There are several types of overvoltage in the power system, including atmospheric overvoltage, power frequency overvoltage, operational overvoltage, and resonant overvoltage.   

Atmospheric overvoltage: Sudden addition of direct lightning strikes or lightning induction to the power system, causing electrical equipment to withstand voltages far exceeding their rated values. Atmospheric overvoltage can be divided into direct lightning overvoltage and induced lightning overvoltage. After the power system is subjected to atmospheric overvoltage, it can cause insulation breakdown or flashover of transmission and distribution lines and electrical equipment, resulting in power outages and endangering human life. The characteristics are short duration, strong impact, directly related to the intensity of lightning strike activity, and independent of equipment voltage level. Therefore, the insulation level of systems below 220KV is often determined by atmospheric overvoltage. For gas overvoltage, measures such as installing lightning rods, lightning conductors, and lightning arresters, reasonably improving the insulation level of the line, and using automatic reclosing devices are usually taken.

Power frequency overvoltage: an overvoltage that occurs in a system during operation or grounding faults, with a frequency equal to or close to the power frequency (50 Hz) and higher than the system operating voltage. The characteristics are long duration, low overvoltage multiple, and generally low risk to equipment insulation. However, it plays an important role in determining the insulation level during high voltage and long-distance transmission. Within a few hundred milliseconds after system operation and grounding tripping, due to the inertia of the generator's automatic voltage regulator, the electromotive force of the generator remains constant. The power frequency overvoltage during this period is called temporary power frequency overvoltage. As time goes on, the automatic voltage regulator of the generator comes into effect, causing a decrease in the electromotive force of the generator. At this time, the power frequency overvoltage is called steady-state power frequency overvoltage. The main reasons for generating power frequency overvoltage are the capacitance effect of long no-load lines, the positive, negative, and zero sequence voltage components caused by asymmetric grounding, and the sudden load shedding of the system causing the generator to accelerate rotation. Special measures should be taken to limit power frequency overvoltage according to specific situations. Common methods include using parallel reactors to compensate for the capacitance effect of no-load long lines, selecting a reasonable system neutral point operation mode, and quickly adjusting the voltage control of the generator.

Operation overvoltage: Overvoltage caused by sudden changes in system parameters due to operation (such as closing and opening of circuit breakers), faults, or other reasons, resulting in the system transitioning from one state to another, and the electromagnetic energy oscillation of the system itself during this transition process. The characteristic is randomness, but under unfavorable conditions, the overvoltage multiplier is relatively high. Reasons for overvoltage operation and avoidance measures

1. The operating overvoltage of the power grid is generally caused by the following reasons

A. Line closing and reclosing; B. Open the no-load transformer and shunt reactor; C. Asymmetric fault tripping and oscillation disconnection of the line; D. Open the circuit without load. The overvoltage caused by line closing and reclosing has a significant impact on the insulation coordination of power grid equipment, and circuit breakers with closing resistors should be used to limit this overvoltage. The lightning arrester can serve as a backup protection device for overvoltage operation of electrical equipment in substations, and it is also a protection device for lightning overvoltage in substations. When designing for Class A and C overvoltage, it should be predicted based on the power grid conditions.

2. Overvoltage during line closing and reclosing operations

When the no-load line is closed, overvoltage will occur due to the oscillation of the line inductance capacitance. When the lines overlap, the high power supply potential and the presence of residual charges on the lines intensify this electromagnetic oscillation process, further increasing the overvoltage. Therefore, circuit breakers should be equipped with closing resistors to effectively reduce closing and reclosing overvoltage. According to the predicted conditions of the power grid, the overvoltage distribution of no-load line closing, single-phase closing, and successful and unsuccessful three-phase closing (such as during operation) should be calculated, and the relative ground and phase to phase statistical operation overvoltage including the line receiving end should be calculated. The conditions for predicting overvoltage in such operations are as follows: A. The no-load line is closed, and before the line circuit breaker is closed, the power bus voltage is higher than the grid voltage; B. Before the successful three-phase reclosing, a single-phase grounding fault occurred at the receiving end of the line; When the three-phase reclosing is unsuccessful, there is a single-phase grounding fault at the receiving end of the line. The relative ground operation overvoltage generated at the receiving end of the no-load line closing, single-phase closing, and successful three-phase closing (if used during operation) should not exceed 2 2UXG.

3. Operation overvoltage of 3-terminal no-load transformer and shunt reactor

The operating overvoltage caused by forced arc extinguishing when the circuit breaker breaks the inductive current of these devices should be determined based on factors such as the circuit breaker structure, circuit parameters, transformer (shunt reactor) wiring and characteristics. The overvoltage of this operation can generally be limited by a lightning arrester installed between the circuit breaker and the transformer (parallel reactor). For transformers, lightning arresters can be installed on the low voltage side or high voltage side, but if the neutral grounding method of the high and low voltage power grids is different, magnetic blow valve type lightning arresters should be used on the low voltage side. When the lightning arrester may frequently operate, it is advisable to use a circuit breaker with a high-value opening resistor.

4. Asymmetric fault tripping and oscillation disconnection operation overvoltage in 4 lines

The weak connection between the transmission and reception ends of the power grid, such as the opening of the circuit due to asymmetric faults or disconnection during grid oscillation, will result in the opening of the circuit due to asymmetric faults or overvoltage caused by oscillation disconnection. To predict the overvoltage caused by asymmetric fault tripping of a power line, the condition of single-phase grounding fault at the receiving end of the line can be selected. The potential power angle difference between the sending and receiving ends of the line during tripping should be selected according to the actual situation. Circuit breakers with opening resistors can reduce the overvoltage caused by asymmetric fault opening and oscillation disconnection in power lines. When this condition is not met, lightning arresters installed on the line should be used to limit it.

5. Open circuit overvoltage for unloaded lines

Attention should be paid to using circuit breakers that do not reignite when opening under the condition of a power to ground voltage of 1.3UXG.

6. Lightning arresters should be installed in substations

Damage to electrical equipment caused by overvoltage during operation. The installation position is as follows: A. On each line entrance side of the outgoing circuit breaker, the lightning arrester installed at that position is called the line lightning arrester; B. The lightning arrester installed at the substation side of the outgoing circuit breaker is called the substation lightning arrester. The specific installation location and quantity of all lightning arresters should be determined in conjunction with 4.4.2. Note: When there is no parallel reactor at the entrance of the line, if predicted (for circuit breaker closing, the condition of one phase failure of the closing resistance needs to be considered)

When the overvoltage at this location is below the level of overvoltage protection for lightning arrester operation, lightning arrester should not be installed here.

7. rated voltage with series gap lightning arrester

It should not be lower than the power grid frequency overvoltage level at the installation point.

8. When using metal oxide surge arresters to limit operating overvoltage

The manufacturer's product manual should be followed to ensure that the long-term operating voltage, power frequency overvoltage, and resonant overvoltage allowable duration meet the requirements of the power grid.

9. Operating overvoltage current capacity of 9 lightning arresters

Allowing energy absorption should meet the requirements of the power grid (considering the condition of one phase failure of the closing resistance when closing the circuit breaker). In addition, it is necessary to verify whether the voltage on the lightning arrester exceeds its specified protection level. When outdated, its impact on insulation coordination should be considered.

10. Monitor the power frequency overvoltage of the operating power grid

It is advisable to install automatic recording devices for overvoltage waveform or amplitude in the substation for resonance overvoltage and operation overvoltage, and collect the measured results properly.

Measures to limit overvoltage during operation include:; Choose high-voltage switches with strong arc extinguishing ability; Improve the synchronicity of switch actions; Install parallel resistors on the switch interface; Using high-performance lightning arresters, such as zinc oxide lightning arresters; Make the neutral point of the power grid directly grounded for operation.

Resonant overvoltage: Some inductance and capacitance components in the power system can form various oscillation circuits when the system is operated or malfunctions. Under certain energy sources, series resonance phenomenon will occur, causing serious overvoltage in some components of the system. There are several types of resonant overvoltage:

(1) The linear resonant overvoltage resonant circuit is composed of inductive elements without iron cores (such as the inductance of transmission lines and leakage inductance of transformers) or inductive elements with iron cores that have excitation characteristics close to linearity (such as arc suppression coils) and capacitive elements in the system.

(2) The ferromagnetic resonance overvoltage resonance circuit is composed of inductive components with iron cores (such as no-load transformers and voltage transformers) and capacitive components of the system. Due to the saturation phenomenon of iron core inductance components, the inductance parameters of the circuit are nonlinear. This type of circuit containing nonlinear inductance components will generate ferromagnetic resonance when certain resonance conditions are met.

(3) Parameter resonance overvoltage is composed of a loop consisting of inductance components with periodic changes in inductance parameters (such as the synchronous reactance of a salient pole generator that varies periodically between Xd and Xq) and system capacitance components (such as no-load lines). When the parameters are matched, energy is continuously transmitted to the resonance system through the periodic changes in inductance, causing parameter resonance overvoltage.

The main measures to limit resonance overvoltage are:

(1) Improving the synchronicity of switch actions is important as many resonant overvoltages are caused under non full phase operating conditions. Therefore, increasing the synchronicity of switch actions and paying attention to non full phase operation can effectively prevent the occurrence of resonant overvoltages.

(2) Installing a small reactance at the neutral point of a parallel high-voltage reactor can block the transmission of power frequency voltage and series resonance during non full phase operation. (3) Disrupting the conditions for the generator to generate self excitation, parameter resonance overvoltage

1. What is a frequency converter?   

A frequency converter is an electrical energy control device that utilizes the on-off function of power semiconductor devices to convert power frequency sources into another frequency.  

2. What are the differences between PWM and PAM?  

PWM is an abbreviation for Pulse Width Modulation, which is a way of adjusting the output and waveform by changing the pulse width of a pulse train according to a certain pattern. PAM stands for Pulse Amplitude Modulation, which is a modulation method that adjusts the output value and waveform by changing the pulse amplitude of a pulse train according to a certain rule.

3. What is the difference between voltage type and current type?

The main circuit of a frequency converter can be roughly divided into two categories: voltage type is a frequency converter that converts the DC of the voltage source into AC, and the filtering of the DC circuit is a capacitor; Current type is a frequency converter that converts the direct current of a current source into alternating current, with a DC circuit filter and inductor.

4. Why does the voltage and current of a frequency converter change proportionally?

The torque of asynchronous motors is generated by the interaction between the magnetic flux of the motor and the current flowing through the rotor. At the rated frequency, if the voltage is constant and only the frequency is reduced, the magnetic flux will be too large, the magnetic circuit will saturate, and in severe cases, the motor will be burned out. Therefore, the frequency and voltage should be changed proportionally, that is, while changing the frequency, the output voltage of the frequency converter should be controlled to maintain a certain magnetic flux of the motor and avoid the occurrence of weak magnetism and magnetic saturation phenomena. This control method is commonly used in energy-saving frequency converters for fans and pumps.

5. When the electric motor is driven by a power frequency source, the current increases when the voltage drops; For variable frequency drive, if the voltage also decreases when the frequency decreases, does the current increase?

When the frequency decreases (at low speed), if the same power is output, the current increases, but under the condition of constant torque, the current remains almost unchanged.

6. What are the starting current and starting torque of the motor when using a frequency converter for operation?

Using a frequency converter for operation, the frequency and voltage increase correspondingly with the acceleration of the motor, and the starting current is limited to below 150% of the rated current (125%~200% depending on the model). When starting directly with a mains power supply, the starting current is 6-7 times, resulting in mechanical and electrical shocks. Using a frequency converter drive can smoothly start (with longer starting time). The starting current is 1.2~1.5 times the rated current, and the starting torque is 70%~120% of the rated torque; For frequency converters with automatic torque enhancement function, they can be started with full load.

7. What does V/f mode mean?

When the frequency decreases, the voltage V also decreases proportionally, as explained in answer 4. The proportional relationship between V and f is predetermined taking into account the characteristics of the motor, and usually there are several characteristics stored in the storage device (ROM) of the controller, which can be selected using switches or dials.

8. How does the torque of the motor change when V and f are proportionally changed?

When the frequency decreases proportionally and the voltage decreases, the decrease in AC impedance while the DC resistance remains unchanged will result in a tendency to reduce the ground torque generated at low speeds. Therefore, given V/f at low frequencies, it is necessary to increase the output voltage slightly in order to obtain a certain starting torque. This compensation is called enhanced starting. Various methods can be used to achieve this, including automatic operation, selecting V/f mode, or adjusting potentiometer.

9. Is there no output power below 6Hz, as the manual states a speed range of 60~6Hz, which is 10:1?

Power can still be output below 6Hz, but depending on the temperature rise and starting torque of the motor, the frequency of use should be around 6Hz. At this time, the motor can output the rated torque without causing serious heating problems. The actual output frequency (starting frequency) of the frequency converter varies from 0.5 to 3 Hz depending on the model

10. Is it possible to require a constant torque for general motor combinations above 60Hz?

Usually it is not possible. When the voltage remains constant above 60Hz (there are also modes above 50Hz), it is generally a constant power characteristic. When the same torque is required at high speeds, attention should be paid to the selection of motor and inverter capacity.

11. What does' open-loop 'mean?

A speed detector (PG) is installed on the motor device used to feed back the actual speed to the control device for control, which is called a "closed loop". If it does not operate with PG, it is called an "open loop". Universal frequency converters are mostly open-loop, and some models can also use options for PG feedback.

12. What should be done when the actual speed deviates from the given speed?

When open-loop, even if the frequency converter outputs a given frequency, the motor's speed varies within the range of rated slip rate (1%~5%) when running with load. For situations where high speed regulation accuracy is required and even load changes require operation close to a given speed, a frequency converter with PG feedback function (optional) can be used.

13. If a motor with PG is used for feedback, can the speed accuracy be improved?

The frequency converter with PG feedback function has improved accuracy. But the accuracy of speed depends on the precision of PG itself and the resolution of the output frequency of the frequency converter.

14. What does stall interception function mean?

If the given acceleration time is too short and the output frequency of the frequency converter changes much more than the speed (electrical angular frequency), the frequency converter will trip and stop running due to overcurrent, which is called stall. In order to intercept stall and keep the motor running, it is necessary to detect the magnitude of the current for frequency control. When the acceleration current is too high, slow down the acceleration rate appropriately. The same applies when decelerating. The combination of the two is the stall function.

15. What is the significance of models with separately given acceleration time and deceleration time, and models with jointly given acceleration and deceleration time?

Acceleration and deceleration can be given separately for different types of machines, which is suitable for short-term acceleration, slow deceleration situations, or situations where a production cycle time needs to be given for small machine tools. However, for situations such as fan transmission, acceleration and deceleration times are relatively long, and both acceleration and deceleration times can be given together.

16. What is regenerative braking?

If the command frequency is reduced during the operation of the electric motor, it will become a asynchronous generator and work as a brake, which is called regenerative (electrical) braking.

1、 Types of commonly used UPS batteries

There are three types of batteries commonly used in UPS power applications: open type liquid lead-acid batteries, maintenance free batteries, and nickel chromium batteries. Factors that affect battery life also have their own advantages and disadvantages. The batteries provided by current UPS manufacturers are generally maintenance free batteries. The following mainly introduces the characteristics of three types of batteries to avoid maintenance:

1: Open type liquid lead-acid battery

This type of battery can be divided into two types based on its structure: 8-10 years and 15-20 years. Due to the corrosive gas generated by sulfuric acid electrolysis of this battery, it should be installed in a well ventilated room away from precision electronic equipment, and the battery room should be covered with anti-corrosion tiles.

Due to evaporation, open batteries require regular measurement of specific gravity and addition of acid and water. This battery can withstand high temperature, high voltage, and deep discharge. The battery room should be smoke-free and use open battery racks.

This battery cannot be transported after charging, so it usually takes 55-90 hours for initial charging after installation on site. The normal voltage for each section is 2V, and the initial charging voltage is 2.6-2.7V.

2: Maintenance free battery

Also known as valve regulated sealed lead-acid batteries, the following principles should be followed in use and maintenance:

a: The allowable operating range for sealed batteries is 15-50 degrees, but using within 5-35 degrees can extend the battery life. The chemical composition of the battery will change below minus 15 degrees Celsius and cannot be charged. Using within the range of 20 to 25 degrees will result in a longer lifespan. Batteries will have a long lifespan but lower capacity when operating at low temperatures, and a higher capacity but shorter lifespan when operating at high temperatures.

b: The relationship between battery life and temperature can be referred to as follows: after the temperature exceeds 25 degrees Celsius, the battery life will be reduced by half for every 8.3 degrees Celsius increase.

c: The design float charge voltage of maintenance free batteries is 2.3V/cell. A 12V battery has a voltage of 13.8V. CSB recommends 2.25-2.3V per cell. When 120 cells are connected in series and the temperature exceeds 25 degrees Celsius, the float charge voltage should be reduced by 3MV for every degree of temperature increase. Similarly, to avoid insufficient charging, the voltage should be increased by 3MV for every degree of temperature increase. The discharge termination voltage is 1.67V per cell at full load (<30 minutes). At low discharge rates (low current long-term discharge), the voltage should be increased to 1.7v-1.8v per section, and the apcsymmetria can adjust the charging voltage according to the load.

d: If the battery is not recharged within 72 hours after discharge. Sulfate will adhere to the electrode plate for insulation charging, causing damage to the battery.

e: When the battery is float charged or evenly charged, the gas generated inside the battery is electrolyzed into water on the negative electrode plate, thereby maintaining the capacity of the battery without the need for external water. But the corrosion of battery plates will reduce the battery capacity.

f: The lifespan of the battery separator is only 5-6 months at an ambient temperature of 30-40 degrees Celsius. Batteries stored for a long time should be charged every 6 months. The battery should be stored in a dry and cool environment. The self discharge rate of maintenance free batteries in a 20 degree environment is 3-4% per month and varies with temperature.

g: Maintenance free batteries are equipped with safety valves. When the internal pressure of the battery rises to a certain level, the safety valve can automatically eliminate excess gas. When the internal pressure is restored, the safety valve will automatically recover.

h: The cycle life (number of charge and discharge cycles) of a battery depends on the discharge rate, depth of discharge, and the method of restorative charging, with the factor being the depth of discharge. When the discharge rate and time are constant, the shallower the discharge depth, the longer the battery cycle life. The maintenance free battery has a cycle life of about 200 times under 25 degrees and 100 deep discharge conditions.

i: When the battery reaches its lifespan, it exhibits capacity decay, internal short circuit, shell deformation, plate corrosion, and reduced open circuit voltage.

j: IEEE defines the end of battery life as a capacity that is less than 80% of the nominal capacity AH. The relationship between nominal capacity and actual backup time is non-linear, and a 20% reduction in capacity will result in a significant decrease in backup time. Some UPS manufacturers define the end of battery life as a decrease in capacity to 50-60% of the nominal capacity.

k: It is prohibited to mix batteries of different capacities and manufacturers, otherwise it will reduce the battery life.

l: If two sets of batteries are used in parallel, the batteries should be connected and the busbar impedance should be the same.

m: Maintenance free batteries mean that there is no need to add liquid, but regular inspections of the casing for cracks and electrolyte leakage are also necessary.

3: Nickel chromium battery

This type of battery is different from lead-acid batteries, as it produces hydrogen and oxygen during electrolysis without producing corrosive gases, making it suitable for installation next to electronic devices. And the consumption of water is minimal, generally requiring no maintenance. The normal lifespan is 20-25 years. Far more expensive than the batteries mentioned earlier. The initial installation cost is about three times that of lead-acid batteries. It will not affect battery life due to high ambient temperature, nor will it affect battery capacity due to low ambient temperature. Generally, the voltage per cell is 1.2V, and UPS requires a higher charger voltage to be designed for the use of such batteries.

2、 Advantages and disadvantages:

1. Open type lead-acid battery:

Advantages: Less investment, longer lifespan of maintenance free batteries, and lower temperature requirements.

Disadvantages: Maintenance is complex and requires a dedicated battery room, with corrosive gases emitted. The initial charging time on site is 50-90 hours, and a dedicated person is needed for maintenance.

2: Maintenance free battery:

Advantages: No need for maintenance such as adding liquid, can be transported in a fully charged state, and does not require dedicated maintenance.

Disadvantages: Failure to recover charging in a timely manner can damage the battery, be sensitive to temperature, have a shorter lifespan, and are more expensive than lead-acid batteries.

3: Nickel chromium battery:

Advantages: Low maintenance requirements, long lifespan, insensitivity to temperature, and no harmful gas emissions.

Disadvantage: The three types of batteries are relatively expensive.

3、 UPS common batteries

Most computer centers nowadays use maintenance free batteries, which are easier to maintain, but the following tasks also need to be carried out:

1: Discharge once every three to four months to prevent electrode oxidation.

2: The ambient temperature should be maintained at 20-25 degrees Celsius.

3: The connection should not be too tight or too loose and should be checked regularly.

4: After three years of use, it is necessary to check and replace it in a timely manner.

There are various forms of interference on the public power grid. Except for noticeable power outages, the vast majority of interferences are not easily noticeable. However, it is precisely this imperceptible interference that poses a serious threat to the normal operation of electrical and electronic devices. For example, the interference induced by lightning on the power grid can cause a voltage of over 20000 volts, which can burn out electrical equipment on the grid.

The interference of high-order harmonics on the neutral line can seriously affect the operation of high-frequency communication equipment, causing misoperation of digital circuits and resulting in serious consequences such as communication interruption and system data loss.

Traditionally, power grid interference is divided into the following categories:

1、 Low frequency interference.

A. Overvoltage: Voltage continuously exceeding 10% of the rated value.

B. Undervoltage: Voltage continuously below 15% of rated value.

C. Power outage: Power interruption exceeding 300ms.

D. Interruption: Power supply interruption of less than 300ms.

E. Surge: Voltage exceeding 10% of the rated value for a duration of 1 to several cycles.

F. Frequency drift: ± 2% of the normal frequency offset.

2、 High frequency interference.

A. Peak: Short term overvoltage several times higher than the rated voltage, sometimes up to several thousand volts, with a duration of milliseconds.

B. Burr: Transient overvoltage several times higher than the rated voltage, sometimes up to tens of thousands of volts, with a duration of microseconds.

C. High order harmonics: Distortion of the power grid waveform caused by the nonlinearity of the load.

D. The main causes of low-frequency interference are: the on/off of large electrical appliances; Excessive changes in power grid load (overload or light load); Load short circuit, etc.

The main reasons for high-frequency interference are:

Nonlinear loads powered by the power grid; Radiation generated by equipment operating in high-frequency mode; Thunder and lightning; Electrical equipment on/off, etc. How to eliminate the impact of various interferences on electrical equipment and provide high-quality and pure power supply for electrical equipment? The current common practice is:

A. Enable UPS to have voltage and frequency stabilization functions, eliminating the effects of high or low voltage and frequency drift.

B. UPS comes with a built-in battery pack, which solves the problems of power grid failures and power outages.

C. Use harmonic filters to effectively filter out high-order harmonics.

D. Use radio frequency interference (RFI) filters to eliminate radio frequency interference.

E. Adopt good shielding measures.

(1) The EVADA UPS adopts DSP control technology for dual isolation online operation, which enables it to have stable voltage, frequency and anti-interference performance. The input voltage range of the IGBT rectifier can be as wide as ± 25% (usually ± 15% for UPS), and the output voltage is within the rated value within this range. Thus solving the problem of input voltage being too high or too low.