zone modification for smother fault recovery is suggested. Moreover, this paper proposes the use of an adjustable parameter into the FRT strategy for a fair compromise of voltage and frequency support. The proposed FRT strategies are applied to a RES that is interconnected with the IEEE 14-bus test system and dynamic electromagnetic transient simulation results are provided.

A new hybrid PLL for interconnecting Renewable Energy Systems to the grid

There is a need to develop new control strategies for interconnecting Renewable Energy Sources (RES) to the power system due to the ever increasing penetration of RES and in particular for wind power systems (WPS). The control strategies are typically based on a fast and accurate detection of the phase angle of the grid voltage which may be estimated by using a Phase-Locked Loop (PLL) control circuit. The performance of the PLL under normal and abnormal operational conditions is a crucial aspect, since the RES is desired to operate to support the power system under grid fault conditions. This paper investigates the performance of three different PLLs: a synchronous reference frame PLL (dqPLL), a stationary reference frame PLL (αβPLL), and a decoupled double synchronous reference frame PLL (ddsrfPLL). The results of this investigation motivate to the development of a new hybrid PLL which is a combination of the abovementioned PLLs and uses the advantages of each PLL. The proposed decoupled stationary reference frame PLL (dαβPLL) may be an appropriate solution to use in an interconnected RES with Ride Through Fault (RTF) capability, since it prevails the other PLLs with regards to its accuracy under unbalanced faults. Further, it has a lower deviation of the estimated phase after the fault occurs. The performance of the new hybrid dαβPLL is verified through simulations and experiments. Further the new PLL is used in an interconnected RES through experiments under normal and RTF operation.

Performance enhancement of MAF based PLL with phase error compensation in the pre-filtering stage

The large scale integration of Renewable Energy Sources (RES) requires sophisticated control techniques for efficient power transfer under faults and/or off-nominal grid conditions. A RES is efficiently integrated to the grid via proper control of the Grid Side Converter (GSC) by accurately estimating the grid voltage phase angle. Moving Average Filter (MAF) based Phase Lock Loop (PLL) techniques provide reduced complexity, however, they present disadvantages under specific grid fault conditions. The most recent MAF based technique is the EPMAFPLL, which provides improved dynamic response and reduces the phase error under off-nominal grid frequencies. However, the EPMAFPLL presents high phase and frequency overshoot at the time of fault. Furthermore, inaccurate harmonic mitigation under off-nominal grid frequencies was not investigated in EPMAFPLL. A modified EPMAFPLL (EPMAFPLL Type 2) is proposed in this paper. The modified EPMAFPLL accurately compensates the offset errors under off-nominal grid frequencies, offers lower frequency overshoot and faster dynamics under faults. In addition, it provides accurate compensation of grid voltage harmonics under off-nominal grid frequencies.