The OZpcs-RS40 is a 40kW Power Conversion System (PCS) intended for battery-based energy storage applications. The PCS is designed to be mounted in a standard 19” rack, and easily paralleled to scale power capability. All hardware interfaces are located on the front panel, utilizing pass-through type terminal blocks for simple daisy chained cable or bus bar power connections. Similarly, the Modbus and digital I/O signals are provided on redundant, high density, 15-pin D-Sub connectors, which also allow for simple daisy chain cabling. When using Modbus to control multiple, paralleled PCS there are several things to consider, including termination, addressing, and broadcast messaging.
In a previous post, I discussed how the Volt/VAR function can be used to provide grid voltage stabilization during over and under-voltage conditions. In addition to stabilizing out of tolerance voltage conditions, over and under-frequency grid conditions can also be mitigated using Frequency/Watt functionality. In a manner similar to Volt/VAR, the Frequency/Watt function will automatically generate real power commands based on grid frequency measurements.
To support grid voltage stabilization during over and under-voltage conditions, UL1741 certified smart inverters, such as the OZpcs-RS40, can be configured to automatically absorb or inject reactive power based on grid voltage measurements. This behavior is commonly referred to as Volt/VAR control and is implemented using a configurable array of points, that when combined, define a linear, piece-wise curve that results in the desired Volt-VAR behavior.
Traditionally, grid energy storage systems (ESS) have been one-off solutions, utilizing proprietary software and hardware components. As such, each installation requires time consuming, custom integration. Often times proprietary vendor hardware or software protocols require "hacks" to get all components to play nice. Ultimately this approach results in higher costs, decreased reliability, and limited scalability and upgrade options.
This post is first in a series discussing the subsystems and key components that go into a grid tie inverter (GTI), and design considerations for development of a practical commercial system. The majority of topics will apply equally to active front-ends (AFEs), and active rectifiers (more on the subtle differences between these two is covered in Active Front-end or Grid Tie Inverter?). We’ll use the term GTI in a general sense for this series, and point out differences from AFEs and active rectifiers when relevant.
Before delving into the details, let’s first examine what a GTI is by comparing it to the more familiar voltage mode inverter (VMI). A VMI is a DC to AC converter. It generally is intended to source power into a load, and thus has a low output impedance to maintain a stable output voltage with varying loads. A GTI is also a DC to AC converter, but it is generally intended to source power into a low impedance utility grid, so its output impedance is therefore high. Addressing these dissimilar operating characteristics requires a different output filter topology, a different control loop implementation, and a different reference implementation. Secondary requirements that arise from things like startup and fault handling impose further distinctions between the two, which we will explore later on.
Topics: Grid Tie Inverter
A common question we field regarding Grid-Tie inverters goes something like "Can I interface to a 480V grid with a 680V DC Link?". To answer this question, lets consider a typical Grid Tie Inverter or Active Front End application as illustrated in the figure below.
In non-isolated, grid-tie inverter applications, it is common practice to connect the neutral point of the 3-phase AC grid to earth ground. Unfortunately, this creates a problem for DC link common mode filtering. Typical control methods utilize space vector modulation to control the power stage, which introduces a common mode voltage in order to maximize the DC link voltage utilization. When you connect a capacitor between DC link and neutral, the space vector common mode voltage is impressed across it. Unless the capacitor is very small, it will adversely effect operation, often leading to control loop instabilities. In addition, this capacitor will tend to resonate with the grid connect filter, causing further behavioral problems.
Topics: Grid Tie Inverter
A few days ago Beacon Power announced that their installation of the world’s first 20MW Flywheel plant is currently operating at 18MW, and is expected to be running at full, 20MW capacity before the end of the month. We’re pretty excited about this at Oztek, as we provide the controls for both the grid tie inverter that interfaces with the grid, and the sensorless permanent magnet motor drive used for the flywheel in Beacon's system. This program posed many design challenges, and it’s always extremely satisfying to see things finally come together, particularly on something as large and complex as this.
One reason engineers specify active rectifiers (a.k.a. active front-ends) for their systems is that they can operate with near unity power factor. Being nearly the same system (see my recent post "Active Front-end or Grid Tie Inverter?"), grid tie inverters share this same beneficial characteristic. However, this does not mean that an active rectifier or grid tie inverter must operate with near unity power factor, and in fact, we can use this to our advantage in certain applications. This also implies that we may not want to specify power factor as a means of quantifying how well the system minimizes AC line current harmonics.
Topics: Grid Tie Inverter