Causes and prevention of voltage sag

1. What is a voltage sag

Voltage sag, also known as voltage sag or voltage dip, refers to a sudden drop or almost complete loss of the effective value of the power supply voltage, and then rises back to near the normal value. Voltage sag is generally described in terms of retention voltage and duration. 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 within a short period of 1 minute. working status. picture

2. The danger of voltage sag

1. Less impact on public users

Voltage sag has little impact on many users, especially those in public and civil buildings, and some are not even aware of its occurrence. Since the occurrence time is very short, a special monitoring instrument needs to be installed on the point network, otherwise it will be difficult to judge whether the voltage sag has occurred.

2. Great influence on IT and semiconductor industry

Some users and facilities that are very sensitive to voltage dips (such as the semiconductor industry, electronic numerical control equipment, variable speed drive motor devices, IT industry equipment companies, etc.) will suffer huge losses once a voltage dip occurs.

   (1) Huge impact on the information industry

Malfunction of automatic control devices, failure of computer systems, etc.

   (2) Harm to sensitive mechanical equipment Harm to DC generators. When the voltage is lower than 80% of normal, a circuit trip accident is very likely to occur.

For PLC controllers, every voltage dip or circuit short circuit will cause control program disorder.

For variable frequency speed regulators, when the voltage continues to be lower than 70% within 120s, it will be taken out of operation.

3 has risen to the most important power quality issue

Each voltage sag or voltage fluctuation will cause economic losses to the national economy ranging from 100,000 to 200,000 yuan to as much as millions.

For sensitive users, more than 90% of power quality problems are caused by voltage sag. Therefore, voltage sag has been considered as one of the most important power quality problems affecting the normal and safe operation of such electrical equipment.

According to statistics, in Europe and the United States, power departments and users are much more concerned about voltage sag than other power quality issues. One of the important factors is that among the many reasons related to power quality, user complaints caused by voltage sag account for more than 80% of complaints about the entire power quality problem, while power quality problems caused by harmonics, switching operation overvoltage, etc. Quality problem complaints accounted for less than 20%. Experts believe that voltage sag has become the most important power quality issue and has become a new challenge to power supply quality posed by the information society. picture

3. Voltage dips are unavoidable

1. No matter how high the power supply reliability is, it cannot be avoided

There are many reasons for voltage sag, such as unexpected lightning strikes on the power system. Accidental contact by animals and foreign objects, line failures caused by strong winds and other natural environmental factors, and phenomena caused by power failure switching, etc. The instantaneous power outage or voltage sag caused by all these faults cannot be completely avoided in the current power system, and the probability of voltage sag occurring is much higher than that of a complete power outage. It is understood that foreign power users. In particular, some enterprises that have relatively high requirements for power quality require power companies to improve power supply reliability in the early stages of voltage sag problems. This approach greatly increased the initial investment cost, but the problem of voltage sag has never been effectively solved. With a power supply reliability of 99.99999% (this is the ultimate goal of the power supply company, it is difficult to achieve such a high reliability in actual power supply), calculated based on 365 days a year, the cumulative power outage time per year is only a few seconds. However, despite this high reliability, voltage sag can still occur up to 10 times a year or so. Although the investment cost is large, the suppression effect on voltage sag is not good.

2. Different governance schemes need to be implemented in a multi-pronged manner

Through years of comprehensive investigation and analysis of power quality issues (especially those caused by voltage sag), sensitive power users have begun to realize that blindly requiring power companies to improve power supply reliability from the power supply side is of little significance in solving voltage sag. As a result, the focus is gradually placed on requiring equipment manufacturers to provide equipment with good resistance to voltage sag, and strives to address the impact of power sag from sensitive equipment within enterprise users.

Through continuous research and gradually deepening the understanding of power quality issues, international standards and industry standards for power quality have begun to be formed, which uniformly bind power companies, equipment manufacturers and manufacturing enterprises. picture

4. Treatment of voltage sags

1. Cost issues of different solutions

In theory, power quality can be improved at any level of equipment, but the cost and time vary. The figure below shows the cost of addressing voltage sags at different levels.

Costs of different governance options

As can be seen from the figure, the management cost of voltage sag drops by an order of magnitude from the control of the power supply source to the control of the entire production line, the equipment level, and then the equipment control level. Therefore, in terms of cost and effectiveness of treatment, it is more advocated to carry out treatment at the end of the equipment, or even to deal with the electrical control components deep inside the equipment. The closer to the end of the treatment, the lower the cost and the same treatment effect can be achieved.

2. Measures taken by the power supply system

1. Reduce the frequency of voltage sag and reduce circuit troubleshooting time. Reducing the time for circuit troubleshooting means reducing the duration of voltage sag, reducing the impact of voltage sag on machinery and equipment, thereby reducing the economic losses caused. In order to reduce troubleshooting time, static short circuiters and current-limiting fuses are generally used in circuits.

2. Change the system design of the circuit network to avoid or reduce voltage disturbance. This measure mainly involves changing the wiring, using special transformers, such as K-type transformers, to improve the performance of the equipment, thickening the wires, using sensitive anti-interference protection measures for sensitive equipment, and using independent return lines to reduce the grounding resistance. Arrester configuration and parameters are improved, etc.

3. Measures taken on the user side

1. Improve the equipment’s ability to withstand voltage dips. Using a motor generator (MG), the inertia of the motor can be used to keep the voltage of the generator stable when a voltage drop occurs. The circuit transformer uses a magnetic resonance transformer (CVT). When the circuit voltage drops to 70% of the normal voltage, it can still It can provide stable voltage. The only disadvantage is that it is slightly larger than an ordinary transformer.

2. Install a voltage compensation device in the circuit. The most typical installation of voltage compensation devices in circuits is the installation of dynamic voltage restorers (DVR) and uninterruptible power supplies (UPS). The UPS grid voltage outputs a 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 power grid is under-voltage or suddenly loses power, the UPS power supply starts to work, 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 UPS can effectively solve the problems of voltage sag and short-term power supply interruption by rectifying the grid voltage and directly supplying power to the load. In addition, all the power of the load must be converted and provided by the UPS, which increases system losses and reduces efficiency.

DVR connects the DVR in series to the sensitive load. When a voltage sag occurs, the DC-AC inverter with pulse width modulation function will synthesize a voltage with controlled amplitude, frequency and waveform. Added to the line voltage, it can respond to voltage dips within 1/4 cycle, that is, increase the output voltage to the voltage level required by the system. The reference source of the PWM modulation wave of the inverter 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 that provides the voltage boost is supplied by DC capacitors.

DVR is currently the most popular device at home and abroad for solving voltage sag. Although the DVR runs in series in the line, since it only needs to compensate the energy of the voltage sag, its design power is only 1/5~1/3 of the full capacity of the load, the price is better than that of UPS with the same capacity, and the loss is far Lower than the latter, it has significant technological advancement.

With the increasing scale of the power system, users' demand for electric energy is increasing at a high speed, and the requirements for power quality are constantly improving, so as to reduce the occurrence of voltage dips in the power system requires the joint efforts of power supply companies and users. In the future electricity market, users may consider not only lower electricity prices but also better power quality when choosing electricity suppliers.

HZGJC Computer Controlled Voltage Breakdown Tester