Reactive power compensation circuit

Abstract

The invention discloses a continuous reactive power compensation circuit, which comprises a power grid, a reactor L, a shunt capacitor bank and an adjustable capacitor, one end of the electric reactor L is connected with a power grid, the other end of the electric reactor L is connected with the shunt capacitor bank and the adjustable capacitor, the shunt capacitor bank comprises n alternating-current capacitors which are mutually connected in parallel, and the adjustable capacitor is composed of four I GBTs (T1-T4), four diodes (D1-D4) and one direct-current capacitor Cdc; by monitoring and analyzing reactive power of a power grid, reactive power compensation quantities in different time periods are decomposed into a constant quantity part and a fluctuation quantity part, and a shunt capacitor bank is adopted to compensate the constant quantity part and an adjustable capacitor is adopted to compensate the fluctuation quantity part respectively; therefore, the device has the advantages of high control precision and high response speed, and is easy to expand, and the compensation capacity is improved, so that the comprehensive compensation capability of the device is improved.

Description

Technical Field

[0001] The invention relates to the technical field of electrical control, in particular to a reactive power compensation circuit. Background Art

[0002] In the power system, the presence of a large number of inductive loads will cause the system power factor to decrease. When the power factor is too low, it will bring a series of adverse effects:

[0003] 1) The energy conversion efficiency of the system will decrease, which will lead to increased energy consumption and reduced system efficiency;

[0004] 2) A power factor that is too low will increase the current in the power grid. The greater the current, the greater the loss on the power grid transmission line, increasing the line loss of the power supply system;

[0005] 3) When the power factor is too low, the internal loss of the power equipment will increase, causing the operating temperature of the equipment to rise, thus affecting the reliability and service life of the equipment;

[0006] 4) If the power factor is too low, the relevant companies will be subject to certain power regulation fines, which will increase electricity costs. In addition, it will also cause a waste of power resources, so it must be avoided as much as possible.

[0007] Using power capacitors for reactive power compensation is one of the common measures to improve power factor. Power capacitors can effectively compensate for reactive power and improve power factor. In actual use, capacitor compensation is very susceptible to harmonics in the power system. Since the impedance characteristic of capacitors is that the impedance decreases with increasing frequency, a small amount of harmonic components in the system will cause the harmonic current flowing into the capacitor to be amplified, which will further damage the capacitor. In actual use, series reactors are generally used to increase the impedance at high frequencies to suppress harmonic currents from flowing into the capacitor.

[0008] The system harmonic current is generally 3rd, 5th, 7th, 11th, and 13th. In order to suppress harmonics and avoid the series resonance point of each harmonic, a 14% (reactance rate) series reactor is required for the 3rd harmonic, and a 7% series reactor is required for the 5th and above harmonics. The use of series reactors has a certain inhibitory effect on harmonics, but it also brings a series of problems:

[0009] 1) The series reactor will offset the actual compensation of the capacitor, resulting in a significant reduction in the effective output capacity of the compensation equipment;

[0010] 2) Currently, the series reactor uses an iron core reactor. Since the magnetic permeability of the iron core is not high, in order to achieve the inductive reactance required for series harmonic suppression, the series reactor coil needs many turns, resulting in a large reactor volume;

[0011] 3) The iron core reactor has high loss, and the heating of the reactor requires special temperature control and heat dissipation, which also increases the power consumption accordingly;

[0012] 4) High cost. Traditional reactors use large amounts of iron cores and copper wires, resulting in costs that are much higher than the capacitor itself, and the cost structure is obviously inverted. Summary of the invention

[0013] The purpose of the present invention is to provide a reactive power compensation circuit in view of the problems mentioned in the background technology.

[0014] The invention provides a reactive power compensation circuit, comprising a magnetic balance coil, a compensation capacitor, an inductor and a transformer, wherein the magnetic balance coil, the compensation capacitor, the inductor and the transformer are connected, and further comprising a reactor L, a parallel capacitor group and an adjustable capacitor; the reactor is connected to the parallel capacitor group and the adjustable capacitor; the parallel capacitor group is composed of AC capacitors C1, C2...Cn, and the AC capacitors C1, C2...Cn are connected in parallel; the adjustable capacitor is composed of T1, T2, T3, T4, diodes D1, D2, D3, D4 and a DC capacitor Cdc, the collectors of T1, T2, T3 and T4 are respectively connected to the cathodes of the diodes D1, D2, D3 and D4, the emitters of T1, T2, T3 and T4 are respectively connected to the anodes of the diodes D1, D2, D3 and D4, T1 and D1, T2 and D2, T3 and D3, T4 and D4 respectively form four groups of switch modules, and T1 D1 and T2D2, T3D3 and T4D4 are connected in series to form two bridge arms. The upper ends of the two bridge arms are commonly connected to the positive electrode of the DC capacitor Cdc, and the lower ends are commonly connected to the negative electrode of the DC capacitor Cdc. The midpoints of the bridge arms are respectively connected to the parallel capacitor group and the neutral line.

[0015] Preferably, the DC capacitor Cdc is much smaller than the AC capacitor Cn, and Cn is one hundred times of Cdc.

[0016] Preferably, the reactive power compensation circuit can be set to the same equivalent circuit structure, and the equivalent circuit structure is applied to a three-phase circuit system and is divided into a three-phase star connection structure and a three-phase delta connection structure.

[0017] A control method for a continuous reactive power compensation circuit comprises the following steps:

[0018] S1: Through monitoring and analysis of the reactive power of the power grid, the reactive compensation amount in different periods is decomposed into two parts: constant amount and fluctuating amount;

[0019] S2: Use parallel capacitor group to compensate the constant value part;

[0020] S3: Use an adjustable capacitor to compensate for the fluctuation.

[0021] Preferably, in step S2, the parallel capacitor group and the adjustable capacitor are connected in parallel to form a capacitor Ceq., that is, the circuit is equivalent to a single-phase TSC, and the capacitance n of the parallel capacitor group is related to the above-mentioned reactive constant. For different occasions with the same reactive fluctuation, by changing the value of n, since the parallel capacitor group is normally uncontrollable, the reactive power compensation control is achieved by controlling the size of Cx.

[0022] Preferably, in step S3, the adjustable capacitor adopts four groups of fully controlled semiconductor switches with anti-parallel diodes to form a typical single-phase dual-bridge arm structure, and is connected in parallel with DC capacitors C1, C2...Cn to form three working conditions of charging, discharging and bypassing. The efficiency of capacitor transfer or transmission of charge is controlled by H-bridge structure design, and the charging or discharging time of DC capacitor is controlled according to the change of line current to achieve compensation for fluctuation of energy transmission efficiency.

[0023] Preferably, the selection of the capacitor n in the parallel capacitor group depends on the reactive power demand at the compensation location, which is used to compensate for the step-type reactive power change in the long time dimension.

[0024] Preferably, the transmission efficiency of the adjustable capacitor depends on the amplitude of reactive power variation of the power grid, which is used to compensate for reactive power fluctuations in a short time dimension.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1. When the fundamental current flows through the traditional capacitor plus series reactor, the actual output capacity of the compensation equipment is far lower than the rated capacity due to the offset of the capacitor capacity by the reactor, which reduces the efficiency of the capacitor. The asymmetric magnetic ring coil will be used, and the impedance of the power frequency current will be close to zero due to the offset effect of the magnetic circuit, which greatly improves the compensation efficiency of the capacitor.

[0027] 2. Use high magnetic permeability magnetic ring inductors to replace traditional iron core reactors, which can greatly reduce the volume, loss, and cost of series reactors. It can greatly improve the integration of capacitor modules, with larger module capacity and lower system loss.

[0028] 3. Good harmonic suppression effect, with more excellent "blocking harmonics and passing fundamental waves" performance, extremely small impedance to industrial frequency fundamental current, high system efficiency, high impedance to high-order harmonic current, and better harmonic suppression effect.

[0029] 4. This reactive power compensation circuit uses a smaller DC capacitor, has a simple circuit structure, is easy to control, and has a higher power density and operational reliability.

[0030] 5. This reactive power compensation circuit adopts a fully controlled semiconductor switch IGBT, which has strong controllability and is beneficial to improving the dynamic reactive power control effect.

[0031] 6. This reactive power compensation circuit realizes segmented compensation of grid reactive power through the combination of parallel capacitor group and adjustable capacitor, combines the advantages of TSC and STATCOM, and improves the compensation capacity and dynamic response effect.

[0032] 7. This reactive power compensation circuit has strong scalability and is easy to connect in parallel or in Y / Δ mode in a three-phase system, with large reactive power compensation capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0034] Figure 1 A circuit diagram of the present invention;

[0035] Figure 2 is an equivalent circuit diagram of the present invention;

[0036] Figure 3 It is the structural diagram of the three-phase system of the present invention;

[0037] Figure 4 It is the principle diagram of the present invention;

[0038] Figure 5 This is the current path diagram of the adjustable capacitor of the present invention;

[0039] Figure 6 This is the electrical schematic diagram of the capacitor compensation circuit.

[0040] In the figure: 1 power grid, 2 reactor L, 3 parallel capacitor group, 4 adjustable capacitor. DETAILED DESCRIPTION

[0041] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. It will be understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for ease of description, only parts related to the present invention, rather than all structures, are shown in the drawings. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present invention. The appearance of the phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0043] like Figure 1 As shown, in the embodiment of the present invention: a continuous reactive power compensation circuit includes a magnetic balance coil H, a compensation capacitor C, an inductor L and a transformer T, wherein the magnetic balance coil H, the compensation capacitor C, the inductor L and the transformer T are connected, as shown in FIG. Figure 6 As shown, it also includes a power grid 1, a reactor L2, a parallel capacitor group 3 and an adjustable capacitor 4; one end of the reactor L2 is connected to the power grid 1, and the other end is connected to the parallel capacitor group 3 and the adjustable capacitor 4; the parallel capacitor group 3 is composed of AC capacitors C1, C2...Cn, and the AC capacitors C1, C2...Cn are connected in parallel; the adjustable capacitor 4 is composed of T1, T2, T3, T4, diodes D1, D2, D3, D4 and a DC capacitor Cdc, and the collectors of T1, T2, T3, and T4 are connected to the cathodes of the diodes D1, D2, D3, and D4 respectively. The emitters of T1, T2, T3 and T4 are connected to the anodes of diodes D1, D2, D3 and D4 respectively. T1 and D1, T2 and D2, T3 and D3, T4 and D4 respectively form four groups of switch modules, and T1D1 and T2D2, T3D3 and T4D4 are connected in series to form two bridge arms. The upper ends of the two bridge arms are commonly connected to the positive electrode of the DC capacitor Cdc, and the lower ends are commonly connected to the negative electrode of the DC capacitor Cdc. The DC capacitor Cdc is much smaller than the AC capacitor Cn, and Cn is one hundred times that of Cdc. The midpoints of the bridge arms are respectively connected to the parallel capacitor group 3 and the neutral line.

[0044] See also Figure 2-3 , the reactive power compensation circuit can be equivalent to Figure 2 The circuit structure shown, namely the power grid 1, the reactance inductor L2, the parallel capacitor group 3 and the adjustable capacitor 4, in a three-phase system, the reactive power compensation method requires three groups of circuit units, including star connection and triangle connection.

[0045] See also Figure 4-5 Another embodiment of the present invention: a control method for a continuous reactive power compensation circuit comprises the following steps:

[0046] Step 1: By monitoring and analyzing the reactive power of the power grid 1, the reactive power compensation amount in different periods is decomposed into two parts: a constant amount and a fluctuating amount;

[0047] Step 2: Use parallel capacitor group 3 to compensate the constant value part; Figure 4QC1+…+QCk in the circuit, the parallel capacitor group 3 and the adjustable capacitor 4 are connected in parallel to form a capacitor Ceq., that is, the circuit is equivalent to a single-phase TSC. The capacitance n of the parallel capacitor group 3 is related to the above-mentioned reactive constant. For different occasions with the same reactive fluctuation, it is only necessary to change the value of n. The parallel capacitor group 3 is normally uncontrollable, that is, the precision control of reactive power is mainly achieved by controlling the size of Cx. The selection of capacitor n in the parallel capacitor group 3 depends on the reactive demand at the compensation position, which is used to compensate for the step reactive change in the long time dimension.

[0048] Step 3: Use adjustable capacitor 4 to compensate for the fluctuation. Four fully controlled semiconductor switches with anti-parallel diodes are used to form a typical single-phase double-bridge arm structure and connected in parallel with the DC capacitor. It can work in three working conditions: charging, discharging and bypassing. Its current path is as follows: Figure 5 As shown, ① and ② are capacitor charging paths, ③ and ④ are capacitor discharging paths, ⑤ and ⑥ are capacitor bypass conditions, and this H-bridge structure realizes the control of the DC capacitor charging and discharging process, so that it exhibits an external characteristic of adjustable capacitance; the capacitance external characteristic of the adjustable capacitor 4 is achieved by controlling the efficiency of the capacitor transferring or transmitting charge (that is, the size of the charge charged and discharged per unit time, defined as: energy transfer efficiency), that is, according to the change of the line current, the DC capacitor charging or discharging time is controlled to achieve the effect of controlling its energy transfer efficiency. For the control of energy transfer efficiency, a composite modulation method and a modulation method of simultaneously coordinating the control of frequency and pulse width are specifically adopted; in a three-phase system, the three groups of compensation units operate independently, and the specific compensation principle is the same as that of single-phase compensation.

[0049] Working principle:

[0050] 1. When the system voltage AC voltage Un is applied, the capacitor branch will generate a capacitive current I1, and the inductor branch will output a voltage inductive current I2 through the transformer. The phases of current I1 and I2 are 90° ahead and behind the phase of the system voltage Un respectively; that is, the phases of current I1 and current I2 are exactly opposite.

[0051] 2. The magnetic balance coil is a magnetic ring coil with high magnetic permeability. The primary winding is connected to the current I1 (I1 = Un / Xc, Xc = 1 / 2πfC), and the secondary winding is connected to the current I2 (I2 = U2 / XL, XL = 2πfL). The turns ratio of the primary and secondary windings is 1:n. We select the parameters of the capacitor, transformer and inductor. In the case of the industrial frequency fundamental wave, f = 50Hz, we select the value of L to match the capacitance of the capacitor, so that the fundamental current I1:I2 = n:1.

[0052] 3. Under the condition of industrial frequency basic current, if I1:I2=n:1 is satisfied, the magnetic flux generated by current I1 in the magnetic ring and the magnetic flux generated by current I2 in the magnetic ring are exactly equal in magnitude and opposite in direction, the magnetic fluxes cancel each other out, and the magnetic circuit is balanced. The impedance of the magnetic balance coil in the capacitance and inductance branches is zero, which is equivalent to zero impedance to the fundamental current.

[0053] In the harmonic state, since the capacitance and reactance change in opposite directions in the harmonic, the capacitive reactance value of the capacitor is Xc'=1 / kXc; the inductive reactance value of the inductor is XL'=kXL (k is the harmonic order). Under the same voltage conditions, the harmonic current I1':I2'=k2:1, the magnetic flux generated by I1' and I2' in the magnetic ring no longer cancel each other out, the harmonic current I1' from the primary winding in the magnetic core is larger, and the magnetic balance coil presents a larger series inductance characteristic to the capacitor compensation branch I1', which suppresses the harmonic current in the capacitor branch.

[0054] In summary:

[0055] The traditional capacitor plus series reactor method, when the fundamental current flows through, the actual output capacity of the compensation equipment is far lower than the rated capacity due to the offset of the capacitor capacity by the reactor, which reduces the efficiency of the capacitor. The asymmetric magnetic ring coil will be used, and the impedance of the power frequency current will be close to zero due to the offset effect of the magnetic circuit, which greatly improves the compensation efficiency of the capacitor.

[0056] Using high permeability magnetic ring inductors to replace traditional iron core reactors can greatly reduce the volume, loss, and cost of series reactors. It can greatly improve the integration of capacitor modules, with larger module capacity and lower system loss.

[0057] It has good harmonic suppression effect and more excellent "blocking harmonics and passing fundamental waves" performance. It has extremely small impedance to the industrial frequency fundamental current, high system efficiency, and high impedance to high-order harmonic current, so the harmonic suppression effect is better.

[0058] This reactive power compensation circuit is a comprehensive solution proposed based on an in-depth analysis of the advantages and disadvantages of STATCOM and SVC. It has the advantages of high control accuracy and fast response speed of the former, and can be easily expanded like the latter to increase the compensation capacity, thereby improving the comprehensive compensation capability of the device.

[0059] The above description is only an implementation mode of the present invention, and does not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A reactive power compensation circuit, comprising a magnetic balance coil H, a compensation capacitor C, an inductor L and a transformer T, wherein the magnetic balance coil H, the compensation capacitor C, the inductor L and the transformer T are connected, characterized in that: Also includes: A reactor (2), a parallel capacitor group (3) and an adjustable capacitor (4); the reactor (2) is connected to the parallel capacitor group (3) and the adjustable capacitor (4); the parallel capacitor group (3) is composed of at least one AC capacitor, and at least one AC capacitor is connected in parallel; the adjustable capacitor (4) is composed of an IGBT, a diode and a DC capacitor, and the IGBT and the diode form a switch module; the parallel capacitor group (3) is composed of C1, C2 ... Cn AC capacitors, and C1, C2 ... Cn AC capacitors are connected to the parallel capacitor group (3); ... C1, C2 ... Cn AC capacitors, and the adjustable capacitor (4) is composed of C1, C2 ... Cn AC capacitors. The adjustable capacitor (4) is composed of T1, T2, T3, T4, diodes D1, D2, D3, D4 and a DC capacitor Cdc. The collectors of T1, T2, T3, T4 are respectively connected to the cathodes of the diodes D1, D2, D3, D4, and the emitters of T1, T2, T3, T4 are respectively connected to the anodes of the diodes D1, D2, D3, D4, and T1 and D1, T2 and D2, T3 and D3, and T4 and D4 respectively form four groups of switch modules, and T1 D1 and T2D2, T3D3 and T4D4 are connected in series to form two bridge arms, the upper ends of the two bridge arms are commonly connected to the positive electrode of the DC capacitor Cdc, the lower ends are commonly connected to the negative electrode of the DC capacitor Cdc, and the midpoints of the bridge arms are respectively connected to the parallel capacitor group (3) and the neutral line; the reactor (2) comprises a reactor L2, one end of the parallel capacitor group (3) is connected to the midpoint of one bridge arm of the adjustable capacitor (4) and then connected to the grid phase line through the reactor (2), and the other end of the parallel capacitor group (3) and the midpoint of another bridge arm of the adjustable capacitor (4) are commonly connected to the neutral line.

2. The reactive power compensation circuit according to claim 1, characterized in that: The capacitance value of the DC capacitor is at least one hundred times smaller than the capacitance value of the AC capacitor.

3. The reactive power compensation circuit according to claim 1, characterized in that: The reactive power compensation circuit can be set to the same equivalent circuit structure, and the equivalent circuit structure is applied to a three-phase circuit system and is divided into a three-phase star connection structure and a three-phase delta connection structure.

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