The amount of distributed energy resources (DERs) has constantly increased worldwide. As the power ratings of DERs have become considerably high, the grid code requirements are necessary to secure reliable power generation and transmission for the public electric network. In order to follow grid codes of the various countries and optimize the function of grid-connected inverters for DERs, a robust phase-locked loop (PLL) is essential for extracting the grid phase information accurately, when the grid voltage is polluted by harmonics and unbalanced. This paper proposes fault ride-through techniques based on a low-pass notch (LPN)-PLL. The LPN-PLL has not only fast and smooth transient responses to a sudden transition of the grid voltage but also has robustness to the distorted and unbalanced grid conditions. Therefore, the stable performance in the grid fault conditions is expected without the system trip. Furthermore, a universal voltage sag generator for the various grid codes with six parameters is proposed for the verification of low voltage ride-through (LVRT) performance. Experimental verifications are presented to show the LVRT performance based on the LPN-PLL with a three-phase grid-connected inverter (10 kV·A) and a prototype of the voltage sag generator (10 kV·A).

Implementation of Fault Ride-Through Techniques of Grid-Connected Inverter for Distributed Energy Resources With Adaptive Low-Pass Notch PLL

A transformer based voltage sag generator to test renewable energy systems is presented. Design considerations concerning overvoltages and overcurrents during switching action are adressed. Measurement results of a 30 kW laboratory prototype testing a line side PWM converter and a DFIG are presented. Both measurement results for investigations on the transformer based sag generator at the line side converter and the DFIG show the superb functionality of the sag generator. Voltage dips of variable magnitude, duration time and fault type can be generated to perform certification of LVRT capability. The device is also able to perform HVRT tests.

Transformer based voltage sag generator to perform LVRT and HVRT tests in the laboratory

The increasing use of complex grid-connected technological components in safety-critical applications like public transportation or distributed power systems requires a precise evaluation of the reliability and robustness of these pieces of equipment during their development. Direct on-field tests of high-power systems are generally not possible. A conventional method of getting around this problem consists in using software-based simulation to analyze the behavior of the electric equipment under test (EUT) under normal and transient operation conditions of the grid. However, signal-level simulation is not able to perfectly reproduce every detail of a physical phenomenon. This publication describes a cost-effective realization of a power-hardware-in-the-loop (PHiL) grid emulation system combining two inverters with different electrical characteristics (type of semiconductor, switching frequency, dc link, etc.). The proposed grid emulator is a controllable power source not only capable of emulating low-frequency (50 Hz) transient faults as voltage sags but also high-order harmonic components usually present in the voltage waveforms of the utility grid (e.g., noise). A downscaled 5-kVA laboratory prototype has been implemented and the experimental results shown in this paper demonstrate the emulator’s ability to combine high power with high dynamics.

Extending the Power Capability With Dynamic Performance of a Power-Hardware-in-the-Loop Application—Power Grid Emulator Using “Inverter Cumulation”

This paper describes a transformer-based voltage sag generator (VSG) suitable to evaluate the susceptibility of electrical equipment to voltage sag. The built VSG utilized one auto-transformer and two solid state relays (SSRs) to provide nominal voltage and sag voltage to the load. The switch statuses of two SSRs are controlled by nominal voltage and sag voltage duration signal produced by electronic circuits. The VSG operating result shows that it enables effective control of sag magnitude, duration, beginning and ending points on output voltage wave. If needed, it can work as a voltage swell generator and a voltage interruption generator. By supplying high voltage (HV) transformer from the primary side, the VSG also can provide HV sag, swell, and interruption. The presented VSG is easier to set up in the lab, and the construction cost is much lower than buying VSG products at the current market.

A single-phase voltage sag generator for testing electrical equipments

This paper describes an experimental setup of a voltage sag generator (VSG) based on impedance switching. Bidirectional switches with IGBTs, snubber and clamping circuits are adopted to obtain fast turn on, turn-off while avoiding undesirable voltages and currents stresses. In addition, with the proposed VSG it is possible to adjust the depth and duration of balanced and unbalanced voltage dips making it possible to observe the behavior of wind turbines (WT) under, for instance, voltage dips indicated in the IEC 61400-21.

Design of a voltage sag generator based on impedance switching

This paper describes a new economical sag/swell voltage generator suitable for the evaluation of high power custom power devices such as DVR (dynamic voltage restorer) and DSTATCOM (distribution static compensator). Proposed system was designed to generate the several power quality disturbances in MVA power ratings-sag, swell, under voltage, over voltage and harmonic distortions. The sag and the under voltage can be generated by voltage drop across a reactor, while its magnitude and durations can be controlled by the firing angle of TCR (thyristor controlled reactor). In the case of the swell and the over voltage, we need a step-up transformer. Output of the step-up transformer is regulated by TCR to obtain nominal voltage level. At any given instant, if the firing angle of TCR is retarded or stopped, then we can obtain the swell voltage or the over voltage disturbance. Also we can generate harmonic current by controlling firing angles of TCR, while keeping the system voltage level within the specified limits. In this paper, two identical three-phase wye-connected TCRs are connected to the secondary windings of two step-down transformers, one operated from wye-wye transformer, the other from delta-wye transformer. Harmonic filters (11/sup th/ and 13/sup th/) are added to reduce the voltage distortion when TCRs are operated. Simulation results are given for several cases of voltage sag and swell generations. Design guidelines are given for the TCR. Finally, conclusions are given.

Voltage sag and swell generator with thyristor controlled reactor

This paper proposes a new SSG (sag and swell generator) with series injected inverter for the KCPP (Korea Custom Power Plaza), which is being constructed with the fund of the National Project in Korea. Main purpose of KCPP is to test and to evaluate several different kinds of high power CPD (custom power device) under the several power disturbance conditions before installing it in real site. We can generate typical power disturbances such as sag, swell, over voltage, under voltage and voltage flicker by using the proposed SSG composed of series inverter, energy storage DC capacitor, rectifier and voltage clamp circuit Also we can generate the distorted voltage waveforms and the phase jumping waveforms by controlling the series inverter In this paper, three kinds of control methods for the proposed 2 MVA SSG are given. Typical voltage sag and swell waveforms are implemented by adopting simple control method. Also the voltage flicker is generated by changing the amplitude of the injected voltage in random. Owing to the limited bandwidth of the proposed SSG, high frequency transient waveforms can be obtained by using the open loop control. The simulation and experimental results are given to verify the operation of the proposed SSG. Finally, conclusions will be given.

Voltage sag and swell generator with series injected inverter for the KCPP

This paper describes the medium voltage high power DVR (dynamic voltage restorer) with the new voltage disturbance detection method and the real-time digital voltage control. The new voltage disturbance detector was implemented by using the delta rule of the neural network control. Through the proposed method, we can instantaneously obtain the peak value of each phase voltage even under the single phase input voltage conditions. Compared to the conventional synchronous reference frame method, the proposed one shows the minimum time delay to determine the instance of the voltage sag or voltage swell event. Also real-time digital voltage control technique was adopted, where the inverter output filter capacitance voltage, the filter reactor current and the load current are sampled to calculate the inverter PWM command for the next sampling interval. By using digital control, the disturbance voltage can be compensated to the reference voltage level within two sampling intervals. The proposed disturbance detector and the voltage compensator were applied to the medium voltage high power DVR (22.9 kV/4 MVA) that was developed for the application of semiconductor manufacture plant. The performances of the proposed DVR control were verified through computer simulation and experimental results. Finally, conclusions are given.

T.B. Park

Papers

Voltage sag and swell generator with thyristor controlled reactor

Voltage sag and swell generator with series injected inverter for the KCPP

Medium Voltage Dynamic Voltage Restorer with Neural Network Controlled Voltage Disturbance Detector