CN108493944B - Public network/isolated network combined operation phase shift angle optimization distribution method based on genetic algorithm - Google Patents

Abstract

A public network/isolated network combined operation phase shift angle optimization distribution method based on a genetic algorithm belongs to the field of power generation. The method comprises the following steps: designing a fitness function; randomly initializing various rectifier unit phase shift angle combinations and ensuring that the initialization value is in an actual operation range; respectively calculating fitness function values under various corresponding harmonic value combinations, selecting the individuals with the superiority from the group, eliminating the inferior individuals, and performing selection operation; performing cross operation on the combination reserved in the previous step, and replacing and recombining partial structures of two parent individuals to generate a new individual; and repeating the steps until the fitness function value is converged to obtain the optimal phase shift angle combination and ensure the minimum harmonic quantity. By adopting the method, the analysis of the distribution mode of the most even phase shift angle of the rectifier unit in the isolated network/public network combined operation state can be realized, the minimum harmonic component is ensured, and the influence on the public network/isolated network related electrical equipment is reduced.

Description

Public network/isolated network combined operation phase shift angle optimization distribution method based on genetic algorithm

Technical Field

The invention relates to the field of power generation, in particular to a public network/isolated network combined operation phase shift angle optimization distribution method.

Background

In an ideally pure power system, both current and voltage are pure sine waves. In practice, when current flows through a load that is not linearly related to the applied voltage, a non-sinusoidal current is formed. In a simple circuit containing only linear components (inductance, capacitance and resistance), the current flowing is proportional to the applied voltage, so if the applied voltage is sinusoidal, the current flowing is sinusoidal. It should be noted that in the case of reactive components, the power factor becomes lower when there is a relative shift in the phase of the voltage and current, but the line is still linear. Any periodic waveform can be divided into a fundamental frequency sinusoid plus a number of harmonics of the frequency. The harmonic frequencies are integral multiples of the fundamental frequency.

The metallurgical electrolytic large-current DC power supply consists of AC power supply, rectifier transformer and silicon rectifierObtained by rectifying means. When the rectifier is connected to a sine voltage power grid to operate, all arms of the rectifier are conducted in turn, and the current waveform passing through the rectifier arms is similar to a rectangular wave, so that the current waveform in each phase coil of the rectifier transformer is not a sine wave, and the waveform contains harmonic current except fundamental current. When phase control (such as thyristor rectification) is performed, the larger the control angle is, the larger the fluctuation of the current is, and the larger the harmonic component in the network side current is. Assuming that the on and off of the silicon rectifier element or the thyristor are instantaneous, the arm current of the rectifier device is rectangular wave, the valve side line current is also rectangular wave, and the network side current waveform has 2 types of rectangular wave and step wave according to the difference of the rectifier device and the connection line. Both rectangular waves and step waves can be decomposed into fundamental and harmonic components by Fourier series, having

And even harmonic components and direct current components are not contained in the harmonic waves, so that the rectifying device is a 1 odd harmonic source and can change the waveform of the network side.

The harmfulness of the harmonics is embodied in the following aspects:

(1) harmonics due to power capacitors are amplified. The capacitance reactance of the capacitor under the action of harmonic voltage is much smaller than that under the action of fundamental voltage due to the intrinsic characteristic of the capacitor, so that the waveform distortion influence of harmonic current is larger than that of harmonic voltage. A very small harmonic voltage can cause a very large harmonic current if the capacitor is also resonating with other components in the grid, resulting in damage to the capacitor due to overcurrent.

(2) Increasing the losses of the rotating electrical machine. Causing additional losses and overheating, followed by mechanical vibrations, noise and harmonic overvoltages. These shorten the life of the motor and in severe cases can damage the motor.

(3) The loss of the transmission line is increased, and the service life of the transmission line is shortened. Harmonic current generates harmonic voltage drop on the transmission line and increases the effective value of the current on the transmission line, thereby causing additional transmission loss. Under the condition of cable transmission, the harmonic wave can cause additional loss and also can make voltage waveform generate peak so as to accelerate the aging of cable insulation, cause partial discharge of impregnated insulation, increase dielectric loss and temperature rise, and shorten the service life of the cable by the harmonic wave treatment and reactive compensation technology and application of the low-voltage large-current rectifier.

(4) Increasing the losses of the transformer. The main effect of the harmonic current flowing into the transformer is to increase its copper losses. Under the action of higher harmonic voltage, the skin effect and proximity effect are more serious, additional copper loss is caused in the winding, and iron loss is correspondingly increased. For a transformer with an asymmetric load, if a load current contains a direct current component, the magnetic circuit of the transformer is saturated, and thus the harmonic component of an alternating excitation current is greatly increased.

(5) Causing the disorder of relay protection and automatic device. The presence of harmonics can alter the behavior of the protective relay, depending on the design characteristics and principles of the relay. Digital relays that operate on sampled data or zero crossings are prone to error when there is distortion in the current or voltage. The harmonic wave can have the influence of refusing action and misoperation on relays such as overcurrent, undervoltage, distance, periodic wave and the like, and the protection device is out of order and unstable in action.

(6) Causing errors in the power measurement. The common measuring instrument is verified under the condition of pure sine wave, and if the waveform of power supply is distorted, the instrument is easy to generate errors. For example, an inductive power meter is insensitive to response to frequencies other than the design parameters, the higher the frequency the greater the error, and the negative error, the meter will stop rotating when the frequency is about.

(7) Interfering with the communication system. The interference of the balanced current in the power grid to the communication system is not large, but if the current is unbalanced current or unbalanced harmonic current, the interference to the communication system is very large, the harmonic interference causes noise of the communication system, the quality of the call is reduced, and when the interference is serious, the loss of signals can be caused, even personal safety accidents can be caused.

(8) Arc extinction is delayed. On the ultrahigh-voltage long-distance power transmission line, the arc extinction can be delayed by larger harmonic current, so that the single-phase reclosing failure is caused, and further the accident is enlarged. The larger harmonic components in the arc suppression coil grounding system also delay or impede the arc suppression action of the arc suppression coil. The harmonic component can also cause the value when the current passes through zero to be overlarge, so that the arc breaking of the circuit breaker is difficult, and the current breaking capacity is influenced.

Besides using two indexes of frequency and voltage, the harmonic wave inevitably becomes another important index for measuring the power quality. Therefore, the problem of harmonic waves can be effectively solved from the perspective of guaranteeing safe, stable and economic operation of the power system and the perspective of safe and normal operation of the user electric equipment, and the harmonic waves are controlled within an allowable range.

Disclosure of Invention

The method aims at the situation that the harmonic content in the isolated network system exceeds the standard, and then the insulation aging and damage of electrical equipment, protection devices and the like in the operation of the isolated network system are caused, so that protection misoperation and the like are caused, and the operation safety of the isolated network system is influenced. The method has the defects that a public network/isolated network combined operation phase shift angle optimization distribution method based on a genetic algorithm is provided.

The technical scheme of the invention is as follows:

in order to solve the problem of matching of the phase shift angles of the rectifier units in a point-to-point operation mode of a power plant and an electrolytic aluminum plant, the invention explores an optimal distribution method of the phase shift angles of the rectifier units in a genetic algorithm optimization mode.

The phase shifting angle optimization allocation method comprises the following specific steps:

step 1: and designing a target optimization value of the fitness function as a harmonic generation amount, wherein the fitness function is a harmonic value corresponding to different phase shift angle distribution modes.

Step 2: and randomly initializing various rectifier unit phase shift angle combinations and ensuring that the initialization value is in an actual operation range.

And step 3: and respectively calculating fitness function values under various corresponding harmonic value combinations, selecting the individuals with the superiority from the group, eliminating the inferior individuals, and performing selection operation. Roulette selection is used, i.e. the selection probability of each individual is proportional to its fitness value, leaving those combinations with smaller harmonic values.

And 4, step 4: and (3) performing cross operation on the combination retained in the step (3), and performing replacement and recombination on partial structures of the two parent individuals to generate a new individual. The adopted crossover operator is single-point crossover, namely a crossover point is randomly set in an individual string, and when the crossover is carried out, the structures of two individual parts before or after the crossover point are interchanged to generate a new combination.

And 5: and (4) repeating the step (3) and the step (4) until the fitness function value is converged, so as to obtain the optimal phase shift angle combination and ensure the minimum harmonic quantity.

The invention has the beneficial effects that: by adopting the method, the analysis of the distribution mode of the most even phase shift angle of the rectifier unit in the isolated network/public network combined operation state can be realized, the minimum harmonic component of the public network/isolated network is ensured, the influence on the electric equipment related to the public network/isolated network is reduced, and the equipment maintenance cost caused by harmonic waves is reduced.

Drawings

Fig. 1 is a schematic diagram of the isolated network harmonic content when different rectifier units are matched in an isolated network operation mode.

Detailed Description

In order to ensure the current on the direct current side to be stable, 7 rectifier units are arranged in an electrolytic aluminum plant, and 6 transformers adopt a winding mode of an edge triangle during design, and each rectifier unit is modulated once to shift the phase, so that 84 pulse waves on the direct current side are uniformly distributed. Under the condition that 3 rectifier units operate in isolated network and 4 rectifier units operate in public network, the 1# rectifier unit +4.285 degrees, 2# rectifier unit-4.285 degrees, 3# rectifier unit +12.875 degrees, 4# rectifier unit-12.875 degrees, 5# rectifier unit +8.751 degrees, 6# rectifier unit-8.751 degrees and 7# rectifier unit 0 degrees are obtained through the algorithm, and as can be seen from the graph 1, under the condition that two rectifier unit filtering devices are put into operation, the harmonic content of the 5#, 6# and 7# rectifier units is obviously lower than the harmonic content of the 1#, 2# and 7# rectifier units during isolated network operation. The fact that the harmonic content in the isolated network system can be reduced by the distributed rectifier unit phase shifting angles during isolated network operation is proved.

Claims (1)

1. A public network/isolated network combined operation phase shift angle optimization distribution method based on a genetic algorithm is characterized by comprising the following specific steps:

step 1: designing a target optimization value of a fitness function as a harmonic generation amount, wherein the fitness function is a harmonic value corresponding to different phase shift angle distribution modes;

step 2: randomly initializing various rectifier unit phase shift angle combinations and ensuring that the initialization value is in an actual operation range;

and step 3: respectively calculating fitness function values under various corresponding harmonic value combinations, selecting the individuals with the superiority from the group, eliminating the individuals with the inferior quality, and performing selection operation by adopting a roulette selection method, namely, the selection probability of each individual is in direct proportion to the fitness value thereof, and keeping the combinations with the smaller harmonic values;

and 4, step 4: performing cross operation on the combination retained in the step 3, replacing and recombining partial structures of two parent individuals to generate a new individual, wherein the adopted cross operator is single-point cross, namely, a cross point is randomly set in an individual string, and when the cross is performed, the partial structures of the two individuals before or after the point are interchanged to generate a new combination;

and 5: and (4) repeating the step (3) and the step (4) until the fitness function value is converged, so as to obtain the optimal phase shift angle combination and ensure the minimum harmonic quantity.

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