Power quality specifications

The technical indicators for measuring power quality mainly include: voltage fluctuations, frequency fluctuations, harmonics, and three-phase imbalance. It is known that the quality of the power supply is affected by many factors, such as changes in the load, the use of a large number of nonlinear loads, and high The influence of the sub-harmonics, the input and cut-off of power factor compensation capacitors, lightning and man-made faults, and public facilities all affect the quality of the power supply, thereby reducing the quality of the power supply.

1.1 Voltage fluctuations

The waveform of the ideal supply voltage sine wave is continuous, smooth, and has no distortion. Its amplitude and frequency are stable. When the load changes, the load increases greatly, especially when a large equipment in the vicinity is in startup. The amplitude of the power supply sine wave is affected, resulting in a low voltage. When the voltage fluctuation of the power supply exceeds the allowable range, it will cause the computer and precision electronic equipment to operate in error, and even cause the power outage detection circuit of the computer to be mistaken for a power outage. The power failure processing signal will affect the normal operation of the computer. The general computer allows the voltage fluctuation range to be: ac380v, 220v ± 5%. The computer is considered as interrupted when the voltage is reduced to 70% of the rated voltage. For this purpose, the "computer The design specification of the engine room "gb50174-93 specifies the voltage fluctuations and divides the voltage fluctuation into three levels: a, b, and c.

Voltage fluctuation level table 1

Voltage Class a Class b Class c

Fluctuating range ± 2% ± 5% + 7% ~ -13%

1.2 Frequency fluctuations

The frequency fluctuation of the power supply is mainly caused by the change of the generator speed due to the overloaded operation of the power grid. The external equipment of the computer mostly uses synchronous motors. The allowable frequency fluctuation range of the computer is generally 50hz±1%. When the power supply frequency fluctuates beyond the allowable range. At this time, the frequency of computer storage will change, resulting in errors and even loss. The "Specifications" clearly stipulates frequency fluctuations, and classify frequency fluctuations into three levels: a, b, and c.

Frequency fluctuation level table 2

Frequency Class a Class b Class c

Fluctuating range ± 0.2% ± 5% + 7% ~ -13%

1.3 Wave distortion

The main cause of distortion of the power supply voltage waveform is due to the non-linear load in the power grid, especially the presence of some high-power controllable rectifier devices can generate hydrocarbons on the voltage waveform of the power supply, and can also adversely affect the relative control part of the computer. This kind of waveform distortion will also make the current on the filter capacitor in the computer's DC power supply circuit significantly increase, and the capacitor will heat up; due to the appearance of the saw-like waveform, the power failure detection circuit of the computer will be mistaken for power failure, and the power outage processing will be issued. Signals, affect the normal operation of the computer. The technical indicator for measuring waveform distortion is the waveform distortion ratio, that is, the ratio of the sum of all higher harmonics of the AC voltage at the input of the power equipment to the rms value of the fundamental wave. Rate rules are divided into a, b, c three levels

Waveform distortion rate table 3

Waveform distortion level a b grade c

Distortion rate 3-55-88-10

1.4 Transient Surge and Transient Falling

Transient surge refers to the rapid increase of the sine wave amplitude of the power supply voltage within one or several weeks of the power frequency. The transient surge is generally expressed by the maximum transient rate. The transient fall, also called the notch, is Refers to the sine wave in the power frequency of a week or a few weeks, the power supply voltage sinusoidal amplitude rapid decline. Transient decline is generally represented by the maximum instantaneous rate of decline. Transient surge and transient decline, instantaneous voltage amplitude rapid increase Or reduce the interference to the computer system, resulting in its operation error or damage the stored data and programs. At present, the domestic transient transient transient rate: ≤ 20%; recovery process dropped to less than 15%, is 50ms; Then within 6%, it is 0.5s. The maximum transient reduction rate is allowed to be ≤30%; Within -20%, it is 50ms; Within -13.3%, it is 0.5s.

1.5 transient pulse

Transient pulse, also known as spike or voltage flicker, refers to a narrow pulse superimposed on the ideal sine wave of the power grid within a period of less than one half of the power grid. There are many causes of transient pulses. Generally, the following are the main factors:

1.5.1 Internal Overvoltage

That is, within the power system, due to the input and removal of heavy loads, inductive loads, compensation capacitors, the operation of switches and fuses, and the occurrence of short-circuit faults, the system parameters will change, causing the transformation of the internal electromagnetic energy of the power system. Transmission, overvoltage in the system. According to statistics, in the entire transient pulse accident due to internal overvoltage caused by 80%.

1.5.2 Thunder

In the range of 1.5km to 2km in the thunder and lightning center, dangerous overvoltage may be generated and the equipment on the circuit may be damaged. When a lightning strike power line or a thunder and lightning strike occurs near the line, a transient overvoltage will be formed by direct or indirect coupled lightning flash discharge. Propagation along the route in the form of flowing waves endangers the safety of the equipment. According to statistics, overvoltages caused by lightning strikes account for about 18% of the total transient pulse accidents.

The signal voltage of computers and precision instruments and equipment is very low, generally only about 10v, so it is extremely sensitive to the lightning pulse overvoltage, and is easily subject to lightning pulse overvoltage interference and damage. The general electrical equipment allows the lightning pulse voltage is 6,000V, and Computers and precision instruments and equipment are estimated to be damaged in the tens of volts to several hundred volts.

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