A method for calculating the remanence of a converter transformer based on inrush current.
By using a converter transformer residual magnetism calculation method based on inrush current, and utilizing the deep saturation excitation characteristic curve and the superposition weight formula of inrush current, the problem of difficulty in quantifying the residual magnetism of converter transformers is solved, and the effective calculation of residual magnetism and evaluation of demagnetization effect are realized, ensuring the normal operation of the equipment.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- CHONGQING UNIV
- Filing Date
- 2023-07-20
- Publication Date
- 2026-05-26
AI Technical Summary
Existing technologies make it difficult to effectively quantify and estimate the residual magnetism of the converter transformer before it is switched on, which leads to inrush current, affects the normal operation of the converter transformer, and makes it difficult to assess the demagnetization effect.
The converter remanence calculation method based on inrush current simplifies the remanence calculation process by obtaining the deep saturation excitation characteristic curve, calculating the theoretical per-unit magnetic flux, and using the superposition weight formula of inrush current to calculate the remanence.
It enables effective calculation of the residual magnetism of the converter transformer and evaluation of the demagnetization effect, reduces the impact on the converter transformer, and ensures the normal operation of the equipment.
Smart Images
Figure CN116953575B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of power systems and relates to a method for calculating the residual magnetism of a converter transformer based on inrush current. Background Technology
[0002] As the core of a DC transmission system, converter transformers often retain significant residual magnetism in their cores after circuit breaker tripping. This residual magnetic flux includes transient decay and steady-state periodic variations, with decay typically lasting for tens of seconds. When the converter transformer is closed under no-load conditions, the superposition of the residual magnetism in the core and the magnetic flux generated by the applied voltage can cause the core to reach deep saturation, leading to a sharp drop in magnetizing inductance and a large inrush current in the transformer windings. The amplitude of this inrush current can reach 6 to 8 times the rated current. The resulting short-term temperature rise and vibration severely threaten the normal operation of the converter transformer, potentially causing malfunctions in protective devices such as gas relays, resulting in unsuccessful closing, or even equipment burnout and grid paralysis.
[0003] Sensing and eliminating residual magnetism before converter transformer closing is a fundamental measure to address inrush current generation. However, existing methods mostly rely on core hysteresis models to calculate converter transformer residual magnetism. These methods require extensive experimentation to determine model parameters, and because the magnetic flux inside the core cannot be directly measured, the demagnetizing effect of the demagnetizing device is often verified by measuring the no-load excitation current. This approach cannot effectively quantify and estimate the residual magnetism of the converter transformer core before closing. The difficulty in verifying residual magnetism estimation methods and quantifying the demagnetizing effect are pressing problems that need to be solved in the current methods for sensing and eliminating residual magnetism in converter transformers.
[0004] To address this, this invention proposes a method for calculating the residual magnetism of converter transformers based on inrush current. This method, based on the deep saturation excitation characteristic curve of the converter transformer, uses the per-unit value of the effective inrush current generated by no-load closing at the peak voltage of the converter transformer as a parameter to calculate the theoretical per-unit value of the magnetic flux. Using the starting point for calculating the per-unit value of residual magnetism, the ratio of the rated voltages of the primary and secondary sides of the converter transformer, and a weighting constant, a weighting formula for superimposing the residual magnetism of the converter transformer with the rated voltage magnetic flux is determined. Finally, the residual magnetism of the converter transformer core is calculated using the calculated per-unit value of the theoretical magnetic flux corresponding to the inrush current and the weighting formula for superimposing the residual magnetism with the rated voltage magnetic flux. The method proposed in this invention is simple to implement, highly operable, and has minimal impact on the converter transformer, providing an effective verification strategy for calculating the residual magnetism of converter transformers and evaluating the effectiveness of residual magnetism elimination. Summary of the Invention
[0005] In view of this, the purpose of the present invention is to provide a method for calculating the remanence of a converter transformer based on inrush current.
[0006] To achieve the above objectives, the present invention provides the following technical solution:
[0007] A method for calculating the remanence of a converter transformer based on inrush current, comprising the following steps:
[0008] S1: Obtain the deep saturation excitation characteristic curve of the converter transformer, and express the primary voltage and current of the converter transformer in the deep saturation excitation characteristic curve in per-unit form. The deep saturation excitation characteristic curve includes the voltage and current values under the overexcitation condition of 1.2 to 1.5 times the voltage.
[0009] S2: Using the measured per-unit current value of the converter transformer under 1.2 to 1.5 times voltage overexcitation as the independent variable and the per-unit voltage value corresponding to the current as the dependent variable, fit the mathematical expression of its theoretical per-unit voltage value.
[0010] S3: Perform a no-load closing test on the converter transformer. Close the circuit at the voltage peak, measure the first waveform of the generated inrush current, and calculate the effective value of the waveform within a single cycle. Calculate the ratio of the effective value of the single cycle to the rated current to obtain the per-unit value of the effective inrush current.
[0011] S4: The per-unit value of magnetic flux is expressed as the per-unit value of voltage. The per-unit value of effective excitation inrush current is substituted into the mathematical calculation formula of theoretical per-unit voltage to calculate the per-unit value of theoretical magnetic flux. The formula for superimposing the remanent magnetization of the converter transformer and the rated voltage magnetic flux is determined by the calculation starting point of the per-unit value of residual magnetism, the ratio of the rated voltage of the primary and secondary sides of the converter transformer, and the weighting constant.
[0012] S5: The remanence of the converter core is calculated by obtaining the theoretical per-unit value of the inrush current and the superposition weight formula of the remanence and the rated voltage flux.
[0013] Optionally, in step S1, the primary voltage and current are expressed in per-unit form, specifically including:
[0014] The rated current on the primary side of the converter transformer can be calculated using equation (1):
[0015]
[0016] In the formula, I N The rated current of the primary side of the converter transformer; S N U represents the rated capacity of the converter transformer. N The rated voltage of the primary side of the converter transformer; k is a constant, when the converter transformer is a three-phase transformer. When the transformer is single-phase, k = 1;
[0017] Calculate the per-unit values of voltage and current on the primary side using equations (2) and (3):
[0018]
[0019]
[0020] In equation (2), U puI is the per-unit value of the primary voltage; U is the voltage on the primary side of the converter transformer in the deep saturation excitation characteristic curve; in equation (3), I pu I represents the per-unit value of the primary side current; I is the primary side current of the converter transformer in the deep saturation excitation characteristic curve.
[0021] Optionally, in step S2, the measured per-unit value of the current under 1.2 to 1.5 times the voltage overexcitation of the converter transformer is used as the independent variable, and the per-unit value of the voltage corresponding to the current is used as the dependent variable. The mathematical formula for calculating the theoretical per-unit voltage value is then fitted, specifically including:
[0022] Select the voltage and current values at at least two points in the overexcitation condition of 1.2 to 1.5 times the voltage, calculate the per-unit value of voltage and current at each point, and fit the mathematical calculation formula of the theoretical per-unit voltage value as shown in equation (4).
[0023] Y = a + bX (4)
[0024] In the formula, Y is the theoretical voltage per unit value, X is the current per unit value under overexcitation conditions, a is the intercept of the fitted straight line, and b is the slope of the fitted straight line.
[0025] Optionally, in step S3, a no-load closing test is performed on the converter transformer. The transformer is closed at the voltage peak, the first waveform of the generated inrush current is measured, and the effective value within a single cycle of the waveform is calculated. Specifically, this includes:
[0026] Apply the rated voltage to the primary side of the converter transformer, and use a phase-selective closing device to close the circuit at the voltage peak. Record the winding current on the primary side of the converter transformer and the peak value of the first waveform of the excitation inrush current. Calculate the effective value of the first inrush current waveform. The formula for calculating the effective value is shown in equation (5).
[0027]
[0028] In the formula, I rms is the effective value of the inrush current; T is the period of the inrush current, which is 0.02s at power frequency; i(t) is the measured inrush current on the primary side of the converter transformer.
[0029] The per-unit value of the commutator variable excitation inrush current obtained from the measurement is shown in equation (6);
[0030]
[0031] In the formula, The per-unit value of the commutator variable excitation inrush current obtained by measurement.
[0032] Optionally, in step S4, the per-unit value of magnetic flux is represented by a voltage per-unit value, which specifically includes:
[0033] The formula for calculating the reference magnetic flux under rated voltage is shown in equation (7);
[0034]
[0035] In equation (7), φ base f is the reference magnetic flux at rated voltage; n The operating frequency of the converter transformer is set to 50Hz.
[0036]
[0037] In equation (8), φ pu U1 is the per-unit value of the magnetic flux of the converter transformer core; U1 is the primary side voltage of the converter transformer; φ is the magnetic flux generated under the primary side voltage U1 of the converter transformer; it is theoretically derived from equation (8) that the per-unit value of the residual magnetism of the converter transformer core can be indirectly represented by the per-unit value of the voltage.
[0038] Optionally, in step S4, the effective inrush current per-unit value is substituted into the mathematical formula for calculating the theoretical magnetic flux per-unit value, specifically including:
[0039] Substitute the effective excitation inrush current per unit value into the mathematical calculation expression of the theoretical voltage per unit value to calculate the theoretical voltage per unit value, and express the core flux per unit value in terms of the theoretical voltage per unit value. The core flux calculation result is shown in equation (9).
[0040]
[0041] In the formula, This represents the theoretical per-unit voltage value corresponding to the inrush current. This represents the theoretical per-unit value of the inrush current.
[0042] Optionally, in S4, the formula for superimposing the remanent magnetization of the converter transformer and the rated voltage flux is determined using the per-unit value of the remanent magnetization calculation starting point, the rated voltage ratio of the primary and secondary sides of the converter transformer, and the weighting constant. This formula specifically includes:
[0043] The formula for superimposed weights is shown in equation (10):
[0044]
[0045] In equation (10), k w The superposition weight of residual magnetism and rated voltage flux is given; N is the ratio of the rated voltage of the primary and secondary sides of the converter transformer; C is the starting point for calculating the per-unit value of residual magnetism; D is the weighting constant, which is set to 0.5. The calculated value of the residual magnetism of the converter transformer core is obtained from the solution required.
[0046] Optionally, in step S5, the remanence of the converter transformer core is calculated by obtaining the theoretical per-unit value of the inrush current and the weighted formula of the superposition of remanence and rated voltage flux. Specifically, this includes:
[0047] Substitute the measured per-unit value of the inrush current Calculate the theoretical magnetic flux in equation (9) Theoretical flux It can also be represented by equation (11);
[0048]
[0049] By combining equations (9), (10), and (11), a simultaneous equation (12) is formed, in which N, C, and D are all known. The unknown is then solved. That is, to solve for the per-unit value of residual magnetism before closing the circuit breaker due to the inrush current generated by the converter excitation.
[0050]
[0051] The beneficial effects of this invention are as follows:
[0052] (1) This invention only requires the deep saturation excitation characteristic curve of the converter transformer and the ratio of the rated voltage of the primary and secondary sides of the converter transformer, and has less dependence on data. Closing the circuit at the voltage peak generates a small inrush current and will not affect the normal operation of the converter transformer.
[0053] (2) This method has the advantages of simple implementation process, low computational complexity and strong operability, and can provide an effective verification strategy for the calculation and elimination of remanence of converter transformer.
[0054] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description
[0055] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:
[0056] Figure 1 This is a flowchart of the method of the present invention;
[0057] Figure 2 The curve shows the saturation excitation characteristics of the converter variable depth.
[0058] Figure 3 This is the waveform of the inrush current generated when the converter transformer is closed under no-load conditions.
[0059] Figure 4 Error diagrams for calculating remanence using the method proposed in this invention under different initial remanence conditions. Detailed Implementation
[0060] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.
[0061] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.
[0062] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.
[0063] Specific embodiments of the present invention are as follows:
[0064] Obtain the excitation characteristic curve under deep saturation of the commutator.
[0065] The overall process of the method of this invention is as follows: Figure 1 A deep saturation excitation characteristic test was conducted on a converter transformer at a converter station. The transformer capacity was 248.6 MVA, and the rated valve-side coil voltage was... The rated grid-side coil voltage is The short-circuit impedance percentage is 16.03%. The per-unit values of its test voltage and current are shown in Table 1.
[0066] Table 1. Per-unit values of voltage and current measured in the experiment.
[0067] U(kV) I(A) U(pu) I(pu) 53 0.04844 0.17485 0.00005873 80 0.07312 0.26393 0.00008865 110 0.1005 0.3629 0.00012185 140 0.128 0.46188 0.00015519 170 0.1554 0.56085 0.00018841 200 0.1827 0.65983 0.00022151 230 0.21 0.7588 0.00025461 260 0.2371 0.85777 0.00028747 290 0.2644 0.95675 0.00032057 320 0.2984 1.05572 0.00036179 350 0.384 1.15470 0.00038907 380 1.315 1.25367 0.00046558 410 3.388 1.35264 0.00159435 428.66 4.939 1.41421 0.00598822 450 6.871 1.48461 0.00833065 480 9.794 1.58358 0.01187460
[0068] The deep saturation excitation characteristic curve of the converter is shown in Figure 2 As shown.
[0069] Mathematical expression for fitting the theoretical voltage per unit value;
[0070] Voltage and current values were selected on the excitation characteristic curve with voltage per unit values ranging from 1.2 to 1.5. The mathematical expression for the theoretical voltage per unit value was fitted, and the fitted mathematical expression for the theoretical voltage per unit value is shown in equation (1):
[0071] y = 1.206165 + 30.2349x (1)
[0072] A no-load closing test was performed on the converter transformer, and the circuit was closed at the voltage peak. The resulting inrush current waveform is shown in the attached figure. Figure 3 The effective value of the inrush current generated during the closing of the converter transformer was calculated by using a simulation model to preset the magnetic flux. The ratio of the effective value in the single cycle to the rated current was calculated to obtain the per-unit value of the effective inrush current, as shown in Table 2.
[0073] Table 2 Per-unit values of effective inrush current
[0074]
[0075] (4) Calculate the theoretical magnetic flux per unit value
[0076] Substitute the effective inrush current per unit value into the formula for calculating the theoretical voltage per unit value, and the calculated theoretical magnetic flux per unit value is shown in Table 3 below.
[0077] Table 3 Results of Calculation of Theoretical Magnetic Flux Perimeter Value
[0078]
[0079] The starting point C for calculating the per-unit value of residual magnetism is set to 0.1 (pu), and the ratio of the rated voltage of the primary and secondary sides of the converter transformer is 3.11. Therefore, the formula for superimposing the weight of the residual magnetism of the converter transformer and the rated voltage flux is shown in equation (2).
[0080]
[0081] The remanence of the converter core is calculated by measuring the theoretical per-unit value of the inrush current and the weighted remanence superposition. Equation (3) and Table 3 are used. As a result, the residual magnetism of the converter transformer core was calculated.
[0082]
[0083] Calculate the remanence Compared with the preset magnetic flux per unit value The results and errors are shown in Table 4, and the error variation graph during the initial remanence change process is shown in [Figure 4]. Figure 4 .
[0084] Table 4 Preset per-unit values of magnetic flux With calculation of remanence Results and errors
[0085]
[0086] As shown in Table 4, the residual magnetism calculation method proposed in this invention has a relatively large error in the preset magnetic flux range of 0.1pu-0.3pu. The calculation error is smaller in the preset magnetic flux range of 0.4pu-0.8pu, with an overall error of less than 7.5%. Furthermore, the residual magnetism calculation error is less than 5% in the range of 0.4pu-0.7pu. Considering that the actual residual magnetism range of converter transformers is mostly in the range of 0.3pu-0.8pu, this invention can better cover the actual residual magnetism scenario of converter transformers, and the calculation error is within the error range of engineering applications. It can effectively calculate the residual magnetism of converter transformers before closing.
[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.
Claims
1. A method for calculating the remanence of a converter transformer based on inrush current, characterized in that: The method includes the following steps: S1: Obtain the deep saturation excitation characteristic curve of the converter transformer, and express the primary voltage and current of the converter transformer in the deep saturation excitation characteristic curve in per-unit form. The deep saturation excitation characteristic curve includes the voltage and current values under the overexcitation condition of 1.2 to 1.5 times the voltage. S2: Using the measured per-unit current value of the converter transformer under 1.2 to 1.5 times voltage overexcitation as the independent variable and the per-unit voltage value corresponding to the current as the dependent variable, fit the mathematical expression of its theoretical per-unit voltage value. S3: Perform a no-load closing test on the converter transformer. Close the circuit at the voltage peak, measure the first waveform of the generated inrush current, and calculate the effective value of the waveform within a single cycle. Calculate the ratio of the effective value of the single cycle to the rated current to obtain the per-unit value of the effective inrush current. S4: The per-unit value of magnetic flux is expressed as the per-unit value of voltage. The per-unit value of effective excitation inrush current is substituted into the mathematical calculation formula of theoretical per-unit voltage to calculate the per-unit value of theoretical magnetic flux. The formula for superimposing the remanent magnetization of the converter transformer and the rated voltage magnetic flux is determined by the calculation starting point of the per-unit value of residual magnetism, the ratio of the rated voltage of the primary and secondary sides of the converter transformer, and the weighting constant. S5: The remanence of the converter core is calculated by obtaining the theoretical per-unit value of the inrush current and the superposition weight formula of the remanence and the rated voltage flux.
2. The method for calculating the remanence of a converter transformer based on inrush current according to claim 1, characterized in that: In S1, the primary voltage and current are expressed in per-unit value form, specifically including: The rated current on the primary side of the converter transformer can be calculated using equation (1): In the formula, I N The rated current of the primary side of the converter transformer; S N U represents the rated capacity of the converter transformer. N The rated voltage of the primary side of the converter transformer; k is a constant, when the converter transformer is a three-phase transformer. When the transformer is single-phase, k = 1; Calculate the per-unit values of voltage and current on the primary side using equations (2) and (3): In equation (2), U pu I is the per-unit value of the primary voltage; U is the voltage on the primary side of the converter transformer in the deep saturation excitation characteristic curve; in equation (3), I pu I represents the per-unit value of the primary side current; I is the primary side current of the converter transformer in the deep saturation excitation characteristic curve.
3. The method for calculating the remanence of a converter transformer based on inrush current according to claim 1, characterized in that: In step S2, the measured per-unit value of the current under 1.2 to 1.5 times voltage overexcitation of the converter transformer is used as the independent variable, and the per-unit value of the voltage corresponding to the current is used as the dependent variable. The mathematical formula for calculating the theoretical per-unit voltage value is fitted, specifically including: Select the voltage and current values at at least two points in the overexcitation condition of 1.2 to 1.5 times the voltage, calculate the per-unit value of voltage and current at each point, and fit the mathematical calculation formula of the theoretical per-unit voltage value as shown in equation (4). Y = a + bX (4) In the formula, Y is the theoretical voltage per unit value, X is the current per unit value under overexcitation conditions, a is the intercept of the fitted straight line, and b is the slope of the fitted straight line.
4. The method for calculating the remanence of a converter transformer based on inrush current according to claim 1, characterized in that: In step S3, an no-load closing test is performed on the converter transformer. The transformer is closed at the voltage peak, the first waveform of the generated inrush current is measured, and the effective value within a single cycle of the waveform is calculated. Specifically, this includes: Apply rated voltage to the primary side of the converter transformer, use a phase-selective closing device to close the circuit at the voltage peak, record the winding current on the primary side of the converter transformer, and record the peak value of the first waveform of the inrush current. Calculate the effective value of the first surge waveform. The formula for calculating the effective value is shown in equation (5). In the formula, I rms is the effective value of the inrush current; T is the period of the inrush current, which is 0.02s at power frequency; i(t) is the measured inrush current on the primary side of the converter transformer. The per-unit value of the commutator variable excitation inrush current obtained from the measurement is shown in equation (6); In the formula, The per-unit value of the commutator variable excitation inrush current obtained by measurement.
5. The method for calculating the remanence of a converter transformer based on inrush current according to claim 1, characterized in that: In step S4, the per-unit value of magnetic flux is represented by a per-unit voltage value, which specifically includes: The formula for calculating the reference magnetic flux under rated voltage is shown in equation (7); In equation (7), φ base f is the reference magnetic flux at rated voltage; n The operating frequency of the converter transformer is set to 50Hz. In equation (8), φ pu U1 is the per-unit value of the magnetic flux of the converter transformer core; U1 is the primary side voltage of the converter transformer; φ is the magnetic flux generated under the primary side voltage U1 of the converter transformer; it is theoretically derived from equation (8) that the per-unit value of the residual magnetism of the converter transformer core can be indirectly represented by the per-unit value of the voltage.
6. The method for calculating the remanence of a converter transformer based on inrush current according to claim 1, characterized in that: In step S4, the mathematical calculation formula for the theoretical voltage per unit value is substituted with the effective inrush current per unit value to calculate the theoretical magnetic flux per unit value, which specifically includes: Substitute the effective excitation inrush current per unit value into the mathematical calculation expression of the theoretical voltage per unit value to calculate the theoretical voltage per unit value, and express the core flux per unit value in terms of the theoretical voltage per unit value. The core flux calculation result is shown in equation (9). In the formula, This represents the theoretical per-unit voltage value corresponding to the inrush current. This represents the theoretical per-unit value of the inrush current.
7. The method for calculating the remanence of a converter transformer based on inrush current according to claim 1, characterized in that: In S4, the formula for superimposing the remanent magnetization of the converter transformer and the rated voltage flux is determined using the per-unit value of the remanent magnetization calculation starting point, the ratio of the rated voltages on the primary and secondary sides of the converter transformer, and the weighting constant. Specifically, this formula includes: The formula for superimposed weights is shown in equation (10): In equation (10), k w The superposition weight of residual magnetism and rated voltage flux is given by: N is the ratio of the rated voltage of the primary and secondary sides of the converter transformer; C is the starting point for calculating the per-unit value of residual magnetism; D is the weighting constant, which is set to 0.
5. The calculated value of the residual magnetism of the converter transformer core is obtained from the solution required.
8. The method for calculating the remanence of a converter transformer based on inrush current according to claim 1, characterized in that: In step S5, the remanence of the converter transformer core is calculated by obtaining the theoretical per-unit value of the inrush current and the weighted formula of the superposition of remanence and rated voltage flux. Specifically, this includes: Substitute the measured per-unit value of the inrush current Calculate the theoretical magnetic flux in equation (9) Theoretical flux It can also be represented by equation (11); By combining equations (9), (10), and (11), a simultaneous equation (12) is formed, in which N, C, and D are all known. The unknown is then solved. That is, to solve for the per-unit value of residual magnetism before closing the circuit breaker due to the inrush current generated by the converter excitation.