WEEK 2
● On-line operation, which means zero switching time from normal
to backup mode and vice versa.
● Low THD sinusoidal input current and unity power factor.
● High reliability.
● Bypass as a redundant source of power in the case of internal
failure.
● High efficiency.
● Low electromagnetic interference (EMI) and acoustic noise.
● Electric isolation of the battery, output, and input.
● Low maintenance.
● Low cost, weight, and size.
- The power to the load is continuously supplied via rectifier/charger and inverter, the double conversion which is AC/DC and DC/AC and at the same time, AC/DC converter also will charger the battery set.
Stored energy mode :
- When the AC input voltage is outside the preset tolerance, the inverter and battery maintain continuity of power to the load. The duration of this mode is the duration of the preset UPS backup time or until the AC line returns within the preset tolerance. When the AC line returns, a phase-locked loop (PLL) makes the load voltage in phase with the input voltage and after that, the UPS system returns to the normal operating mode.
Bypass mode :
- The UPS operates in this mode in case of an internal malfunction such as overcurrent. This mode is also used for fault clearing. It should be mentioned that the output frequency should be the same as the AC line frequency in order to ensure the transfer of power. In some cases, there can be a maintenance bypass as well. A manual switch usually operates it
WEEK 3
for this week, I am still doing my literature study on On-line UPS system. In order to design my own On-line UPS system. I am doing online research by using Google Scholar to find the related article.
One of the articles that I found is :
A High-Frequency Isolated Online Uninterruptible Power Supply (UPS) System with Small Battery Bank for Low Power Applications.
From this research, we understand that On-line UPS systems are the most popular and common configuration, as they provide isolation to the load from the grid and have negligible switching times. A conventional online UPS system consists of a rectifier for PFC, battery bank, and an inverter connected with the load. Grid frequency transformers are normally employed to reduce the battery bank voltage and provide isolation from the transients and spikes generated inside the grid. Since the transformer is operating at the grid frequency, it increases the volumetric size and weight of the system significantly.
The objective of this research based on the supervisor are :
- Reducing the size of the system as well as providing suitable galvanic isolation between the
load and grid. - From the isolation, it can ensure our safety if we want to do maintenance.
- These UPS systems have high power factor correction, zero voltage switching
(ZVS) of the chopper and converter stage, and low-cost design.
The UPS operates in two modes, i.e., grid mode and battery mode, as shown in the figure below,
Grid mode :
When the grid voltage is stable and there is no power failure, the UPS system operates in grid mode. The single-stage AC-DC converter regulates the voltage across the Capacitor. Meanwhile; the boost converter steps up battery bank voltage across Capacitor and provides it to the inverter. The inverter provides a regulated output voltage to the connected load.
Battery mode :
In case of utility power disruption or any voltage sag, the magnetic contractor (MC) is opened, and the single-stage AC-DC converter stops operation. the battery supplies power to the load via the boost converter and inverter connected to it.
WEEK 4
For this week, I am still doing the research for a high-frequency transformer isolated online UPS system configuration. The proposed UPS system for my current research consists of a single-stage AC-DC converter, DC-DC boost converter, and H-bridge inverter. The single-stage AC-DC converter provides galvanic isolation, high input power factor, continuous flow of input and output current, with minimum semiconductor devices within the circuit. A high-gain DC-DC converter is introduced to step up the low battery bank voltage to high dc-link voltage in order to feed it to the inverter. This allows the use of a low voltage battery bank in the UPS system.
For this week, I have identified 3 potential research problem of interest is:
- Batteries. As the heart of the system, batteries require regular checkups to ensure they remain fit to safeguard critical systems.
- Capacitor. UPS contains a dozen or more capacitors, which are responsible for smoothing out and filtering voltage fluctuations. However, because capacitors degrade over time, annual inspection helps to optimize their operation and extend their lifespan.
- Design. To design a UPS online system that can require safety and really compact design but mostly UPS system has a complex design.
For the possible research question for each problem above:
- Can we use any type of battery or size of battery for any UPS system design or the battery specification depend on our design to avoid losses?
- How to determine or used a good capacitor to make your UPS system become much better because the capacitor is one of the important components in UPS?
- How do we need to do in order to achieve an online UPS system that has a compact design but at the same time it also has a safety for people for maintenance?
In order to solve all of this question, I need a lot of literature study from an article on the internet or library. Also a guide from my supervisor.
WEEK 5
For this week, I still do a literature study for the characteristic of my online UPS system design. it either 3-phase or single-phase online UPS, but it depends on the type of power we want which is either high power application or low power application. For me, I think I want to do a low power application so we need to use the single-phase online UPS system. Then, for a safety application, we need to use a high-frequency transformer. the main advantages of using high frequency are:
- Reducing the size of the system
- Providing suitable galvanic isolation between the load and grid
- Low-cost design
Then, we also need to consider the type of switch such as the inverter has been designed using MOSFET as the power electronics component due to its easy handling capability. If IGBT is chosen as the power electronic component, additional diodes will be needed to pair it together.
Pulse Width Modulation is a technique used to control the inverter output voltage. In this project, Sinusoidal PWM is chosen as the technique to switch the inverter power devices. There are two types of SPWM which are Bipolar SPWM and Unipolar SPWM.
The type and position of the battery are also being considered. Due to high dc-link voltage, several batteries are connected in a series arrangement, which reduces the reliability of the system. If the UPS system transformer-less, it can have high efficiency and small weight. Series battery arrangement has major drawbacks and limitations in charging and discharging. A small imbalance in voltages occurs across the battery cells during charging and discharging because the voltages across each cell are not equal. Considering this issue, overcharging and deep discharge may result in overheating and even destruction of battery. Thus, this method is not suitable for this project since there are more disadvantages compared to advantages.