SPWM power supply for transformer temperature rise test

Abstract

A SPWM power supply for transformer temperature rise test, characterized in that: the power supply provides suitable voltage for the SPWM modulation unit and the low-voltage amplification unit; the main signal generation unit outputs the main signal to the SPWM modulation unit and the adaptive filter unit respectively, and the SPWM modulation unit outputs the SPWM power supply signal with error to the adaptive filter unit and the isolation subtraction unit through the voltage division unit; the adaptive filter unit performs adaptive filtering operation based on the main signal and the divided voltage signal to generate an adaptive expected signal and output it to the isolation subtractor; the isolation subtractor subtracts the adaptive expected signal from the divided voltage signal, and obtains the error signal with the restored amplitude of the divided voltage through the low-voltage amplification unit; the adder superimposes the SPWM power supply signal with error and the error signal to eliminate the error in the SPWM signal, and provides a variable frequency power supply signal for the distribution transformer in the temperature rise test.

Description

Technical Field

[0001] This invention relates to the field of power distribution equipment, and more specifically, to an SPWM power supply for transformer temperature rise testing. Background Technology

[0002] In high-voltage electrical tests of 10kV distribution transformers, the time consumed per unit of time for temperature rise testing is significantly longer than in other common tests. To further accelerate the testing process while applying test power exceeding the rated voltage for an extended period without damaging the transformer, a low-frequency AC power supply is typically applied to the transformer before the total loss test phase to preheat it, thereby shortening the time it takes for the distribution transformer to reach a steady state. This time is commonly referred to as the preheating process in transformer temperature rise testing. Only after the preheating process is completed and the transformer reaches the required temperature will the formal temperature rise phase begin.

[0003] Currently, conventional temperature rise tests typically use voltage regulators or large frequency converters to power distribution transformers. However, the former is bulky and can only be conducted in specific test sites. While the latter is smaller and can achieve frequency conversion, its output has an isolated core voltage regulator or core transformer structure, resulting in a higher designed output frequency range and a longer temperature rise time.

[0004] Generally, large-scale variable frequency power supplies are designed with an output frequency range of 40-300Hz, which results in a long temperature rise time. To further improve test efficiency, considering non-destructive testing, the frequency needs to be reduced to provide a larger temperature rise current. However, using a conventional power supply structure in this type of power supply, if the frequency is to be reduced to at least 1Hz, the cross-sectional area of ​​the output core transformer needs to be increased by 50 times compared to a 50Hz power frequency output, which will greatly increase the size and weight of the test power supply.

[0005] To address the issue of excessive size and weight of test power supplies, some solutions attempt to use SPWM (Sinusoidal PWM) frequency converters based on DSP (Digital Signal Processing) as test power supplies. However, if the isolation transformer in the original frequency converter structure cannot be used simultaneously, it will lead to waveform distortion in the output power supply and the presence of several mV DC bias in the output signal. In addition, this type of power supply also suffers from low open-loop power gain.

[0006] On the other hand, because the DC input internal resistance of the transformer under test is low (usually several hundred μΩ to a few Ω), the DC bias current can cause the core to become magnetized, or even saturate. If the integral of the DC current component over time reaches 10 A·s, it can easily cause the core of a 400 kVA transformer to saturate, and the test will be terminated due to overcurrent tripping.

[0007] Due to the low open-loop power gain, the output transient peak current is reduced, which also lowers the tripping point. Taking a conventional analog high-pass filter as an example, assuming the passband is between 1Hz and 50Hz, the filter's steady-state settling time constant must be greater than 1 second. If higher transient current / power requirements are needed, the size and weight of the power supply must be increased, which will not meet the requirements for power supply miniaturization.

[0008] In addition, according to the requirements of the temperature rise test, the test power supply is in a wide frequency range, with the main operating frequency between 1 Hz and 50 Hz. The implementation of the frequency selective filter with variable center frequency is more complicated, and it is also difficult to balance fast harmonic suppression and DC suppression.

[0009] To address the aforementioned issues, there is an urgent need for an SPWM power supply for transformer temperature rise testing. Summary of the Invention

[0010] To address the shortcomings of existing technologies, this invention provides an SPWM power supply for transformer temperature rise testing, which eliminates errors in the SPWM power supply signal through an SPWM modulation unit, a voltage divider unit, an adaptive filtering unit, an isolation subtraction unit, and a low-voltage amplification unit.

[0011] The present invention adopts the following technical solution.

[0012] An SPWM power supply for transformer temperature rise testing includes a power supply, a main signal generation unit, an SPWM modulation unit, a voltage divider unit, an adaptive filter unit, an isolation subtraction unit, a low-voltage amplification unit, and an adder. The power supply provides appropriate voltages to the SPWM modulation unit and the low-voltage amplification unit. The main signal generation unit outputs a main signal to both the SPWM modulation unit and the adaptive filter unit. The SPWM modulation unit outputs an error-laden SPWM power signal to the adaptive filter unit and the isolation subtraction unit via the voltage divider unit. The adaptive filter unit performs adaptive filtering operations based on the main signal and the divided voltage signal to generate an adaptive desired signal, which is then output to the isolation subtraction unit. The isolation subtraction unit calculates the difference between the divided voltage signal and the adaptive desired signal, and obtains the error signal, the restored voltage amplitude, through the low-voltage amplification unit. The adder superimposes the error-laden SPWM power signal with the error signal to eliminate the error in the SPWM signal and provide a frequency conversion power signal to the distribution transformer during the temperature rise test.

[0013] Preferably, the power supply provides a fixed-frequency, fixed-power 500V voltage to the SPWM modulation unit and a fixed-frequency, fixed-power 15V low-voltage voltage to the low-voltage amplification unit.

[0014] Preferably, the SPWM modulation unit includes a loop error feedback unit for comparing the main signal and the output of the SPWM modulation unit with a pre-generated SPWM modulation signal to generate an SPWM modulation signal. The loop error feedback unit includes first to third resistors R31, R32, and R33, first to third capacitors C31, C32, and C33, a first operational amplifier 31a, and a comparator 32. R31 is connected in parallel across the branch formed by R33 and C32 to form a first RC unit, and C33 is connected in parallel across the branch formed by C31 and R32 to form a second RC unit. The first RC unit and the second RC unit are connected in series. The output of the SPWM modulation unit is connected to the non-inverting input of the comparator 32 via the first RC unit and the second RC unit. The non-inverting input of the first operational amplifier 31a is connected to a preset reference level, its negative input is connected to the connection point, and its output is connected to the non-inverting input of the comparator 32.

[0015] Preferably, the voltage divider unit includes proportional balancing capacitors 52a and 52b, series voltage divider resistors 51a and 51b, a voltage divider arm resistor 51, and a voltage divider arm follower 53; wherein, the proportional balancing capacitor 52a is connected in parallel across the two ends of the series voltage divider resistor 51a, with one end connected to the output terminal of the SPWM modulation unit, and the other end connected to the positive input terminal of the voltage divider arm follower 53 through the voltage divider arm resistor 51; the proportional balancing capacitor 52b is connected in parallel across the two ends of the series voltage divider resistor 51b, with one end grounded, and the other end connected to the positive input terminal of the voltage divider arm follower 53 through the voltage divider arm resistor 51; the negative input terminal of the voltage divider arm follower 53 is connected to the output terminal.

[0016] Preferably, the isolation subtraction unit includes a proportional subtractor 61, a double-sideband modem 62, and an output driver 63.

[0017] Preferably, the proportional subtractor 61 includes proportional resistors R61 and R62, a proportional capacitor C61, a -1 subtractor U1, and a proportional digital-to-analog converter; wherein, one end of the proportional resistor R61 is connected to the voltage divider signal, and the other end is connected to the input terminal of the double-sideband modem 62; the proportional digital-to-analog converter, the -1 subtractor, the proportional capacitor C61, and the proportional resistor R62 are sequentially connected to the input terminal of the double-sideband modem 62, and the proportional digital-to-analog converter receives the adaptive desired signal.

[0018] Preferably, the dual-sideband modem 62 includes an amplifier U2, resistors R63 and R64, a capacitor C62, first to third multipliers, a high-frequency oscillator, first to fourth RF transformers, first and second synchronous isolation local oscillators, and an output resistor; wherein, the positive input terminal of amplifier U2 is connected to the output terminal of proportional subtractor 61, and the negative input terminal is connected to the output terminal of amplifier U2 through capacitor C62, and is connected to the first synchronous isolation local oscillator through resistor R63 and grounded through resistor R64; the output terminal of amplifier U2 and the high-frequency oscillator are connected to the unbalanced port of the fourth RF transformer through the first multiplier, and the unbalanced port of the fourth RF transformer is... The balance port is connected to the primary high end of the second RF transformer and the primary low end of the third RF transformer, respectively, and is used as an RF balun; the high-frequency oscillator is also connected to the first synchronous isolation local oscillator through the second multiplier and the secondary output of the third RF transformer; the high-frequency oscillator is also connected to the third multiplier and the second synchronous isolation local oscillator in sequence through the first RF transformer and the secondary output of the second RF transformer, and is used as the output terminal of the double-sideband modem 62; the output resistor is connected between the output terminal of the second synchronous isolation local oscillator and the secondary low end of the second RF transformer, the low ends of the first and second RF transformers are both grounded, and the secondary low end of the third RF transformer is grounded.

[0019] Preferably, the low-voltage amplification unit includes a zero-bias compensation amplifier 41, a power amplifier 42, a first amplification resistor, a second amplification resistor, and an amplification capacitor; wherein, the non-inverting input terminal of the zero-bias compensation amplifier 41 is connected to the output terminal of the isolation subtraction unit, and the output terminal is connected to the non-inverting input terminal of the power amplifier 42 through the first amplification resistor; one end of the amplification capacitor is connected to the non-inverting input terminal of the zero-bias compensation amplifier 41, and the other end is connected to the non-inverting input terminal of the power amplifier 42; the negative input terminal of the power amplifier 42 and the negative input terminal of the zero-bias compensation amplifier 41 are respectively connected to the output terminal of the power amplifier 42 through the second amplification resistor; the output terminal of the power amplifier 42 is connected to the input terminal of the low-voltage amplification unit.

[0020] Preferably, the SPWM power supply is used to supply power to a 10kV oil-immersed three-phase distribution transformer or a 10kV dry-type three-phase distribution transformer; and the SPWM power supply is suitable for the temperature rise voltage and preheating voltage of a 10kV oil-immersed three-phase distribution transformer with a rated capacity of 630kVA and below and an impedance voltage of not more than 4%; or, suitable for the temperature rise voltage of a 10kV oil-immersed three-phase distribution transformer with a rated capacity of 1600kVA and below; or, suitable for the temperature rise voltage and preheating voltage of a 10kV dry-type three-phase distribution transformer with a rated capacity of 630kVA and below and an impedance voltage of not more than 4%.

[0021] Preferably, when the output frequency of the SPWM power supply is greater than 1Hz and the output current is 1.5 times the rated current of the distribution transformer, the distortion of the output current of the SPWM power supply is less than the preset requirements of the transformer temperature rise test.

[0022] The beneficial effects of this invention are that, compared with the prior art, the SPWM power supply for transformer temperature rise testing in this invention uses an isolation subtractor with analog circuitry to obtain the DC bias error signal of the frequency converter, and then eliminates the error signal in the output voltage through the cooperation of a low-voltage high-response power amplifier. Therefore, it can effectively drive the transformer to achieve efficient temperature rise by the low-frequency power supply, solving the current bias problem caused by the elimination of the isolation transformer. This invention has a relatively simple circuit design, and in the process of implementation, it features low signal frequency, wide range, and high power quality. The low-frequency frequency converter is small in size and highly flexible. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the circuit structure of an SPWM power supply for transformer temperature rise testing according to the present invention.

[0024] Figure 2 This is a schematic diagram of the circuit structure of the SPWM modulation unit in an SPWM power supply for transformer temperature rise testing according to the present invention.

[0025] Figure 3 This is a schematic diagram of the circuit structure of the voltage divider unit in an SPWM power supply for transformer temperature rise testing according to the present invention.

[0026] Figure 4 This is a schematic diagram of the circuit structure of the isolation subtraction unit in an SPWM power supply for transformer temperature rise testing according to the present invention.

[0027] Figure 5 This is a schematic diagram of the circuit structure of the low-voltage amplification unit in an SPWM power supply for transformer temperature rise testing according to the present invention.

[0028] Figure 6 This is a schematic diagram of the circuit structure of the main signal generation unit in an SPWM power supply for transformer temperature rise testing according to the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this invention are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, all other embodiments not described in this invention obtained by those skilled in the art based on the embodiments described in this invention without creative effort should fall within the protection scope of this invention.

[0030] A distribution transformer is a power transformer that reduces a higher voltage to the final distribution voltage, directly used for power distribution. Since power consumption in my country is typically 220V on the consumption side or 230V on the transmission side, the low-voltage line voltage of a three-phase distribution transformer is commonly 400V. Existing technologies specify various specifications for distribution transformers. For example, 10kV oil-immersed three-phase double-winding off-grid voltage regulating distribution transformers and 10kV dry-type three-phase double-winding off-grid voltage regulating distribution transformers each have different energy efficiency ratings.

[0031] There are various testing methods for temperature rise testing of oil-immersed distribution transformers. This invention primarily considers the short-circuit test method for two-winding transformers. This test method typically consists of two stages: applying total losses and applying rated current, to separately calculate the multi-point temperature rise under steady-state conditions. Prior to this, the preheating process can be considered a preparatory process to accelerate the steady-state establishment time after applying total losses.

[0032] The main requirements for the output of the frequency converter test power supply include ensuring that the output voltage waveform is a steady-state sine wave. Specifically, the voltage should be a sine wave with a distortion of less than 5%, the difference between the effective value and the average value should be less than 3%, and the long-time integral DC component should be ultra-low.

[0033] During preheating, the maximum applicable voltage should be referenced to the transformer's rated voltage. Based on common core hysteresis loops and general design experience, distribution transformers are not suitable for prolonged use at 1.1 times the rated voltage. Otherwise, it can cause significant localized near-saturation and nonlinear heating in the core, resulting in uneven temperature distribution, affecting temperature rise measurement, and potentially damaging the transformer over time. In principle, the total input power should be increased as much as possible to improve the heating rate and reduce the heating time. In this case, a current of 1.5 times the rated voltage at the power frequency is permissible.

[0034] It's not difficult to see that, based on the inductive load characteristics of the test transformer, a lower frequency can be used to apply 1.5 times the rated current at a lower voltage, thereby increasing the power applied to the load. In this case, applying a frequency of approximately 30Hz or lower would yield 1.5 times the rated current without exceeding the rated voltage. Experiments have shown that when the distribution transformer is at a frequency not lower than 1Hz, applying a steady-state sinusoidal voltage still allows the load current to meet sinusoidal characteristics (distortion less than 5%). However, as the frequency decreases further, it becomes difficult to obtain a sinusoidal rated current that meets the distortion requirements.

[0035] However, reducing the frequency to a minimum of 1Hz would greatly increase the size and weight of the test power supply. In order to reduce the overall size and weight, it is necessary to modify the power supply characteristics to accelerate the heating steady-state settling time.

[0036] Considering that the present invention needs to apply a test voltage at a frequency greater than 1Hz and as low as possible, and to meet the maximum loss allowed by the test procedure with the same 1.5 times rated current through a lower power supply volt-ampere capacity, the present invention uses a DSP-based SPWM type frequency converter power supply.

[0037] After the improvement, the steady-state settling time of heating is accelerated. Under normal test conditions, the steady-state settling time is about 5 hours when the total loss is directly applied. When 1.5 times the rated current and about 2.2 times the load loss are applied, the steady-state settling time is about 0.8 hours.

[0038] In addition, during the test of applying total loss, the rated frequency and rated loss should be applied to the transformer under test. At this time, it should be applied together through the frequency converter and the capacitor compensation circuit. After compensation, the power factor of the frequency converter is generally not less than 0.7.

[0039] To achieve the above requirements, the present invention improves the SPWM type frequency converter power supply.

[0040] Figure 1 This is a schematic diagram of the circuit structure of an SPWM power supply for transformer temperature rise testing according to the present invention. Figure 1 As shown, an SPWM power supply for transformer temperature rise testing is characterized by comprising a power supply, a main signal generation unit, an SPWM modulation unit, a voltage divider unit, an adaptive filter unit, an isolation subtraction unit, a low-voltage amplification unit, and an adder. The power supply provides appropriate voltages to the SPWM modulation unit and the low-voltage amplification unit. The main signal generation unit outputs a main signal to both the SPWM modulation unit and the adaptive filter unit. The SPWM modulation unit outputs an error-laden SPWM power signal to the adaptive filter unit and the isolation subtraction unit via the voltage divider unit. The adaptive filter unit performs adaptive filtering operations based on the main signal and the voltage divider signal to generate an adaptive desired signal, which is then output to the isolation subtraction unit. The isolation subtraction unit calculates the difference between the voltage divider signal and the adaptive desired signal, and obtains the error signal, whose voltage divider amplitude has been restored, through the low-voltage amplification unit. The adder superimposes the error-laden SPWM power signal with the error signal to eliminate the error in the SPWM signal and provides a frequency conversion power signal to the distribution transformer during the temperature rise test.

[0041] As is easily understood, the power supply is a standard high-voltage AC power supply. Through voltage division, it can also power other components in the circuit, such as the low-voltage amplifier unit. The main signal generation unit can provide an AC fixed-amplitude signal with an adjustable frequency within a certain range, which is the main signal mentioned earlier, according to the specific needs of the experiment. In one embodiment, the power supply provides a fixed-frequency, fixed-power 500V voltage to the SPWM modulation unit and a fixed-frequency, fixed-power 15V low-voltage voltage to the low-voltage amplifier unit.

[0042] Both the SPWM modulation unit and the adaptive filtering unit receive this signal. The SPWM modulation unit outputs a stable SPWM power signal according to a common SPWM modulation method. The adaptive filtering unit performs adaptive LMS filtering and outputs a reference signal component related to the feedback voltage divider, which this invention refers to as the adaptive desired signal.

[0043] Subsequently, the isolation subtractor calculates the difference between the voltage divider signal and the adaptive desired signal to obtain an error signal isolated from the influence of the main circuit. Finally, an adder eliminates the error in the SPWM signal and provides the frequency conversion power signal to the distribution transformer during the temperature rise test.

[0044] Figure 2 This is a schematic diagram of the circuit structure of the SPWM modulation unit in an SPWM power supply for transformer temperature rise testing according to the present invention. Figure 2 As shown, the SPWM modulation unit includes a loop error feedback unit, used to compare the main signal and the output of the SPWM modulation unit with a pre-generated SPWM modulation signal to generate an SPWM modulation signal. The loop error feedback unit includes first to third resistors R31, R32, and R33, first to third capacitors C31, C32, and C33, a first operational amplifier 31a, and a comparator 32. R31 is connected in parallel across the branch formed by R33 and C32 to form a first RC unit, and C33 is connected in parallel across the branch formed by C31 and R32 to form a second RC unit. The first RC unit and the second RC unit are connected in series. The output of the SPWM modulation unit is connected to the non-inverting input of the comparator 32 via the first RC unit and the second RC unit. The non-inverting input of the first operational amplifier 31a is connected to a preset reference level, its negative input is connected to the connection point, and its output is connected to the non-inverting input of the comparator 32.

[0045] It is understandable that, such as Figure 2 This circuit is used to convert 500V AC voltage to DC voltage. During the voltage conversion process, the circuit uses the SPWM method to reasonably control the magnitude of the output voltage, thereby ensuring that the amplitude of the output voltage is within the range of 350V RMS voltage.

[0046] The first and second RC units in the loop error feedback unit can be used to eliminate ripple in the output feedback voltage. The main signal is compared with the output feedback to obtain a comparison signal. The loop error feedback unit provides three poles 2ΠR31C31, 2ΠR33C32, and 2ΠR32C33, and two zeros 2Π(R31+R33)C32, 2ΠR32C31 (where C31>>C33), which, together with the error amplifier 31a, achieve a low DC open-loop gain and a linear broadband response. At this time, the output feedback voltage linearly tracks the main signal and ensures the stability of the power supply design; therefore, the comparison signal can be a periodic cyclic AC signal. In one embodiment, the error amplifier 31a is a low open-loop gain, high-response operational amplifier, preferably an LM6171.

[0047] Comparator 32 is a high-speed push-pull comparator with hysteresis, which can suppress repeated switching at the transition point; preferably, it is an ADCMP572. In this invention, the pre-generated SPWM modulation signal is an 8kHz modulated triangular wave. tw After passing through comparator 32, the SPWM modulated wave is realized.

[0048] The output signal of the SPWM modulator 32 is amplified by the switching transistor driver 33 with high-side drive and low-side drive, in conjunction with the IGBT switching transistor 34, and then output to the LC filter 35 to obtain the output signal u. SPWM Following the change in the frequency of the main signal, the u of the modulated wave... SPWM The frequency can also be adjusted accordingly.

[0049] However, u obtained in this way SPWM There is some error. For example, because the SPWM output signal amplitude is large, it differs from the original main signal u. ref The amplitude of (t) is amplified by A1 times. However, due to its inherent characteristics, an additive DC bias voltage error will occur. This invention assumes that this additive DC bias voltage error is e(t). This DC bias voltage error mainly consists of the DC current and distortion current in the switching power supply and the operating input power supply.

[0050] Since the main operating frequency during the heating test is around 1Hz and it has amplitude modulation characteristics, the test voltage needs to maintain constant power (total loss) during the process. If the error is large, it will make it difficult to use this type of power supply in the heating test process.

[0051] If an analog high-pass filter is used in series in the output power stage, the required bandwidth is very low, and the settling time is extremely long, resulting in poor transient waveforms and causing the test power supply to be large and inefficient. If an IIR or FIR filter is used, there will be a signal lag between the output and the input, making it difficult to eliminate the error by subtraction, resulting in low cancellation efficiency.

[0052] Furthermore, if LMS adaptive filtering is used to directly calculate the error signal, even when operating at a lower frequency, the voltage divider signal 1 / R·u SPWM The error component in (t) is extremely small. To improve the resolution, the digital sampling interval in the ADC and DAC needs to be significantly reduced. At this time, the time deviation in the LMS adaptive filtering process will lead to a decrease in the accuracy of error estimation, and directly solving for the error signal is difficult to meet practical requirements.

[0053] On the other hand, the error signal obtained directly from the LMS adaptive filter is not isolated from the main circuit. Therefore, when the main circuit oscillates or malfunctions, it can also lead to errors in the calculation of the error signal.

[0054] However, the output amplitude of frequency converters is usually in the range of 30V to 600V, while the DC error component is usually required to be controlled within 10uV and the total distortion should be controlled within 2%. This means that the sampling stability parameter ENOBs needs to reach 25.8 bits, which makes it impossible to directly use mixed signal devices to extract accurate error signals at a controllable cost.

[0055] Therefore, this DC bias voltage error signal cannot be eliminated by conventional filtering methods. To address this, the improvements of this invention also include the following.

[0056] Figure 3 This is a schematic diagram of the voltage divider unit in an SPWM power supply for transformer temperature rise testing according to the present invention. Figure 3 As shown, preferably, the voltage divider unit includes proportional balancing capacitors 52a and 52b, series voltage divider resistors 51a and 51b, voltage divider arm resistor 51, and voltage divider arm follower 53; wherein, the proportional balancing capacitor 52a is connected in parallel across the two ends of the series voltage divider resistor 51a, with one end connected to the output terminal of the SPWM modulation unit, and the other end connected to the positive input terminal of the voltage divider arm follower 53 through the voltage divider arm resistor 51; the proportional balancing capacitor 52b is connected in parallel across the two ends of the series voltage divider resistor 51b, with one end grounded, and the other end connected to the positive input terminal of the voltage divider arm follower 53 through the voltage divider arm resistor 51; the negative input terminal of the voltage divider arm follower 53 is connected to the output terminal.

[0057] By using SPWM precision voltage division, the divided voltage can be obtained as follows: R represents the voltage division ratio of the voltage divider resistors. Specifically, the series voltage divider resistor 51 adopts an upper and lower bridge arm structure, with the resistance ratio of the upper bridge arm 51a to the lower bridge arm 51b being R:1 to achieve a high-precision, low-noise-figure R-fold voltage division. Each resistor is preferably a 1206 metal film resistor. The proportional balancing capacitor 52 is a low-temperature coefficient capacitor with a ratio of the reciprocal of the resistance ratio (1:R) to achieve broadband response consistency across the voltage divider arms. The voltage divider resistor 51 and the proportional balancing capacitor 52 are used to control the voltage distribution of u... SPWM The voltage is divided, and then driven by voltage divider follower 53 to track the divided voltage signal, avoiding the influence of subsequent stages on the divided voltage signal. Voltage divider follower 53 is preferably a chopper amplifier like the ADA4522-1. The final output...

[0058] On the other hand, the adaptive filter performs adaptive LMS filtering to eliminate unknown interference contained in the basic signal in a kind of optimized manner. Unlike the standard LMS output, the LMS filter in this design does not output an error signal but instead outputs the reference correlation component from the SPWM power supply output feedback voltage divider, obtaining a signal related to the reference signal u. ref (t) tends towards A1 / R·u ref The d1(t) signal (t-t0).

[0059] Specifically, in an adaptive filter, the input signal x(n) passes through a parameter-adjustable digital filter to generate an output signal y(n). This output signal y(n) is compared with the desired signal d(n) to form an error signal e(n). The filter parameters are then adjusted using an adaptive algorithm to minimize the mean square value of e(n). In this invention, the main reference signal DAC1 is output and converted to obtain a high-resolution sine wave as the desired signal d(n). This error signal is obtained by subtracting the output signal y(n) of the parameter-adjustable digital filter. Through iterative recursion using the LMS algorithm, the filter coefficients are adjusted to make d(n) infinitely close to y(n). At this point, the coefficients Δω of the multiple parameter-adjustable digital filters in the adaptive filter loop are adjusted. n Normalized to Δω n / ∑Δω n Furthermore, by using a filter to filter x(n) and output y(n), A1 / R·u mixed in x(n) can be extracted. ref (t-t0) signal.

[0060] Figure 4 This is a schematic diagram of the circuit structure of the isolation subtraction unit in an SPWM power supply for transformer temperature rise testing according to the present invention. Figure 4 As shown, the isolation subtraction unit includes a proportional subtractor 61, a double-sideband modem 62, and an output driver 63.

[0061] Preferably, the proportional subtractor 61 includes proportional resistors R61 and R62, a proportional capacitor C61, a -1 subtractor U1, and a proportional digital-to-analog converter; wherein, one end of the proportional resistor R61 is connected to the voltage divider signal, and the other end is connected to the input terminal of the double-sideband modem 62; the proportional digital-to-analog converter, the -1 subtractor, the proportional capacitor C61, and the proportional resistor R62 are sequentially connected to the input terminal of the double-sideband modem 62, and the proportional digital-to-analog converter receives the adaptive desired signal.

[0062] Specifically, after the first capacitor C61 of the subtractor filters out the DC error of the DAC output, the DAC output is a more ideal d1(t) → A1 / R·u ref (t-t0). t0 is the output delay. The proportional subtractor 61 has a -1 subtractor for matching with the DAC. Through the subtractor, the first subtractor resistor R61 (which divides the input voltage), the second subtractor resistor R62, and the first subtractor capacitor C61 (which filters out DC errors from the DAC output), 1 / R·u is finally obtained. SPWM (t)-d1(t). The preferred DAC is AD5541, and the subtractor is OP07.

[0063] Preferably, the dual-sideband modem 62 includes an amplifier U2, resistors R63 and R64, a capacitor C62, first to third multipliers, a high-frequency oscillator, first to fourth RF transformers, first and second synchronous isolation local oscillators, and an output resistor; wherein, the positive input terminal of amplifier U2 is connected to the output terminal of proportional subtractor 61, and the negative input terminal is connected to the output terminal of amplifier U2 through capacitor C62, and is connected to the first synchronous isolation local oscillator through resistor R63 and grounded through resistor R64; the output terminal of amplifier U2 and the high-frequency oscillator are connected to the unbalanced port of the fourth RF transformer through the first multiplier, and the unbalanced port of the fourth RF transformer is... The balance port is connected to the primary high end of the second RF transformer and the primary low end of the third RF transformer, respectively, and is used as an RF balun; the high-frequency oscillator is also connected to the first synchronous isolation local oscillator through the second multiplier and the secondary output of the third RF transformer; the high-frequency oscillator is also connected to the third multiplier and the second synchronous isolation local oscillator in sequence through the first RF transformer and the secondary output of the second RF transformer, and is used as the output terminal of the double-sideband modem 62; the output resistor is connected between the output terminal of the second synchronous isolation local oscillator and the secondary low end of the second RF transformer, the low ends of the first and second RF transformers are both grounded, and the secondary low end of the third RF transformer is grounded.

[0064] In this invention, resistors R61 to R64 form a resistor network, which can be implemented using an LT5400. By setting R61 = R62 and R63 = R64, and C61 * R62 >> 1s, compensation for C62 compression of the high-frequency response is achieved. Amplifier U2 uses an AD8675.

[0065] The output of amplifier U2 is multiplied by the sinusoidal local oscillator of the high-frequency oscillator to obtain the modulated carrier u with the local oscillator as the center frequency. DSB1 (t)=2u1-(t)=2u 1+ (t)→-1 / R·e(t), where u 1- (t) and u 1+ (t) represents the upper and lower sidebands, and u 1+ (t)=1 / 2·(1 / R·u SPWM (t)-d1(t))→-1 / 2·1 / R·e(t). After further isolation by the RF isolation transformer, the signal is multiplied by the synchronous isolation local oscillator on the secondary side of the RF transformer to restore the output signal u of the same size. DSB1 (t)=u DSB2 (t)→-1 / R·e(t), thereby achieving isolation.

[0066] Additionally, feedback from the third RF transformer and the high-frequency oscillator is received at the negative feedback of amplifier U2 to achieve deep feedback closed-loop adjustment of the output. Based on the circuit structure, u can be obtained. DSB2 (t) is isolated from the main circuit and has the characteristics of high-speed linear response.

[0067] Figure 5 This is a schematic diagram of the circuit structure of the low-voltage amplification unit in an SPWM power supply used for transformer temperature rise testing according to the present invention. Figure 5 As shown, preferably, the low-voltage amplification unit includes a zero-bias compensation amplifier 41, a power amplifier 42, a first amplification resistor, a second amplification resistor, and an amplification capacitor; wherein, the non-inverting input terminal of the zero-bias compensation amplifier 41 is connected to the output terminal of the isolation subtraction unit, and the output terminal is connected to the non-inverting input terminal of the power amplifier 42 through the first amplification resistor; one end of the amplification capacitor is connected to the non-inverting input terminal of the zero-bias compensation amplifier 41, and the other end is connected to the non-inverting input terminal of the power amplifier 42; the negative input terminal of the power amplifier 42 and the negative input terminal of the zero-bias compensation amplifier 41 are respectively connected to the output terminal of the power amplifier 42 through the second amplification resistor; the output terminal of the power amplifier 42 is connected to the input terminal of the low-voltage amplification unit.

[0068] By using a low-voltage amplification unit, and through compensation control and adding bias from the power amplification input to the output, the output signal of the isolation subtraction unit is amplified, thereby accurately amplifying the -1 / R·e(t) signal to its original error level of -e(t). Finally, through an adder, u can be realized. SPWM (t)=A1·u ref The cancellation of the e(t) part in (t-t0)+e(t) yields the ideal A1·u. ref (t-t0) Output power signal.

[0069] Figure 6This is a schematic diagram of the circuit structure of the main signal generation unit in an SPWM power supply for transformer temperature rise testing according to the present invention. Figure 6 As shown, the digital signal processing unit hardware includes a digital signal processor (DSP), a main signal DAC1 (Digital-to-Analog Converter), a voltage divider output acquisition ADC (Analog-to-Digital Converter), an adaptive filter output DAC2, and a sampling clock generator. Since the inverter power supply output is a sine wave, the main signal generation section uses a cascaded phase accumulator, a CORDIC (Coordinate Rotation Digital Computer) module, and a linear interpolation module to output from the main signal DAC1 and convert it into a high-resolution sine wave as a reference waveform. This reference waveform is then used as the reference signal for the adaptive filter and output from the adaptive filter output DAC2.

[0070] In one embodiment, the digital signal processor is selected as RT1052. The main signal DAC is selected as AD5541. The ADC is selected as 16-bit AD7688-5. The adaptive filter DAC2 is selected as AD5541. The sampling clock generator is selected as AD9548 to provide low-jitter sampling clock for the ADC and DAC.

[0071] In this example circuit system, the 1 / R·u of the voltage divider output is taken into account. SPWM (t) The peak value of the signal is controlled within 5Vp so that the ADC can sample. At this time, the DC error is about several hundred uV to tens of mV. After suppression, the DC resolution needs to reach several hundred nV to several uV. Therefore, the resolution of the input ADC and the output DAC needs to reach more than 23-bit effective linear resolution. In actual circuits, it is difficult to implement with relatively simple circuits. Therefore, in this example, instead of using the LMS adaptive filter to directly output the error cancellation signal -e(t), an isolation subtractor is used to obtain -e(t).

[0072] In addition, the mixing multiplicative error signal generated by factors such as frequency converter power switch is generally distributed in the frequency band above several kHz, which can be filtered out by LC filter. At the same time, considering that the input impedance of the distribution transformer test sample has a large inductive component, high-frequency current cannot pass through, so its influence on the test measurement results can be ignored.

[0073] Preferably, the SPWM power supply is used to supply power to a 10kV oil-immersed three-phase distribution transformer or a 10kV dry-type three-phase distribution transformer; and the SPWM power supply is suitable for the temperature rise voltage and preheating voltage of a 10kV oil-immersed three-phase distribution transformer with a rated capacity of 630kVA and below and an impedance voltage of not more than 4%; or, suitable for the temperature rise voltage of a 10kV oil-immersed three-phase distribution transformer with a rated capacity of 1600kVA and below; or, suitable for the temperature rise voltage and preheating voltage of a 10kV dry-type three-phase distribution transformer with a rated capacity of 630kVA and below and an impedance voltage of not more than 4%.

[0074] The preheating process is not considered a test process, but rather an operation to accelerate the establishment of a steady-state temperature rise. In principle, the total input power should be increased as much as possible to improve the heating rate and reduce the heating time; therefore, it is found that it is difficult to significantly increase the speed at the power frequency. Applying a frequency of approximately 30Hz or lower can achieve 1.5 times the rated current while the applied voltage does not exceed the rated voltage. However, according to actual tests, when the distribution transformer is at a frequency not lower than 1Hz, the load current can still meet the sinusoidal characteristics when a steady-state sinusoidal voltage is applied. Therefore, under operating conditions with a frequency greater than 1Hz and the frequency reduced as much as possible, applying the test voltage allows for heating with the same 1.5 times rated current corresponding to the maximum loss allowed by the test procedure, using a lower power supply capacity (power supply volt-ampere capacity). This invention achieves faster heating steady-state establishment time by modifying the power supply characteristics.

[0075] Preferably, when the output frequency of the SPWM power supply is greater than 1Hz and the output current is 1.5 times the rated current of the distribution transformer, the distortion of the output current of the SPWM power supply is less than the preset requirements of the transformer temperature rise test.

[0076] In addition, the tests applying total losses and rated current also need to be conducted jointly by the frequency converter and the capacitor compensation circuit. After compensation, the power factor of the frequency converter is generally not lower than 0.7. Taking into account the test requirements, the power supply of this invention can be applied to the temperature rise test and preheating of the above-mentioned three-phase distribution transformers of different capacities.

[0077] The beneficial effects of this invention are that, compared with the prior art, the SPWM power supply for transformer temperature rise testing in this invention uses an isolation subtractor with analog circuitry to obtain the DC bias error signal of the frequency converter, and then eliminates the error signal in the output voltage through the cooperation of a low-voltage high-response power amplifier. Therefore, it can effectively drive the transformer to achieve efficient temperature rise by the low-frequency power supply, solving the current bias problem caused by the elimination of the isolation transformer. This invention has a relatively simple circuit design, and in the process of implementation, it features low signal frequency, wide range, and high power quality. The low-frequency frequency converter is small in size and highly flexible.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. An SPWM power supply for transformer temperature rise testing, the SPWM power supply comprising a power supply, a main signal generation unit, an SPWM modulation unit, a voltage divider unit, an adaptive filtering unit, an isolation subtraction unit, a low-voltage amplification unit, and an adder; characterized in that: The power supply provides appropriate voltages to the SPWM modulation unit and the low-voltage amplification unit, respectively. The main signal generation unit outputs the main signal to the SPWM modulation unit and the adaptive filtering unit respectively, and the SPWM modulation unit outputs the SPWM power signal with error to the adaptive filtering unit and the isolation subtraction unit respectively through the voltage divider unit. The adaptive filtering unit performs adaptive filtering operations based on the main signal and the voltage divider signal to generate an adaptive desired signal and output it to the isolation subtraction unit. The isolation subtraction unit calculates the difference between the voltage divider signal and the adaptive desired signal, and obtains the error signal whose voltage divider amplitude has been restored through the low-voltage amplification unit; The adder superimposes the error signal with the SPWM power signal containing the error to eliminate the error in the SPWM signal and provides a frequency conversion power signal to the distribution transformer during the temperature rise test.

2. The SPWM power supply for transformer temperature rise testing according to claim 1, characterized in that: The power supply provides a fixed-frequency, fixed-power 500V voltage to the SPWM modulation unit and a fixed-frequency, fixed-power 15V low-voltage voltage to the low-voltage amplification unit.

3. The SPWM power supply for transformer temperature rise testing according to claim 1, characterized in that: The SPWM modulation unit includes a loop error feedback unit, which is used to compare the main signal and the output of the SPWM modulation unit with a pre-generated SPWM modulation signal to generate an SPWM modulation signal. The loop error feedback unit includes first to third resistors R31, R32, and R33, first to third capacitors C31, C32, and C33, a first operational amplifier 31a, and a comparator (32). R31 is connected in parallel at both ends of the branch formed by R33 and C32 to form the first RC unit, and C33 is connected in parallel at both ends of the branch formed by C31 and R32 to form the second RC unit. The first RC unit and the second RC unit are connected in series. The output of the SPWM modulation unit is connected to the non-inverting input of the comparator (32) via the first RC unit and the second RC unit in sequence. The positive input terminal of the first operational amplifier 31a is connected to a preset reference level, the negative input terminal is connected to the connection point of the first capacitor C31, and the output terminal is connected to the positive input terminal of the comparator (32).

4. The SPWM power supply for transformer temperature rise testing according to claim 1, characterized in that: The voltage divider unit includes proportional balancing capacitors 52a and 52b, series voltage divider resistors 51a and 51b, voltage divider arm resistor (51), and voltage divider arm follower (53). The proportional balancing capacitor 52a is connected in parallel across the two ends of the series voltage divider resistor 51a, with one side connected to the output terminal of the SPWM modulation unit and the other side connected to the positive input terminal of the voltage divider follower (53) through the voltage divider arm resistor (51). The proportional balancing capacitor 52b is connected in parallel across the two ends of the series voltage divider resistor 51b, with one side grounded and the other side connected to the positive input terminal of the voltage divider follower (53) through the voltage divider arm resistor (51). The negative phase input terminal of the voltage divider follower 53 is connected to the output terminal.

5. The SPWM power supply for transformer temperature rise testing according to claim 1, characterized in that: The isolation subtraction unit includes a proportional subtractor (61), a double-sideband modem (62), and an output driver (63).

6. The SPWM power supply for transformer temperature rise testing according to claim 5, characterized in that: The proportional subtractor (61) includes proportional resistors R61 and R62, proportional capacitor C61, a -1 subtractor U1, and a proportional digital-to-analog converter. Wherein, one end of the proportional resistor R61 is connected to the voltage divider signal, and the other end is connected to the input terminal of the double-sideband modem (62); The proportional digital-to-analog converter, the -1 subtractor, the proportional capacitor C61, and the proportional resistor R62 are sequentially connected to the input terminal of the double-sideband modem (62), and the proportional digital-to-analog converter receives the adaptive desired signal.

7. An SPWM power supply for transformer temperature rise testing according to claim 5, characterized in that: The dual-sideband modem (62) includes an amplifier U2, resistors R63 and R64, a capacitor C62, first to third multipliers, a high-frequency oscillator, first to fourth RF transformers, first and second synchronous isolation local oscillators, and an output resistor; Wherein, the positive input terminal of the amplifier U2 is connected to the output terminal of the proportional subtractor (61), the negative input terminal is connected to the output terminal of the amplifier U2 through capacitor C62, and is connected to the first synchronous isolation local oscillator through resistor R63 and grounded through resistor R64; The output of the amplifier U2, together with the high-frequency oscillator, is connected to the unbalanced port of the fourth RF transformer through the first multiplier. The balanced port of the fourth RF transformer is connected to the primary high end of the second RF transformer and the primary low end of the third RF transformer, respectively, and is used as an RF balun. The high-frequency oscillator is also connected to the first synchronous isolation local oscillator via a second multiplier and the secondary output of the third radio frequency transformer. The high-frequency oscillator is also connected to the third multiplier and the second synchronous isolation local oscillator in sequence through the second radio frequency transformer and the first radio frequency transformer, serving as the output terminal of the double-sideband modem (62). The output resistor is connected between the output terminal of the second synchronous isolation local oscillator and the low secondary side of the second RF transformer. The low secondary sides of the first and second RF transformers are both grounded, and the low secondary side of the third RF transformer is grounded.

8. The SPWM power supply for transformer temperature rise testing according to claim 1, characterized in that: The low-voltage amplification unit includes a zero-bias compensation amplifier (41), a power amplifier (42), a first amplification resistor, a second amplification resistor, and an amplification capacitor; wherein, The non-inverting input terminal of the zero-bias compensation amplifier (41) is connected to the output terminal of the isolation subtraction unit, and the output terminal is connected to the non-inverting input terminal of the power amplifier (42) through the first amplification resistor; One end of the amplifying capacitor is connected to the non-inverting input terminal of the zero-bias compensation amplifier (41), and the other end is connected to the non-inverting input terminal of the power amplifier (42). The negative phase input terminal of the power amplifier (42) and the negative phase input terminal of the zero bias compensation amplifier (41) are respectively connected to the output terminal of the power amplifier (42) through a second amplification resistor; The output terminal of the power amplifier (42) is connected to the input terminal of the low-voltage amplifier unit.

9. An SPWM power supply for transformer temperature rise testing according to claim 1, characterized in that: The SPWM power supply is used to power a 10kV oil-immersed three-phase distribution transformer or a 10kV dry-type three-phase distribution transformer; and... The SPWM power supply is suitable for the temperature rise voltage and preheating voltage of 10kV oil-immersed three-phase distribution transformers with a rated capacity of 630kVA and below and an impedance voltage of no more than 4%. Alternatively, it applies to the temperature rise voltage of 10kV oil-immersed three-phase distribution transformers with a rated capacity of 1600kVA and below. Alternatively, it applies to the temperature rise voltage and preheating voltage of 10kV dry-type three-phase distribution transformers with a rated capacity of 630kVA and below and an impedance voltage of no more than 4%.

10. An SPWM power supply for transformer temperature rise testing according to claim 9, characterized in that: When the output frequency of the SPWM power supply is greater than 1Hz and the output current is 1.5 times the rated current of the distribution transformer, the distortion of the output current of the SPWM power supply is less than the preset requirements of the transformer temperature rise test.

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