Grid-Connected Voltage Source Inverter (VSI) to improve Low Short Circuit Ratio

This tech spec was submitted by Nikheel Sharma as part of the University Technology Exposure Program.

Problem / Solution

Electricity plays a vital role in modern society, not just in science and development but also in our daily life. However, the increasing number of end-users affects its demand, which is supplied using power generation units that run on gas turbines, fuel cells, PV systems, and wind turbines connected to the power grid. One critical engineering issue in the generation process is the control of power converters. Since most plants generate direct current (DC), converting it to alternating current (AC) must be met so consumers can utilize the generated energy. Maintaining the Short Circuit Ratio within the acceptable value leads to efficient power distribution and transmission.

Using a two-level grid-connected voltage source inverter in DC-AC conversion, improving a low short circuit ratio is within reach. This inverter also includes the configurations of additional power generation facilities like the PV system (Solar) and a designed boost converter that increases the DC voltage in the system. Numerous preparations, tests, and simulations are met to ensure its effectiveness.

Design

As mentioned, the DC output generated from solar energy plants must be converted to AC to maximize its utilization. In AC, voltages can be increased or decreased using transformers. As a result, AC transmission allows for both high-voltage and low-voltage distribution. The energy loss in AC is less than the DC voltage, which makes it more suitable in transmitting power. The VSI considered these factors, making its components feasible and vital in the grid.

VSI Switches

The Current in power electronic equipment is unidirectional, unlike the traditional switch. Switches installed in electrical circuits control the powering of the VSI, where there must be no room for error during switching. The universal bridge topology, placed in the middle of two switches, helps increase the input inductance, which reduces the current harmonics.

Control Strategies

The VSI has distinctive features that can be useful in power grids. The closed loop strategy used depends on the desired or referenced current, signalling the inverter to switch its process. The PWM or Pulse Width Modulation Control Strategies uses the induction motor drives to control the rectifier or inverter model. In this control method, the direct axis currents and the quadrature axis currents regulate the active and reactive power injected into the grid, respectively.

The synchronous reference frame depends on the Voltage oriented control or the VOC. It is almost similar to the controller closed loop, comparing results with the reference current calculated using the formula in the paper. The error generated upon comparison is regulated by the PI-controller and added to the feed-forward term and the measured d and q-axis grid voltages. PI controllers usually provide zero steady-state error. However, sinusoidal references are converted to synchronous reference frames (DC quantities) where the operation of PI controllers produces reliable results utilizing Clarke and Park transformations.

Another method of control is the stationary reference frame using the periodic signal, such as a PR controller that tracks AC signals. It uses the PID approach wherein the variables are adjusted to reach the setpoint or reference. The general integrator acts as the primary controller of the loops to maintain their instability and achieve zero error tracking.

There are cases wherein the controller itself is directly integrated into the DC Link. This practice is common in the double-stage configuration, wherein the regulation and smoothing of the DC input power are done at the DC link before being injected into the grid via an inverter.

Weak Grid Configurations

One of the problems in a weak grid is the low SCR or Short Circuit Ratio. Neglecting it may result in a significant loss in utilized power in grids. The Resonant Harmonic Compensator is a technique that can control power converters connected to this kind of grid. The Sinusoidal Pulse Width Modulation (SPWM) is the technique that will be used in the modulation technique due to its out to be compatible with the IGBT switches explained above.

Grid Synchronization and PV system Integration

The VSI used the PLL or the Phase-Locked Loop, consisting of the Phase Detector, Loop Filter, and Voltage Controlled Oscillator. The phase errors determined through a series of calculations determine the configuration of the closed loop of the PLL. The researcher added the SOGI filter to identify the positive component of voltage, which identifies the phase error. The boost converter, interfaced between the PV system and the VSI, allowed a double-stage configuration. Thus, the dual-stage setup also provides greater freedom than the single-stage topology.

SIMULATION RESULTS

Findings show that the DOSGI-PLL technique and SPWM in grid synchronization is efficient. The two proposed reference frames, the synchronous and stationary, prove to be promising in the cases of strong and moderate grid conditions. However, the VSI fails to accomplish the weak grid condition on optimal SCR. On the other hand, the harmonic compensators that reduce into 5th and 7th harmonics produce the THD, which is slightly higher than the required standards of the IEEE but is the best candidate for the VSI.