Analyzing the potential for using active filters to reduce voltage nonsinusoidality in the electric power supply system of a coal open-cut in Vietnam

Coal mining is one of the important economic sectors in Vietnam. Power supply systems of coal mines and open-cuts are complex. They have multiple levels of voltages and different types of loads. New electrical equipment usually works alongside with the equipment that is obsolete and worn-out. Power supply systems of industrial areas of Vietnam where coal is mined are characterized by the low power quality. In Vietnam the indices which characterize the distortion of both voltage and current waveforms have standard values. The article presents the findings upon the analysis of a power supply system of a coal open-cut in Vietnam and the electrical equipment of the coal sorting plant along with the results of the experimental studies of power quality indices and the non-sinusoidal mode parameters. The measurement analysis has shown that the indices of the 5-th and 7-th harmonics of voltage were over the limits. There are interharmonics in voltages and currents. The article analyzes the characteristics of active filters. It is possible to improve the power quality by using an active filter which generates both reactive power to increase the load power factor and harmonic and interharmonic currents to reduce the degree of voltage and current waveform distortion.


Introduction
Coal mining is one of the important economic sectors in Vietnam. Power supply systems of coal mines and open-cuts are complex. They have multiple levels of voltages and different types of loads. New electrical equipment usually works alongside with the equipment that is obsolete and worn-out. There are high-power loads including non-linear ones which distort the parameters of the electric network modes and, thus deteriorate the power quality. Power supply systems of industrial areas of Vietnam where coal is mined are characterized by the low power quality.
In Russia the degree of voltage waveform distortion is limited to the standard values specified for the KU and KU(n) indices in [1]. In Vietnam the indices which characterize the distortion of both voltage and current waveforms have standard values specified in [2,3].
This article analyzes a power supply system scheme of one of the coal open-cuts in Vietnam, its loads, the power quality based on the measurements of power quality indices and harmonics mode parameters. It also analyzes the characteristics of active filters that could be used to improve the power quality in the coal opencut.

Power supply system of a coal opencut
The power supply system of a coal open-cut is given on Fig. 1. The diagram makes it clear that the power is supplied to the coal open-cut from a 110 kV twotransformer substation, which is powered from two independent sources via 110 kV transmission lines. One of the transformers is in working condition and the other is in reserve. There are two 22 kV busbar sections (6604 and 6643 nodes) on the low-voltage side of the transformers. The electric power is supplied to the coal open-cut from the 22 kV busbar section (node 6643) through a 22/0.4 kV step-down transformer.
Coal is mined using an excavator and then transported by conveyors to a sorting plant for processing. The sorting plant has two coal-sorting shops and an electric power and water supply shop. The total length of power lines of all voltages (110, 22 and 0.4 kV) is about 60 km. Reactive power sources -450 kVAr capacitors -are connected to the 0.4 kV buses.
The sorting plant has various processing equipment: vibration generators, breaker machinery, conveyors, winches, pumps, etc. Most of this equipment is powered by asynchronous motors with power ranging from 4 to 185 kW.  The electric power and water supply shop which provides lighting to the sorting shops, supplies water to the sorting plant and ensures coal transportation to a warehouse has 23 units of electrical equipment. The power range of electric motors is from 4 to 160 kW.
In general frequency regulated asynchronous motors are used at the sorting plant. Fig. 3 shows the asynchronous motor diagram.
Frequency regulated asynchronous motor consumes non-sinusoidal current. It is a non-linear load, i.e. a source of harmonic and interharmonic currents causing the power quality degradation. To assess the power quality the power quality indices and the nonsinusoidal mode parameters were measured. The measurement results are provided below.

Results of the power quality and mode parameters measurement
The measurements were carried out using a PQ-Box 150 tool made in Germany. The measurements of the power quality indices and mode parameters were being taken from the low-voltage side of a 22/0.4 kV stepdown transformer at node 4143 in the course of 24 hours with a one second interval.  standard value. The Table shows that although the 450 kVAr capacitors are installed for reactive power compensation, the average value of the power factor in phase B is 0.83 which is less than 0.85 (the standard value) specified in [2]. This is a reason why the company that owns the coal open-cut has to pay a fine every month.     6 shows a graph of the change in the power quality index -the total harmonic distortion of voltage KU in phase A and the standard value KUS. The figure shows that most of the measured KU values exceed the standard value of 6.5% specified in [2]. The largest KU value is 16.7%, the smallest value -3.8%, the average -9.2%.  The standard value for KU(n) equal to 3% is exceeded on the 5-th and 7-th harmonics, the sources of which are three-phase 6-pulse rectification circuits that feed the frequency regulated asynchronous motors.  7 shows the graphs of changes in the measured KU (5) and KU (7) indices in phase A. It is clear that KU (5) and KU (7)

Results of current harmonics measurement
Tables 4 and 5 provide statistical estimates of coefficients of the n-th current harmonics KI(n) for some harmonics in phase A. The tables show that KI(n) values do not exceed the standard value of 12% specified in [3]. The largest coefficients values are on the 3-rd, 5-th and 7-th harmonics, notably, during the break in work.

Results of voltage and current interharmonics measurement
Unlike harmonics, interharmonics have fractional numbers. Their frequency is not multiple to the fundamental frequency of 50 Hz. Interharmonics appear when working with loads such as static frequency converters. They have a major influence on the electrical network mode since they cause additional electric power losses [4]. The Russian [1] and Vietnamese [2,3] regulatory documents on the power quality do not provide any indices characterizing interharmonics nor any limits for their values. When measuring the power quality indices and the non-sinusoidal mode parameters, the voltage and current values of interharmonic centered subgroups were determined [12]. The frequency of the interharmonic centered subgroup is defined as the average of two multiple frequencies, i.e. fisg,n =(fn+fn+1)/2, where na harmonic number, isgis interharmonic centered subgroup. Tables 6-9 provide the statistical estimates of voltage and current values for interharmonic centered subgroups in phase A for both the operation time and break time. The interharmonic number, represented, for example, as 1-2, means that the interharmonic number of the centered subgroup is n=(1+2)/2=1. 5.
From the data provided in tables 6-9, it follows that during working hours the interharmonic voltage and current values exceed the values of the interharmonics during a break in work. The  the current harmonics and interharmonics, which cause the voltage harmonics and interharmonics.

KU(5)
and KU (7) indices exceed the standard value of 3%, established in [2] both during the operation time and break time. To reduce the KU (5) and KU (7) values to the standard value, it is necessary to apply special measures for example to install an active filter.

Characteristics of active filters
The operating principles of active filters were developed in the 1970s. Recently, a closer attention has been paid to these filters due to the introduction of bipolar transistors with insulated gate and digital signal processors [5,6].
The operating principle of the active filter is as follows: the power electronics of a filter generates current harmonics into the electrical network, which are equal in magnitude to the harmonics of the currents consumed by the non-linear load but with the opposite phase. The load current is measured by a current transformer and then analyzed by a digital signal processor to determine the spectrum of current harmonics, their magnitude and phase. The obtained information is used by a generator of current harmonics to produce current harmonics into the network in the next period. These harmonics will have the values and phases required to compensate for the current harmonics and the reactive power consumed by a nonlinear load. Considering that the active filter is controlled using the data received from the current transformer, it dynamically adapts to the changes in load current harmonics. The software controls the processes of analysis and current harmonics generation, so the active filter is easily programmed to compensate for any harmonics.
The active filters are classified according to the output power value [7]. The filter selected for harmonic currents compensation should match the non-linear load power and the reactive power it consumes.
The rated power of low power active filters is under 100 kVA. These filters are intended for three-phase networks of residential areas, business buildings, hospitals, for small and medium factory loads and enterprises using electric drives. The response rate of the low power active filter to the change in the load current harmonics is within tens of microseconds and milliseconds. During this time active filters produce current harmonics and reactive power into the network in compliance with the needs of the electrical network at the filter installation site [5,8].
The rated power of average power active filters ranges from 100 kVA to 10 MVA. They are intended for use in three-phase distribution networks of medium and high voltage, including under a minor voltage unbalance. For economic reasons, high-voltage distribution networks do not use active filters for reactive power compensation, since insulation problems arise. The response rate of the average power active filter to the change in the load current harmonics is tens of milliseconds.
The rated power of high power active filters is over 10 MVA. They are very expensive since currently there is no electronic equipment, which operates at high voltage and at high frequencies. To ensure the operation of these filters, they should be provided with special current sensors and instrument transformers, which should operate at high voltages and high currents. The response rate of the high power active filter is tens of seconds. This is necessary for the operation of electric contactors and circuit breakers once the optimal decision about switching the transistors is made.
Based on the grid connection method with regard to a non-linear load, series and parallel active filters are distinguished. A parallel active filter is used both to compensate for current harmonics and for reactive power of a non-linear load [9]. Fig. 8a shows a configuration of a parallel filter connection to the network. Fig. 8b demonstrates a connection configuration of a series active filter. A voltage source is the electrical network and a current source is a nonlinear load.
In the figures: C U -a network voltage vector, Series active filters [10] are used when only current harmonics compensation is required. They are connected between the mains voltage source and the non-linear load through three-phase transformers or three single-phase transformers as shown in Fig. 8b.
The main disadvantage of the series active filter is the need to withstand large currents of load in the secondary winding of the connecting transformer, which increases the series filter rated current compared to a parallel filter. The main advantage of the series filters over parallel filters consists in the fact that they could be used for voltage harmonics cancellation and three-phase voltage balancing [11].

Conclusion
The analysis of the coal open-cut power supply scheme, the electrical equipment composition at the sorting plant, the measurement results for power quality indices and the non-sinusoidal mode parameters along with the characteristics of active filters suggest that an active filter can be used to reduce voltage and current nonsinusoidality so that it meets the requirements specified in the power quality regulations accepted in Vietnam. When choosing the type, power and parameters of the active filter, it is necessary to carefully analyze the measurement results, mode parameters of the network, where it is going to be installed, both at the harmonic and interharmonic frequencies and at fundamental frequency.
This work is conducted in the framework of the research projects № AAAA-A17-117030310438-1 of the program of fundamental research of SB RAS III.17.4.