Estimation and reduction of harmonic currents from power converters

Power Electronics is entering more and more products that inevitably increase the number of non-linear loads installed on the power system. The major concern of the non-linear loads is the emission of non-sinusoidal currents in the supply. Circulation of the harmonic currents in power systems creates losses, thus determining overrating of the power system. Furthermore, the harmonic currents cause harmonic voltage distortion, which is detrimental for all connected equipments to the power system, such as capacitors, ac-machines, control and protection equipments, measuring devices and electronic power supplies. Although their design takes into account a certain level of harmonic voltage distortion, there are many real-life cases when the equipments experience abnormal operation, malfunction or failure.

One such case appeared at a local company in Denmark, a Heat Power Station where due to the existing large power Adjustable Speed Drives (ASD), a fault-trip is reported at one of the control equipments. The fault-trip does not allow the operation of the plant at a loading further than 80 % of the total designed power.

This case shaped the research directions of the project, to study methods of estimation and reduction of the harmonic currents from industrial Adjustable Speed Drives, in order to come up with an optimized solution for customers. The work is structured in two main directions. The first direction consists of analyzing the mechanism of the harmonic current generation from ASD's. The second direction presents different passive and active harmonic parallel filters for harmonic current.

Regarding the estimation of the harmonic currents, extensive investigations are done to establish a practically usable prediction method. Among four different methods of harmonic predictions, i.e. theoretical, table-based, simulated and numerical methods, the most suitable for the actual project purpose was found the table-based method. It provides the results in a fast way; it needs no laborious implementation and it can be customized for different applications, in this case ASD's. However, the table-based method depends very much on the amount and accuracy of collected data in the development stage. The outcome of this investigation is a Harmonic Calculation Software compiled into a Graphical User Interface PC-software application, which can be applied for fast estimations of the harmonic currents in typical industrial applications.

The Harmonic Calculation Software was compared against harmonic measurements, to asses its estimation precision. It is confirmed in a good agreement with both simulated and measured data that the harmonic voltage distortion created by the existing ASD's in the Heat Power Station case is beyond the standard immunity limits (in this case of 8 % Total Harmonic Voltage Distortion), which is the cause of the occurring fault-trip. It is concluded that the table-based approach used for the Harmonic Calculation Software gives relatively accurate harmonic results, in the range of 5-10 % errors compared to other state of the art simulators and also the measured data.

The second direction in this project follows the red-line given by this real-life case, to find a suitable harmonic mitigation solution to meet the terms existing international harmonic standards. On this ground, different parallel harmonic filters are investigated to decrease the actual level of the harmonic current distortion below the standard limits. The analysis includes passive power filters (PPF), active power filter (APF) and hybrid power filters (HPF).

PPF's are the main candidates, as they are very common in harmonic current mitigation. Although their mitigation efficiency is lower, they are compact, simple to build, and relatively inexpensive. As it is confirmed here, the design of a PPF is very sensitive to the system impedance, but this can be decoupled with a large line reactance installed on the line. Such a topology, in essence a harmonic trap filter customized for ASD's installation, is investigated and analytically modelled here. Its design and performance are analyzed, which indicate a better stability compared to classical PPF's. Based on simulations it is found, that for the given Heat Power Station case the trap filter is sufficient to comply with the harmonic standards. This solution is currently installed and proves good harmonic mitigation performance, as the faulttrip does not occurs and the installation can operate in safe conditions up to the nominal rated power.

Continuing the investigation of harmonic current mitigation, the project moves the focus on active filtering methods as a way of improving the harmonic mitigation performance. The design of the APF's is reviewed and recommendations are given regarding different harmonic detection methods: instantaneous power theory, fundamental and harmonic rotating frame transformations, generalized harmonic integrators. Extensive simulations are employed in Matlab/Simulink and a laboratory stand is built to test the shunt APF topology. The actual implementation is done in fundamental rotating frame, where a new selective harmonic integrator is developed to reduce the required number of harmonic controllers, which allows a faster execution speed with the same Digital Signal Processors. It is also concluded that such harmonic controllers have improved stationary performance and better dynamic behavior compared to similar existing control methods.

Although the purpose of APF study was not finding a solution directly applicable for the Heat Power Station, there was an influence in the way the research line is further approached. As the cost of the APF is still relatively high, its utilization becomes more efficient for mediumor high-power applications. However, at a higher power the inverter cannot operate at a very high switching frequency because of the considerable increase of switching losses. On the other hand, a lower switching frequency is not desirable in APF's because of the reduced controller bandwidth, which determines improper harmonic current compensation, even unstable operation. In this respect several potential utilizations of APF's of moderate switching frequency (in the range of 10 kHz) were investigated to tackle the mitigation of higher power harmonic currents. Several design steps for the passive components and the APF control of the proposed topologies are given together with laboratory tests.

One harmonic current mitigation solution found is to connect (two) smaller power APF's in parallel, sharing the same ac- and dc-bus. It is proven that parallel APF's may have lower passive components although other issues arises, like circulation currents, which is removed here by common mode coils.
Another harmonic solution is to use cascade connection of (two) independent APF's that cooperatively share the task of the harmonic mitigation. Two cooperative control methods are proposed called load-sharing and harmonic-sharing, which give an increased overall performance, in terms of dynamic response and harmonic compensation.
A third harmonic solution consists in using a smaller power inverter in a HPF topology fitted with a proper control to reduce the power flow through the inverter. It is found that this active harmonic solution may be a possible candidate for the actual plant, as the power inverter in HPF is less than 10 % of an APF's and the HPF provides also reactive power, which is useful in this case.

The application of Discontinuous PWM's in APF's is investigated, and a new discontinuous modulator is proposed, which proves that although it gives higher distortion compared to a continuous PWM, it assures adaptive clamping of the power switch that conducts the largest current at any instant, thus reducing the switching losses to less than 60 %.

It is concluded in this thesis that a passive power filter designed for ASD applications was sufficient to provide compliance with harmonic standards for the studied case. A better efficiency for medium- to high-power applications may be obtained with active harmonic filtering, which can be implemented with parallel connected shunt APF's or hybrid power filters.

The research of in this project deals with harmonic currents from ASD's. The models developed help predicting the harmonic currents and propose solutions for their mitigation. The project makes it possible to compare different harmonic mitigation solutions and reveals their demands in respect to the performance. The project gives also a good basis for future analysis.