A direct-current electric energy meter calibrating device and method applied to a large direct-current ripple factor working condition

Abstract

At present, under the working condition of the DC power grid based on power electronic devices, the DC current ripple factor flowing through the power electronic devices greatly exceeds the standard requirement, and the DC electric energy meter based on different metering principles may produce large errors under the environment of large DC ripple factor, and the traditional DC electric energy meter calibration method is not suitable for the working condition of large ripple factor. The application discloses a DC electric energy meter calibration device and method applied to the working condition of large DC ripple factor. The DC electric energy meter calibration device comprises a voltage standard source, a current standard source, a standard DC electric energy meter, a measured DC electric energy meter and an error comparator; the calibration method based on the DC electric energy meter calibration device combines the voltage DC standard source and the AC standard source to generate smooth DC voltage, DC current and the reactive current component contained in the DC current and the harmonic component of the DC voltage ripple under the working condition of the simulation power electronic device, so that the components are superimposed on the DC electric energy meter, the error of the standard DC electric energy meter and the measured DC electric energy meter is compared, the error grade of the meter is determined, and it is ensured that the measured DC electric energy satisfies the accuracy requirement under the working condition of large ripple factor.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of direct current electric energy meter calibration, and particularly relates to a direct current electric energy meter calibration device and method applied to a large direct current ripple factor working condition. BACKGROUND

[0002] In recent years, flexible direct current power transmission technology has developed greatly, and direct current power transmission and distribution is applied more and more. Direct current electric energy metering technology under a direct current working condition is a key in the process of direct current power transmission and distribution. In 2013, the State Grid Corporation of China issued the technical standard of direct current electric energy meter technology specification (Q / GDW1825-2013), which stipulates the accuracy requirements, electrical performance, anti-interference and other technical requirements and test methods of the direct current electric energy meter. In 2017, the General Administration of Quality Supervision, Inspection and Quarantine issued the national metrological verification regulation of electronic direct current electric energy meter (JJG 842-2017), which standardizes the verification of the electronic direct current electric energy meter. The standards both require that the ripple factor of the measured direct current voltage and current is not greater than 2%. However, under the working condition based on a power electronic device, the direct current ripple factor flowing through the power electronic device will exceed the standard requirement, and the ripple factor is greater than 2%. Under this environment, if the direct current electric energy meter qualified based on the above standard is measured by different metering methods (such as instantaneous integration method or average value method), the measurement error will be different. The existing verification method cannot verify the error difference, and serious errors will cause unqualified electric energy meters to flow into the market.

[0003] The application is aimed at the parameter characteristics of direct current based on a power electronic device, that is, the direct current has a large direct current ripple factor. By analyzing the existing direct current electric energy metering method and existing problems, a direct current electric energy meter calibration device and method applied to a large direct current ripple factor working condition are proposed, so that the direct current electric energy meters produced by different manufacturers can meet the error requirements under the large direct current ripple factor working condition. SUMMARY

[0004] The application is aimed at the parameter characteristics of direct current based on a power electronic device, that is, the direct current has a large direct current ripple factor. By analyzing the existing direct current electric energy metering method and existing problems, a direct current electric energy meter calibration device and method applied to a large direct current ripple factor working condition are proposed, so that the direct current electric energy meters produced by different manufacturers can meet the error requirements under the large direct current ripple factor working condition.

[0005] The above technical problems are solved by the following technical solutions:

[0006] The present invention provides a DC electric energy meter calibration device for use in conditions with a large DC ripple factor. The device comprises a voltage standard source, a current standard source, a standard DC electric energy meter, a DC electric energy meter to be tested, and an error comparator. The device is characterized in that the voltage standard source comprises a controllable DC voltage source and a controllable AC voltage source, the accuracy of the controllable voltage source being less than 0.5%, the output ripple factor of the DC voltage source being no greater than 1%, and the maximum output voltage U of the controllable DC voltage source being less than 0.5%. DCmax The DC power meter is powered by 1.2 times the rated DC voltage U0. The output voltage frequency of the controllable AC voltage source is adjustable from 50 to 300 Hz, and the amplitude U a The maximum is 2% of the rated DC voltage U0 of the DC power meter; the current standard source consists of a controllable DC current source and a controllable AC current source. The accuracy of the controllable current source is less than 0.5%, the output ripple factor of the controllable DC current source is not more than 1%, and the maximum output current I max The output current frequency of the controllable AC current source is adjustable from 50 to 300 Hz, which is 1.2 times the rated voltage I0 of the DC energy meter. The standard DC energy meter consists of voltage and current sensors, low-pass filters, high-speed AD, digital filters, voltage accumulation and averaging processing modules, multipliers, and voltage-frequency U / F pulse converters. The voltage and current sensors convert the measured voltage and current into small voltage signals within a certain range. The low-pass filter filters the voltage signal. The high-speed AD uses a fixed sampling time T s = 33.3μs to sample, convert the voltage and current analog signals into digital signals, remove the disturbance signal with digital filter, and accumulate the sampled signals by voltage averaging processing module until N = 1201, and calculate the average value of voltage and current respectively. and The multiplier will pass N*T s The average voltage, average current and time within a time period are multiplied together to obtain the electric energy W within that time period. The voltage-frequency U / F pulse converter outputs the obtained electric energy as the number of pulses. The error comparator compares the electric energy pulses generated by the standard DC electric energy meter with the pulses generated by the DC electric energy meter under test to determine the error of the tested electric energy meter.

[0007] Based on a DC energy meter verification device applied to large DC ripple factor working conditions, the verification method is:

[0008] Step 1: The DC voltage standard source closes KD1 and opens KA1. If the rated voltage of the DC energy meter is U N , the rated current of the DC energy meter is I N , adjust the controllable DC voltage source, the voltage standard source output voltage U is U N, DC current standard source is closed KD2, open KA2, adjust the controllable DC current source, the current standard source output current I is respectively: 0.5I N , 0.8I N , I N , voltage and current are applied to the standard DC energy meter and the measured DC energy meter. The standard DC energy meter measures the energy at each point respectively E 11 , E 12 , E 13 ; the measured DC energy meter measures the energy at each point respectively W 11 , W 12 , W 13 , the absolute error of the energy is Δe 11 , Δe 12 , Δe 13 .

[0009] Second step: DC voltage standard source is closed KD1, open KA1, adjust the controllable DC voltage source, the voltage standard source output voltage U is U N , DC current standard source is open KD2, closed KA2, adjust the controllable AC current source, the current standard source output current I = I N cos(ωt), f = 50Hz, voltage and current are applied to the standard DC energy meter and the measured DC energy meter. The standard DC energy meter measures the energy at U = U N , I = I N cos(ωt) in the specified time, the measured energy is E 21 ; the measured DC energy meter measures the energy as W 21 , the absolute error of the energy is Δe 21 .

[0010] Third step: voltage and current in-phase error; DC voltage standard source is open KD1, closed KA1, adjust the controllable AC voltage source, the voltage standard source output voltage U = 0.1U N cos(ωt), ω = 50Hz, DC current standard source is open KD2, closed KA2, adjust the controllable AC current source, the current standard source output current I = I N cos(ωt), voltage and current are applied to the standard DC energy meter and the measured DC energy meter. Change the frequency f = 100Hz, 150Hz, 200Hz, 250Hz and 300Hz, repeat the third step test. The standard DC energy meter measures the energy at U = 0.1U N cos(ωt), I = I N cos(ωt) in the specified time, the standard energy meter measures the energy as E 31i , the measured DC energy meter measures the energy as W 31i , the absolute error of the energy is Δe31i where i = harmonic number.

[0011] Fourth step: error of opposite phase of voltage and current; disconnect KD1 of DC voltage standard source, connect KA1, adjust controllable AC voltage source, output voltage U = 0.1U of voltage standard source N cos(ωt), ω = 50 Hz, disconnect KD2 of DC current standard source, connect KA2, adjust controllable AC current source, output current I = I of current standard source N cos(ω(t+π)), apply voltage and current to standard DC energy meter and measured DC energy meter. Change frequency f = 100 Hz, 150 Hz, 200 Hz, 250 Hz and 300 Hz, repeat fourth step test. When U = 0.1U N cos(ωt), I = I N cos(ωt+π), the standard energy meter measures energy E within a specified time 41i , the measured DC energy meter measures energy W 41i , the absolute error of energy is Δe 41i where i = harmonic number.

[0012] Fifth step: superimpose errors affected by reactive current measured in first, second and third steps, respectively, to obtain:

[0013] (e 11 ) i = Δe 11 + Δe 21+ Δe 31i ,

[0014] (e 12 ) i = Δe 12 + Δe 21+ Δe 31i ,

[0015] (e 13 ) i = Δe 13 + Δe 21+ Δe 31i

[0016] where Δe 11 , Δe 12 , Δe 13 are errors measured in first step at 0.5I N , 0.8I N , I N , respectively, and i represents different harmonic numbers corresponding to third step. Use formula to calculate 0.5I NThe forward comprehensive error under each harmonic component is obtained.

[0017] Step 6: superimposing the errors affected by the reactive current measured in the first, second and fourth steps, respectively, to obtain:

[0018] (*e 11 ) i = Δe 11 + Δe 21+ Δe 41i ,

[0019] (*e 12 ) i = Δe 12 + Δe 21+ Δe 41i ,

[0020] (*e 13 ) i = Δe 13 + Δe 21+ Δe 41i

[0021] Where Δe 11 , Δe 12 , Δe 13 are the errors measured in the first step at 0.5I N , 0.8I N , I N , and i represents the different harmonic numbers corresponding to the fourth step. The formula is used to calculate the negative comprehensive error under each harmonic component at 0.5I N .

[0022] Step 7: if the errors of each point obtained above are not greater than the maximum error allowed by the corresponding instrument grade, it is determined to be qualified. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a principle block diagram of a DC electric energy meter calibration device applied to a larger DC ripple factor working condition of the present application;

[0024] Figure 2 is a standard DC electric energy meter function block diagram;

[0025] Figure 3 is a typical DC power distribution system structure schematic diagram;

[0026] Figure 4 is a working condition schematic diagram of a DC / AC power electronic device connected in a DC power distribution network;

[0027] Figure 5 is a DC electric energy meter current waveform diagram placed before the DC / AC device;

[0028] Figure 6 DC / AC inverter DC terminal voltage and current waveform for photovoltaic grid-connected;

[0029] Figure 7 DC / AC inverter DC terminal voltage and current waveform for voltage source type. DETAILED DESCRIPTION

[0030] The application will be further described below in conjunction with the drawings and examples.

[0031] Figure 3 The schematic diagram of the typical DC distribution system structure provided by the national electric power industry standard DL / Z 1697-2017 (Technical Guidelines for Voltage Source Converters for Flexible DC Distribution System) is shown in the figure. The flexible DC distribution system adopts two independent AC power sources to realize the loop operation mode of double power supply, and the AC / DC conversion is realized through two voltage converters VSC1 and VSC2. In the DC power grid, different types of distributed power sources, energy storage devices and AC / DC load devices, as well as AC or DC microgrid systems are connected. By Figure 1 It can be seen that the DC distribution is connected between the AC system, the load, the distributed power source and the energy storage device through the power electronic device, which can conveniently realize the control and operation of the DC distribution system. From the perspective of electric energy metering, the DC electric energy meters need to be installed at the nodes of "No. 2", "No. 3", "No. 4" and "No. 5" in the figure for energy metering. The voltage and current at this point are important parameters for the DC electric energy meter to collect. At these junctions, the DC current at the grid end flows through the load through the power electronic device, and the DC current signal collected by the DC electric energy meter is the input current at the power electronic device end. The input DC current parameters of the power electronic device are different due to different control modes.

[0032] Figure 4 The working condition diagram of the DC / AC power electronic device connected in the DC distribution network is shown in the figure. E represents the DC power grid, the box represents the DC / AC power electronic device, RL represents the DC / AC external load, M represents the DC electric energy meter connected before the DC / AC power electronic device, which is used to measure the electric energy after the device, U represents the DC grid voltage, and I represents the current flowing into the DC / AC module.

[0033] Assuming that the DC / AC power electronic device outputs power frequency sine voltage, the load RL is a resistive and inductive load, and the inverter module adopts SPWM modulation method. According to the modulation principle of the inverter module, in the positive half wave of the output signal, VI is turned on, and V3 and V4 are turned on alternately. As can be seen from the figure, when VI is turned on, V3 is turned off, and V4 is turned on, the current I flows from the power grid end, forms a closed loop through VI, RL, and V4, and at this time the current I is positive; when VI is turned on, V3 is turned on, and V4 is turned off, due to the presence of inductive devices in the load, the current forms a closed loop through D2 and VI, and at this time the current I flowing into the electric energy meter M is 0. Therefore, in the modulation process, the current flowing into the direct current electric energy meter is actually a pulse current, and the width of the pulse changes with the change of the modulation signal. The complete period current is shown in Figure 5 As can be seen, the current through the direct current electric energy meter is a group of pulse currents, and the waveform has obvious modulation characteristics, and the ripple factor is affected by the modulation control strategy.

[0034] Figure 6 is the current waveform diagram of the direct current end when the photovoltaic grid-connected inverter works. As can be seen from the figure, the entire direct current is composed of a direct current component and a 100Hz alternating current component. Obviously, the 100Hz alternating current component is the reactive current caused by the DC / AC power device, and this reactive current cannot be calculated in active energy metering.

[0035] Figure 7 is the direct current waveform when the independent DC / AC inverter works. As can be seen from the figure, the current is not a constant direct current waveform, but has a large mutation, which can also be regarded as being affected by the alternating current component.

[0036] From the above theoretical analysis and actual experimental waveforms, under the direct current working condition based on the power electronic device, the direct current voltage fluctuation is small, but the direct current always has an alternating current component, so that the direct current fluctuates according to a certain rule. How do these fluctuations affect the metering in the direct current active energy metering? The following further analyzes.

[0037] The national standard defines the ripple factor of direct current as the difference between the peak and valley values of the fluctuating direct current quantity divided by the average value of the direct current quantity. From the above examples of several working conditions, the current waveform of the direct current electric energy meter placed in front of the DC / AC device has a large ripple factor. The current national standard requires that the ripple factor of the direct current to be detected in the detection of the direct current electric energy meter is not greater than 2%. Obviously, the ripple factors of the above several working conditions are obviously greater than 2%. Therefore, can the direct current electric energy meter that is qualified under the condition that the ripple factor is not greater than 2% accurately measure the direct current energy when the ripple factor is greater than 2%? The following further analyzes.

[0038] There are three methods for the measurement of DC power: average value method, effective value method and instantaneous power integration method. The calculation formula of average value method is as follows:

[0039]

[0040] Among them respectively represent the average DC voltage and DC current in Δt time.

[0041] The calculation formula of effective value method is as follows:

[0042] W b = P b · Δt = U rms · I rms · Δt (2)

[0043] Among them U rms , I rms are the effective values of voltage and current in Δt time, and the product of the two multiplied by the sampling period is the electric energy of this period.

[0044] The calculation formula of instantaneous power integration method is as follows:

[0045]

[0046] Among them u(t), i(t) are the values of instantaneous voltage and current in the sampling period.

[0047] Suppose the DC power grid is an ideal DC voltage source, that is, u = U0, u represents the DC bus supply voltage. According to the above analysis, the DC current through the ammeter contains large ripples, which can be regarded as an AC component superimposed on the DC steady quantity. Let the current through the DC power meter be:

[0048] i = I0+ I a sin(ω a t) (4)

[0049] In the formula, I0 is the DC steady quantity provided to the load, and I a sin(ω a t) is the AC component, that is, the ripple.

[0050] 1) Using average value method

[0051] If the average time Δt is an integer multiple of the period of the AC current component, then If the average time Δt is not an integer multiple of the period of the AC current component, the average value of the current signal in Δt will be biased, but as time goes on, suppose that after N times of Δt average value calculation, N*Δt = M*T a , M represents M T athe time of N times Δt is equal to an integer times the period of the alternating current component, also has the electric energy after the time of N times Δt is:

[0052]

[0053] 2) using the effective value method, then:

[0054] In a certain time Δt, according to the definition of effective value, U rms = U0, I rms = I0+I arms the electric energy after the time of N times Δt is:

[0055]

[0056] 3) using the instantaneous power integration method

[0057] the electric energy after the time of N times Δt is:

[0058]

[0059] Obviously, when Δt i is an integer times the period T a of the alternating component in the measured current or N*Δt=M*T a , there is:

[0060]

[0061] then:

[0062]

[0063] The above analysis can be seen that for the direct current with large ripple factor, the effective value method will accumulate large system error, while the average value method and the instantaneous power integration method can accurately measure the direct current energy. In fact, the alternating component superimposed on the direct current can be regarded as a reactive current component due to the internal control of the power electronic device. For the measurement of active energy, the average value method and the instantaneous power integration method will not produce system error, but the effective value method will produce large measurement error, which will greatly increase with the accumulation of measurement time. It can be seen that the effective value method cannot be used in the measurement of direct current energy.

[0064] It is well known that the current DC power is generally obtained by rectifying AC voltage. The rectifiers of these DC systems have pulse numbers of 6, 12, 24, etc. which will generate rich h=Nk characteristic harmonics (N is the pulse number, k=l, 2, 3...) on the DC side, and some non-characteristic harmonics. These harmonics appear on the DC power grid in the form of ripples, i.e. these ripples can be regarded as a series of AC components superimposed on the DC voltage.

[0065] When considering the DC bus voltage containing harmonics, let the DC bus voltage be:

[0066]

[0067] where U n is the amplitude of the harmonic voltage, nω is the harmonic frequency, n is the harmonic number, and ω is the fundamental frequency.

[0068] If the average method is used in the DC energy meter, the average value of the current is still

[0069] The average value of the voltage is

[0070] Obviously, after the average value calculation of N times Δt, the latter term of the above formula tends to zero, i.e.:

[0071]

[0072] Then:

[0073] It can be seen that the DC energy meter using the average method can eliminate the influence of the ripple.

[0074] If the instantaneous power integration method is used, after the time integration of N times Δt, the measured electric energy is W c :

[0075]

[0076] In the above formula, the first term is the DC electric energy of the DC quantity after N times of calculation time Δt, and its size is N*U0I0. The second and third terms are zero in the time period of N times Δt, i.e. N*Δt is approximately equal to an integer multiple of the harmonic period. The fourth term is the integral value of the electric energy, and the amplitude and frequency of each harmonic voltage are changing. Only when the voltage harmonic frequency is equal to the current harmonic frequency, i.e. nω=ω a , the integral of this term is not zero. In a period, the average power of I a sin(ω a t)(U n sin(nωt+θ n )) is Ia U n cos(θ n ). By W c From the expression, it can be seen that when the harmonic frequency contained in the DC grid voltage is equal to the AC current frequency of the ripple component in the DC current, there is a part of additional electric energy in the measurement of electric energy based on instantaneous power integration. This part of electric energy is caused by the reactive current generated by the power electronic device. The electric energy size within the accumulation time Δt is:

[0077] ΔW c =I a U n cos(θ n )·Δt (13)

[0078] In general, the amplitude of the harmonic component U n Small, when the DC current ripple factor is less than 2%, I a is also smaller, so I a U n Small, ΔW c The generated electrical energy is negligible. However, for DC current based on power electronic devices, the ripple factor is large, I a The amplitude is large. According to the DC current ripple investigation of the above-mentioned field conditions, I a The amplitude is even comparable to the DC component I0, so ΔW c The existence of will cause the energy measurement error to increase sharply. In addition, the phase difference between current and voltage θ n It is not determined by the load characteristics. The AC component of the voltage ripple is determined by the voltage rectification technology, and the AC component of the current ripple is generated by the power conversion modulation. The phase difference between the two is also random. When the phase is exactly 90 degrees, ΔW c When the phase is exactly 0 degrees, ΔW c Maximum phase change ΔW c It also changes, but what can be determined is ΔW c The existence of is due to a systematic error generated when the instantaneous power integration method is used for electric energy measurement. The size of this systematic error is variable and random.

[0079] At present, the design of electric energy meter is based on metering chip, such as ADE71 series and ADE75 series electric energy metering chip of ADI company and CS54 series electric energy metering chip of Cirrus Logic company. These chips integrate voltage and current signal filtering, ADC sampling, voltage and current signal instantaneous power multiplication and digital frequency pulse conversion inside, which simplifies the structure of electric energy meter and improves the measurement accuracy of electric energy meter. From the design principle of chip, they are all used for electric energy metering by instantaneous power integration method, which is suitable for the measurement of alternating current energy, i.e. voltage and current are all alternating signals with the same frequency, and the accuracy of electric energy metering is high.

[0080] From the current circulating direct current electric energy meter in the market, most of the direct current electric energy meters still use the metering chip of instantaneous power integration method. According to the above analysis, the direct current electric energy meter using the instantaneous power integration method is not suitable for application in the occasion with large ripple factor. The current national standard direct current electric energy meter is calibrated in a certain range of ripple factor, and the calibrated direct current electric energy meter cannot guarantee the measurement accuracy of these electric energy meters under the working condition of power electronic device and direct current with large ripple factor. Therefore, the calibration of direct current electric energy meter must have a standard electric energy meter based on average value method.

[0081] Figure 1 It is a novel structure block diagram of direct current electric energy meter calibration device, which comprises (1) voltage standard source, (2) current standard source, (3) standard direct current electric energy meter, (4) direct current electric energy meter to be calibrated and (5) error comparator. Its characteristics are that the voltage standard source (1) is composed of a controllable direct current voltage source and a controllable alternating current voltage source, the accuracy of controllable voltage source is less than 0.5%, the ripple factor of direct current voltage source output is not more than 1%, the controllable direct current voltage source outputs maximum voltage U DCmax 1.2 times of direct current rated voltage U0 of direct current electric energy meter, the controllable alternating current voltage source outputs voltage frequency of 50-300Hz adjustable, and the amplitude U a 1% of direct current rated voltage U0 of direct current electric energy meter; the current standard source (2) is composed of a controllable direct current source and a controllable alternating current source, the accuracy of controllable current source is less than 0.5%, the ripple factor of controllable direct current source output is not more than 1%, the maximum output current I maxThe controllable AC current source outputs current frequency adjustable between 50-300Hz; the standard DC electric energy meter (3) is composed of voltage and current sensors, low-pass filter, high-speed AD, digital filter, voltage cumulative average processing module, multiplier and voltage frequency U / F pulse converter, the voltage and current sensors transform the measured voltage and current into voltage small signal in a certain range, the low-pass filter filters the voltage signal, the high-speed AD samples the voltage and current analog signals into digital signals with fixed sampling time T s =33.3μs, the digital filter removes the disturbance signal, the voltage cumulative average processing module accumulates the sampling signals until N=1201 to obtain the average voltage and current and The multiplier multiplies the average voltage, average current and time within N*T s to obtain the electric energy W within the time, the voltage frequency U / F pulse converter outputs the obtained electric energy in pulse number, the internal structure diagram of the standard electric energy meter is shown in Figure 2 The error comparator (5) compares the electric energy pulses generated by the standard DC electric energy meter (3) and the pulses generated by the measured DC electric energy meter (4) to determine the error of the measured electric energy meter.

[0082] The design basis of the inventive testing device will be explained below.

[0083] Firstly, the voltage standard source of the inventive device is composed of a controllable DC power supply and a controllable AC power supply in parallel, the controllable DC voltage source is set to output maximum voltage U DCmax 1.2 times of the DC rated voltage U0 of the DC electric energy meter, which can output various voltages in the measured range to supply the DC electric energy meter; in addition, according to the transformer and filter technology of flexible DC transmission and distribution, the output voltage ripple factor of the DC grid voltage after filtering is not greater than 2%, and the total amount of harmonics above 300Hz is less than the specified threshold, therefore, the harmonic components generating ripple mainly distribute below 300Hz, the controllable AC voltage source of the inventive device outputs voltage frequency adjustable between 50-300Hz, and the amplitude U a 1% of the DC rated voltage U0 of the DC electric energy meter, through the combination of the AC and DC power supplies, various given voltages can be output to supply the DC electric energy meter for testing.

[0084] Secondly, the current standard source of the inventive device is composed of a controllable DC current source and a controllable AC current source. According to the analysis of the current components containing large ripple factor, the AC component contained in the DC current has an amplitude even comparable to that of the DC current component, and the reactive current based on the power electronic device will only produce error power ΔW cTherefore, the maximum output current of the controllable direct current source of the device is 1.2 times of the rated current of the measured direct current energy meter, and the maximum output alternating current amplitude of the controllable alternating current source is 1.2 times of the rated current of the measured direct current energy meter, and the output frequency is adjustable between 50-300Hz.

[0085] The combination of the above-mentioned AC and DC voltage sources and current sources can conveniently output various standard voltages and currents with large ripple factors.

[0086] The direct current energy meter based on the average method is theoretically more accurate in average value calculation with longer average time, but the dynamic performance of the meter is reduced and the response is slow with longer average time. The maximum harmonic frequency in the analyzed direct current voltage and current ripple is 300Hz, and its period is about 3.33ms. If the voltage and current signals are sampled at a fixed time of 33.3us, 10 points can be sampled per cycle for the 300Hz harmonic, and more sampling points are obtained per cycle for other low-order harmonics, so that the signal sampling is not distorted and the AD device with a sampling rate of 33.3us is easy to obtain. In average value calculation, the flexible direct current grid not only has high-order harmonics, but also contains inter-harmonics. In order to better eliminate the influence of alternating components, the device takes the AD sampling rate as 33.3us and the average time as 40ms, i.e. the average value of voltage and current is calculated when N=1201 sampling points are obtained, and the average time is:

[0087] T=N*T s =1201*33.3us=40ms. The average value of 40ms in the device of the standard direct current energy meter can effectively remove the alternating components, and the meter has a faster refresh time.

[0088] A direct current energy meter calibration method applied to a larger direct current ripple factor working condition is used in the above-mentioned calibration device to achieve:

[0089] First step: close KD1 and open KA1 of the direct current voltage standard source, if the rated voltage of the direct current energy meter is U N , if the rated current of the direct current energy meter is I N , adjust the controllable direct current voltage source, and the output voltage U of the voltage standard source is U N , close KD2 and open KA2 of the direct current current standard source, adjust the controllable direct current source, and the output current I of the current standard source is respectively: 0.5I N , 0.8I N , I N , and the voltage and current are applied to the standard direct current energy meter and the measured direct current energy meter.

[0090] Assume that the standard DC energy meter is at U=U N , I are 0.5I N , 0.8I N , I N When the electric energy at each point is measured within the specified time, it is E 11 、E 12 、E 13 ; The DC energy meter under test is at U=U N , I are 0.5I N , 0.8I N , I N When the electric energy at each point is measured within the specified time, it is W 11 、W 12 、W 13 , and their absolute errors in electric energy are Δe 11 , Δe 12 , Δe 13 The purpose of this step of verification is to check the measurement error of the DC energy meter under steady DC power, under the condition that the DC ripple factor is not greater than 1%.

[0091] Step 2: The DC voltage standard source closes KD1, opens KA1, and adjusts the controllable DC voltage source. The output voltage U of the voltage standard source is U N , the DC current standard source disconnects KD2, closes KA2, adjusts the controllable AC current source, and the current standard source outputs current I=I N cos(ωt), f=50Hz, apply voltage and current to the standard DC energy meter and the DC energy meter under test. Assume that the standard DC energy meter is at U=U N , I=I N When cos(ωt), the electric energy at each point is measured to be E within the specified time. 21 ; The DC energy meter measured the energy is W 21 , the absolute error of electric energy is Δe 21 .

[0092] The purpose of the second step of verification is to make the DC voltage a stable DC current and the current simulated as the AC reactive current under the power electronic device. At this time, the active energy of the DC energy meter should be zero, and the energy meter using the effective value method will produce a large error.

[0093] Step 3: Disconnect KD1 from the DC voltage standard source, close KA1, and adjust the controllable AC voltage source. The voltage standard source outputs a voltage of U = 0.1U N cos(ωt), f=50Hz, the DC current standard source disconnects KD2, closes KA2, and adjusts the controllable AC current source. The current standard source outputs current I=I Ncos(ωt), the voltage and current are applied to the standard DC energy meter and the measured DC energy meter. Change the frequency f = 100 Hz, 150 Hz, 200 Hz, 250 Hz and 300 Hz, repeat the third step test. Set the standard DC energy meter at U = 0.1 U N cos(ωt), I = I N cos(ωt), the standard energy meter measures the energy E 31i , the measured DC energy meter measures the energy W 31i , the absolute error of the energy is Δe 31i , where i = harmonic number.

[0094] The purpose of the third step is to apply a same frequency and same phase AC signal to the DC energy meter, the AC voltage simulates the harmonic voltage component contained in the ripple on the DC voltage, the AC current simulates the harmonic current component contained in the DC current ripple under the condition of larger ripple factor, at this time the DC electric energy meter based on instantaneous integration method will produce a positive active power system error.

[0095] Fourth step: disconnect KD1 of the DC voltage standard source, close KA1, adjust the controllable AC voltage source, the voltage standard source output voltage U = 0.1 U N cos(ωt), f = 50 Hz, disconnect KD2 of the DC current standard source, close KA2, adjust the controllable AC current source, the current standard source output current I = I N cos(ωt), the voltage and current are applied to the standard DC energy meter and the measured DC energy meter. Change the frequency f = 100 Hz, 150 Hz, 200 Hz, 250 Hz and 300 Hz, repeat the fourth step test.

[0096] Set the standard DC energy meter at U = 0.1 U N cos(ωt), I = I N cos(ωt), the standard energy meter measures the energy E 41i , the measured DC energy meter measures the energy W 41i , the absolute error of the energy is Δe 41i , where i = harmonic number.

[0097] The purpose of the fourth step is to apply a same frequency and opposite phase AC signal to the DC energy meter, at this time the DC electric energy meter based on instantaneous integration method will produce a negative active power system error.

[0098] Fifth step: superimpose the errors affected by the reactive current measured in the first, second and third steps, respectively, to obtain:

[0099] (e 11 )i = Δe 11 + Δe 21+ Δe 31i ,

[0100] (e 12 ) i = Δe 12 + Δe 21+ Δe 31i ,

[0101] (e 13 ) i = Δe 13 + Δe 21+ Δe 31i

[0102] where Δe 11 , Δe 12 , Δe 13 are the errors measured at 0.5I N , 0.8I N , I N respectively in the first step, i represents the different harmonic orders in the third step, the forward comprehensive error under the action of each harmonic component is calculated by using the formula at 0.5I N .

[0103] The purpose of the fifth step is to superimpose the errors of the first, second and third steps, and the basis is to consider the comprehensive action of each component on the input signal on the basis of the first step, and the result is a positive error.

[0104] Step 6: Superimpose the errors measured in the first, second and fourth steps affected by the reactive current, respectively:

[0105] (*e 11 ) i = Δe 11 + Δe 21+ Δe 41i ,

[0106] (*e 12 ) i = Δe 12 + Δe 21+ Δe 41i ,

[0107] (*e 13 ) i = Δe 13 + Δe 21+ Δe 41i

[0108] where Δe 11 , Δe 12, Δe 13 The first step is to measure the error of 0.5I N , 0.8I N , I N The measured error, i represents the different harmonic numbers corresponding to the fourth step. Using the formula to calculate 0.5I N Down Get the negative comprehensive error under the action of each harmonic component.

[0109] The purpose of the sixth step is to superimpose the errors of the first, second and fourth steps. Based on the first step, the comprehensive effect of each component when superimposed on the input signal in reverse is considered, and a negative error is obtained.

[0110] Step 7: According to the above obtained error of each point is not greater than the corresponding instrument grade allowed maximum error, then it is determined to be qualified.

Claims

1. A DC energy meter calibration device for use in conditions with a large DC ripple factor, comprising a voltage standard source, a current standard source, a standard DC energy meter, a DC energy meter to be tested, and an error comparator; characterized in that The voltage standard source consists of a controllable DC voltage source and a controllable AC voltage source. The accuracy of the controllable voltage source is less than 0.5%, the output ripple factor of the DC voltage source is not greater than 1%, and the maximum output voltage of the controllable DC voltage source is U DCmax The DC power meter is powered by 1.2 times the rated DC voltage U0. The output voltage frequency of the controllable AC voltage source is adjustable from 50 to 300 Hz, and the amplitude U a The maximum is 2% of the rated DC voltage U0 of the DC power meter; the current standard source consists of a controllable DC current source and a controllable AC current source. The accuracy of the controllable current source is less than 0.5%, the output ripple factor of the controllable DC current source is not more than 1%, and the maximum output current I max The output current frequency of the controllable AC current source is adjustable from 50 to 300 Hz, which is 1.2 times the rated voltage I0 of the DC energy meter. The standard DC energy meter consists of voltage and current sensors, low-pass filters, high-speed AD, digital filters, voltage accumulation and averaging processing modules, multipliers, and voltage-frequency U / F pulse converters. The voltage and current sensors convert the measured voltage and current into small voltage signals within a certain range. The low-pass filter filters the voltage signal. The high-speed AD uses a fixed sampling time T s =33.3uS for sampling, converting the voltage and current analog signals into digital signals, removing the disturbance signal with a digital filter, and accumulating the voltage and taking the average value processing module to accumulate the sampling signals until N=1201, respectively, to obtain the average values ​​of the voltage and current. and The multiplier will pass N*T s The average voltage, average current and time within a time period are multiplied together to obtain the electric energy W within that time period. The voltage-frequency U / F pulse converter outputs the obtained electric energy as the number of pulses. The error comparator compares the electric energy pulses generated by the standard DC electric energy meter with the pulses generated by the DC electric energy meter under test to determine the error of the tested electric energy meter.

2. A DC energy meter calibration method for a large DC ripple factor operating condition, which is used with the calibration device of claim 1 to implement: Step 1: The DC voltage standard source closes KD1 and opens KA1. If the rated voltage of the DC energy meter is U N , the rated current of the DC energy meter is I N , adjust the controllable DC voltage source, the voltage standard source output voltage U is U N , the DC current standard source closes KD2, opens KA2, adjusts the controllable DC current source, and the output current I of the current standard source is: 0.5I N , 0.8I N , I N , the voltage and current are applied to the standard DC energy meter and the DC energy meter under test. The standard DC energy meter measures the energy at each point within the specified time, which is E 11 、E 12 、E 13 The DC energy meter under test measures the energy at each point in W. 11 、W 12 、W 13 , and the absolute errors of electric energy are Δe 11 , Δe 12 , Δe 13 ; Step 2: The DC voltage standard source closes KD1, opens KA1, and adjusts the controllable DC voltage source. The output voltage U of the voltage standard source is U N , the DC current standard source disconnects KD2, closes KA2, adjusts the controllable AC current source, and the current standard source outputs current I=I N cos(ωt), f=50Hz, apply voltage and current to the standard DC energy meter and the DC energy meter under test. The standard DC energy meter is at U=U N , I=I N When cos(ωt), the electric energy measured within the specified time is E 21 , the DC energy meter measured the energy is W 21 , the absolute error of electric energy is Δe 21 ; Step 3: Disconnect KD1 from the DC voltage standard source, close KA1, and adjust the controllable AC voltage source. The voltage standard source outputs a voltage of U = 0.1U N cos(ωt), f=50Hz, the DC current standard source disconnects KD2, closes KA2, adjusts the controllable AC current source, and the current standard source outputs current U=0.1U N cos(ωt), apply voltage and current to the standard DC energy meter and the DC energy meter under test, change the frequency f=100Hz, 150Hz, 200Hz, 250Hz and 300Hz, repeat the third step test, the standard DC energy meter is at U=0.1U N cos(ωt), I=I N When cos(ωt), within the specified time, the standard electric energy meter measures the electric energy as E 31i , the DC energy meter measured the energy is W 31i , the absolute error of electric energy is Δe 31i , where i = harmonic order; Step 4: Disconnect KD1 of the DC voltage standard source, close KA1, and adjust the controllable AC voltage source. The voltage standard source outputs a voltage of U = 0.1U. N cos(ωt), f=50Hz, the DC current standard source disconnects KD2, closes KA2, and adjusts the controllable AC current source. The current standard source outputs current I=I N cos(ω(t+π)), apply voltage and current to the standard DC energy meter and the DC energy meter under test, change the frequency f=100Hz, 150Hz, 200Hz, 250Hz and 300Hz, repeat the fourth step test, the standard DC energy meter is at U=0.1U N cos(ωt), I=I N When cos(ω(t+π)), within the specified time, the electric energy measured by the standard electric energy meter is E 41i , the DC energy meter measured the energy is W 41i , the absolute error of electric energy is Δe 41i , where i = harmonic order; Step 5: Superimpose the errors affected by reactive current measured in the second and third steps to obtain (e 11 ) i =Δe 11 +Δe 21+ Δe 31i , (e 12 ) i =Δe 12 +Δe 21+ Δe 31i , (e 13 ) i =Δe 13 +Δe 21+ Δe 31i where Δe 11 , Δe 12 , Δe 13 The first step is 0.5I N , 0.8I N , I N The measured error, i represents the different harmonic orders corresponding to the third step, using the formula Get 0.5I N The forward comprehensive error under the action of each harmonic component; Step 6: Superimpose the errors affected by reactive current measured in steps 2 and 4 to obtain (*e 11 ) i =Δe 11 +Δe 21+ Δe 41i , (*e 12 ) i =Δe 12 +Δe 21+ Δe 41i , (*e 13 ) i =Δe 13 +Δe 21+ Δe 41i where Δe 11 , Δe 12 , Δe 13 The first step is 0.5I N , 0.8I N , I N The measured error, i represents the different harmonic orders corresponding to the fourth step, using the formula Get 0.5I N The negative comprehensive error under the action of each harmonic component; Step 7: If the errors of each point obtained above are not greater than the maximum error allowed by the corresponding instrument level, it is judged to be qualified.

Images