UPS circuit structure and performance characteristics

1. Types of UPS circuit structures
At present, various UPS with mature technology and products have been formed. In terms of their main circuit structure and uninterruptible power supply operation mechanism, there are four main categories:
· Backup type
· Online interactive type
· Dual inverter online type
· Dual inverter voltage compensation online type (delta inverter)

Figures (I), (II), (III) and (IV) are the basic circuit structures of the above four types of UPS.
1. The backup type is the original form of static UPS. Because it was applied early and widely, the technology and products are very mature. The functions of each link of the circuit in Figure (I) are as follows:
Charger: When the mains is available, the battery is charged and float-charged. If it is a long-delay UPS, it is required to have a strong charging capacity, or an additional charger of corresponding capacity is added.
DC-AC inverter: When the mains is available, the inverter does not work; when the mains is off, it converts the DC voltage (battery supply) into an AC voltage that meets the load requirements. The voltage waveform has three forms: square wave, quasi-square wave and sine wave.

Output conversion switch: When the mains is on, the input power is connected to supply power to the load; when the mains is off, the grid is disconnected, the inverter is connected, and the load continues to be supplied with power. Intelligent voltage regulation: When the mains is on, the output voltage can be adjusted and stabilized.
The performance characteristics of backup UPS are:
·When the mains is available, the efficiency is high, reaching more than 98%;
·When the mains is available, the input power factor and input current harmonics depend on the load current, and the UPS itself does not generate additional input power factor and harmonic current distortion;
·When the mains is available, the output capacity is strong, and there are no strict restrictions on the load current peak factor, surge current factor, output power factor, overload, etc.;
·When the mains is off, the output has a conversion time, which can generally be around 4ms, which is enough to meet the load requirements;
·When the mains is available, the output voltage stability accuracy is poor, but it can meet the load requirements;
·When the mains is available, the whole machine relies on additional filtering circuits to improve the UPS bidirectional anti-interference function;
·The circuit is simple, low cost and high reliability;
·Because the output has a conversion switch, it is difficult to increase the UPS output power due to the limitations of switching current capacity and action time. Most of the backup UPS currently on the market are below 2KVA.
2. Online interactive means that the inverter is in hot backup state, while taking into account the function of battery charging, which increases the power capacity of the backup UPS, reduces the conversion time when the mains power is off, and improves the filtering effect on the output voltage.

The functions of each link in the circuit in Figure (II) are as follows:
· Input switch: When the mains power is lost (referring to the grid voltage loss), the switch is disconnected to prevent the inverter from feeding power to the grid;
· Intelligent voltage regulation: When the mains power is present, the output voltage can be adjusted to stabilize;
· DC/AC inverter: This inverter can perform bidirectional inversion. When the mains power is present, the inversion direction is AC-DC, charging the battery and floating charge; after the mains power is lost, the inversion direction is DC-AC, powered by the battery, keeping the UPS continuing to supply power to the load.
The performance characteristics of online interactive UPS are:


· When the mains is available, the efficiency is high, which can reach more than 98%;
· When the mains is available, the input power factor and the input current harmonic components depend on the load current, and the UPS itself does not generate additional input power factor and harmonic current distortion;
· When the mains is available, the output capacity is strong, and there are no strict restrictions on the load current peak factor, surge current factor, output power factor, overload, etc.;
· When the mains is off, because the input switch has an on-off time, the UPS output still has conversion time, but it is smaller than the backup type;
· When the mains is available, the output voltage stability accuracy is poor, but it can meet the load requirements;
· When the mains is available, because the inverter is directly connected to the output end and is in hot backup status, it has a filtering effect on the output voltage spike interference;
· The circuit is simpler, the cost is low, and the reliability is high;
· The inverter also has a charging function, which saves the additional charger of the backup UPS. Its charging capacity is much stronger than that of the additional charger. When long-delay power supply is required, there is no need to add external charging equipment;
·Since the inverter is directly connected to the output and there is no restriction on the conversion switch, the output power can be increased to 5-10KVA.
In order to further improve the function of the online interactive type, an inductor can be connected in series between the input switch and the intelligent voltage regulation. The purpose is that when the city power is off, the inverter can immediately supply power to the load, because the series inductor has a strong inhibitory effect on the current fed back to the grid by the inverter output, avoiding the danger of short-circuiting the inverter output when the input switch is not disconnected. This can reduce the conversion time of the online interactive type to zero, making it fully equipped with the conversion function of the dual-inverter online type, and at the same time increase the anti-interference ability of the entire UPS. However, this has the adverse consequence of reducing the UPS input power factor.
3. Dual-inverter online type
At present, most online types, especially high-power online UPS, mostly adopt a dual-inverter circuit structure. The functions of each link of the circuit in Figure (III) are as follows:

·Inverter (Ⅰ): This inverter is an AC-DC unidirectional inverter. When the mains is available, it completes charging the battery and supplies power to the load through the inverter (Ⅱ). This inverter is mostly a rectifier or controllable rectifier circuit;

·Inverter (Ⅱ): This inverter is a DC-AC unidirectional inverter. When the mains is available, it obtains power from the inverter (Ⅰ) and then sends it to the output end, and ensures that high-quality power is provided to the load; when the mains is off, the battery supplies power to the load through the inverter (Ⅱ);
·Bypass switch: It is usually in the disconnected state. When the inverter circuit fails, or when the load is impactful (for example, when starting the load) or the fault is overloaded, the inverter stops outputting, the bypass switch is turned on, and the power grid directly supplies power to the load. The bypass switch is mostly an intelligent contactless switch with a very high power capacity.
The performance characteristics of dual-inverter online UPS are as follows:
·Because all the load power is provided by the inverter regardless of whether there is AC power, it can provide high-quality power to the load, such as output voltage stability accuracy, frequency stability, output voltage dynamic response, waveform distortion and other indicators, which are all relatively high;
·When the AC power is off, the output voltage is not affected in any way and there is no conversion time;
·Because all the load power is provided by the inverter regardless of whether there is AC power, the power margin of UPS is limited and the output capacity is not ideal, so restrictions are imposed on the load, such as output current peak coefficient (generally only 3:1), overload capacity, output power factor (generally 0.8), output active power less than the calibrated KVA number, and the ability to cope with impact loads;
·Since the inverter (Ⅰ) is mostly a rectifier circuit, it will cause current harmonic interference to the power grid and the input power factor is low. After filtering, the minimum harmonic current component is about 10%, while the input power factor is only about 0.8. If power factor correction technology is used in the inverter (Ⅰ), the input power factor can be increased to close to 1, and the input current harmonic component will be greatly reduced;
·When the mains is available, since both inverters bear 100% of the load power, the overall efficiency is low. The efficiency of UPS below 10KVA is about 80%, that of 50 KVA can reach 85-90%, and that of more than 100 KVA can reach 90-92%.
4. The dual-inverter voltage compensation online technology was first proposed by APC Silcon in the United States and developed into a product in three-phase high-power UPS. It also has a dual-inverter circuit structure and works online. However, since it successfully applies the voltage compensation principle in AC voltage stabilization technology to the main circuit of UPS, compared with dual-inverter online UPS, Silcon DP300E series high-power UPS has made new breakthroughs in adapting to the power grid environment and not interfering with or damaging the power grid, and in many other UPS main indicators such as output capacity and reliability. APC Silcon calls this circuit structure Delta inverter technology.

The functions of each link in Figure (IV) are as follows:
· Inverter (Ⅰ): It is a set of DC-AC and AC-DC bidirectional inverters. The secondary side of its output transformer (high frequency) is connected in series in the UPS main circuit. It has three functions:
First: to compensate the input power factor of the UPS input and suppress the input current harmonics;
Second: together with the inverter (Ⅱ), it completes the compensation of the input voltage. When the input voltage is higher than the rated value of the output voltage, the inverter (Ⅰ) absorbs power and

compensates the difference between the input and output voltages with reverse polarity. When the input voltage is lower than the rated value of the output voltage, the inverter (Ⅰ) outputs power and compensates the difference between the input and output voltages with positive polarity;
Third: together with the inverter (Ⅱ), it completes the charging and floating charging functions of the battery.
Inverter (II): This inverter is also a DC-AC and AC-DC bidirectional inverter, and it has four functions:
First, together with inverter (I), it completes the compensation of the input and output voltage difference;
Second, together with inverter (I), it completes the battery charging and voltage floating charge functions;
Third, it monitors the output voltage at any time to ensure the stability of the output voltage, and compensates for the output voltage waveform distortion and output current harmonic components so that they do not affect the power grid; Fourth
, when the mains power is cut off, all output power is given by inverter (II), and the output voltage is guaranteed to be uninterrupted and the conversion time is zero.

Figure (V) is a schematic diagram of current and power transmission of dual inverter voltage compensation online UPS under various conditions (here UPS efficiency is assumed to be 100%).

The performance characteristics of dual inverter voltage compensation online UPS are as follows:
Because the inverter (II) monitors and controls the output voltage at any time, and participates in the adjustment of the main circuit voltage through the inverter (I), it can provide high-quality power to the load regardless of whether the mains is available. For example, the output voltage stability, frequency stability, output voltage dynamic response, waveform distortion and other indicators are relatively high.
When the mains power is off, the output voltage is not responded to and there is no conversion time. In addition, when the load current is distorted, it is also adjusted and compensated by the inverter (II), so it is a typical online working mode.
When the mains is present, the inverters (I) and (II) only adjust and compensate for the difference between the input voltage and the output voltage. The maximum power borne by the inverter (when the input voltage is at the upper and lower limits) is only 20% of the output power (equivalent to the input voltage variation range), so the power intensity is very small (1/5) and the power margin is large, which greatly enhances the output capacity of the UPS. Compared with the dual-inverter online type, the overload capacity is enhanced (200%, 1 minute), and the load current peak factor is no longer restricted, so the impact load can be handled calmly. The load power factor is no longer restricted, and the output active power can be equal to the calibrated KVA value.
Inverter (I) also completes the power factor correction function of the input end, making the input power factor equal to 1 and the input harmonic current drop below 3%.
When the mains is present, since the maximum power borne by the two inverters is only 1/5 of the output power, the overall efficiency can reach 96% in a large power range.
When AC power is available (99% of the UPS's continuous operation time is with AC power), the inverter power intensity is only 1/5 of the design value (Inverter II), so the life and reliability of components and even the entire machine will inevitably be greatly improved.

2. Performance characteristics of
four types of UPS with circuit structures The coexistence of four types of UPS structures is the reality of current UPS technology and market. This phenomenon is formed in the process of continuous improvement of UPS use requirements and continuous progress of UPS technology. If we make a comprehensive comparison from the aspects of technological advancement, advantages and disadvantages of main performance indicators (adaptability to the power grid, output capacity and reliability), output power level, production cost, different use occasions, etc., we can be sure that although these four types of UPS will coexist, they will inevitably continue to improve technology and performance during use. Of course, the survival of the fittest is inevitable.
However, due to the change in the concept of UPS function, higher requirements are put forward for UPS hardware technology. This requirement can be summarized by the four words "practical and reliable". In other words, although the various performance indicators of the current UPS can basically meet the requirements of users, there are still many limitations and shortcomings.
As a primary power supply device, what power supply equipment (user) requirements should UPS meet in terms of performance indicators, and what kind of commitment should it make to the load? As far as the current situation is concerned, this issue is clear and seems to be self-evident, but there is a phenomenon of misleading users, resulting in the phenomenon of not distinguishing between good and bad and uneven evaluation of the performance of a UPS.
The following tendencies and viewpoints are worthy of attention from manufacturers and users:
1. High standards for conventional indicators should not be excessively pursued
. The so-called conventional indicators refer to output voltage stability accuracy, distortion, frequency stability accuracy, phase difference (three-phase), voltage balance (three-phase), conversion time (backup UPS and online to bypass conversion), dynamic response, etc. These indicators represent the quality of UPS output voltage. In fact, the current UPS of various brands have reached very high standards in these indicators, which are more than enough to meet the load requirements. Can it be said that these conventional indicators are necessary but not harsh conditions for the transmission voltage performance of power supply equipment? However, what is worrying is that some UPS manufacturers currently use the high standards of these indicators as the quality mark of their own brand of UPS, and users often use the advantages and disadvantages of these specifications as the criteria for evaluation and decision whether to purchase.

2. The inspection of UPS output capacity and reliability should not be underestimated or even ignored.
As a primary power supply device, UPS is first required to be able to operate normally in a complex power grid environment and not cause interference and damage to the power grid; second, it is necessary to comprehensively improve the quality of the power grid; third, its output capacity and reliability should be inspected. Especially in the first and third points, the current UPS still has many shortcomings. For the power grid environment, the input voltage variable range is not enough (for example, ±15%), which cannot adapt to
the actual situation of large fluctuations in the voltage of China's power grid. The UPS input power factor is low (for example, about 0.8), and the input current harmonics are large (≥10%), which interferes with and destroys the power grid voltage. As for the output capacity, the limitations are more obvious, which is much worse than the real power supply capacity of the power grid. For example, for special loads, such as strong capacitive loads, strong inductive loads, non-periodic impact loads, periodic impact loads, load failures and even human misoperation failures, the destructive threat to UPS output. If the power grid is used directly, the power grid capacity and distribution components (transformers, switches, fuses, etc.) can be tolerated. However, there is no guarantee for using UPS for power supply. In fact, UPS is often damaged during the startup process and the above-mentioned special loads and faults. In use, users need to use the front-stage additional AC voltage stabilization equipment to solve the problem that the UPS (especially high-power UPS) allows the grid voltage variation range to be not large enough. As for the low input power factor and large harmonic current, they can only let it exist helplessly. In terms of output capacity, UPS manufacturers have put forward various restrictions on the load, such as output current peak coefficient (generally not more than 3:1), non-periodic impact load (increase UPS power distribution capacity), load power factor (generally around 0.8, so that the UPS calibrated KVA≠KW)
, etc. Compared with the real power grid, these restrictions should not exist. These restrictions reflect the limitations of UPS input and output capabilities. It should be said that output capacity and reliability are the most critical indicators of UPS hardware technology.
3. About the conversion time
The manufacturer's propaganda and the media's misleading have made users mistakenly believe that online UPS has no conversion time, and the performance of UPS without conversion time is superior. This is a prejudice. Please see the following two facts:

First: The backup UPS only has conversion time when the mains power is off, while the online UPS not only has conversion time from inverter to bypass when the inverter fails, but also has conversion time to bypass when the load fails, is overloaded, and starts (there is a surge current) due to the limited output capacity of the inverter.
Second: When there is a rectifier filter circuit (such as a computer) at the input end of the load, the load input current only exists at the peak of the sine wave voltage, and the current pulse width is only 3-4ms (related to the load and circuit parameters), that is, every 10ms, there is a 6-7ms power outage, and the power is cut off 100 times per second. In this sense, the uninterruptible power supply does not exist, or is unnecessary. The problem lies in the size of the UPS conversion time. Generally, the energy storage of the computer input rectifier filter circuit can maintain the load power supply for tens of milliseconds. At present, the conversion time of the backup UPS can be controlled below 4ms, which basically has no effect on the load.
4. About the input power factor and input current harmonics of UPS
For dual-inverter online UPS, its AC/DC inverter is mostly a rectifier filter circuit. Its input power factor is low, generally only around 0.8, and the input current harmonics are large, up to 30%. After adding special filtering measures, it can only be reduced to 10%. Low input power factor means large input reactive power, and input harmonic current interferes with and damages the power grid, especially three-phase high-power UPS. These two indicators are very harmful, forming the so-called power pollution, causing transformers, motors, capacitors, etc. powered by the same power grid to produce additional harmonic losses, overheating, and accelerated insulation aging; causing asynchronous motor torque reduction, increased vibration, and increased noise; causing relays and automatic devices to malfunction; high-order harmonics to radiate interference to communication lines and measuring instruments; affecting the accuracy of electric energy metering, etc. Therefore, the input power factor and input harmonic current of UPS should be regarded as one of the important performance indicators, and the input power factor>0.95 and input current harmonics<5% should be used as one of the criteria for judging whether the UPS performance indicators are qualified. 5. Frequency stability of UPS
Among the conventional indicators of UPS output, there is a frequency stability accuracy, especially for dual-inverter online UPS, which is calibrated to <±1% (or even 0.1), but this is only the case when the battery is powered after the mains power is lost. 99% of the time, the UPS is running under the condition of mains power, and at this time, the UPS has no frequency stability. In order to prevent the online UPS from being damaged due to inverter output short circuit when switching from inverter to bypass, the inverter operating frequency and phase are required to track the input grid voltage during normal operation. Therefore, it is meaningless to mark the high indicator of UPS frequency stability. Moreover, the normal operation of general electronic equipment is not affected at all when the input power frequency changes within the range of ±3%.
6. Efficiency and reliability
When the UPS works efficiently, it means saving energy, which is one of the signs of green power. However, it should also be noted that efficiency and reliability are closely related. High efficiency means advanced circuit technology and good selection of components, which means that the power device has low power loss, low power intensity and low temperature, which will inevitably enhance the life and reliability of components and even the whole machine.
7. Output capacity and reliability
The output power factor, output current crest factor, output overload capacity, and the ability to output unbalanced loads directly reflect the output capacity of the UPS. The restrictions on these indicators illustrate the limitations and fragility of the UPS output capacity. Although the load should meet the requirements of the UPS as much as possible when configuring the UPS capacity, and even leave a large margin, these indicators directly reflect the reliability of the UPS. It is indisputable that the reliability of a UPS with strong overload capacity, high output current crest factor, and small load power factor restrictions must be high when operating under the same power grid environment and load conditions.
The following table compares the operating parameters of various UPSs in the presence of mains power. When the mains power is lost and switched to battery power, all types of UPSs are powered by DC-AC inverters, and there is no essential difference in the operating parameters.