APPLICATION OF AN ACTIVE RECTIFIER USED TO MITIGATE CURRENTS DISTORTION IN 6-10 KV DISTRIBUTION GRIDS

The paper addresses issues of using the active rectifier in partially loaded variable frequency drive as active filter in the conditions of non-sinusoidal current and voltage disturbances caused by the presence of high-power non-linear load in the grid. The topology of transformless three-level converter for 6-10 kV suitable for proposed solution has been presented and its mathematical model has been de-rived. Based on the model, the direct power control algorithm with ability to compensate non-linear currents has been designed. The investigation of active rectifier efficiency was performed depending on the relation between linear and non-linear load currents of the grid node, as well as on active power load of the active rectifier. Efficiency analysis was based on the developed computer model of the grid node with connected non-linear load simultaneously with the variable frequency drive with active rectifier.

As present mines still contain numbers of non-linear loads based on diode rectifier, the presented study considers case of using the AR to mitigate current harmonic distortion, caused by traditional VFD operation. To maintain such functions special control algorithm is required. There are two basic control system topologies for the AR, both came from similar algorithms used to control motor inverters: voltage oriented control (VOC) and direct power control (DPC). VOC is an analogue of well-known field-oriented control systems for motor inverters [8]. It is based on calculations of active and reactive current components in rotational reference frame, synchronized with grid voltage vector [12]. VOC system contains outer control loops of DC-link voltage and reactive power, which provide reference for active and reactive currents to inner loop used to produce modulating voltages for pulse-width modulation (PWM) block. Every loop contains 2 PID controllers to reduce feedback error that leads to tuning problems for such systems.
DPC, in contrast, provides great simplicity due to direct power calculations according to the instantaneous power theory and its control with hysteresis regulators in inner loop that provide high dynamics and absence of necessity for fine tuning. The main drawback of DPC algorithm is variable switching frequency, which leads to increased high-order harmonics injection in consumed currents and provides additional complexity to input filter design.
While DPC due to its dynamics is the most suitable algorithm for active filters, its drawbacks become a severe problem especially in megawatts rated application, causing noticeable losses and degrading of grid power quality. To eliminate this problem, several authors propose DPC improvements in recent papers, which consider achieving constant switching frequency of hysteresis controllers [10], replacement of hysteresis controllers by direct [14] or predictive [4] calculations of voltage references that further applied to the input of space-vector modulation block. Therefore, modified DPC becomes a viable solution as control algorithm for AR operating as AF.
The study considers issues and performance analysis of AR application for mitigating harmonic distortion of grid currents, cause by the presence of high power non-linear load. The 3-level neutral-clamped inverter topology is proposed for the application and based on the given mathematical model of converter, the DPC algorithm with currents filtration is derived.
The model and control systems are implemented in MATLAB/Simulink, and analysis of AR efficiency is preformed depending on the active load of AR.
Grid topology. The use of AR for non-sinusoidal currents filtering is considered on the example of a mine node with radial power supply scheme, which is shown in the Fig.1. The mine power is supplied by 70 km overhead line 110 kV connected to the main grid, which is considered as an ideal voltage source. All mine loads are connected to the bus of the step-down transformer 110/6 kV that is considered as point of common coupling further (PCC). The initial distribution network has two considered load types: linear distributed load connected to the PCC via the stepdown transformers 6/0.4 kV and cable lines with length of 12 km, and non-linear load considered as VFD of a power rating equal to 2 MW with non-controlled 2-level rectifier connected to PCC via 10 km cable line and step-down transformer 6/0.4 kV. The further mine development led to appearance of another mine shaft, where the ventilation fan has to be placed. As a presence of non-linear load negatively affects grid currents and voltages, the VFD with AR was chosen to be placed to the new shaft, where AR can most of its lifetime implement filtering functions.
The currents waveform absorbed by the mine node depends on the ratio between the non-linear and linear loads. The non-sinusoidal currents in the considered grid are mainly consumed by the 2-level VFD with diode rectifier. The AR in such case is supposed to suppress mainly 5-th and 7-th harmonics, however it is also important, that AR also injects higher order harmonics, related to the IGBT switching.
The grid currents and voltages harmonic content is regulated according to the IEEE 519. The total voltage harmonic distortion for 1-69 kV class of grids shouldn't exceed 5 %, while harmonic content for any particular voltage harmonic shouldn't exceed 3 % of the fundamental. The currents harmonic content is regulated according to its ration between short-circuit value at PCC and maximum load current. The PCC considered as second bus of transformer, and calculated I scr /I l = 74, that brings the grid to 50-100 kV class.
Active rectifier mathematical model. The paper considers AR closely connected to the node with non-linear load, which in general allows to consider AR as AF with variable power rating.
Three-level topology of the VFD becomes a viable solution in 6-10 kV distribution networks thanks to development of semiconductor devices. In comparison with two-level topology, threelevel VFD allows to: connect load of several MWs directly to the grid; distribute load between power switches that leads to decreased switching losses; increase of number of output voltage levels leads to decrease of harmonic distortion caused by switching, which becomes severe problem on MW rates of VFD due to exponentially increased losses on the input filter [3].
At the same time 3-NPC topology remains relatively simple and cheap to implement in comparison with other multilevel topologies. However, the disadvantage of multilevel inverter is the unbalanced voltage in DC-link, which requires advanced control algorithms [11].
Structure of 3-NPC AR in VFD is shown in the Fig.2. Its model can be described as follows: where u g -phase-to-ground voltages at the PCC; i -phase currents; R f , L f -resistance and inductance of input filter; u AR -output inverter voltages, which are calculated as follows: 3-level active rectifier of NPC-type

3-level rectifier of NPC-type
Non-linear load DC-link power balance equations are written as follows: where C dc -capacitance of each DC-link capacitor; i 1 -load current. Active rectifier control system. The DPC control algorithm is used to control AR in presented solution. Structure of the proposed AR control system is shown in the Fig.3. The control system contains following blocks: 1 -calculation of the instantaneous power on the grid side; 2 -fundamental harmonic extraction of the grid currents and voltages; 3 -calculation of the angle of the grid voltage vector; 4 -determination of the sector of the grid voltage vector; 5 -calculation of the instantaneous power of the load harmonics; 6 -DC-link voltage control circuit; 7 -saturation for the AR output apparent power; 8 -hysteresis controllers; 9 -switching table with DC-link voltage balancing.
All calculations in the DPC systems are maintained in stationary two-phase plane, transform to which from the abc-frame is obtained by Clarke transform equations:

Switching states of the 3-level NPC inverter
The equation (3) is useful for balanced and sinusoidal voltage and currents. Otherwise the calculated powers will be also distorted.
In such case power can be written as a sum of average p , q and oscillating components q p, : Assuming that AR consumes near sinusoidal active currents from the grid, power balance for the grid node shown in the Figure 1 is written in terms of pq-components as follows: To achieve grid node sinusoidal power consuming, the p g should have no oscillating power components. To enhance grid power quality, the reactive power exchange of the grid and grid node should be also reduced to zero. Therefore, active filtering functions of the AR are obtained when it compensates oscillation caused by the diode rectifier nl p and full reactive power of the grid node q g .
AR should provide constant level of the DC-link voltage by absorbing active power from the grid, that is achieved by PI-control law Power references for the AR control system to provide active filtering therefore become: The nl p component is calculated according to (7): The average power nl p usually obtained by filtering the oscillations of nl p from the oscillating component of nl p value. Low-pass Butterworth filter was used in presented work for that purpose [13]. To maintain following obtained reference signals, the AR switches are controlled by the switching table, which is also used to balance DC-link capacitors voltages, similarly to as it was done in the paper [1,14].
Simulation. The waveforms and spectrum of currents, consumed by the grid node before connecting the VFD with AR to the node are shown in the Fig.4. It can be seen that currents waveform is far from sinusoidal, which is caused by presence of 5-th and 7-th harmonic components, injected by diode rectifier. THD i in this case equals to 16.11 %, which in turn leads to voltage distortions: THD u = 7.29 % that is not acceptable level according to IEEE 519 (<5 % is allowed for such applications).
Simultaneous work of 2 drives without harmonic compensation. The next simulation case considers connected VFD with AR operating to consume only active sinusoidal currents from grid. The grid current waveform and spectrum was obtained for AR loaded with active power at 80 % of  (Fig.5, a, b) and 30 % (Fig.5, c, d). It can be seen from the figures that connecting of VFD with AR to the grid node improves the waveform of consumed currents: THD i = 9.07 % in case of 80 % loaded AR; however, when AR is loaded with active currents only for 30 %, currents waveform is more distorted: THD i = 12.72 %. This is explained by the fact that as AR consumes nearly sinusoidal currents, the magnitude of the fundamental component increases -from 245A before placing the VFD with AR to 310A for 30 % loaded and 412A for 80 % loaded AR. It also should be noticed that currents spectrum contains higher harmonics caused by high switching frequency of AR IGBTs, however their magnitude is much smaller than magnitudes of 5-th and 7-th harmonics, injected by the diode rectifier.  Fig.6 shows results of simulation for AR with active filtering function enabled. Two cases of AR load were considered. In the case of AR loaded at 80 % the THD i = 8.72 %, which is achieved by the use of available 20 % of AR rated power to filter currents distortion. Magnitude of fundamental harmonic component equals to the case with no active filtering function, while magnitudes of 5 and 7-th harmonics decrease from 8 and 6 to 4 and 3 %, which shows effectiveness of active filter algorithm. Decrease of AR active load to 30 % leads to further improvement of THD i to 3 %, while 5 and 7 harmonics decrease to 1 % value.
Active filtering performance. As it was indicated in particular simulations, the active filtering function of AR allows to greatly reduce currents distortion. However, the filtering effectiveness hardly depends on the amount of available current to perform this function and accordingly on the AR active current load. To investigate that dependency, the series of simulation with different