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2016 China International Conference on Electricity Distribution (CICED 2016) Xi’an, 10-13 Aug, 2016 Design and Implement Harmonics Injection Source for Voltage Measurement Transducer Guo Liwei Guangdong Power Grid Corporation Communication Company Abstract—Nowadays, the fast rising of the industries and the power system development cause the harmonic source increasing. More and more high orders and power harmonic is injecting into the power system, which may cause equipments damage. The equipment has to be test whether its quality and performance is enough reliable when harmonic is existing at operating. Therefore, the harmonic injection method is used as the harmonic test bed. A new harmonic test bed is be proposal in the paper and implemented harmonic range from 100Hz to 5000Hz in High voltage laboratory. The new harmonic injection method is separately injection the harmonic source and the fundamental main supply via different step-up transformer. And the filter is designed and installed at the harmonic side in order to block the fundamental frequency which may cause the negative effect to the power amplifier. Index Terms—Power Quality, Reliability, Harmonic I. INTRODUCTION With the development of the power system, the modern equipments, such as communication systems and electronic equipment would be disturbed not only by power interruption or voltage sag, but also the distorted voltage or current source [1]. Although the concept of power quality is introduced at early 1990s, the interest of the harmonics becomes the key issue in recently year. As the definition of Harmonics, it is a periodic sinusoidal waveform having a frequency that is an integer multiple of a fundamental frequency (50Hz or 60Hz). [2] In power system, the waveform is expected as perfectly sinusoidal wave at rated voltage and one specific system frequency (50Hz or 60Hz). However, different kinds of power electronic equipment and sources, which built in the power system, cause the ‘pollution’ to the network [3]. As the harmonic propagation cannot be eliminated spontaneously, relevant suppression equipment has to be installed. The relevant equipment is expensive so that increases investment cost. The impact of harmonics is extremely harmful to the grid. For example, Capacitor or transformer would fail because of the resonance, overheating the transformers and motors causes low efficiency, the protection devices would maloperate and so on [3]. Therefore, the aim of the harmonics CICED2016 Session 2 Paper No CP0842 measurement in industry is verifying the compliance with the harmonic distortion requirement and investigating flawed design. Monitoring and measurement of the harmonics is necessarily considered in the power system operation. According to IEEE Standard 519 [4], the harmonic measurement should be taken up to 50th harmonics. However, it is required 100th harmonics in some industry customers. For monitoring and measurement of harmonics, the harmonic source designs can be classified as 3 types. First one is single power source [7], which is the source generation both fundamental and different orders frequencies. And the output signal is amplified by the power amplifier to the desired power. The signal can be directly injected to the test object. Second one is combined power source with main supply and harmonic generation [8] [9]. And there are two filters to block unwanted signals, one is located at the main source side to block harmonic signal and another one is located at harmonic side to block the fundamental signal. The main power and the harmonic generation are superimposed and injected into the test objective directly or via the step-up transformer. In the previous two designs, both of the main supply and harmonic injection way is either directly inject or use step-up transformer. There is a new design, which is separately located the main supply and the harmonic source by the step-up transformer. The main supply rises to the desired voltage level by the step-up transformer. And the harmonic source is amplified by power amplifier to desired power. This design is given another way to injection the harmonic to measurement the frequency response of the test object. II. PROPOSAL AND DESIGN OF THE HARMONIC INJECTION SOURCE A. Review the existing Test Rig The Medium Voltage Transducer Frequency Response Test bed was built in the high voltage laboratory. As Fig.1 shown, the test bed consists of Power Source, the High Voltage Reference, the VT test object and the data acquisition system. Page /5 2016 China International Conference on Electricity Distribution (CICED 2016) Fig.1 block diagram of frequency response test bed Step-Up Transfor mer Power Source HV Referenc e VT Test Object Data Acquisition System PC This test bed was used to do tests of the 33kV wound voltage transformer. PC generates the combined signal with fundamental and harmonic by controlled PCI-6251[10]. And the signal is amplified to desired level by a power amplifier. And there are two resistors R1 and R2. The R1is used as a bridge resistor that ensures the amplifier can output maximum power. And R2 is used as stabilizer. Next, the amplified signal is step up to the desired voltage level of the test object via the step-up transformer. The high voltage reference, North Star VD-60 [10], provides a reference voltage of the secondary side of the step-up transformer which also is the primary side of the test object. Finally, the reference of high voltage and response of the transformer is recorded by the PCI-6251. The PCI-6251 works as control system for the generation and data acquisition system. The test bed can inject 1% smooth harmonic at the high voltage for all harmonic range from 100Hz to 5000Hz. And the new injection method is based on this existing test bed. B. Proposal and Design As mentioned at previous chapter, the single power source, which contains with both fundamental and harmonic signal, via power amplifier and injection through one step-up transformer, is used in the existing test rig. Therefore, the new harmonic injection method is based on the existing test rig. The configuration of new injection method is shown as Fig. 2. Fig.2 the proposal test bed HV Reference 50Hz Main Supply From PC AMP Load V2 V1 ST1 WVT V3 High Pass Filter AMP Main supply Side V4 Harmonic Supply from PC ST2 Harmonic Side DAQ Device PC the fundamental signal and the harmonic signal separately inject into the system. The main supply and the harmonic supply are parallel connection. And there are two amplifiers connected with both of sources in order to amplify the signal to the required power level. And the amplified signals are step up by voltage transformers which are located at the main supply side and the harmonic side separately. In order to avoid the amplifier break down by the large current, a high pass filter is designed to give infinite impedance at 50Hz to block the fundamental signal so that the power of main supply can flow into the test object. At the same time, it gives low impedance for CICED2016 Session 2 Paper No CP0842 Xi’an, 10-13 Aug, 2016 harmonic supply to let harmonic power inject into the test object. Whether the new harmonic injection method gives a better or a worse performance than the original test rig is depends on the high pass filter performance. The reason why it is not necessary to install a low pass filter at the main supply side is the step-up transformer performs as a low pass filter in the system. The harmonic power consumes on both the harmonic side and main supply side transformers. The effect on the main supply side components is weak from the harmonic power. C Simulation of the High Pass Filter As mentioned previously, the load impendence is nonlinear in the system. In addition, the simulation model of step-up transformer and the WVT are not enough accurate while the computational complexity is difficult. Therefore, the simulation is roughly tuning the value of the filter. The tuning aiming performance is listed as follow Step.1.According to the superposition, the main supply is supping rating voltage, V1, voltage on the burden. Tuning the value of the filter, till the V3, is lower than 1V and the current, I, is very small when connected a small burden at the harmonic side, which is seem the voltage source as a short circuit with a small internal resistance, which means the filter blocks the 50Hz fundamental frequency. Thus, there is no negative effect for the amplifier at the harmonic side. Step.2.Short circuit the main supply and connected a small burden. The harmonic supply injects 1V at whole frequency range from second order to one hundredth orders (100Hz to 5000Hz) into the network. Tuning the filter, till the voltage at the HV reference, V2, can show an acceptable quantity of the harmonic power injected into the test object. And the harmonic voltage at the main supply side, V1, should keep at a very low voltage level. Step.3.Keep tuning the filter leads the frequency response of High voltage reference side to smooth and balance the cost of the filter components and the performance. D High Pass Filter Simulation Results In the simulation, the third order filter has following advantages 1 Much better attenuation at fundamental frequency 50Hz. 2.Three peak values in the V2 frequency response make the injection power lower from 300Hz to 2000Hz. 3.Smooth frequency response with the resistor installed. The values of components are fixed. The filter circuit diagram is shown in Fig.3 and parameter of components are listed in TABLE 1 Page /5 2016 China International Conference on Electricity Distribution (CICED 2016) transformer and the amplifier. The C1 is parallel connection by two 20uF capacitors each side. And the C3 is parallel connection by four 20uF capacitors each side. The circuit diagram and the actual filter are shown in Fig.6 Fig 3 the filter circuit diagram C1 C3 L2 Main supply side Xi’an, 10-13 Aug, 2016 Harmonic injection side Rr Fig.6 he circuit diagram of ST and filter and the actual filter and ST TABLE 1 the filter parameters C1 L2 C3 Rr 20uF 1mH 40uF 1Ω A.The System According to the proposed design of the harmonic branch, the whole Voltage transducer frequency response test bed was implemented in the HV laboratory of the University of Manchester. The harmonic branch consists of harmonic source, the power amplifier, the high pass filter and the step-up transformer. The whole block diagram is shown in Fig. 4. B.Validation the performance of the High Pass Filter The purpose of validating the high pass filter finds out whether the filter can block the fundamental frequency from the main supply side. In other word, the voltage at the amplifier, caused by the main supply, is so low that not negative effect on the amplifier. In addition, the high pass filter can pass though acceptable voltage level of high order harmonic injection. The procedure of validation the performance of the high pass filter is as similar as the tuning steps discussed. The circuit diagram is shown in Figure 7. Connect the filter with the current limit resistor, the amplifier and the harmonic source. Switch on the amplifier. Measurement the HV side voltage, the inductor voltage and the amplifier voltage. Fig. 4 the circuit diagram of whole system Fig.7 The circuit diagram of filter performance III. IMPLEMENTATION THE HARMONIC INJECTION SOURCE HV Reference 50Hz Main Supply From PC Load V2 V1 ST1 V4 WVT L2 AMP VLV C1 Vm C3 Rr V3 HS ST2 DAQ Device PC The configuration of the Harmonic supply is similar as the power source in the existing system. It consists of a Harmonic Source (supply by PC and share channel with the main supply), an EP2500, and two resistors R1 and R2. The circuit diagram of the power source and the actual power source. Injection fixed harmonic level 0.1% from the harmonic side into the system. Validation the harmonic can be measured at the high voltage side with an acceptable level. And the injection level in order to keep 1% harmonic level into the high voltage side is calculated. Fig.5 the circuit diagram of harmonic source and the actual power source Fig.8 Injection 0.1% from harmonic source Although, the VLP2500 amplifier only is amplify the harmonic source not both of fundamental frequency and harmonic. As the diagram shown, the circuit is balance circuit. The R1 and the R2 is current limit resistor, which make sure no tripping the amplifier protection by high current. The 3rd order high pass filter is designed as balance circuit in order to fit the voltage injection circuit. The filter is installed middle of the second step-up CICED2016 Session 2 Paper No CP0842 As the Figure shown, the highest harmonic level is 2.56% at 34th order harmonic (1700Hz). The smallest harmonic level is 0.01% at 2nd order harmonic (100Hz). However, the first four order harmonic is not in consideration as the chapter 3 mentioned that the high pass filter blocked the low frequency signals. So the smallest harmonic level is 0.14% at the 100th order harmonic (5000Hz). It proves the high pass filter is working as expected and the performance is acceptable. It means there is 10% of the high order harmonic power can be injected into the high voltage Page /5 2016 China International Conference on Electricity Distribution (CICED 2016) side. The high order harmonic power is absorbed by the step-up transformer. C.The determination steps of the voltage injection level at the harmonic side The voltages mention in the equations is represented as the normalized voltage, which is calculated by Equation 1 V NormanizedVoltage V_50Hz Injection fixed the harmonic level, V3_fixed, from the harmonic source. Measurement and record the voltage V2 at the high voltage side. The recorded data will be extracted the different order value by FFT in Matlab programming. According to the spectrum of V2, the transfer function (TF) between the V2 and V3 at whole frequency range from 50Hz to 5000Hz is calculated by Equation 2. V NormanizedVoltage Xi’an, 10-13 Aug, 2016 the result of the existing system. The case is to test the balance configuration filter whether can be injected desired level harmonic from 100Hz to 5000Hz. and the harmonic injection is sustaining 0.4 second. As mention in chapter 4, the fixed level harmonic 0.1% is injected from the harmonic side into the system. And according to the V2 spectrum, the voltage injection level is calculated in order to keep 1% harmonic at the HV side. The spectrum of harmonic injection at harmonic side is shown and the measured harmonic is shown in Figure 9. Fig.9 the spectrum of harmonic injection at Harmonic source and the spectrum of measured harmonic at HV side V3_fixed It is expected keeping 1% harmonic level at the high voltage side. The corresponding harmonic injection is calculated by Equation 3. (1%) TF Reinjection according to the spectrum of harmonic injection. Measurement and record the voltage at high voltage side V2 and the test object side V4. NormanizedVoltage It was expected smooth 0.7% level of harmonic injection at the HV side of the test bed for all harmonic. However, it is only 0.4% level of harmonic injection at the HV side at 5000Hz and it is more than 1% level of harmonic at around 1050Hz. The injection spectrum has to be recalculated based on the result, which is shown in Figure 10. Fig.10 Reinjection spectrum of harmonic injection at the harmonic source and spectrum of measured smooth harmonic at HV side IV. TEST AND COMPARISON OF THE NEW INJECTION METHOD AND RESULTS DISCUSSION The ultimate goal of this project is develop a new injection method, which is the harmonic source injection from another step-up transformer. And the test bed can injection 1% harmonic into the test object and get the frequency response The fundamental frequency 50Hz and the harmonic signal from 100Hz to 5000Hz (2nd order to 100th order) are generated by the PC via PCI-6251 by two different channel. The 50Hz signal is amplified by EP2500 at channel 1 to desired level (main voltage is 63.5rms). The harmonic signal is amplified by VLP2500 at channel 2. And the VLP2500 is set at a suitable amplify level. Both signals are injected into the network by each side transformer and superimposed at the high voltage side. And the HV reference voltage V2 and the 33kV WVT output V4 is measurement and recorded.. And the frequency response is shown by the voltage ratio between its secondary side voltage (V4) and the primary side voltage (V2), and it is calculated by Equation 4 as below. Nomanized _ voltage _ ratio As the Fig.10 shown, the percentage of harmonic injection at the harmonic side is higher than the first time, especially, the high order harmonic orders. It has to injection 120% of harmonic at the harmonic side in order to keep 1% harmonic at high voltage side. However, the smooth 0.7% harmonic level is obtained at the high voltage side. There is resonance around the 1050Hz which causes the harmonic level is higher than others. In a word, this method used for the harmonic injection is confirmed by the results As the mention at the introduction, the test bed is used for the test object frequency response. The response is compared with the existing system shown and the differences between the new method and the existing system is shown in Fig.11. V4 V2_50Hz × V2 V4_50Hz In the case studies, the result will be compared with CICED2016 Session 2 Paper No CP0842 Page /5 2016 China International Conference on Electricity Distribution (CICED 2016) Fig.11 frequency response and the difference and the existing system As the Figure 11 shown, there is no significant difference between the no-smooth injection and the existing system except the first few orders. However, there is significant difference between the smooth injection and the existing system at high order harmonic. The frequency response of the smooth injection is little higher, 5%, than the existing system. The reasons may cause this problem is listed in below: 1. The configuration of filters 2. The capacitor is unstable when it is operating at high voltage and high frequency 3. the charge or the discharge is not stable when the data is collected 4. The environment effect V. Conclusions The new harmonic injection method, injection harmonic and main supply separately, is implemented and validation. The harmonic injection is proved by the results, which is shown that the injection is successful and controllable. However, the accuracy of this method is not ideally as expected when the frequency response of the test object is measure. The frequency response of this injection method is higher compared with the existing system, which may be caused by the operation environment. And the harmonic injection is not enough stable, which may be caused by the quality of the capacitance. It is changing the capacitance when the voltage is varying. Moreover, the injection power is too high at the 100th order harmonic in order to keep 1% harmonic level at the high voltage side. Besides, this method may be improved by the following further research: 1. Better quality capacitor which has a stable value at the high voltage condition 2. Given much space between the isolation cage and the equipment. 3. Increase the value of C3 which can change the self-impulse of the filter and decrease the injection power at high order harmonic. Xi’an, 10-13 Aug, 2016 ter. [4]Recommendation Practices and Requirements for Harmonic Control in Electric Power Systems,IEEE Std.519-1992,1992 [5]IEC 61000-4-7 (1993), Electromagnetic compatibility (EMC) – Part 4-7:Testing and measurement techniques – General guide on harmonics and interhamonic measurements and instrumentation for power supply systems and equipment connected thereto. [6]IEC 61000-4-30 (2008), Electromagnetic compatibility (EMC) – Part 4-30: Testing and measurement techniques –Power quality measurement methods. [7] M.I.Samesima,J.C. de Oliveria and E.M.Dias,”Frequency response analysis and modeling of measurement transformers under distorted current and voltage supply ” IEEE Transaction on Power Delivery,vol.6,No.4,Oct 1991. [8] L. Cristaldi,A Ferrero and R.Ottoboni, “A VI-Based Equipment for the frequency-Domain Characterization of Voltage Transformers”, IEEE Instrumentation and Measurement Technology Conference,May,1997 [9] M.Tanaskovic,A Nabi, and S.Misur, “Coupling Capacitor Voltage Transformer as Harmonic distortion Monitoring Devices in Transmission Systems”, International Conference on Power Systems transien, paper Bo.IPST05-031,2005 [10] M series user manual. Han 15,2009.[Online]. Available: http://www.ni.com/pdf/manuals/371022k.pdf. Guo liwei was born in China, in 1987. Received the B.Sc. degree in Electrical and electronic Engineering from University of Strathclyde , the M.Sc. degree in Power System Engineering from University of Manchester. Currently working at Guangdong Power Grid Corporation Communication Company. Contact Number : 0086-13925025612 Contact Email: [email protected] REFERENCES [1] “Power quality measurement”. [Online]. Available: http://ww.allaboutcircuits.com/vol_2/chpt_12/4.html. [2]J.Arrillaga,N.R.Watson,”Power System Harmonics”,2nd ed. New York,John Wiley and Sons 2003 [3]PQ Course Brochure Manchester. [Online].Available: http://www.scribd.com/doc/24804266/PQ-course-brochure-manches CICED2016 Session 2 Paper No CP0842 Page /5