WU+Changjie SUN+Lingyan XU+Xiaoyan
Abstract： A realtime harmonic detection and compensation waveform generation circuit is designed for the harmonic parameter measurement system in ship power network. The design applies the distributed data processing method instead of the traditional centralized data processing method， and is equipped with a Digital Signal Processor （DSP） for online measurement of harmonic parameter， which can promote the realtime data processing ability. Then， a shunt active power filter is designed in a simulation experiment platform of isolated power network with power electronic load. The experimental result shows that the designed realtime harmonic detection and compensation waveform generation circuit works effectively for the ship power network.
Key words： ship power network； harmonic detection； harmonic suppression； shunt active power filter； simulation experiment
摘要：针对船舶电网谐波参数测量系统，设计电网谐波实时检测与补偿波形发生电路.该设计用分布式数据处理方法代替常规的集中式数据处理方法，采用数字信号处理器（Digital Signal Processor，DSP）进行谐波参数在线检测，提高其实时数据处理能力.在带电力电子负载的孤立电网仿真实验平台上设计一并联有源电力滤波器.实验结果表明，所设计的电网谐波实时测量与补偿波形发生电路在应用于船舶电网时能有效进行谐波检测和滤除.
关键词： 船舶电网； 谐波检测； 谐波抑制； 并联有源电力滤波器； 仿真实验
The development of electric power technology in the area of electric power production， transmission and distribution results in wide application of electric power technology. But the application of the phase control leads to significant deformation of source current and increment of demanded reactive power， which are very difficult to be suppressed. While the ship power quality is mentioned， the harmonic is the most popular topic. Recently， power electronic devices are applied to marine systems wider， which can be seen from the fact that the number of the power electronic devices equipped in ships increases quickly and the capacity of these devices in ships becomes larger， especially the obvious trend of substitution of the electrical propulsion system for the traditional diesel propulsion system can be observed since the electrical propulsion system is of superior operation characteristics. Because of the importance of the ship power quality， it is now considered as a special phenomenon and attracts a considerable attention， which in return makes the parameters of the ship power quality be under even stricter restriction compared with those of onshore power quality.
The measurement of ship power quality parameters and the improvement of power quality have now become a pressing task due to the fact that more and more troubles are caused by problems of the electromagnetic compatibility of ship environment.  That is， the effective reduction of harmonic pollution of ship power network is an efficient way to energy saving and ship power network safety.
1 Design of realtime power quality parameter measurement system for ships
One of the most significant parts of the research on power quality is the measurement of power quality parameters. The accurate measurement of harmonic together with the concise assessment on power quality parameters is considered as the necessary precondition of solving power quality problems. The most traditional method of multiorder harmonic component measurement is to adopt band rejection filters， which is simple and visualized in principle. But this method is no longer popular since it is very sensitive to the parameters of the filters and the core frequencies of signals， the hardware realization is difficult， and its precision is rather poor.
Generally speaking， there are only two major algorithms for power quality parameter measurement in practical application： one is based on series expansion of Fourier Transform； the other is based on the theory of Instantaneous Reactive Power of threephase circuit， also called pq method. The former method is able to detect a series detailed order of harmonics or the harmonics of a specified frequency band， which qualifies it for fault diagnosis and protection of power network， but its realtime operation property of current detection is unsatisfying. The latter method has quite good property in realtime detection， but it is not able to work effectively when source voltage is distorted. Nevertheless， the theory of Instantaneous Reactive Power of threephase circuit has been the most used theory till nowadays in the control of active power filters. Of course， there are quite a number of other types of harmonic detection methods， e.g.， the adaptive harmonic current detection method and the predictive current detection method. The fact is， all these proposed methods are still in the stage of research and development and lack of practicability. A current detection method based on the improvement of Instantaneous Reactive Power theory of threephase circuit， which is called ipiq method （or the current component filtering method）， is developed to effectively detect the load current and divide the load current into the fundamental active component， the fundamental reactive component and the harmonic component with accuracy and realtime property even when source voltage is distorted or threephase unbalanced.
There are various methods that can be selected to fulfill the task of power quality parameter measurement. The signals to be captured are normal electric power signals. There are mainly 2 methods of data obtaining， the centralized data processing method and the distributed data processing method， and the latter one is suitable for power quality monitoring of ship power network， as shown in Fig. 1. Furthermore， a realtime measurement system suitable for the detection of power quality parameters in an isolated small capacity power network is designed based on the application of Digital Signal Processor （DSP）. That is， the strong calculation ability of DSP is utilized to fulfill online detection of power network parameters， and to provide precise measured values for ship power network to improve its power quality.
1.1 Overall design of hardware system
The power quality measurement system for ships is required to have strong properties （sufficiently high accuracy， high speed and high antiinterference） in measurement of a variety of the power quality parameters concerning the ship power network. At the same time， the cost of the measurement system can not be high.
Since synchronous sampling is able to reflect the nature of the variety of AC signal and keep the sampling signal and the measured synchronous， it is widely used in the power quality measurement system. The most important point in using synchronous sampling is how to make sure that the sampling frequency and the signal frequency are strictly synchronous. Meantime， because the measurement unit is required to process large quantities of data， the utilization of the traditional serial port for data transmission can not satisfy the requirement of realtime data analysis in the power system. The following hardware circuit is designed to satisfy the aim of synchronous sampling and quick transmission （e.g. RS232C） of the sampled data. The block diagram of hardware composition is shown in Fig. 2. The designed system includes the analog quantity input module， the data acquisition unit， the DSP data processing unit， the synchronous phaselocked module and the analog quantity output module.
The analog quantity input circuit consists of the electric potential transformer， the current transformer， the current converter and the voltage converter. Both the current converter and the voltage converter take effect not only in the antiinterference but also in the isolation between the power system and the low voltage microcomputer system.
In addition， the transformers and the converters in the analog quantity input circuit make the sampling signal be mixed with the components of various frequencies before the signal entering the data acquisition unit. But in practical application， the system must satisfy that the aliasing of the frequency spectrum should be avoided. So the method that combines the analog lowpass filter with the digital filtering is used to improve the effect of antialiasing， as well as reduce the measurement error due to the nonstraight bandwidth characteristic while using the analog filter only. In order to make the filtering characteristics closer to the ideal situation， the analog antialiasing lowpass filter adopts a second order circuit， which is shown in Fig. 3.
The acquired signals enter the instrument based on the data acquisition unit， and then the instrument carries out the operation of data processing. Therefore， the measurement precision is related to the quality of this unit. That is， the structure of this channel should be selected rationally according to the technical requirement of the instrument， and the relevant chip should be chosen correctly and be connected with the mainframe circuit. This unit consists of the signal preconditioning circuit （as shown in Fig. 4）， the A/D conversion circuit， the synchronous phaselocked circuit， etc. Considering that the A/D converter produced as a DSP plugin board can not satisfy the system design requirement， a 14bit A/D converter is adopted.
The synchronous phaselocked circuit is applied in the designed system as shown in Fig. 5. Compared with the software synchronous sampling， though this method increases the hardware spending， it reduces the workload of the software greatly， and the reliability and error characteristics are better than the software method. That is， the hardware synchronous sampling is adopted to produce the sampling pulse. The clock/counter and the digital phaselocked loop are used together to complete the implement.
A DSP is selected as the data processing unit. After data processing and data storage， in order to display the excursion between the real voltage waveform and the standard voltage waveform more directly， a D/A converter is added to the system to convert the digital quantities stored in the DSP into the analog quantities displayed directly in the oscilloscope. The circuit of the analog quantity output module is shown in Fig. 6.
2 Design of shunt active power filter for ships
The designed physical model of the shunt active power filter in a simulated ship power network （the experiment platform） is shown in Fig. 7. In the simulated ship power network， a transformer is installed as an overland electrical power separator， which makes the power network connected to the transformer be an isolated power network. In the laboratory， the isolated power network is a small capacity network with a resistiveinductive nonlinear load. The shunt active power filtering circuit consists of two parts， the harmonic detection circuit and the harmonic suppression circuit that includes a threephase inversion IGBT bridge.
Fig. 8 shows the scheme of the precise connection of DSP and IGBT bridge， as well as the way of connecting the coupling reactors between the IGBT bridge and the power network. The snubber is installed for suppressing overvoltage and it is a nonpolar capacitor.
In the experiment， the phase voltage supplied from the transformer is 70 V. The measured waveform of load current is shown in Fig. 9. Figs. 10 and 11 show the incircuitemulation results operated in the emulator of the DSP. Fig. 10 shows the compensation current， which is the output of the shunt active power filter. Fig. 11 is the waveform of network side current. The waveforms of the incircuitemulation results show that the designed shunt active power filter is able to fulfill the harmonic current suppression and keep the current of network side close to a sinusoidal wave in a simulated ship power network.
3 Final remarks
The simulation research on measurement of the load harmonic current is carried out and a physical model of the shunt active power filter is built in a simulated ship power network. In the measurement part of the system， an analog antialiasing filter is designed to avoid the spectrum aliasing phenomenon of the measured signals and reduce measurement error. In the compensation wave generation part， a shunt active power filter for the ship power network is developed， which is realized by means of a transformer as an overland electrical power separator that makes the power network connected to the transformer be an isolated small capacity network， together with a resistiveinductive nonlinear load. The designed shunt active power filter is able to fulfill the harmonic current suppression and keep the current of network side sinusoidal. The simulation experimental results show that the designed method can be used to detect and compensate the harmonic current of network side in the ship power network effectively.
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（Editor ZHAO Mian）