In recent years, the cable television industry has developed rapidly in my country, providing viewers with a wide variety of television programs and enriching our cultural lives. In particular, the newly developed multi-functional value-added services and data services of cable television networks have injected new vitality into my country's informatization construction. Therefore, ensuring the safe operation and high-quality broadcasting of cable television networks is crucial. The cable television headend is the heart of the system. To ensure the stable operation of this core, a UPS (Uninterruptible Power Supply) is essential. A UPS can solve the problems caused by poor mains power quality, such as power outages, voltage sags, power surges, amplitude oscillations, power interference, power spikes, power fluctuations, switching transients, and harmonic distortion, all of which can damage headend equipment. This article discusses my experience and insights on how to design and maintain a UPS.
I. How to Design a UPS Power Supply System 1. Working Principle of an Uninterrupted UPS A UPS (uninterrupted power supply) consists of two parts: the main unit and the battery. Based on its operating mode, it can be divided into standby and online types. In a standby UPS, when the mains power is normal, the mains power directly supplies power to the load through an AC bypass channel. At this time, the inverter on the main unit does not work. Only when the mains power fails does the battery supply power, driving the load through the inverter. Therefore, it has essentially no impact on the quality of the mains power.
Online UPS power supplies differ from traditional UPS systems. When the mains power is normal, they first convert AC to DC, then perform pulse width modulation and filtering, before converting the DC back to AC to power the load. If the mains power fails, they immediately switch to battery inverter power supply. Therefore, online UPS power supplies output a pure sine wave power supply that is completely isolated from the mains power grid, greatly improving power quality and protecting the load for safe and efficient operation. The working principle of an online UPS power supply is shown in the figure.
2. Selection of UPS Rated Output Capacity: First, calculate the power of the upstream load. To ensure high system efficiency and extend the UPS lifespan as much as possible, the load power should generally meet 60-70% of the UPS rated power. For example, our upstream equipment requiring uninterrupted power includes satellite receivers, modulators, mixers, optical transceivers, computers, and network switches, with a total power of 4500VA. Therefore, we choose 4500÷75%≈6428VA, resulting in a 6500W online UPS.
3. Calculate the battery's operating time. Basic battery parameters: voltage (2V, 6V, 12V, etc.), capacity (65AH, 100AH, etc.).
Through practical experience, we have summarized the following formula, which can be used to calculate the working time of a storage battery:
Battery pack capacity × voltage / main unit rated power × 0.75 (power factor) = battery working time at full load. For example, in our UPS system, the main unit has a rated power of 6500W, 3 battery cabinets with a total of 30 lead-acid batteries, each with a voltage of 12V and a capacity of 100AH.
Therefore, the battery pack voltage = 12V × 10 = 120V
Battery pack capacity = 100AH × 3 = 300AH
The battery operating time of the system under full load is: 300AH×120V/6500W×0.75= 7.4 hours, indicating that the battery can operate for at least 7.4 hours when the power is off.
Calculated based on actual load power:
300AH × 120V / 4500W = 8 hours. Our front-end UPS system can last up to 8 hours during a power outage. This result has been proven correct through our multiple discharge experiments. If we reverse the above formula and determine the required working time of the battery based on local conditions, we can then determine the required battery capacity and voltage.
4. Choosing the right wiring is crucial. If the wire diameter is too thin, the current will be too high, easily causing overheating and a fire; if the wire diameter is too thick, it will be wasteful. Based on the electrical characteristics of metal conductors, multi-strand copper wire generally has a capacity of 6A/mm², and aluminum wire has a capacity of 4A/mm². After determining the power of the main unit, you can refer to the table below to select the wiring and circuit breaker.
5. Choose a brand
When purchasing a UPS power supply, the following factors should be considered: input voltage range, output voltage range, output frequency range, bypass inverter zero-switching, and the ability to resist surges, interference, and harmonic distortion. After-sales service is also very important. Extensive practical experience has shown that if the "interference" potential between the neutral wire and ground wire at the UPS output is too high, it will lead to an increase in the bit error rate of data communication in the computing network. If a high-frequency model is used, high-frequency radiation can affect the computer network. Therefore, these issues should be considered when selecting a model.
At the same time, while using a UPS as your power protector, you should also pay attention to routine maintenance and design, and choose a UPS power supply that suits your needs. This will help maintain your equipment and extend its lifespan.
