How do you design a control strategy for a grid-connected converter with multiple objectives?

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Grid-connected converters are power electronic devices that can interface renewable energy sources, energy storage systems, or flexible loads with the utility grid. They can provide various benefits, such as power quality improvement, voltage regulation, frequency support, and active filtering. However, designing a control strategy for a grid-connected converter is not a trivial task, as it involves multiple objectives and trade-offs. In this article, we will discuss some key aspects of grid-connected converter control, such as grid synchronization, power control, current control, and modulation techniques.

本文章的要点总结

Define control objectives:

Tailoring the control strategy based on specific goals, such as maximum power injection for photovoltaic systems or grid support for battery energy storage systems, ensures the converter aligns with its intended purpose.
Integrate techniques:

Using a combination of grid synchronization methods like phase-locked loop (PLL) and synchronous reference frame (SRF) can provide accurate tracking and adaptability to various grid conditions.

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1 Grid synchronization

One of the first steps in grid-connected converter control is to synchronize the converter with the grid voltage and frequency. This is essential for ensuring stable and reliable operation, as well as minimizing power losses and harmonics. There are different methods for grid synchronization, such as phase-locked loop (PLL), synchronous reference frame (SRF), or droop control. The choice of method depends on the application requirements, the grid conditions, and the converter topology. For example, PLL is widely used for single-phase or three-phase converters, as it can track the grid phase angle and frequency accurately. SRF is suitable for three-phase converters with balanced or unbalanced grid voltages, as it can transform the grid voltages into a rotating reference frame. PLL and SRF, however, are not necessarily distinct techniques since their processes often involve both methods. Droop control is a decentralized method that can coordinate multiple converters without communication, as it adjusts the frequency and voltage according to the active and reactive power. (This section has been updated by LinkedIn editors based on member feedback.)

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Mohit Sinha

Power Electronics and Control Engineer
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I don't think SRF is a separate technique, rather it is always used in conjunction with PLL. We have different PLL designs that use one or more synchronous reference frames. Even for single-phase PLLs, the current and voltages are transformed into a synchronous reference frame ( synchronous to the grid frequency) via the Park's transform. (made possible by augmenting a predicted quadrature component)

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Ian Forsyth

former Senior Network Management Officer at Hydro One
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Pseudo PLL achievement is contingent on many things. Firstly a very articulate data image of the source and load waveforms must be made whether AC or DC, all elements of energy transfer. Analysis must be made with respect to Voltage, Current, Frequency and Power Factor. The analysis must take into account instantaneous, short term and longer term desired performance. The sequencing of the sample to analysis to implementation is very important and "obvious to someone skilled in the art". Metering, protection, grid interface and all other influences are secondary. Possible if the waveforms captured in high granularity. Less than 0.5 V on a 120V base and less than 1/10th of an electrical degree on a 60 Hz basis.

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2 Power control

Another important aspect of grid-connected converter control is to regulate the active and reactive power exchanged with the grid. This can be achieved by controlling the converter output voltage magnitude and phase angle, or by controlling the converter output current magnitude and phase angle. The power control strategy depends on the converter operating mode, the grid connection point, and the control objectives. For example, if the converter is connected to a weak grid or a microgrid, it may need to operate as a voltage source and maintain a constant voltage magnitude and frequency at the point of common coupling (PCC). If the converter is connected to a strong grid or a transmission system, it may need to operate as a current source and inject or absorb active and reactive power according to the grid demand or the renewable generation.

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Luccas Soares

Estagiário na Andritz Hydro | Eletr?nica de Potência - Máquinas Síncronas - Sistema Elétrico de Potência
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The power control of the converter depends on the purpose of this equipment. For example, PV systems work by injecting the maximum active power generated by the array, however, BESS systems work by helping the grid with active and reactive power at critical moments. In this sense, to define the power converter control strategy, its goals must be taken into account. However, control strategies such as VSM (virtual synchronous machine) can be adopted to provide a range of auxiliary services to the grid.

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3 Current control

Current control is a crucial component of grid-connected converter control, as it determines the quality and stability of the converter output current. Current control can be implemented in different ways, such as hysteresis control, proportional-integral (PI) control, proportional-resonant (PR) control, or model predictive control (MPC). The choice of current control method depends on the converter switching frequency, the grid impedance, and the harmonic performance. For example, hysteresis control is a simple and fast method that can switch the converter on and off according to a predefined current error band. However, it may cause variable switching frequency and high switching losses. PI control is a common and robust method that can regulate the current error to zero using feedback and feedforward terms. However, it may suffer from steady-state errors and poor performance under grid distortions. PR control is an improved version of PI control that can eliminate steady-state errors and enhance harmonic rejection by adding resonant terms at the grid frequency and its multiples. MPC is an advanced method that can optimize the converter switching states based on a predictive model of the converter and the grid.

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4 Modulation techniques

Modulation techniques are the methods that generate the switching signals for the converter based on the desired output voltage or current. They can affect the converter efficiency, harmonic distortion, and dynamic response. There are different modulation techniques for grid-connected converters, such as pulse width modulation (PWM), space vector modulation (SVM), or selective harmonic elimination (SHE). The choice of modulation technique depends on the converter topology, the switching frequency, and the harmonic spectrum. For example, PWM is a simple and widely used technique that can vary the duty cycle of the switching signals according to a reference signal and a carrier signal. However, it may produce high harmonic distortion and switching losses at low switching frequencies. SVM is a more efficient and flexible technique that can generate the switching signals based on the location of the reference vector in a space vector diagram. However, it may require more computational resources and complex algorithms. SHE is a sophisticated technique that can eliminate specific harmonics by solving a set of nonlinear equations. However, it may have limited applicability and stability issues.

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Ian Forsyth

former Senior Network Management Officer at Hydro One
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Problem with algorithms is they depend on the performance ranges of sampling and implementing devices. And they depend on the parameters of the electrical installation. Whether in a factory or in a grid install. Just think how many of those parameters change and how frequently. A unique Pseudo PLL makes use of phase sequencing and highly articulate waveforms. Capture, Analysis, Implementation, Redo.

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How do you design a control strategy for a grid-connected converter with multiple objectives?

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1 Grid synchronization

2 Power control

3 Current control

4 Modulation techniques

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