Line-interactive UPS on Arduino - Part 1

Arduino Nano

Arduino IDE

<html><body><h3>Introduction</h3>I've been thinking about this project for long time - since I disassembled my first UPS trying to figure out how to make it smarter. It was a cheap noname line-interactive UPS with just 3 LED on it indicating outlet power presence, charge and battery fault. It did save me on short power interrupts but was performing rather poorly during the &gt; 2hr blackouts, switching off my server ungracefully. My first motivation was to develop a connector between this UPS and my server so that it could shutdown properly. My first thought was to use Arduino Micro for this task, since it can be easily connected to the PC through its onboard USB port and has analog inputs, which could be employed to measure the battery voltage and current. As a result, the <a href="https://projecthub.arduino.cc/abratchik/hid-compliant-ups-with-arduino-3e597a">HID-compliant UPS simulator project </a>has emerged. To my surprise, that project got attention - <a href="https://www.mail-archive.com/nut-upsuser@alioth-lists.debian.net/msg02506.html">some people used it for their own UPS and battery designs </a>. I was also thinking to use it for my UPS but quickly found one main issue - the UPS had its battery and the main board logic indirectly connected to the AC mains with no proper galvanic isolation. Hence, my initial idea of using analog Arduino inputs for battery measurements would not really work since my server could become "hot" or even toasted by the AC power current through the USB connection. Since implementing the insulation for all the required sensors would have been a complex task, I've came to an idea of having 2 Arduino talking to each other over the serial bus insulated with help of optocouplers. One would be a UPS controller doing heavy lifting for managing the UPS logic and feeding its state over the I2C to the second Arduino, whose role would be to maintain the the HID-compliant communication with the host. <figure><img src="https://projects.arduinocontent.cc/ce4a5653-f4ef-4bf2-adc6-95660fdb1c7e.png"/><figcaption></figcaption></figure>My plan was to make this project very simple and cheap: no expensive or rare parts, minimal amount of discrete components. <h3>Theoretical part</h3>Let us take a step back and discuss some theory. For those of you who don't know what "line-interactive UPS" (or LI UPS, for brevity) really means, it is the most widespread type of a UPS, inexpensive and energy efficient, compared to other types, such as online and offline. If you are interested to know more about them and how they differ from each other, <a href="https://easyelectronics.co.in/ups-types-and-application/#3_Line_Interactive_UPS_System">this article </a>should help. The key advantages of the LI UPS are its low cost and energy efficiency but one of the tradeoffs is absence of the insulation between the mains and the load, which I mentioned above in the intro. There are many different types and options how LI UPS can be implemented. I'm not going to consider all possible scenarios, just stick to the one I encountered most of the time in the consumer-grade LI UPS. The schematic diagram is represented below: <figure><img src="https://projects.arduinocontent.cc/f542229a-b9a2-4013-8cec-47651e2a4d03.png"/><figcaption></figcaption></figure> The core part of the LI UPS is the Big Iron Transformer. If the input voltage is within the acceptable limits (230V +/- 10%) then the UPS passes it to the load with no changes, just applying EMI/RFI filters. But once the inout is deviating for more than 10% from the nominal value, the relays RY2 and RY3 are switching thus adding or subtracting the L1 booster winding of the transformer to ensure voltage back or boost, depending on the sign of deviation. In the event when the input voltage is falling beyond the limits of back/boost regulation then the input relay RY1 is disconnected and the DC/AC inverter kicks in to ensure the uninterrupted power feed to the load. The RY4 relay is mainly for protecting the load from the under-/overvoltage or overload scenario. Since the inverter is taking power form the battery, the latter needs to be charged when the load is powered from the mains. There are different options how the charger can be implemented but the most recent ones involve some sort of fast-switching voltage convertor maintaining the required battery charging current and voltage by pulse-width modulation. By looking at the schematic diagram, here is what the LI UPS controller should be able to do (minimal requirements): <ol><li>Control the relays RY1-RY4 </li><li>Measure input and output AC voltage and output AC current </li><li>Measure the battery voltage and current </li><li>Manage the DC/AC inverter switch on/off </li><li>Manage the battery charging process. </li><li>Detect the exception cases like under/over voltage, overload, battery failure etc and execute the corresponding alarm scenarios. </li></ol>Additional requirements may include: <ol><li>maintaining display indication of the LI UPS mode and key parameters like battery level, load level, input and output voltage etc. </li><li>supporting different buzzer alarms when the UPS is on battery or some issue happened. </li><li>support the load shutdown based on external command </li><li>support automatic restore of the load output when the AC mains are within the acceptable limits </li><ol><li>support self-test </li></ol></ol><h3>Implementation prerequisites and considerations</h3>In order to get something working quickly I did not want to start from the ground zero and decided to re-use a cheap commercial LI UPS as a starter kit (let's call it the Donor). You may ask - why to build a new UPS from the existing one? The main reason is the quality - a cheap commercial LI UPS is rarely reliable, doesn't manage the battery properly thus reducing its life span. Also, cheap UPS are usually producing square approximation of the sine in the output when on battery, while I wanted to have a pure sine. Commercial pure-sine UPS may be quite expensive. So, after exploring the Donor UPS I learnt that it matches the diagram above very closely. The inverter in the Donor was built on H-bridge in the power part. Since I did not want to keep the original inverter controller of the Donor, I removed all the circuitry associated with it, only keeping the H-bridge. For pure sine, I used the popular module <a href="x Pure Sine Wave Inverter Driver Board EGS002 AliExpress https://www.aliexpress.com › item">EG002 </a>. It is a inexpensive board, which provides a complete solution for H-Bridge pure sine inverter controller, with wide range of output power. <figure><img src="https://projects.arduinocontent.cc/b2ae985a-5f4d-4cc6-b2c0-e8e26d8f98fd.jpg"/><figcaption></figcaption></figure>According to the datasheet, "EGS002 is a driver board specific for single phase sinusoid inverter. It uses ASIC EG8010 as control chip and IR2110S as driver chip. The driver board integrates functions of voltage, current and temperature protection, LED warning indication and fan control. Jumper configures 50/60Hz AC output, soft start mode and dead time". It is pretty much all we need for building the UPS inverter, however, some tweaking will be required. <figure><img src="https://projects.arduinocontent.cc/01a6c645-9884-40a4-a402-8725fd8374b7.png"/><figcaption></figcaption></figure> First, the board comes pre-configured for the soft start as a default. This means that the output voltage will reach the nominal target not instantly but in 3 seconds, once powered on. This is not working for the line-interactive where we expect the backup power to come up instantly when the input AC goes out. In order to disable the soft start, the JP2 jumper needs to be removed while JP6 soldered short. Second, we will need to enable/disable the output automatically based on the signal from the UPS Controller. The EGS002 board doesn't have the pin for managing the output. Fortunately, the EG8010 chip has the SPWMEN leg (#6) so we can use it precisely for that. There are also other modifications required for managing the charging, which we omit for now, will discuss later. The H-bridge schematic for using along with EGS002 can be found in the EG8010 datasheet. I'm adding it here just in case. The schematic may differ depending on the desired output power and MOSFETs used. My donor UPS was matching it pretty closely:<figure><img src="https://projects.arduinocontent.cc/6ab56521-c01e-4f2f-8533-1ad4b5f6079e.png"/><figcaption></figcaption></figure><h3>To be continued ...</h3> </body></html>

Line-interactive UPS on Arduino - Part 1

How to build a fully functional pure-sine line-interactive UPS on Arduino