CN103887805B - The asymmetrical voltage control method of small current neutral grounding system - Google Patents

Abstract

An asymmetrical voltage control method for an ungrounded neutral point or grounded power system through an arc suppression coil can control the asymmetrical voltage caused by the unbalanced three-phase parameters of the line to approach zero. According to the constraint relationship between the system asymmetric voltage and the power frequency zero-sequence current injected through the system neutral point or any one of the three phases, when the system parameters are known, the method can directly calculate the injected current to control the asymmetric voltage to Zero; when the system parameters are unknown, starting from the injection current of any amplitude and phase, first changing the phase of the injection current, and then changing the amplitude of the injection current can make the asymmetrical voltage approach zero. The method can adapt to the change of the line parameter structure, can flexibly and effectively control the asymmetrical voltage of the small current grounding system, maintain the three-phase voltage balance of the power grid, and effectively improve the safety and economy of the power grid operation.

Description

Asymmetry voltage control method for small current grounding system

technical field

The invention belongs to the field of three-phase voltage balance control of electric power system, and relates to an asymmetrical voltage control method of a small current grounding system.

The invention particularly relates to a control method for controlling the asymmetrical voltage of a small current grounding system to approach zero by using an active compensation technology.

Background technique

my country's medium-voltage distribution network widely adopts the low-current grounding method, that is, the neutral point is not grounded and the neutral point is grounded through the arc suppression coil. Due to the influence of factors such as overhead line manufacturing process, actual construction, and poor transposition, the three-phase-to-ground capacitance of the general power grid is not equal to each other. In addition, the power grid sometimes uses non-full-phase power supply to supply power to single-phase loads, which will also cause the asymmetry of the three-phase ground capacitance of the distribution network. For the medium-voltage distribution network using the low-current grounding method, the neutral point will generate a certain value of ground voltage. In the arc-suppression coil grounding system, the inductance of the arc-suppression coil and the three-phase-to-ground capacitance of the grid form a voltage resonance circuit, which will also increase the neutral point voltage and may seriously damage the balance of the three-phase voltage.

Although the three-phase voltage imbalance caused by the zero-sequence voltage will not affect the normal power supply of users, it will cause many harms to power lines, transformers, transformers, lightning arresters and other equipment, and at least it will reduce the efficiency and performance of electrical equipment If it is serious, it will shorten the service life of electrical equipment or cause damage accidents, and reduce the safety and economic performance of power grid operation, which must be limited.

"Overvoltage Protection and Insulation Coordination of AC Electrical Installations" (DL\T620-1997) stipulates that under normal operating conditions, the long-term voltage displacement of the neutral point shall not exceed 15% of the rated phase voltage of the system. %. Traditionally, the methods to limit the asymmetric voltage of the system are mainly line transposition, connecting coupling capacitors, tuning arc suppression coils and appropriately increasing their damping rate, etc. However, these methods cannot effectively limit the asymmetric voltage due to the disadvantages of difficult construction, increased equipment investment, and inability to adapt to changes in line parameters and structures. At present, there is still a lack of flexible and effective asymmetric voltage control methods in this field.

Active compensation technology based on active power electronic devices and pulse width modulation technology controls the zero sequence voltage of the system by injecting zero sequence current into the distribution network. This technology has been applied in the single-phase ground fault arc suppression of the neutral point non-effectively grounded distribution network. It can compensate the residual current at the fault point, improve the probability of arc extinction, and control the recovery speed of the voltage at the fault point after arc extinction. Prevent arc reignition. Its superior zero-sequence voltage control performance provides an idea for seeking new asymmetric voltage control principles.

Contents of the invention

The technical problem solved by the invention is: the asymmetrical voltage control technology of the small current grounding system. When the low-current grounding system has a large voltage (ie asymmetrical voltage) at the neutral point due to insufficient line transposition, non-full-phase power supply, etc., it can flexibly and effectively eliminate the asymmetrical voltage of the system and maintain the three-phase voltage of the system balance.

The technical solution adopted by the present invention to solve the technical problem is: to inject a power frequency zero-sequence current with a value and a phase control into the system through the neutral point of the system or any one of the three phases, so as to control the asymmetrical voltage of the system to approach zero and maintain The three-phase voltage of the system is balanced. The process is:

1. When an asymmetrical voltage is detected in the system, the initial injection current is injected into the system immediately (amplitude is I ₁ , phase is ). Selection method of initial injection current:

The initial injection current It can be a current with any amplitude and phase, or it can be manually preset to a fixed current. In addition, according to theory, the final injected power frequency current of the method of the present invention can be deduced for:

${\stackrel{<span\; class="notranslate">\; \&Center\; Dot;</span>}{\mathrm{<span\; class="notranslate">\; I</span>}}}_{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>{\stackrel{<span\; class="notranslate">\; \&CenterDot;</span>}{\mathrm{<span\; class="notranslate">\; u</span>}}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; j\&omega;</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; A</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; )</span>$

In the formula, is the asymmetrical voltage of the system when no current is injected, C _A , C _B , and C _C are the three phase-to-ground capacitances of the system. Therefore, for a system with the condition of measuring three-phase-to-ground capacitance, this calculated value can also be used as the initial injection current Then use subsequent steps to eliminate errors caused by measurement and calculation, so as to reduce the injection current adjustment time.

2. Fix the injection current amplitude to I ₁ and change the current phase to determine the phase that makes the asymmetrical voltage of the system reach a local minimum

a. Set the counter j=1, record the initial injection current (amplitude is I ₁ , phase is ) system asymmetrical voltage U ₀₁ ;

b. Set the counter j=2, keep the amplitude of the injected current I ₁ unchanged, in the phase Based on a certain step size (set to ) to increase the injection current phase to Record its corresponding system asymmetrical voltage U ₀₂ , if U ₀₂ < U ₀₁ , then record the phase change direction coefficient k=0 and go to step d;

c. Keep the counter j=2, keep the amplitude of the injected current I ₁ unchanged, in the phase step size Reduce the injection current phase Record the corresponding system asymmetric voltage U ₀₂ , if U ₀₂ > U ₀₁ , go to step e, otherwise record the current phase change direction coefficient k=1;

d. Add 1 to the counter j, according to the phase change direction, according to the phase step keep changing phase Record the corresponding system asymmetrical voltage U _0j , if U _0j <U _0(j-1) , return to step d;

e. Determine the phase that minimizes the asymmetrical voltage of the system for:

3. Fixed injection current phase Change the current amplitude to determine the current amplitude I _f that makes the asymmetrical voltage of the system reach the minimum:

a. Set the counter j=1, record the amplitude as I ₁ , and the phase as The system asymmetrical voltage U ₀₁ corresponding to the injected current;

b. Set the counter j=2 to maintain the injection current phase unchanged, on the basis of the amplitude I ₁ , increase the amplitude of the injection current with a certain step size (set as ΔI) to I ₂ =I ₁ +ΔI, and record the corresponding system asymmetrical voltage U ₀₂ , if U ₀₂ <U ₀₁ , then record the amplitude change direction coefficient k=0 and turn to step d;

c. Keep the counter j=2, keep the injection current phase No change, reduce the injection current amplitude I ₂ = I ₁ -ΔI with a step size ΔI on the basis of the amplitude I ₁ , record the corresponding system asymmetric voltage U ₀₂ , if U ₀₂ > U ₀₁ , go to step e , otherwise the coefficient of the change direction of the recorded current amplitude k=1;

d. Add 1 to the counter j, continue to change the amplitude I _j = I _(j-1) + (-1) ^k ΔI according to the amplitude change direction according to the amplitude step size ΔI, and record the corresponding system asymmetrical voltage U _0j , If U _0j <U _0(j-1) , return to step d;

e. Determine the amplitude I _f that minimizes the system asymmetry voltage as: If = I ( _{j -1)} .

4. Continuously inject current into the system (the amplitude is I _f and the phase is ), the control asymmetry voltage tends to 0. If the magnitude of the asymmetrical voltage changes, the injected current at the moment as the initial injection current Go to step 2.

Compared with prior art, the beneficial effects of the present invention are:

The three-phase voltage imbalance of the medium-voltage distribution network caused by the asymmetry of the three-phase parameters of the line will shorten the service life of electrical equipment and easily cause safety accidents, which must be restricted. However, the existing methods for limiting system asymmetric voltage cannot flexibly and effectively control asymmetric voltage due to various shortcomings: (1) reasonable transposition, but it is difficult to transpose the power grid in operation, and the distribution line changes (2) Make the arc suppression coil compensation device operate properly away from the resonance point, or increase its damping rate appropriately to reduce the aggravation effect of the arc suppression coil on the asymmetrical voltage, but the effect is limited and cannot fundamentally (3) Add a special three-phase coupling capacitor to make the capacitance of the three phases equal to the ground, but it needs to increase equipment investment, and when the line structure and parameters change, they cannot be changed immediately, which may aggravate the three-phase The degree of voltage imbalance.

The asymmetrical voltage control principle of the small current grounding system proposed by the present invention gradually adjusts the phase and amplitude of the injected current according to the constraint relationship between the injected current and the asymmetrical voltage, and controls the asymmetrical voltage to approach zero. The method of the present invention can overcome various errors in actual operation, has high accuracy and stability, and can adapt to changes in line parameters and structures. When the asymmetrical state of the power grid changes, it only needs to readjust the injected current. The three-phase balance state of the power grid can be maintained, and the economy, safety performance and power quality of the power grid can be effectively improved.

Description of drawings

Attached Figure 1 shows the constraint relationship between the phase of the injected current with different amplitudes and the neutral point voltage in a specific three-phase-to-ground capacitance asymmetry of the resonant grounding system (the system is different under the condition of three-phase-to-ground capacitance asymmetry, its same trend);

Attached Figure 2 shows the constraint relationship between the magnitude of the injection current of different phases and the neutral point voltage in a specific three-phase-to-ground capacitance asymmetry of the resonant grounding system (the system is different under the condition of three-phase-to-ground capacitance asymmetry, and same trend);

Accompanying drawing 3 is the oscillogram that utilizes method described in the present invention to control neutral point voltage to approach 0;

Accompanying drawing 4 is a system block diagram of the active compensation device.

Detailed ways

The specific embodiment of method of the present invention is:

(1) Active compensation device

The active full compensation control device is composed of data acquisition system, rectification and inverter power supply, filter circuit, access circuit, control unit and other parts (as shown in Figure 4). The data acquisition system is responsible for collecting various power system parameters required for control, such as asymmetric voltage, three-phase-to-ground capacitance, etc., and sending them to the control unit as feedback; the control unit calculates the required injection current according to the algorithm, and through the drive The circuit controls the conduction of the active power electronic devices in the inverter circuit to output a suitable PWM voltage wave; the filter circuit is responsible for filtering out the high-frequency components in the PWM wave, and the filtered voltage provides the system with all the necessary power through a suitable access circuit. The required injection current is used to control the asymmetric voltage. The active compensation device can be connected in parallel or in series with the arc suppressing coil and then connected to the neutral point of the system, or can be connected in parallel or in series from any one of the three phases.

(2) Acquisition of neutral point voltage

The neutral point voltage can be obtained directly through the three-phase or zero-sequence voltage transformer (TV) connected to the neutral point of the system.

(3) Initial injection current s Choice

initial injection current It can be a current with any amplitude and phase, or it can be manually preset to a fixed current. In addition, according to theory, the final injected power frequency current of the method of the present invention can be deduced for:

${\stackrel{<span\; class="notranslate">\; \&Center\; Dot;</span>}{\mathrm{<span\; class="notranslate">\; I</span>}}}_{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>{\stackrel{<span\; class="notranslate">\; \&CenterDot;</span>}{\mathrm{<span\; class="notranslate">\; u</span>}}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; j\&omega;</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; A</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; )</span>$

In the formula, is the asymmetrical voltage of the system when no current is injected, C _A , C _B , and C _C are the three phase-to-ground capacitances of the system. In actual operation, the current can be calculated by using the measured asymmetric voltage and the three-phase-to-ground capacitance when no current is injected, but due to errors in measurement and calculation, there will be certain errors in the calculated value. Therefore, for a system with the condition of measuring three-phase-to-ground capacitance, this calculated value can be used as the initial injection current Then, the injection current is adjusted by using the method of the present invention, so that the adjustment range of the injection current can be significantly reduced, and the time required for adjustment can be reduced.

(4) Measurement of the capacitance value of the three phases of the system

If you choose to use the calculated injection current as the initial injection current, you need to measure the capacitance values of the three phases of the system. Generally, the signal injection method is used: inject different frequency signals into the neutral point of the system through the microcomputer measurement and controller, and receive feedback signals from the power grid at the same time , according to the corresponding equivalent circuit and algorithm, calculate the three-phase-to-ground capacitance value of the power grid. The injection point of the current signal injection method can be selected at the open triangle of the TV secondary side, or at the neutral point of the transformer and parallel capacitor bank at the grounding station.

(5) The asymmetrical voltage of the control system approaches zero and maintains the three-phase voltage balance

From the initial injection current At the beginning, first fix the injection current amplitude as I ₁ and change the current phase, and determine the phase that makes the system asymmetrical voltage reach the local minimum Fix injection current phase Change the current amplitude to determine the current amplitude I _f that makes the system asymmetrical voltage reach the minimum; continue to inject current into the system (the amplitude is I _f and the phase is ), control the asymmetric voltage to approach 0, if the amplitude of the asymmetric voltage changes, the injection current at this moment as the initial injection current Readjust the injection current.

The above is only a preferred embodiment of the present invention, and is not intended to limit the present invention to other forms. Any skilled person who is familiar with this profession may use the technical content disclosed above to change or modify the equivalent of equivalent changes. Example. However, any simple modifications, equivalent changes and modifications made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still belong to the protection scope of the technical solution of the present invention.

Claims (2)

1. small current neutral grounding system asymmetrical voltage control method, by injecting the power frequency zero-sequence current of certain amplitude, phase place to system neutral, control system asymmetrical voltage levels off to zero, maintains system three-phase voltage balance, it is characterized in that:

(1) from the initial Injection Current of any amplitude and phase place start, amplitude be expressed as I ₁, phase place is expressed as first fixing Injection Current amplitude is I ₁change current phase, determine to make system asymmetrical voltage reach the phase place of Local Minimum implementation procedure is:

A. establish counter j=1, record initial Injection Current time system asymmetrical voltage U ₀₁, wherein amplitude be I ₁, phase place is

B. establish counter j=2, keep Injection Current amplitude I ₁constant, in phase place with a fixed step size on basis increasing Injection Current phase place is record the system asymmetrical voltage U of its correspondence ₀₂if, U ₀₂< U ₀₁, then record phase place change direction coefficient k=0 and proceed to steps d;

C. keep counter j=2, keep Injection Current amplitude I ₁constant, in phase place with step-length on basis reduce Injection Current phase place record the system asymmetrical voltage U of its correspondence ₀₂if, U ₀₂> U ₀₁then proceed to step e, otherwise record current phase place change direction coefficient k=1;

D. counter j adds 1, according to phase place change direction, according to phase step continue to change phase place the system asymmetrical voltage U that record is corresponding _0jif, U _0j< U _{0 (j-1)}then return steps d;

E. the phase place making system asymmetrical voltage minimum is determined for: ;

(2) fixing Injection Current phase place is again change current amplitude, determine the current amplitude I making system asymmetrical voltage reach minimum _f, implementation procedure is:

A. establish counter j=1, record amplitude is I ₁, phase place is system asymmetrical voltage U corresponding to Injection Current ₀₁;

B. establish counter j=2, keep Injection Current phase place constant, at amplitude I ₁basis increases Injection Current amplitude for I with certain step delta I ₂=I ₁+ Δ I, records the system asymmetrical voltage U of its correspondence ₀₂if, U ₀₂< U ₀₁, then record amplitude change direction coefficient k=0 and proceed to steps d;

C. keep counter j=2, keep Injection Current phase place constant, at amplitude I ₁basis reduces Injection Current amplitude I with step delta I ₂=I ₁-Δ I, records the system asymmetrical voltage U of its correspondence ₀₂if, U ₀₂> U ₀₁then proceed to step e, otherwise record current amplitude change direction coefficient k=1;

D. counter j adds 1, according to amplitude change direction, continues to change amplitude I according to amplitude step delta I _j=I _(j-1)+ (-1) ^kΔ I, the system asymmetrical voltage U that record is corresponding _0jif, U _0j< U _{0 (j-1)}then return steps d;

E. the amplitude I making system asymmetrical voltage minimum is determined _ffor: I _f=I _(j-1).

2. method according to claim 1, is characterized in that:

The system asymmetrical voltage of making is tending towards the final injection power frequency zero-sequence current of 0 for:

In formula, for the asymmetrical voltage of system during non-Injection Current, C _a, C _b, C _cfor system three-phase ground capacitance.