Modelling and Simulation of Three Level Inverters for Main Drive of the Plate Mill Rolling Stand

Abstract. This paper addresses a modelling and a simulation of three level inverters for main drive of the plate mill rolling stand. The main purpose of the article is to develop of a mathematical description of the three level voltage source inverter based on a space vector pulse width modulation and to simulate a consumption current by the main electric drive based on 18-pulse rectification circuit. A twelve-megawatt synchronous motor as a load has been used. The modelling results were simulated at the Matlab/Simulink program. It can allow us to estimate an electromagnetic compatibility of the main electric drive based on transients of the consumption current. The research results can be used to assess the electromagnetic compatibility of powerful three level natural point clamped converters with different types of the space vector pulse width modulation on a load or a grid.


Introduction
Nowadays medium voltage reversible electric drives based on synchronous motors and power converters, which consist of active front end rectifiers (AFEs) and voltage source inverters (VSIs), are widely used in electrotechnical systems of rolling stands.Such consumers are able to create a bidirectional power flow at a low negative effect on the grid.Usually medium voltage drives have a high rated power, so a modelling and a researching of these drives are more effective to increase an energy-efficiency of real industrial objects.Power quality and the energy efficiency are very important factors to keep the production costs [1].
Very often the medium voltage AFEs and VSIs have a neutral point clamped (NPC) stricture.A parallel connecting of the AFEs or the VSIs is commonly used to create more than six-pulse rectifier circuits.For example, several transformers with a shift angle of secondary voltages on 20°, 0° and -20° for 18-pulse circuit and 15°, 0°, -15° and 30° for 24-pulse circuit can be applied [2].
A structural feature of powerful AFEs and VSIs is three level NPC topology.It allows one to improve an electromagnetic capability with the grid or a load at a low swathing frequency of semiconductors modules.Total harmonic distortional (THD) of output current or voltage for three level NPC converters is a small magnitude due to the increasing of output voltage levels.High voltage insulate gate bipolar transistors (HV IGBTs), integrated gate-commutated thyristors (IGCTs) and injection enhanced gate transistors (IEGTs) are generally used as the semiconductors modules for AFEs and VSIs [4].
A control of the modules is done by the pulse width modulation (PWM).The most common PWM technology is a space vector (SVPWM) and a selective harmonics elimination (SHE PWM).In AFEs operate based on the SHEPWM and in VSIs are used the SVPWM [5,6].

Problem definition
The main purpose of this paper is to develop a mathematical model of a power circuit of the electric drive of the plate mill rolling stand based on three level NPC inverters with SVPWM.Additional purpose is to evaluate the electromagnetic compatibility of consumption current by the synchronous motor.

Typical features of the main drive of the plate mill rolling stand
Generally, a main electric drive of plate mill rolling stands is system, in which synchronous motor of each mill roll are connected to the grid through several medium voltage three level NPC converters.The NPC converters consist of AFEs and VSIs.The maximal power of each converter can achieve more than 20 MVA.18-pulse or 24-pulse rectification circuits are typically used to improve the quality of direct voltage and to reduce the harmonic level of the consumption current.The inductor with high voltage short circuit are applied at the input of motors [7][8][9][10][11].The characteristic power circuit for main electric drive of plate mill rolling stand is shown in Figure 1.

Modelling of a three-level inverter
There are a lot of control methods of three-level inverters, but at present the SVPWM technology is widely used.
The exponential increase of digital signal processors (DSPs) has allowed one to develop the complex algorithms SVPWM for multilevel NPC inverters.It is possible to reduce switching losses using low switching frequency of semiconductor modules [12][13][14].The threelevel VSI based on the NPC structure is shown in Figure 2.  The three level VSI in Figure 1 contains twelve HV-IGBT or IEGT transistors (VT1…VT12) and eighteen diodes (VD1…VD18).Two equivalent capacitors CDC1 and CDC2 create a neutral point 0. Snubbers consist of the resistors (RS1…RS6) and the capacitors (CS1…CS6), which are parallel connected the semiconductor modules [15].
An output phase voltage of a VSI has magnitudes +Udc/2, 0, and -Udc/2.If the symmetric phase system is considered, all curves of the phase voltages have an odd harmonic spectrum.
According to Kirchhoff's laws, the system equations have been given and written based on snubbers current iS1 and iS2, and current i1, i2, i0 flowing through the semiconductor modules for all operating conditions: , where RS = RS1 = RS2active resistance of the first and the second snubbers; Ron = RonVT1 + RonVT2 = RonVD1 + RonVD2total active resistance of the first and the second transistors or diodes; iS1 and iS2snubber currents; i1current flowing through the first and the second transistors or diodes; uDC1, uDC2direct current (DC) voltages of the equivalent capacitors CDC1 and CDC2; uS1 and uS2snubber voltages; iload current of the phase А.
If first harmonic of the system (1) is considered, the space vector theory can be used to write the following system equation [16]: Based on the system equation ( 2) we can use the space vector theory and apply the following expression: The equation ( 3) allows us to determine a reference space vector for an each switching state of the three-level VSI [17].In Figure 3, the space vector diagram of three level NPC inverters is presented.The space vector diagram in Figure 3 is a symmetric hexagon with shift on 60° for the each sectors.The angle θ determines the sector number based on the following conditions: The times calculation T1, T2, T3 for each sector and region are as the following equations [18,19]:   A ratio of the times T1, T2, and T3 regulate the space vector depth and the angle by using the following expression:

DOI: 10 Figure 1 .
Figure 1.Characteristic power circuit for main electric drive of plate mill rolling stand

Figure 2 .
Figure 2. Three level VSI based on NPC structure

Figure 3 .
Figure 3. Space vector diagram of three level NPC inverters The example of space vector formation in the Sector 1 and the region 1 shown in Figure4.02001-p.3

2 60°Figure 4 .
Figure 4. Space vector formation space vector angle.4Simulation of the main drive of the plate mill rolling standThe simulation model of the main drive of the plate mill rolling stand based on the three level NPC inverters with SVPWM in Matlab/Simulink was developed.The block diagram of this model is shown in Figure5.

Figure 5 .
Figure 5. Simulation model of the main drive of the plate mill rolling

Figure 6 .Figure 7 .
Figure 6.Consumption phase current by the synchronous motor