Research of Mining STATCOM Based on Hybrid Multilevel H-bridge Inverter *

The paper presents a new STATCOM system based on H-bridge inverter. It can be used in mine power network. It has been commonly verified for the positive effects of SVG on the reactive power compensation and voltage fluctuation suppression. This paper focuses on a generalized structure of multilevel power converter where individual voltage sources are not necessarily the same. The cascade H-bridge consists of two cells, high-voltage cell and low-voltage cell. The high-voltage cell is responsible for voltage lifting, while the low-voltage cell is responsible for PWM modulation. If two cells are cascaded with DC voltages in a ratio of 2:1, the single-phase output voltage can reach 7 levels. Increasing voltage levels of output waveform can bring up AC current quality, optimize harmonic spectrum and enhance converter efficiency. The hybrid multilevel is characterized by per-phase series connection of a high-voltage H-bridge converter and a low-voltage H-bridge converter. Due to the different capacitor voltage, it is a key problem as to how to maintain the capacitor’s voltage at a reference level. Independent DC source can effectively ensure the DC voltage. Through the reactive power compensation technology, the three-phase voltage and current can remain at the same phase.


Introduction
Early reactive power compensation is mainly for passive device, which is using a certain capacity of the capacitor or reactor.It is directly connected in parallel or series in a circuit.This kind of device is operated by mechanical switch cut.So the application is mainly used for mechanical cast cut capacitor (MSC).But it has no rapidity, continuity, repeatability characteristics.However, when the SVG come to the application, it can continuous control the impedance of the device and adjust for inductor or capacitor.
With the rapid development of modern coal mine, the use of high power motor brings up a lot of reactive power.This kind of situation is very outstanding in the mining coal face.It becomes obvious especially on-peak demand.Therefore, the dynamic reactive power compensation technology research is of important significance in coal mine underground.It is ensuring the production, saving energy and reducing consumption.
Multilevel power conversion has drawn heavy attention in the area of medium-voltage industrial applications and high-power applications [1][2][3].In [3], the high-voltage H-bridge used IGCT instead of IGBT.It has been proven that the voltage blocking capability of faster devices, such as IGBT, and the switching speed of high voltage thyristor-based devices, such as IGCT, is limited.So the combination of the two types of thyristors can bring each own advantages to the circuit.
When the DC voltages have various ratios, the number of output levels is also different.An extension of conventional multilevel topology with the series connection of unequal DC voltage sources is adopted in literature [4] [5].The topology effectively output a higher voltage levels compared with traditional H-bridge converter.Literature [5] provides a motor drive direct torque control (DTC) scheme for electric vehicles (EVs) or hybrid EVs.The limitation is that the control method is not applicable to STATCOM in which capacitors are replaced in dependent DC power.
Compared with conventional STATCOM, some paper proposed a new topology from hybrid cascading one NPC inverter and several conventional H-Bridge inverters.A control method is also proposed to realize DC voltage regulation of series-connected multiple cells in the STATCOM operation, making it possible to remove DC sources from all H-bridge cells [6].It will eliminate the cumbersome transformers and meanwhile all the advantages of H-Bridge cascade inverter can be kept.But when the number of cascade H-bridge increases, the control method will also become more complex [7,8].In order to keep the balance of DC voltages, many DC sources are consist of isolate power supplies [9], independently powering each cell.
The aim of this paper is to clarify the advantages of the 7-level output voltage.Y-type cascaded Hybrid single-phase H-bridge topology was used for its modularity and simplicity.A hybrid frequency carrier-based PWM control strategy is presented in this paper.Every cluster is comprised of a 7-level hybrid inverter, high-voltage H-bridge converter and low-voltage H-bridge converter.The DC bus voltages were configured in the ratio of 2:1.And they also work in unequal switching frequency.

Configuration of the STATCOM System
Figure 1 shows the principle of parallel SVG configuration.For application in coal mine, the majority of the load is inductive.So the SVG should compensate the inductive reactive.
Figure 2 shows current vector of the Figure 1.Where Us is the grid side voltage; Is the grid side current; U L is the voltage drop in the inductance; U I is the output voltage of three-phase inverter bridge.We can measure the magnitude of the power line voltage Us and current Is, as well as their phase α, β.In the ideal case, the connecting reactor is regarded as inductance.We do not consider the various losses.Inductance on the direction of the current lags the voltage of 90o.So Us and U I are maintained in the same phase.The magnitude and direction of the voltage drop in the inductance can be controlled by changing the amplitude of the U I , whereby the magnitude and direction of the current in the inductance be controlled.According to Kirchhoff's voltage law, U I =Us-U L .Inductive reactive power that SVG absorbed from the grid is equivalent to the capacitive reactive power output to grid.As a result, we need to keep the reactive current ILref equal to the reactive current of the load.This is called direct current control theory.
Figure 3 illustrates the three-phase cascaded H-bridge circuit used in this paper.For cascade H-bridge inverter, there are two main topology connections, Delta-type and Y-type.In this paper, we use Y-type cascade topology.

ower Detection
Assume that e a , e b , e c and i a , i b , i c are the three-phase tage and current values of the circuit.To facilitate analysis, the voltage current instantaneous value of the three-phase static coordinate system a-b-c can be transformed into two-phase static coordinate system αβ.This is called Clarke transformation.
The instantaneous active current and instantaneous reactive current of the system can be calculated through Park transformation.This current is expressed as: As is shown in Figure 6, the three-phase grid voltage an he grid.The output of the PI Compare the reactive current of the inductance with the reactive current of the grid, then put the subtraction Id* into PI regulator.In this way, the reactive current of the grid can be tracked and compensated.The circuit is regarded as an ideal model that does not deliver active power to t regulator is the three-phase input voltage of the cascade inverter, which are U aref , U bref and U cref .

H-bridge STATCOM
order to validate the propose verter structure and the control algorithm, a hybrid seven level cascade inverter simulation has been built in Simulink.Fig    oltage and cureent of the three-phase grid after the alysis of the compensation cu king situation of the active and reactive current is sh t DC-level of hybrid cascaded multi-level transform circuit, this paper proposed a new PWM dual phase control, it is faster to of COM.With the right control strategy to di v compensasion when t = 0.04 s.Before compensation, as the connecting reactor is regarded as inductance in the ideal case, the voltage phase is therefore ahead of the current phase by 90 o .When the reactive power compensation is integrated in the system, the two waveforms maintain in the same phase.
Figure 8 shows the FFT an rrent.The THD = 1.81% with a better harmonic spectrum.
Trac own as in Figure 9.

Conclusions
Based on the inpu This paper studies a kind of applied DC power hybrid cascade STAT rectly control the voltage of the capacitance, cost will be lower and operation more practical.This will greatly promote the application of reactive power compensation.

Figure 1 .
Figure 1.Downscale of Reactive power compensation system.

Figure 4 .
Figure 4. Square waveform of Cell1 and PWM waveform of Cell 2.

In d hybrid cascaded in- ure 7
illustrates the phase difference between the Desired output Low-voltage d current are assumed as U a , U b , U c and I a, I b and I c .I al , I bl and I cl are the three-phase current of the inductance.After 3-2 Clarke and Park transformation of current, we can get the inductance reactive current I ql and grid reactive current I q .

Figure 6 .
Figure 6.The strategy of reactive power detection.

Figure 7 .Figure 8 .
Figure 7. Phase comparison of voltage and current after compensation.

Figure 9 .
Figure 9.The tracking waveform of active and reactive current.
control method.The two cascaded H-bridge have different DC voltages.The Matlab simulation results have shown that by stacking the output current, the proposed operation strategy is feasible.It effectively improves the system output voltage level and simultaneously reduces the harmonic content.Due to the fact that cascade control of multilevel inverter is based on indivi fset current reaction speed by direct current control of the converter.