A NPC IGBT BASED MEDIUM VOLTAGE INVERTER APPLIED TO SALIENT POLE WOUND ROTOR SYNCHRONOUS MACHINE

Gilberto da Cunha, Diogo Brum Cândido, Adriano da Silva Dias, Márcio Sari and Paulo José Torri

WEG Drives & Controls Av. Prefeito Waldemar Grubba, 3000 – 89256-900 – Jaraguá do Sul – SC – Brazil

gilbertoc@weg.net, diogobc@weg.net, adrianosd@weg.net, msari@weg.net and paulojt@weg.net

Abstract – This paper presents the results of an implemented control strategy applied to an IGBT based Medium Voltage inverter with NPC topology driving a salient-pole synchronous AC machine. Tests with a 1000 HP synchronous machine and a three level inverter (4.16 KV / 120 A) were performed. The control system provides a high dynamic performance with a fast torque control. The additional functionalities necessary to drive a synchronous AC machine are depicted. The results also show the behavior of a synchronous bypass transfer of the machine from the inverter to the line supply.

Keywords – Converter control, dynamic model,

medium voltage drive, multi-level inverter, synchronous machine.

I. INTRODUCTION Due to technology advancements in semiconductor

devices such as insulated gate bipolar transistors (IGBTs), modern medium voltage (MV) drives are increasingly used in petrochemical, mining, steel and metal, transportation industries among others, to conserve electric energy, increase productivity and improve product quality. The development of MV drives was also motivated by the proved improvement in the efficiency, reduced weight and volume of the motor and in the reduced installation costs in cabling, cable trays etc. [1].

Available MV drives cover power ratings from 0.4 MW to 40 MW at the medium voltage level of 2.3 kV to 13.8 kV. The power rating can be extended to 100 MW where synchronous motor drives with load commutated inverters are used. However, most of the installed MV drives are in the 1 to 4 MW range with voltage ratings from 3.3 kV to 6.6 kV [1].

The basic structure of a MV drive consists of an active or passive input rectifier, a decoupling DC link, an IGBT, SCR or thyristor based voltage source inverter (VSI) and optional line and/or motor filters, Fig. 1.

The MVW01 is an IGBT MV inverter based on Neutral Point Clamped (NPC) topology, with a standard output voltage of 4.16 kV and its structure and characteristics are presented in next section.

Nowadays, in low speed applications, such as rolling mills, mine hoists, ship propulsion, etc., MV drives fed salient pole wound rotor synchronous machine (WRSM) are generally used. For high speed applications (such as ID fans, pumped storage plants etc) MV drives fed cylindrical rotor WRSM is preferred [2]. Furthermore in oil and gas industry, particularly in high power compressor trains for liquefied natural gas, drives act as starter and helper for the prime mover of the compressor trains [3].

The modeling and analysis of the synchronous machine has always been a challenge. In field oriented control (FOC), the machine is generally modeled in the flux reference frame in which the control is done. The synchronous AC machine modeling the d-q reference frame is well defined and therefore control is straightforward. But in most of the synchronous machine drives, stator flux oriented control in an additional reference frame is preferred compared to d-q axis reference frame. The reasons are the capability to control power factor and the possibility of optimal sizing of the stator side converter [2]. In this paper the additional reference frame is called m-t frame.

This paper describes the results of a medium voltage inverter driving a salient-pole wound rotor synchronous machine. The performance analysis of a speed step response is shown.

II. MEDIUM VOLTAGE INVERTER A Neutral Point Clamped (NPC) multilevel topology is

shown in Fig. 2 [4]. It employs clamping diodes and cascaded DC capacitors connected to the floating neutral point. It produces AC voltage waveforms with multiple levels.

The higher the number of step levels, the higher the number of switching devices. The NPC inverter showed in

Fig. 1. Medium voltage variable-speed drive (VSD) functional blocks.

978-1-4799-0272-9/13/$31.00 ©2013 IEEE 852

Fig. 2 produces three level voltages between each phase to neutral point and five level voltages between phase to phase at motor terminals. This topology is suitable for motors rated up to 4.6 kV and applies standard 6.5 kV semiconductors. It is possible to increase the voltage to 6.9 kV connecting two NPC legs in an H-bridge form. In this configuration the phase to neutral voltage contains five voltage levels and nine levels at motor terminals. The main features of the NPC inverter includes reduced dv/dt and THD on its AC output voltages in comparison to the 2-level-voltage source inverter topology. This inverter can be used in MV drives to reach a certain voltage level without series connection of switching devices. Thus, the efficiency levels can reach 99%. The switching frequency should be as low as possible due to the power losses and it is usually limited to a few hundred hertz. Normally, special modulation techniques are needed to produce the minimum harmonic distortion in the motor current in all range of speed and torque. In some applications the floating neutral point requires control of voltage deviation [5].

The MVW01 NPC single leg is presented in the Fig. 3. Basically, it consists of four 6.5 kV IGBT modules, two clampling diodes, a laminated busbar and DC link capacitors. An Active Front End (AFE) converter or a motor side converter consists of three such modules of Fig. 3.

For the synchronous machine applications some other features are required, different of the induction machine where just the speed feedback is enough to implement the vector control strategy. In the synchronous machine control the rotor position measurement is required. For this reason an absolute encoder is used.

When the synchronous machine is used as a motor, concerns about the overvoltage protection in the field winding must be taken. During the start of the machine there is an interval where the excitation winding current is not yet established by the exciter. Since the PWM voltage already exists on the stator winding induced currents in the field winding can result on insulation damage and even damages to the exciter. For this reason a circuit protection, called Crowbar, was implemented to act when an overvoltage situation appears in the field winding.

Fig. 2. Medium voltage VSD setup.

Fig. 3. MVW01 NPC single leg.

III. SYNCHRONUS MACHINE MODEL The Fig. 4 shows the reference frames. The - reference

frame is stator fixed, where an axis is in the direction of phase R. The d-q is aligned with the rotor where d axis is in the direction of the excitation winding. The m-t reference frame is defined in the direction of stator flux.

The machine model used for the control design is shown in the Fig. 5.

It consists of mutual inductances, and , the correspondent fluxes and and the correspondent currents and ; the leakage inductances of the stator

and , that for mathematical simplicity are considered equal and also the dumper ( , , and

) and excitation circuit ( and ) values.

Fig. 4. Reference frames

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Fig. 5. Synchronous AC machines model in d-q reference

frame. The dynamic mathematical model of a synchronous

machine operating as a motor is shown below:

(1)

(2)

(3)

(4)

(5)

0 00 0 0

0 00 0

0 0 0

iiiii

(6)

Where u and u are d and q axis components of the

stator winding phase voltage, u is the field winding voltage, i and i are d axis and q axis components of the stator winding current, i , i and i are respectively the field winding d axis and q axis components of damping winding currents, R is the per phase stator winding electrical resistance, R , R and R are field winding electrical resistance, the d axis and q axis components of damping winding electrical resistance respectively, ω is the rotor electrical angular speed, ψ and ψ are the stator winding d and q axis phase linkage fluxes per second (Voltages). ψ , ψ , and ψ are respectively, the field winding, the d axis and q axis damping winding linkage fluxes per second (Voltage).

The system equations 6 represent the relation between the fluxes and currents, where L , L and L are the inductances in d and q axis and field inductance, respectively. L and L are the damping inductances.

Table 1 presents a list of the main synchronous machine parameters used for get experimental results.

TABLE I Synchronous machine parameters

Stator phase resistance Rs = 0,00867 pu Stator leakage inductance Xlσ = 0,078 pu

Motor time constant Tr = 1,866 s Damper D leakage inductance LDσ = 0,088 pu Damper Q leakage inductance LQσ = 0,08 pu

Damper D resistance RD = 0,067 pu Damper Q resistance RQ = 0,04 pu

Number of stator poles p = 4 Stator D inductance LD = 1,587 pu Stator Q inductance LQ = 0,83 pu

Field inductance Lf = 1,597 pu Field resistance Rf = 0,00227 pu

Nominal motor voltage Vmotor = 4000 V Nominal motor current Imotor = 110 A Nominal motor speed Nmotor = 1800 rpm

Nominal motor frequency fmotor = 60 Hz

In general, due to economic reason, the synchronous

machine operates in the saturated part of the magnetizing curve and so the variables are sensitive to load and excitation current amplitude conditions [6].

IV. CONTROL STRUCTURE An overview of the control structure is given in Fig. 6.

The components will be explained in the following sections.

A. Torque and power factor control Depending on the demand by the process control, the

velocity controller calculates the t component of the torque current it using the torque T (7) equation. The demand current im, is used to adjust the power factor of the model [7].

Additionally a flux set point Ψ is calculated depending on the speed and the demand of torque and power factor.

. . . (7)

B. Excitation controller and flux observer The flux is regulated by a closed loop controller and it is

estimated by a flux observer. Also the saturation effects are taken into account making use of stored magnetization curves. The observers estimate the stator flux, damping flux vector and also the estimated torque angle values that are needed in the control.

C. Fundamental current estimation The method of optimal synchronous PWM is the most

effective approach to minimize current harmonic distortion, especially for PWM inverters of higher power rating operating at high modulation index [8], [9].

The medium voltage inverter operates at low switching frequency around 300Hz, which affects the direct current measurement since the current presents a spread harmonic content and so the estimation of the fundamental current value becomes necessary to extract out the current ripple.

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Fig. 6. Synchronous machine vector control scheme

High-dynamic closed-loop control is enabled by extracting the fundamental component of the stator flux linkage vector from its distorted trajectory. The process is based on the reconstruction of the harmonic content of the actual pulse pattern.

The Fig. 7 presents the real and estimated iq current.

V. EXPERIMENTAL RESULTS This section presents the measurements made in a 1000

HP machine coupled to a dynamometer. The main characteristics of the medium voltage drive are presented in table II.

TABLE II

MVW01 inverter parameters Rated output voltage(RMS) 4.16kV Rated output current(RMS) 120A

Repetitive peak off-state voltage 6.5kV Max. turn off current 1kA

Max. switching frequency 500Hz

A. Steady state operation (Power Factor = 1) The Fig. 8 presents machine phase u current and voltage

waveforms in steady state operation with power factor controlled to unit when the machine was loaded at a stator current of 92Arms.

B. Transient operation When imposed a speed step from 100 to 900 rpm the

controller set the torque reference at a maximum value. In Fig. 9 is presented the torque step response as consequence of the speed variation. The real total current is used to evaluate the torque response and the delay time value is around 5 ms.

Fig. 7. Fundamental current estimation. Real iq (black) and

estimated iq (grey).

Fig. 8. Unit power factor operation with load. Output voltage vu

(kV) (grey) and output current iu/50 (black).

1

2

3

4

5

Time (500ms/div)

-3

-2

-1

0

1

2

3

Time (10ms/div)

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Fig. 9. Torque step response. Total current (Black) and torque

reference (grey).

C. Synchronism with the line supply (synchronous bypass)

Some applications require the drive to start the motor and transfer it to the line supply without any interruption or system disturbance. In Fig. 10 and Fig. 11 we can see the synchronism process and in Fig. 12 is possible to observe the moment when the bypass from the inverter to the line supply occurs. The transfer is done very smoothly without any current disturbance.

Fig. 10. The asynchronous operation of the motor. Drive output

voltage (kV) (black) and sampled line supply voltage (grey).

Fig. 11. The synchronous operation of the motor. Drive output

voltage (kV) (black) and sampled line supply voltage (grey).

Fig. 12. Synchronous bypass of the motor to the line supply. Ch1 and Ch2 represent drive (black) and line (grey) current, Ch3 and

Ch4 represent drive (black) and line (grey) voltages.

VI. CONCLUSION This paper described the control strategy of a MV

inverter driving a synchronous machine. The measured results obtained on a medium voltage setup (1000 HP 4.16 kV machine + NPC inverter) proved the good functionality of the implemented control scheme. Although the inverter uses a very low switching frequency the measured response time of few milliseconds demonstrate the high performance of the torque control. The drive implemented also a power factor control feature. The results showed that unity power factor was achieved and may be well controlled during operation. Smooth synchronous bypass transfer of the synchronous machine from the inverter to the line supply was also demonstrated.

The drive system is suitable for high-power high-performance application.

ACKNOWLEDGEMENT The authors gratefully acknowledge the contributions and

support of their colleagues at WEG.

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[3] J. Böcker, J. Janning, H. Jebenstreit, “High Dynamic Control of a Three-Level Voltage-Source-Converter Drive for a Main Strip Mill”, IEEE Transactions on Industrial Electronics, vol. 49, no. 5, pp. 1081-1092, October 2002.

[4] A. Nabae, I. Takahashi, H. Akagi, “A New Neutral Point Clamped PWM Inverter”, IEEE Transactions on Industry Applications, vol. 17, no. 5, pp. 518-523, September/ October 1981.

[5] G. Cunha, P.J. Torri, “Neutral Point Potential Balancing Algorithm at High Modulation Index for Three-Level Medium Voltage Inverter”, 9º COBEP, pp. 523-527, October 2007.

0

1

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6

Time (10ms/div)

-3

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-1

0

1

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3

Time (10ms/div)

-3

-2

-1

0

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Time (10ms/div)

Time (10ms/div)

Ch1,Ch2

Ch3,Ch4

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[6] E. Ruppert, F.L. Nunes, S.O. Nunes, “Synchronous Machine Field Current Calculation Taking into Account the Magnetic Saturation”, Revista Controle & Automação, vol. 13, no. 2, pp. 165-170, May/June/July/August 2002.

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[8] J. Holtz, N. Oikonomou, “Synchronous Optimal Pulsewidth Modulation and Stator Flux Trajetory Control for Medium Voltage Drives”, IAS, pp1784-1791, 2005.

[9] P. Kjaer, T. Kjellqvist, C. Delaloye, “Estimation of Field Current in Vector-Controlled Synchronous Machine Variable-Speed Drives Employing Brushless Asynchronous Exciters”, IEEE Transactions on Industry Applications, vol. 3, pp. 1703-1710, 2003.

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