Power Quality: Analysis of Voltage Sag Problems

Electricity is now widely used by people. Electric energy has become the most common energy used by people. Electric energy is also an important symbol of a country's comprehensive national strength and development level. Ensuring the stable supply of electric energy is related to people's normal life and production, the operation of the national economy and the safety of power grid electrical equipment. The problem of power quality has attracted widespread attention, and the quality standards for electric energy have been promulgated one after another. However, compared with developed countries such as Europe, the United States and Japan, we still have a certain gap. There has been no systematic research on the voltage problem, and the understanding of electric energy still remains at the traditional voltage and frequency deviation.

The voltage drop in the power distribution system is a common voltage disturbance phenomenon. No matter how to improve the reliability of the power system, the voltage drop phenomenon still exists. For example, the startup of large generators, various short-circuit faults in the circuit, circuit protection action caused by lightning strikes, the operation of switching switches, etc. These phenomena are very likely to cause the circuit voltage to drop. With the development of science and technology and information technology, people are paying more and more attention to the circuit voltage drop.

1. Causes of voltage sag

One of the important indicators of electric energy is voltage sag. Voltage sag is an important component of electric energy indicators. The definition of voltage sag in the Institute of Electrical and Electronics Engineers is that the power frequency effective value of the voltage in the power supply system suddenly drops to 10%-90% of its rated value, and then returns to normal working state within a short period of 1 minute.

There are many factors that cause voltage sags, the main reasons are: when high-power electrical equipment is suddenly started or loaded, the power system network is struck by lightning, the circuit is short-circuited, the circuit protection measures fail to detect, etc. Compared with power line outages, voltage sags occur more frequently and are difficult for people to detect, so they are relatively difficult to handle.

2. Hazards caused by voltage sag

Since voltage sag occurs frequently, it is the main factor affecting the power quality of the power system, which can affect every power user in the power system. The hazards of power sag in the power system are mainly: first, it has a huge impact on the information industry, malfunction of automatic control devices, failure of computer systems, etc. Second, it causes harm to sensitive mechanical equipment, such as the harm to DC generators. When the voltage is lower than 80% of the normal level, it is very likely to cause a circuit trip accident. For PLC controllers, each voltage sag or circuit short circuit will cause control program disorder. For variable frequency speed regulators, when the voltage is continuously lower than 70% within 120s, it will be shut down. Fourth, it affects people's normal production and life. Fifth, each voltage sag or voltage fluctuation will cause economic losses of 100,000 to 200,000 yuan to the national economy, and up to millions of yuan.

3. Measures to solve voltage sag

From the above, we can see that voltage sag brings many harms to our lives, so measures must be taken to solve the losses caused by voltage sag. The following measures are generally taken to solve the problem of voltage sag:

(I) Reduce the frequency of voltage sags and reduce the time for circuit troubleshooting. Reducing the time for circuit troubleshooting means reducing the duration of voltage sags and reducing the impact of voltage sags on machinery and equipment, thereby reducing the economic losses caused.

In order to reduce the time of troubleshooting, static circuit breakers and current limiting fuses are generally used in circuits.

(ii) Change the system design of the circuit network to avoid or reduce voltage disturbances. This measure mainly involves changing the wiring, using special transformers, such as K-type transformers, improving the performance of equipment, thickening the wires, using sensitive anti-interference protection measures for sensitive equipment, using independent return lines, reducing grounding resistance, and improving the configuration and parameters of lightning arresters.

(III) Improve the ability of circuit equipment to resist voltage sags. For example, using a motor generator set (MG), the inertia of the motor can keep the generator voltage stable when a voltage sag occurs. The circuit transformer uses a magnetic resonance transformer (CVT), which can still provide a stable voltage when the circuit voltage drops to 70% of the normal voltage. The only disadvantage is that the volume is slightly larger than that of an ordinary transformer.

(IV) Install voltage compensation devices in the circuit. The most typical voltage compensation devices installed in the circuit are dynamic voltage recovery (DVR) and uninterruptible power supply (UPS). The grid voltage outputs DC voltage through the AC-DC inverter, supplies the DC-AC inverter, and outputs a stable AC voltage to supply the load. At the same time, the grid voltage charges the energy storage battery. When the grid is undervoltage or suddenly loses power, the UPS power supply starts working, and the energy storage battery supplies the power required by the load to maintain normal production. Due to production needs, when the load is seriously overloaded, the grid voltage is rectified and directly supplied to the load. UPS can effectively solve problems such as voltage drops and short-term power outages. In addition, all the power of the load must be provided after conversion by the UPS, which increases system losses and reduces efficiency.

When the DVR is connected in series to a sensitive load and a voltage sag occurs, the DC-AC inverter with pulse width modulation function will synthesize a voltage with controlled amplitude, frequency and waveform, and add this voltage to the line voltage through a series-connected step-up transformer, which can respond to the voltage sag within 1/4 cycle, that is, increase the output voltage to the voltage level required by the system. The reference source of the inverter's PWM modulation wave is a standard sine wave. By collecting the voltage waveform and comparing it with the standard wave, the voltage harmonics can be effectively compensated. The energy for providing voltage boost is supplied by the DC capacitor.

DVR is the most popular device for solving voltage sag at home and abroad. Although DVR is connected in series in the line, it only needs to compensate for the energy of voltage sag, so its design power is only 1/5~1/3 of the total capacity of the load. Its price is better than UPS of the same capacity, and its loss is much lower than the latter, which has significant technological advancement.

IV. Conclusion

From the above, we can see that voltage sag is a particularly prominent problem in power quality issues. The key to reducing power system losses is to be able to quickly and timely carry out effective control and processing when voltage sag occurs. The voltage quality detection network should be continuously improved and perfected, the comprehensive detection capability of power quality should be improved, and the correct understanding of voltage sag issues by power users should be improved. Power users who are more sensitive to power sag issues should help improve circuits and other measures to reduce the harm caused by voltage sag. Reducing voltage sag is not only of great significance to the safety of the power grid, but also of great significance to the country's economic operation.

References:

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