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, functionality, and fire safety of buildings. The more modern the building, the higher its dependence on electricity. However, power outages are beyond human control; once a disaster occurs, power outages or subsequent disasters will directly threaten people's lives and property. 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 primary loads must be equipped with dual power supplies. Currently, commonly used backup power supplies on the market include generator sets, UPS, and EPS products. However, these three power supply solutions and their derivatives cannot guarantee 100% uninterrupted power supply. The content of this article is a summary based on the author's several years of experience working on EPS and UPS projects.
1. Problem-solving methods
Whether frequency control, vector control, or torque control type, frequency converters all have a soft-start function. This means that when the motor starts, because both the output voltage and frequency can start from zero, the starting current of the motor is limited, and it can even start normally with a current lower than the rated current. Frequency converters with capacities ranging from 2.2 to 1500kW at the 380V voltage level can cover almost all current applications.
Currently, commonly used frequency converters are AC-DC-AC type voltage source frequency converters, with the intermediate DC link voltage being approximately 510-620V, obtained by three-phase AC voltage after three-phase uncontrolled rectification. If another 510-620V DC power supply can be provided to the intermediate link of the frequency converter after a mains power outage, its IGBT inverter can continuously output a three-phase sinusoidal AC voltage, with a voltage range of 0-380V and a frequency range of 0-50Hz, achieving soft starting of the load or reaching an output voltage of 380 (1±3%)V and an output frequency accuracy of 50 (1±1%)Hz. For example, a set of batteries can provide uninterrupted power supply to the load. Based on this idea, a new type of UPS has been developed, which can become a variable frequency AC uninterruptible power supply, enabling the frequency converter to be used in new application areas. Because the UPS output is a three-phase sinusoidal wave with stable voltage and frequency, a transformer and an LC low-pass filter are added to the output of the frequency converter to increase the reliability of the equipment and avoid interference to the load.
Depending on the load characteristics, this variable frequency UPS can be designed as a standby or online UPS, just like a regular UPS. The power supply has an overload capacity of 150% for 3 seconds and an overall efficiency of over 98%.
This article uses Siemens' general-purpose frequency converter products as an application example to illustrate the working principle, architecture, and design method of this power supply.
2. System Composition and Working Principle
The main units of this power supply include: a vector inverter, a battery pack, a DC/DC converter step-down charging module, a control logic board, a DC/DC step-down power supply (+24V) module, an output isolation transformer and LC filter, a digital panel meter and a semiconductor energy-saving lamp human-machine interface unit. The structural diagram is shown in Figure 1.
Feasibility and application of converting frequency converters into uninterruptible power supplies
2.1 Vector type frequency converter
This article still uses Siemens products as an example, and their technical parameters are as follows:
Input voltage: 3-phase 380～460V±10% (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
Valve-regulated, fully sealed, maintenance-free lead-acid batteries are selected, typically 2V per cell for batteries with a capacity of 200A·h or higher.
2.2.1 Determining the number N in series
The number of batteries in series, M, depends on the maximum and minimum allowable DC voltage in the intermediate circuit of the general-purpose frequency converter. During normal operation, the uninterruptible power supply (UPS) is in a float charge state, and the number of batteries, N, should be determined based on the feasibility and application of converting a frequency converter into an UPS.
In the formula: N is the number of batteries connected in series, Ue is the rated voltage of the intermediate DC link of the frequency converter, and Uf is the float charging voltage of a single battery.
Taking a 12V single battery as an example, the float charge voltage Uf = 13.5V (the float charge voltage of a single battery is Uf = 2.25V). Taking a Siemens frequency converter as an example: Ue = 510～620V, that is, Ue(min) = 510V × 0.9 = 459V, Ue(max) = 620 × 1.1 = 682V, which are the upper and lower limits of the voltage at which the frequency converter can work normally. Taking the average value: Ue = (459V + 682V) / 2 = 570.5V.
Then N = Ue/6Uf = 570.5V/(6x2.25V) = 42.25, so we take N = 42 units.
During float charging, the battery terminal voltage Ud = 42 × 2.25V × 6 = 567V, which 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 minimum voltage value required for the battery pack to discharge to the inverter after a mains power outage, when it disconnects from the charging module. Uz can be calculated using formula (2):
Feasibility and application of converting frequency converters into uninterruptible power supplies
Taking Siemens products as an example, if Ue = 510V, then Uz = (0.875 x 510V) / (6 x 42) = 1.77V. Considering the reliability of the battery and inverter, the battery discharge termination voltage Uz should not be less than 1.75V, and is usually taken as Uz = 1.8V. That is, the termination voltage of a single battery Uz = 1.8 x 6 = 10.8V, and the battery pack voltage Ud = 10.8 x 42 = 453V, which is slightly less than the minimum allowable voltage value of the inverter Ue(min) = 459V, and still meets the operating requirements of the inverter.
2.2.3 Determination of Battery Pack Capacity Q
The battery pack capacity Q (A·h) depends on the rated power supply current I of the load, the time T of delay after a mains power outage when the load is powered by the battery, and the termination voltage Uz after the battery pack discharges. It can be selected and calculated based on the battery discharge curve or discharge table provided by the battery manufacturer. Specifically, the calculation can be based on empirical formulas. Taking a 15kW load and a power outage duration of 1 hour as an example, the calculation result is: select 42 50A·h batteries.
2.3DC/DC converter step-down charging board
The charging module works by using IGBT power electronic devices to form a Buck (step-down) isolated DC-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 an adjustable output voltage of 274-300V, which can float charge battery packs below 400A·h. Dividing the battery packs into several groups of equal quantity (for example, dividing 42 batteries into two groups of 21 each, with a charging voltage of only 287-289V) can reduce the output voltage of the charging module, simplify the module structure, and thus reduce costs. This type of charging board can also be used in parallel.
2.4 Rectifier Diode
A DC isolating switch composed of high-power rectifier diodes is used. When the mains power is normal, the diodes are in the off state, cutting off the path between the battery pack and the inverter. However, a drawback is that the battery pack may not be fully charged during float charging due to the high terminal voltage and the DC voltage output from the rectified mains power. When the mains power fails, the diodes instantaneously turn on, and the battery pack discharges instantly. This allows for instantaneous switching between mains power and battery power for the load, which is a key component of online uninterruptible power supplies.
2.5DC/DC step-down power supply (+24V) module
A Buck step-down DC/DC converter can be used to convert the DC point voltage of a general-purpose frequency converter into the +24V voltage required by the logic control board, which is used for digital panel meters and human-machine interface units for semiconductor energy-saving lamps.
2.6 Digital panel meter and semiconductor energy-saving lamp human-machine interface unit
A monitoring system with a human-machine interface is composed of a digital panel meter and a semiconductor energy-saving lamp human-machine interface unit. Its functions include: displaying the voltage value of the charging board and battery, the output current value, the input mains voltage value, and the output voltage value; power start and stop operation; displaying operating parameters; displaying the power working status; fault status alarm; and realizing four remote functions through RS485 communication with the host computer.
2.7 Battery Balance Manager and Battery Detection System
Because uninterruptible power supplies (UPS) use a large number of batteries, and based on past lessons learned where UPS failures often led to system-wide crashes, we added a battery monitoring device (developed in-house) to the battery pack to detect the voltage, internal resistance, discharge current, and ambient temperature of each battery. This device can monitor up to 128 batteries. Furthermore, to maintain consistent voltage across all batteries during charging and discharging, we independently developed a battery balance manager. This ensures battery consistency during charging and discharging, allowing us to identify lagging batteries and detect problems early.
2.8 Output isolation transformer and LC filter
Because the inverter's output voltage waveform is a high-frequency stepped wave, to ensure a perfect sine wave, an LC filter is added after the isolation transformer, with an inductance of 1.5mH and a capacitance of 20μF. Simultaneously, to increase the load-carrying capacity and reliability of this uninterruptible power supply (UPS) and avoid disturbing the load, a Δ/Y isolation transformer with a turns ratio of 300:380 is connected to the inverter's output. The choice of turns ratio is crucial to the output voltage regulation accuracy (because when the DC voltage gradually decreases to DC 459V during battery discharge, the actual output voltage of the inverter is AC 325V). This is a critical aspect of online UPS systems, but the disadvantage is that it reduces the inverter's power output.
3 Application Examples
Based on the working principle described above, the author successfully developed a prototype of a 22kW online variable frequency AC uninterruptible power supply in 2002. The principle is shown in Figure 1. The experiment was successful, and it was successfully applied in practice.
3.1 Application Example 1
The emergency lighting system for the multi-functional conference and canteen building of the Beijing Architectural Design Institute has a load power of 22kW and a backup delay time of 90 minutes. Power outages are unacceptable for the emergency lighting corridors and fire-fighting facilities throughout the year, as such an outage would have serious consequences. The machine has been operating well since its installation, although there have been several power outages due to construction accidents, but the machine has provided uninterrupted power to ensure the building's normal operation.
3.2 Application Example 2
At the Bosch construction site in Nanjing Xingang Development Zone, an emergency power supply (18.5kW) was used to drive the roller shutter doors. Powered by a Siemens Micro-Master 440 frequency converter (22kW), the system required a backup power supply to ensure uninterrupted operation of the warehouse roller shutter doors and the normal shipment of stored appliances after a mains power outage. In 2005, our company designed and manufactured an online uninterruptible power supply (UPS) for them, using 42 65Ah batteries with a 60-minute backup time.
4 Conclusion
Variable frequency AC uninterruptible power supplies (UPS) are suitable for various loads, possessing the dual functions of a regular AC UPS and a frequency converter. It is understood that many domestic and international power supply manufacturers have already converted this power supply into an emergency power supply, but there are very few articles on this topic. This article is written in the hope of contributing to the improvement of domestic power supply technology. For loads where power outages are unacceptable, choosing this type of power supply offers a much higher performance-price ratio than choosing a regular UPS. Therefore, it is a power supply device worthy of widespread application.
