Uffe Borup

A new control structure for grid-connected LCL PV inverters with zero steady-state error and selective harmonic compensation

The PI current control of a single-phase inverter has well known drawbacks: steady-state magnitude and phase error and limited disturbance rejection capability. When the current controlled inverter is connected to the grid, the phase error results in a power factor decrement and the limited disturbance rejection capability leads to the need of grid feed-forward compensation. However the imperfect compensation action of the feed-forward control results in high harmonic distortion of the current and consequently noncompliance with international standards. In this paper a new control strategy aimed to mitigate these problems is proposed. Stationary-frame generalized integrators are used to control the fundamental current and to compensate the grid harmonics providing disturbance rejection capability without the need of feed-forward grid compensation. Moreover the use of a grid LCL-filter is investigated with the proposed controller. The current control strategy has been experimentally tested with success on a 3 kW PV inverter.

Transformerless Photovoltaic Inverters Connected to the Grid

Renewable energy sources are getting more and more widespread, mainly due to the fact that they generate energy by keeping the environment clean. Most of these systems have an isolation transformer included, which if excluded from the system would increase the efficiency and decrease the size of PV installations, furthermore it would lead to a lower cost for the whole investment. But there are some safety issues regarding the missing galvanic isolation. This paper is aiming to analyze and compare the most common single-stage transformerless PV inverter topologies for single-phase and three-phase with respect to the leakage current generation. The best results, both for single-phase and three-phase systems, are obtained when the middle point of the input capacitors is connected to the neutral point, thereby minimizing the voltage fluctuations present at the terminals of the PV panel.

A digital controlled PV-inverter with grid impedance estimation for ENS detection

The steady increase in photovoltaic (PV) installations calls for new and better control methods in respect to the utility grid connection. Limiting the harmonic distortion is essential to the power quality, but other requirements also contribute to a more safe grid-operation, especially in dispersed power generation networks. For instance, the knowledge of the utility impedance at the fundamental frequency can be used to detect a utility failure. A PV-inverter with this feature can anticipate a possible network problem and decouple it in time. This paper describes the digital implementation of a PV-inverter with different advanced, robust control strategies and an embedded online technique to determine the utility grid impedance. By injecting an interharmonic current and measuring the voltage response it is possible to estimate the grid impedance at the fundamental frequency. The presented technique, which is implemented with the existing sensors and the CPU of the PV-inverter, provides a fast and low cost approach for online impedance measurement, which may be used for detection of islanding operation. Practical tests on an existing PV-inverter validate the control methods, the impedance measurement, and the islanding detection.

Implementation and test of an online embedded grid impedance estimation technique for PV inverters

New and stronger power quality requirements are issued due to the increased amount of photovoltaic (PV) installations. In this paper different methods are used for continuous grid monitoring in PV inverters. By injecting a noncharacteristic harmonic current and measuring the grid voltage response it is possible to evaluate the grid impedance directly by the PV inverter, providing a fast and low-cost implementation. This principle theoretically provides an accurate result of the grid impedance but when using it in the context of PV integration, different implementation issues strongly affect the quality of the results. This paper also presents a new impedance estimation method including typical implementation problems encountered, and it also presents adopted solutions for online grid impedance measurement. Practical tests on an existing PV inverter validate the chosen solution.

Evaluation of the voltage support strategies for the low voltage grid connected PV generators

Admissible range of grid voltage is one of the strictest constraints for the penetration of distributed photovoltaic (PV) generators especially connection to low voltage (LV) public networks. Voltage limits are usually fulfilled either by network reinforcements or limiting of power injections from PVs. In order to increase PV penetration level further, new voltage support control functions for individual inverters are required. This paper investigates distributed reactive power regulation and active power curtailment strategies regarding the development of PV connection capacity by evaluation of reactive power efforts and requirement of minimum active power curtailment. Furthermore, a small scale experimental setup is built to reflect real grid interaction in the laboratory by achieving critical types of grid (weak and sufficiently stiff).

A combined two-method MPPT control scheme for grid-connected photovoltaic systems

In order to increase the output efficiency of a grid-connected photovoltaic (PV) system it is important to have an efficient maximum power point tracker (MPPT). In the case of low irradiation, the perturb and observe (PO) and incremental conductance (IC) methods have a poor efficiency, because of the poor resolution in the acquired signals, when a fixed point implementation is done. A cost-effective two-method MPPT control scheme is proposed in this paper to track the maximum power point (MPP) at both low and high irradiation, by combining a constant voltage (CV) method and a modified PO algorithm. Simulation and experimental results are presented to verify the performance of the proposed method

Online grid measurement and ENS detection for PV inverter running on highly inductive grid

Photovoltaic (PV) and other sources of renewable energy are being used increasingly in grid-connected systems, for which stronger power quality requirements are being issued. Continuous grid monitoring should be considered so as to provide safe connections and disconnections from the grid. This letter gives an overview of the methods used for online grid measurement with PV inverters. Emphasis is placed on a method based on the injection of a noncharacteristic harmonic in the grid. Since this injection is regarded as a disturbance for the grid, different issues, i.e., the influence on total harmonic distortion (THD), the accuracy of line impedance measurement and the ENS (German abbreviation of Main Monitoring units with allocated Switching Devices) detection are studied. Laboratory results conducted on an existing PV inverter are presented to demonstrate the behavior of the PV inverter under different grid conditions. Some of the injection parameters are tuned in order to get an accurate measurement of line impedance.

A new method of on-line grid impedance estimation for PV inverter

The recent increase in photovoltaic (PV) installations calls for new and better power quality requirements with respect to connection to the grid supply. Therefore, different methods are typically used for continuous grid monitoring, usually by using external devices. In this paper a new method for on-line measuring the grid impedance is presented. The presented method requires no extra hardware being accommodated by typical PV inverters, sensors and CPU, to provide a fast and low cost approach of on-line impedance measurement. By injecting a noncharacteristic harmonic current and measuring the voltage grid response it is possible to evaluate the grid impedance. Practical test on an existing PV inverter validate the new method.

Online grid impedance measurement suitable for multiple PV inverters running in parallel

Due to the increased use of photovoltaic (PV) installations, new and stronger grid connection requirements are issued in order to protect the utility network. One of the most severe demand applying to PV installations is the ability to detect islanding conditions. Hence, the continuous grid monitoring is always a matter to be considered in order to fulfill this requirement. The present paper gives a description of one method used for on-line grid impedance estimation by means of signal injection and focus is put on the problems issued by the situation when two or more inverters run in parallel on the same network. The paper reveals a new method to detect a second inverter connected to the grid and the proposed solution is discussed. Practical tests on an existing PV inverter validates the second inverter detection algorithm.

Quasi 12-pulse rectifier for adjustable speed drives

This paper presents a quasi 12-pulse rectifier scheme for standard adjustable speed drives (ASDs). An analysis of the standard diode rectifier shows that the harmonic currents to some degree are independent of the load. As a result, near true 12-pulse rectifier line currents for a wide load range can be achieved with the quasi 12-pulse scheme. Furthermore, it is shown that the harmonic performance of the quasi 12-pulse scheme actually is superior to the true 12-pulse rectifier if the supply voltage is pre-distorted. Both simulations and experimental results are verifying the analysis.