The feasibility of converting a frequency converter into a UPS

With the rapid development of science and technology and the continuous improvement of people's living standards, people have put forward higher requirements for the environment, use functions, fire safety, etc. in buildings. The more modern the building, the higher its dependence on electricity, but power failure is not subject to human will. Once a disaster occurs, it will lead to power outage or disaster after power outage, and the safety of people's lives and property will be directly threatened. Therefore, the "Code for Fire Protection Design of High-rise Civil Buildings" and the "Code for Electrical Design of Civil Buildings" strictly stipulate that particularly important equipment in the first-level load must be equipped with a second power supply. At present, the commonly used backup power supplies on the market include generator sets , UPS, EPS and other products. As for their three power supply schemes and derivative schemes, they cannot guarantee 100% uninterrupted power supply. The content of this article is summarized by the author based on his work in EPS and UPS projects for several years.

　　1. Solutions to the problem

　　The inverter, whether it is frequency control type, vector control type or torque control type, has the function of variable frequency soft start, that is, when the motor starts, because the output voltage and frequency can start from zero, the starting current of the motor is limited, and it can start normally even if it is less than the rated current. The capacity of the inverter, at the voltage level of 380V, the power range of products from 2.2 to 1500kW can cover almost all current application ranges.

　　At present, the commonly used frequency converters are AC-DC-AC type voltage source frequency converters, in which the voltage of the intermediate DC link is about 510~620V, which is obtained after the three-phase AC voltage is rectified uncontrolled by three phases. If another 510~620V DC power supply can be provided for the intermediate link of the frequency converter after the city power outage, its IGBT inverter can continuously output three-phase sinusoidal AC voltage, and its voltage is 0~380V and the frequency is 0~50Hz, which is continuously adjustable, so as to realize the soft start of the load or achieve the output voltage of 380(1±3%)V and the output frequency of 50(1±1%)Hz. For example, a group of batteries can realize uninterrupted power supply to the load. Based on this idea, a new type of UPS is developed, which can become a variable frequency AC uninterruptible power supply, so that the frequency converter can be used in new application fields. Because the UPS output is a three-phase sine wave with stable voltage and frequency, in order to increase the reliability of the equipment and avoid interference with the load, a transformer and an LC low-pass filter are added to the output of the frequency converter.

　　According to the load characteristics, this variable frequency UPS can also be designed as backup and online like ordinary UPS. The overload capacity of this power supply is 150% for 3s, and the overall efficiency is over 98%.

　　This article uses Siemens' general-purpose inverter products as an application example to illustrate the working principle, architecture composition and design method of this power supply.

　　2 System composition and working principle

　　The main units of the power supply include: vector inverter, battery pack, DC/DC converter step-down charging module, control logic board, DC/DC step-down working power supply (+24V) module, output isolation transformer and LC filter, digital panel meter and semiconductor energy-saving lamp human-machine interface unit. The structure diagram is shown in Figure 1.

　　Feasibility and application of converting frequency converter into uninterruptible power supply

　　2.1 Vector Inverter

　　This article still takes Siemens products as an example, and its technical parameters are:

　　Input voltage 3-phase 380~460V±10%f inverter);

　　Output voltage 3-phase 0~380V or 380(1±3%)V;

　　Input frequency 50/60(1±6%)Hz:

　　Output frequency 0~600Hz or 50/60(1±1%)Hz.

　　2.2 Battery Pack

　　Use valve-regulated fully sealed lead-acid maintenance-free batteries, generally 2V/single battery for batteries above 200A·h.

　　2.2.1 Determination of the number of series connections N

　　The number of series connections M depends on the maximum and minimum allowable values ​​of the DC voltage in the intermediate link of the general inverter. When the uninterruptible power supply is in normal operation, the system is in a floating charge state, and the number of batteries N should be the feasibility and application of converting the inverter into an uninterruptible power supply

　　Where: N is the number of battery packs connected in series, Ue is the rated voltage of the intermediate DC link of the inverter, and Uf is the floating charge voltage of the single battery.

　　Taking 12V/single battery as an example, the floating charge voltage Uf=13.5V (the floating charge voltage of a single battery Uf=2.25V). Taking Siemens inverter as an example: Ue=510～620V, that is, Ue(min)=510V×0.9=459V, Ue(max)=620x1.1=682V, which are the upper and lower limits of the voltage for the inverter to work normally. The average value is: Ue=(459V+682v)/2=570.5V.

　　Then N=Ue/6Uf=570.5V/(6x2.25V)=42.25, so N=42.

　　During float charge, the battery terminal voltage Ud=42×2.25V×6=567V, and the voltage is within the allowable range of the equipment.

　　2.2.2 Determination of battery discharge termination voltage Uz

The battery　　discharge termination voltage Uz depends on the value of the termination voltage when the battery pack is disconnected from the charging module and turns to the inverter after the mains power outage. The termination voltage value must meet the minimum voltage value for the normal operation of the inverter. Uz can be calculated according to formula (2):

Feasibility and application of 　　converting frequency converter into uninterruptible power supply

　　Still taking Siemens products as an example, if Ue=510V, then Uz=(0.875x510V)/(6×42)=1.77V. Considering the reliability of the battery and the inverter, the battery discharge termination voltage Uz should not be less than 1.75V, and Uz=1.8V is usually taken. That is, the termination voltage of a single battery is Uz=1.8x6=10.8V, and the battery pack voltage is Ud=10.8×42=453V, which is slightly less than the minimum voltage value Ue(min)=459V allowed by the inverter, and can still meet the inverter working requirements.

　　2.2.3 Determination of battery pack capacity Q

　　The battery capacity Q (A·h) depends on the rated power supply current I of the load, the delay time T of the load being powered by the battery after the mains power outage, and the termination voltage Uz of the battery after discharge. The battery discharge curve or discharge table provided by the battery manufacturer can be used for selection and calculation. The specific calculation can be based on the empirical formula. Taking 15kW load and power outage maintenance time of 1h as an example, the calculation result is: select 42 50A.h batteries.

　　2.3DC/DC converter step-down charging board

　　The working principle of the charging module is to use IGBT power electronic devices to form a Buck (step-down) isolated DC converter with a withstand voltage of 1200V. The current is charged at 0.1C (10) according to the battery capacity. The single board can output a current of 10~20A, and the output voltage is adjustable from 274~300V. It can float charge the battery pack below 400A·h. Dividing the battery pack into several groups of equal number (for example, 42 batteries are divided into two groups, each with 21 batteries, and the charging voltage is only 287~289V) can reduce the output voltage value of the charging module, simplify the module structure and reduce costs. This charging board can also be used in parallel.

　　2.4 Rectifier diode

　　The DC isolation switch is composed of a high-power rectifier diode. When the mains power is normal, the diode is in the off state, cutting off the path between the battery pack and the inverter. However, the disadvantage is that the battery pack may not be fully charged during floating charge due to the high terminal voltage and the DC voltage output after rectification of the mains power. When the mains power is cut off, the diode is turned on instantaneously, and the battery pack is discharged instantly, which can achieve instantaneous conversion of the load from the mains power supply to the battery pack, and is a key link in the online uninterruptible power supply.

　　2.5DC/DC step-down working power supply (+24V) module

　　Buck step-down DC/DC converter can be used to convert the DC point voltage of general inverter into the voltage +24V required by the logic control board for digital panel meter and semiconductor energy-saving lamp human-machine interface unit.

　　2.6 Digital panel meter and semiconductor energy-saving lamp human-machine interface unit

　　A monitoring system with a human-machine operation display interface is composed of a digital panel meter and a semiconductor energy-saving lamp human-machine interface unit. Its functions are: charging board and battery voltage value, output current value, input mains voltage value, output voltage value; power supply start and stop operation; operating parameter display; power supply working status display; fault status alarm and four remote functions realized through RS485 and host computer communication.

　　2.7 Battery Balance Manager and Battery Detection System

　　Because the uninterruptible power supply uses more batteries, based on the lessons learned from the previous UPS that most of the battery failures affected the entire system crash, a battery inspection instrument (developed by our company) was added to the battery pack to detect the voltage, internal resistance, discharge current and ambient temperature of each battery. This type of inspection instrument can detect up to 128 batteries. In addition, in order to keep the voltage of each battery consistent during charging and discharging, our company independently developed a battery balance manager. This can maintain the consistency of the battery during charging and discharging, pick out the lagging batteries, and discover problems early.

　　2.8 Output isolation transformer and LC filter

　　Because the output voltage waveform of the inverter is a high-frequency step wave, in order to make the output waveform a perfect sine wave, an LC filter is added after the isolation transformer, and its inductance value is selected as 1.5mH and the capacitance value is 20μF. At the same time, in order to increase the load capacity and reliability of this uninterruptible power supply and avoid disturbing the load, a △/Y isolation transformer is connected to the output end of the inverter, and its ratio is 300:380. The selection of the ratio is related to the output voltage regulation accuracy (because when the DC voltage gradually drops to DC459V when the battery is discharged, the actual voltage output by the inverter is AC325V), which is the key link of the online uninterruptible power supply, but the disadvantage is that it will reduce the power of the inverter.

　　3 Application Examples

　　Based on the above working principle, the author of this article successfully developed a 22kW online variable frequency AC uninterruptible power supply prototype in 2002. The principle is shown in Figure 1. The test was successful and it was successfully applied in practice.

　　3.1 Application Example 1

　　The emergency lighting system of the multifunctional building for conference and canteen of Beijing Architectural Design Institute has a load power of 22kW and a backup delay time of 90 minutes. The emergency lighting channel and fire-fighting facilities cannot be powered off during the year, otherwise serious consequences will occur. The machine has been running well since it was installed. There have been power outages due to construction accidents many times, but the machine has provided uninterrupted power supply to ensure the normal operation of the building.

　　3.2 Application Example 2

　　The BSH construction site in Nanjing New Port Development Zone uses an emergency power supply to drive the rolling shutter door, with a power of 18.5kW, powered by a Siemens MICRO-MASTER440 inverter (power 22kW). It is required that after the mains power outage, a backup power supply must be connected to ensure that the company's warehouse rolling shutter door can operate uninterruptedly and that the home appliances in stock can be shipped normally. In 2005, our company designed and manufactured an online uninterruptible power supply for it, with a battery capacity of 65A·h, 42 pieces, and a backup delay of 60 minutes.

　　4 Conclusion

　　The variable frequency AC uninterruptible power supply is an application power supply suitable for various loads. It has the dual functions of ordinary AC uninterruptible power supply and inverter. According to investigation, many power supply manufacturers at home and abroad have converted this power supply into emergency power supply, but there are very few articles in this regard. I wrote this article hoping to do my best to improve domestic power supply technology. For loads that do not allow power outages, choosing this power supply has a higher performance-price ratio than choosing ordinary UPS. Therefore, it is a power supply device worthy of promotion and application.