Transient characteristics verification method for DC transformer used in flexible HVDC system

1 Introduction

High-voltage,direct current (HVDC) transmission technology has been widely employed owing to its advantages such as long-distance transmission,large capacity,no synchronization,and broader application prospect [1-3].Flexible DC transmission is a new generation DC transmission technology with improved technical advantages in island power supply,urban distribution network capacity transformation,AC system interconnection,and large-scale integration of wind farms.The control and protection signal of a flexible DC transmission system requires a faster sampling speed and wider bandwidth owing to the large instantaneous fault current caused by low-loop impedance on the DC side.Therefore,higher requirements have been proposed for the transient characteristics of a DC transformer,particularly for fast response of the transient voltage and current during fault conditions [4-5].

A DC transformer is the key equipment for protection and control in a flexible HVDC system; its transient characteristics directly affect the safe and stable operation of the power grid,and the verification of the transient characteristics of a DC transformer can effectively improve its performance and ensure safe and stable operation of the flexible HVDC system [6-7].

The Chinese National Standard GB/T 26217-2010 and GB/T 26216.1-2010 provide clear requirements on the transient characteristics of DC transformers such as the step response characteristics and frequency response characteristics; however,transient tests for a DC transformer are rarely conducted locally due to lack of suitable test method and equipment.Many studies have been conducted on the verification of transient characteristics of an AC electronic transformer [8-10]; however,few studies have been conducted on the verification of transient characteristics of a DC transformer.The step response time of transient characteristics of a DC transformer have been verified by using a transient controllable current source and oscilloscope in a field test [11] and by establishing the transfer function model of a DC comparator [12].

In this study,a transient characteristics verification method for a DC transformer used in a flexible HVDC system is proposed based on resampling technology and LabVIEW measurement technology.Laboratory experiment was conducted to determine the transient characteristics of a full-fiber electronic DC transformer using the proposed verification method.Experimental results show that the verification method can achieve frequency response and step response verification of a DC transformer.

2 Key techniques for DC transformer transient characteristics verification

The transient characteristics test method of an electronic transformer can be used as reference for a DC transformer as the latter is a special type of electronic transformer [13].Related studies have been carried out by several scholars on measurement methods for the delay time of an electronic transformer [9,14-15].At present,the steady-state verification technology of a DC transformer is relatively mature,and diverse results have been obtained [16-18];however,there are relatively few studies on transient characteristics verification.The differences between transient and steady-state verification of a DC transformer are as follows:

(1) Transient characteristics verification requires the use of a transient source including a wideband source and a step source,which has a higher technical threshold than a steadystate source.

(2) A higher sampling rate is required for a transient characteristics verification system to meet the requirements of higher sampling rate for a flexible DC transformer and ensure accuracy and reliability of transient response verification.

(3) There are more stringent requirements on synchronization and higher clock synchronization accuracy in transient verification.

(4) In transient verification,uncertainty of the error introduced by discrete digital signal processing in step response verification is reduced.

According to the Chinese National Standard GB/T 26216.1-2010 and GB/T 26217-2010,the signal frequency range is 50 Hz-1200 Hz in the frequency response test,and the amplitude should be 10% of the rated current and voltage.The step response test should be conducted using at least 10% of the rated current and voltage.In this study,a scheme suitable for transient characteristics verification of small voltage and current (10% of rated voltage and current) is proposed based on analysis of key technologies of transient characteristics verification.

For the transient source,the frequency response verification requires the use of a wideband source in the frequency range of 50 Hz-1200 Hz.A simple solution is to use a combination of a signal generator,a power amplifier,and a current or voltage booster to provide wideband current or voltage.The step source can consist of a signal generator and a power amplifier to produce a continuous square wave signal for step response verification.

The sampling rate of a flexible DC transformer is usually 50 kHz or 100 kHz.The standard sampling module in the transient characteristics verification system is a highspeed acquisition card with mega-level sampling rate,to ensure sampling signal integrity and verification accuracy.

System on a programmable chip (SOPC) technology based on field programmable gate array (FPGA) with NIOS soft core processor as the core can be used for highprecision clock synchronization to meet the stringent requirements of clock synchronization in step response verification.

Discrete digital signal processing introduces technical difficulty in the step response verification,and the verification result is closely related to the sampling rate of a DC transformer or verification system regardless of analog output or digital output from the DC transformer.The sampling rate of a flexible DC transformer is typically 50 kHz at a sampling interval of 20 µs,the typical delay time of step response is usually less than 100 µs,and the rise and fall time is typically approximately 30 µs.When the sampling rate of the tested DC transformer is low,the verification results have poor reproducibility due to the large uncertainty introduced by discrete digital signal processing when calculating the delay time and the rise or fall time of the step response.Two solutions can be implemented to address this problem.First,the sampling rate of the tested DC transformer can be increased,but this is obviously unrealistic.Second,the sampling rate can be increased using resampling technology in the case of a fixed sampling rate of the DC transformer to reduce the uncertainty introduced by the large sampling interval and improve the measurement accuracy and stability.

The resampling principle is shown in Fig.1.Fig.1(a)shows the sampling data mapped to the same time period with several rising edges,whereas Fig.1(b) shows the resampled data.The principle of the resampling technology is described below.Under the condition that the clock of the high-speed acquisition card and calibrator is strictly synchronized,the time for 50% of the time in the rising or falling edge of the standard channel is set as a reference point.Then,sampling data of multiple rising or falling edges overlap with each other at the same time period,which improves the sampling rate of the DC transformer and solves the problem of low accuracy and poor reproducibility of measurement result due to low sampling rate.

Fig.1 Principle of resampling technology

3 Transient characteristics verification scheme of DC transformer

3.1 Design of transient characteristics verification system

The transient characteristics verification scheme of a DC transformer is shown in Fig.2.The scheme is suitable for analog and digital output from a DC transformer.The transient source consists of a signal generator,a power amplifier,and a current or voltage booster.The signal generator outputs AC signal in the frequency range of 50-1200 Hz for frequency response verification and square wave signal at a frequency of 50 Hz for step response verification.The frequency range of a power amplifier is up to 10 kHz with phase accuracy of 0.006°,which can meet the transient verification requirements of a DC transformer.

Fig.2 Transient characteristics verification scheme of DC transformer

During analog verification,the analog signal outputs of the tested and standard DC transformer are fed to the high-speed acquisition card for sampling,and then to the transient verification system for verification.During digital verification,the FT3 protocol message output of the tested DC transformer is analyzed by the message parsing device;this device outputs the measurement signal through the Ethernet to the transient verification system for processing.

In this study,LEM's ITZ 2000-50-PR ULTRASTAB and FLUKE's A40B-100A shunt was used as the standard DC transformer,the shunt has a DC measurement uncertainty of 15 × 10-6 and a phase offset of 0.001°,and is suitable for transient verification.NI's PXI-5922 acquisition card was used as the high-speed acquisition card,with a sampling rate set to 2 MHz and sampling resolution of 16 bits,which meets the high sampling requirements of a standard DC transformer.

The clock synchronization device is used for synchronizing data acquisition.The frequency and duty cycle of the synchronization signal are adjustable.The time values of the rising and falling edge of the pulse are all less than 300 ns,the phase errors of the positive and negative output signals are less than 10 ns,which meets the highprecision clock synchronization requirements for transient verification.

During the step response verification,the signal generator outputs square wave signal and the rise and fall time was set to 20 µs,whereas the duty cycle was set to 50%.Fig.3 shows the output waveform of the signal generator and standard DC transformer measured with an oscilloscope using the traditional recording method.The error introduced by the power amplifier,current or voltage booster,and standard DC transformer is very small,the step delay is approximately 1 µs and rise and fall time is approximately 1.5 µs,which meets the requirements of step response verification for a DC transformer.

The transient verification system shown in Fig.2 is based on LabVIEW platform and was used to process the transient verification data.The PC interface of the transient characteristics verification system is shown in Fig.4.

Fig.3 Output waveform of signal generator and standard DC transformer

Fig.4 Transient characteristics verification system

3.2 Step response verification

Frequency response verification is relatively simple as it involves calculating only the ratio error and phase error using Fourier algorithm.Step response verification involves the calculation of step-time parameters in microseconds and implementing the process for discrete digital signal,making the verification more difficult.When calculating the delay time of the step response,the time for 50% of the actual maximum sampling data measured using the standard and tested DC transformers can be calculated using an interpolation algorithm,and set as the start and end time for calculating the delay time.The following precautions are necessary when using the interpolation algorithm:

(1) The rise time of the step source should not be less than 2 times the sampling period to ensure accuracy of the interpolation.

(2) When the high-speed acquisition card and message parsing device are not strictly synchronized,the offset cumulative error of the sampling points of two adjacent pulses gradually increases.Thus,the first transition edge is more appropriate for calculating the delay time to avoid the influence of accumulated error.

When the high-speed acquisition card and message parsing device are strictly synchronized,the sampling rate can be improved using resampling technology to ensure that the rise time of the step source is not less than 2 times the sampling period.Fig.5 shows the delay time measurement principle,whereas Fig.6 shows LabVIEW interface for delay time measurement of the step response.

Fig.5 Delay time measurement principle

The red and blue curves in Fig.5 represent the standard and tested sampling data,respectively,which are discrete sampling points.The values y0 and y0' are equal to 50% of the actual maximum sampling data measured using the standard and tested DC transformers,and their corresponding time values are t0 and t0'.There are two sampling points before and after the sampling points mentioned above,namely (y1,t1) and (y2,t2) in the red curve and (y1',t1') and (y2',t2') in the blue curve,respectively and the sampling intervals of the standard and tested sampling data are Δt and Δt',respectively.Assuming that the curve approximates a straight line at 50% of the rising edge,the time at 50% of the rising edge can be calculated as given in equations (1) and (2):

where Δt= 1/ f 0,Δ t '= 1/f0',f0 and f0' are respectively the sampling frequency of the standard and tested DC transformers.

The delay time tD of the step response can be obtained using the above formulas as follows:

Fig.7 shows LabVIEW interface for the rise and fall time measurement of the step response.

During the calculation of the rise and fall time of the step response,the sampling data is processed using the resampling technique; then,the time values at 10% and 90%of the rising edge are calculated using equation (2).The rise and fall time is the time difference of these two values.

Fig.6 LabVIEW interface for delay time measurement of step response

4 Test for transient characteristics verification

In this study,the transient characteristics verification system of a DC transformer was used to verify the transient characteristics of eight full-fiber electronic DC current transformers from a specific manufacturer.The setup for the field test is shown in Fig.8.The tested DC transformer consists of a sensing unit and an acquisition unit,with a rated current of 3000 A,accuracy of 0.2,and sampling rate of 100 kHz.During the step response verification,10% of the rated primary square wave current (300 A) was applied,and the rise and fall time of the square wave was set to 20 µs and the duty cycle was set to 50%; during the frequency response verification,20% rated primary wideband current was applied.Fig.9 shows the sampling waveform (red curve:current waveform of tested transformer; white curve:current waveform of standard transformer).The sampling data in Fig.9 was processed using resampling technique,and the processing result is shown in Fig.10.Fig.10(a) shows the sampling waveform for multiple rising or falling edges mapped to the same time period,whereas Fig.10(b) shows the final waveform of the rising edge processed using resampling technique,and the sampling rate increases equivalently,which has more sampling points at the rising edge.

Fig.7 LabVIEW interface for rise and fall time measurement of the step response

Fig.8 Setup for field test of transient characteristics

Fig.9 Sampling waveform of the standard and tested DC transformers

Fig.10 Resampling waveform of the tested DC transformer

Fig.11 shows the delay time of the step response at the rising and falling edge,respectively.It can be observed from the measurement results that the delay time of the step response essentially changes in the range of 15-20 µs,and the maximum delay time does not exceed 25 µs.The test results are essentially consistent with the actual situation.

Fig.12 shows the rise and fall time of the step response of the DC transformer.

Fig.11 Delay time of step response

Fig.12 Rise and fall time of step response

The test results show that the rise and fall time of the step response is mostly in the range of 17-25 µs,and the maximum rise or fall time does not exceed 30 µs.Some portions of the rise and fall time in the test results are less than 20 µs because of the following:the test was conducted using 10% of the rated step current,noise in the current signal was relatively large,and a large uncertainty factor was easily introduced in the calculation of the start and stop time of 10% and 90% of the rising or falling edge.To address these problems,the start and stop time was set to 15% and 85% of the rising or falling edge so that the actual rise and fall time of the step response is smaller; this can be achieved by improving the verification algorithm or increasing the primary voltage and current.

Fig.13 and 14 show the ratio error and phase error of the frequency response of eight full-fiber electronic DC current transformers.It can be observed that the error of one of the transformers is larger than the tolerance,and the error ratio of the remaining transformers has the same trend as the frequency.The curve of the error ratio is almost linear and minimal variations occurred in the phase error.The measurement results reflect the actual frequency response characteristics of a DC electronic transformer by comparing the product reports.

Fig.13 Error ratio of frequency response

Fig.14 Phase error of frequency response

5 Conclusions

In this study,a transient characteristics verification method for a DC transformer used in a flexible HVDC system is proposed based on resampling technology and LabVIEW measurement technology after analyzing the key technology of transient characteristics verification for a DC transformer.The transient characteristics verification system of the DC transformer was designed and verified through experiments.The following conclusions are drawn from the study:

(1) Experimental results show that the transient source consisting of a signal generator,a power amplifier,and a current or voltage booster can be used for transient characteristics verification at a small voltage or current (10%of the rated value) of the DC transformer.

(2) Resampling technology was used to improve the sampling rate and reduce the uncertainty error introduced by discrete digital signal processing in the step response verification in order to address the problem of low sampling rate of the DC transformer during transient verification.

(3) Measurement results of the frequency response and step response for eight full-fiber electronic DC transformers show that the proposed method can verify the transient characteristics of a flexible DC transformer at a sampling rate of 100 kHz.The measurement results are consistent with the actual situation,indicating the capability of the proposed method to verify the transient characteristics of a flexible DC transformer.