Development of AC voltage stabilization power supply using instantaneous comparison method

Precision electronic instruments and equipment are widely used in the fields of home, industrial production, office automation, experimental research, communication engineering, medical treatment, etc., and are generally powered by the city power grid. The rated voltage of the city power grid is single-phase 220V, which is an ideal voltage stipulated by humans. Since the voltage at the head end of the power supply line from the transformer is (220+10%)V and the voltage at the tail end is (220-10%)V, electrical equipment is required to have a certain ability to resist grid voltage fluctuations. Although most electrical equipment has the ability to resist power supply voltage fluctuations, according to statistics, power supply voltage fluctuations are one of the causes of electrical equipment failures. Therefore, these electrical equipment need to use a stabilized power supply to ensure their normal operation.

The voltage-stabilizing power supply is a device that stabilizes the voltage of the power grid and has been used in my country for many years, such as the sliding contact type, variable reactor type, and magnetic saturation type automatic voltage-stabilizing power supply driven by a servo motor to drive the voltage regulating transformer.

The above-mentioned voltage-stabilized power supplies are based on the principle of sampling the grid voltage, and after comparison processing and adjustment using a servo motor or a contactless switch, the AC voltage is output stably. This type of voltage-stabilized power supply generally has a poor voltage-stabilizing effect, with an accuracy of 1% to 0.5%.

The current AC regulated power supply mainly has the following problems:

(1) The use of nonlinear reactors often brings harmonics, which does not improve the quality of the grid voltage waveform. Some reactors even bring additional waveform distortion, causing harmonic pollution to the grid.

(2) The sampling time and actuator action time are too long.

These two links have a great impact on the performance of the voltage stabilizer. The sampling method generally adopts effective value or average value sampling, which requires at least several cycles; and the servo motor generally takes several seconds to tens of seconds to perform the adjustment action, which is too long and has no effect on the voltage waveform. These voltage stabilization methods cannot react and execute in time to the fast-changing interference in the grid voltage, such as surge, sag, pulse interference and high-frequency interference, causing the interference to pass through the voltage stabilization device to reach the electrical equipment, which cannot protect the electrical equipment and may also cause the electrical equipment to not operate normally or even be damaged.

It can be seen from this that when the grid voltage changes and is disturbed, making the output voltage of the voltage-stabilized power supply quickly return to near the rated value and providing the electrical equipment with a sinusoidal AC voltage with a good waveform and stable amplitude will be of great benefit to ensuring its safe and normal operation.

2. Instantaneous comparison AC voltage stabilization method

The following introduces a new method of AC voltage stabilization that can improve the shortcomings of the above-mentioned AC voltage stabilization power supply, that is, using the instantaneous comparison method - waveform repair technology to produce an AC voltage stabilization power supply. The basic principle is shown in Figure 1.

Grid voltage uin = rated voltage uS + ⊿u

Grid voltage uin + control voltage uC = output voltage uout

Figure 1 Schematic diagram of waveform repair technology

The rated voltage uS is the ideal voltage set by humans, and ⊿u is the deviation of the grid voltage from the rated voltage. Regardless of the size, as long as it deviates from the rated voltage, the artificially made control voltage uC can be superimposed on the grid voltage uin. If the artificially made control voltage uC is equal to -⊿u, when ⊿u changes, -⊿u also changes accordingly, and the input voltage uin and the control voltage uC are superimposed to equal the rated voltage uS. In the urban power grid, the effective value of the single-phase rated voltage is 220V.

The schematic diagram of obtaining the control voltage is shown in FIG2 .

For the changes caused by the input voltage, the input voltage is first sampled with a transformation ratio of 1/A.

uin/A=（uS+⊿u1）/A = uS/A+⊿u1/A

A reference voltage ur is artificially created, which has the same frequency and phase as the grid voltage, an effective value of US/A, and a good waveform. The difference between the sampled value of the input voltage uin and ur is ⊿u1/A, that is, uin-ur=⊿u1/A,

This voltage difference and power are then amplified by β times.

Let β = A, then the amplified value is ⊿u1, and then ⊿u1 is inverted and superimposed on the circuit through the coupling transformer. At this time, the output voltage is equal to the rated voltage, that is, uout = uS = Aur = βur. At this time, since β is an amplification factor, the performance of uout is only related to ur.

ur is a real voltage generated artificially, which can be obtained by analog or digital circuits and has good performance indicators. The stability of ur determines the stability of the output voltage.

From the above analysis, it can be seen that the essence of the instantaneous comparison AC voltage stabilization method is: use the input voltage and the reference voltage for instantaneous comparison to find out the shortcomings of its waveform, and improve and repair the input voltage waveform by superimposing the control voltage, so as to achieve the purpose of stabilizing the output voltage. The power quality of the output voltage is determined by ur, with a good waveform and stable amplitude; while the input voltage only provides energy for the internal operation of the power supply and stable voltage output, and a low-power control voltage is used to obtain a high-power stable output voltage. In this way, the output energy is provided by the power grid, and the control voltage is only used to repair the fluctuation part of the power grid that deviates from the rated voltage.

Similarly, when the input voltage remains unchanged but the output voltage changes due to changes in the load, sampling is performed from the output voltage end and the control voltage uC2 is adjusted in a similar way to change the control voltage uC2 to maintain the output voltage stable without affecting the input voltage.

In the implementation of the scheme, for economic considerations and to simplify the circuit, the acquisition of the control voltage uC1 and the acquisition of the control voltage uC2 are combined in a specific circuit to obtain the principle block diagram shown in FIG3 .

As shown in Figure 3, the comparison value between the input voltage and the reference voltage and the comparison value between the output voltage and the reference voltage are added by the adder, then amplified by the voltage and power amplifier circuit, and the control voltage uC is obtained by the coupling transformer and superimposed between the input voltage and the output voltage. The control voltage uC is mainly used to repair the waveform, adjust the power supply voltage, and also isolate the input power supply and the load.

Figure 3 Simplified circuit block diagram

3. Comparison with conventional regulated power supplies

Compared with the voltage-stabilized power supply made by the conventional principle, the voltage-stabilized power supply made by the above principle has the following characteristics:

(1) Fast response speed. Since all high-speed linear electronic devices are used, instantaneous sampling and instantaneous execution are achieved, and the control response speed is extremely fast. The adjustment can be completed in less than milliseconds, so that the output voltage quickly returns to the rated voltage. Therefore, it has a suppressive effect on high-frequency interference and noise, and a purifying effect on millisecond-level interference, which is something that ordinary voltage-stabilized power supplies cannot do.

(2) The input voltage has a wide applicable range. The input voltage can vary by 30% to 50% or even more, and can be adjusted symmetrically. The wider the range, the more repair energy is required. The control voltage value is mainly determined by demand. From an economic and practical point of view, (8-10)% is appropriate.

(3) High voltage stabilization accuracy. Depending on the way the reference voltage is generated, the voltage stabilization effect can reach 1%, 0.1%, and 0.01%. Voltage stabilization with different accuracy is suitable for different occasions. 1% is used in general voltage stabilization requirements; 0.1% is used in laboratories or important industrial equipment; 0.01% can be used for instrument calibration.

(4) It has the characteristics of green power supply. This method first corrects the waveform of the grid voltage into a good sine wave, and then supplies power to the load. The amount of repair energy is determined by the needs. Since the essence of this method is to correct the grid waveform, the distortion of the corrected waveform is generally less than 1% ~ 0.5%, so this voltage stabilization method is green.

(5) It has certain environmental protection properties. If the input voltage remains unchanged, the output voltage changes due to different load properties. The output voltage can be kept unchanged by changing the control voltage within a certain harmonic range. Since the control voltage has an isolation effect and does not affect the input voltage, this voltage stabilization method has certain environmental protection properties.

(6) High working efficiency. The working principle of this power supply is to control high power with low power, which has high efficiency. The capacity of the output voltage is mainly taken from the power grid, and the control voltage is generally the part of the power grid voltage that deviates from the rated voltage, so only the power required to manufacture the control power supply is consumed, so the efficiency is extremely high.

Take a 300 VA output power supply as an example: if the grid voltage fluctuates by +10%, it is only necessary to control the +10% power fluctuation. If the efficiency of manufacturing the control power supply is 30%, it is necessary to manufacture the power consumption of the control power supply to be 100 VA, and the overall efficiency is 300/(300+100)=75%. And the higher the efficiency of the control power supply manufacturing method, the higher the overall working efficiency of the machine.

Take the inverter method as an example: when making a 0.01% high-stability power supply, due to the AC/DC, DC/AC conversion and the efficiency limitations of the power amplifier device, without considering other losses, its overall efficiency is below 30%. When using the instantaneous comparison method to make a voltage-stabilized power supply of the same power, only the power of the control power supply used to compensate for the 10% fluctuation is needed. Even if the control power supply is also made using the inverter method, the power consumed is only equivalent to 1/10 of the inverter method. Obviously, when making a high-stability voltage-stabilized power supply, the efficiency of the voltage-stabilized power supply using the instantaneous comparison method is much higher than that using the inverter method.

(7) It does not use low-frequency filtering components such as large inductors and large capacitors. It has a small size, light weight, good output waveform, and the general waveform distortion is between 1% and 0.5%.

(8) This type of voltage stabilizer can be cascaded with other voltage stabilizers. When in use, the narrower the stability accuracy range is set, the lower the energy consumption. For example, if the stability accuracy of the previous stage voltage stabilizer is 2%, if a power output of 5000 VA is required, only 100 VA of control power supply is required, and the stability can reach more than 0.1%.

(9) A fast protection circuit can be used. When a short circuit fault occurs at the load end, the control power supply will immediately stop working. At this time, the coupling transformer is equivalent to a reactor (the coupling transformer is the transformer that introduces the control power supply), which has the function of limiting the growth of the short-circuit current. After the fault is eliminated, the control power supply will resume working automatically.

4. Application effect

Initially, this control method was used to develop a compensated voltage-stabilized power supply. The main idea was to perform negative feedback control on the grid voltage, with a stability of 0.1%, a waveform distortion of 1%, and a power of 100VA. Due to the limited selection of devices at the time and the inability to keep up with the protection speed, this voltage-stabilized power supply failed to be promoted and applied. Later, the instantaneous comparison and waveform repair technology were used to optimize the original design, device selection, and fast protection input circuit. After many improvements and tests, the AC voltage-stabilized power supply with an output power of 300VA produced for the calibration of electric energy meters has practical functions, and the actual measurement has achieved the following indicators:

When the input grid voltage changes by +10%, measured with a digital voltmeter, the maximum jump in output voltage stability does not exceed +0.03%/3 minutes, and the output waveform distortion is <0.5%.

The regulated power supply has the following features:

(1) The circuit is composed entirely of analog devices, which are easy to select and inexpensive;

(2) The working principle of this power supply is that small power controls large power, and it has high efficiency. Only 30 VA of control power is needed to achieve a power output of 300 VA;

(3) The output power tube does not need a combination tube. When the output power of the whole machine is 300VA, since only 30VA of control power is needed, only a pair of high-power tubes are needed for output, and no air cooling is required;

(4) Strong anti-interference ability. During the test, three-phase welding was performed on the same power supply line in the same room with the power supply, and the output voltage did not jump;

(5) The voltage-stabilized power supplies independently made by assistants according to this design have the same technical indicators, indicating that the design method has good consistency.

5. Conclusion:

The basic principle of the instantaneous comparison method - waveform repair technology to make a voltage-stabilized power supply is to compare the sampling value of the input voltage with the reference voltage to find out the shortcomings of its waveform, and then improve and repair the input voltage waveform and stabilize the amplitude by changing the control voltage to achieve the purpose of output voltage stability. Its essence is to use a small-power control power supply to obtain a large-capacity stable voltage output. It is an AC voltage stabilization method that combines green, environmental protection, purification, high quality and high efficiency. The AC voltage-stabilized power supply developed using this technology has the characteristics of low cost, high indicators, low cost and easy control, and can also be expanded to a high-power voltage-stabilized power supply as needed.

This voltage stabilization method can provide high-quality AC regulated power supply for scientific research, computer rooms, medical equipment, industrial automation equipment, communication equipment, lighting systems, audio and video equipment, etc.

This AC voltage stabilization method has been applied for a national patent.