The 2014 International Power Electronics Conference
Device Loading of Modular Multilevel Converter MMC in Wind Power Application L.Popova,J.Pyrh6nen
K.
Department of Electrical Engineering Lappeenranta University of Technology Lappeenranta, Finland liudmila.popova@lutfi; juhapyrhonen@lutfi
Department of Energy Technology Aalborg University Aalborg, Denmark
[email protected],
[email protected]
Abstract- Modular multilevel converter (MMC) is a recently emerged
multilevel
applications.
topology
However,
in
for
high-voltage
wind
power
the literature [4], [5], not many papers are published about the application of this topology for wind turbines. A thorough analysis of the MMC converter is required in order to investigate the suitability of the topology for wind power systems. Reliability and power density aspects of the converter are of great importance for high power wind turbines, which tend to be installed off-shore in a remote and harsh environment as the power is increasing. These aspects are affected by the loading and the thermal performance of the switching power devices, which will be studied in this paper.
high-power
application
performance of the MMC has not been deeply investigated.
the In
this paper the application of MMC in wind energy systems is studied. The converters used to connect the wind turbine to the grid having the rated active powers of 2MW and lOMW are designed and investigated. Electrical losses and thermal loading of the power devices in the proposed converter solutions are analyzed. The efficiency of the MMC converter under different PIQ
boundaries
defined
by
grid
codes
is
Ma, F. 81aabjerg
investigated
and
compared with two-level and three-level NPC converters. It is concluded that it is possible to use the MMC in wind power
[I.
application and the losses are distributed evenly between the sub modules of the MMC converter.
However, inside a sub-module
IN WIND APPLICATION
the losses of the power devices are not equal that can lead to the
A simplified schematic of the MMC used in wind power application is presented in Fig. 1. The topology is based on series connection of sub-modules (SM). The number of SMs in each phase arm is equal. The circuit diagram of a half bridge SM which contains a floating dc capacitor and two [GBTs with free-wheeling diodes is also presented in Fig. 1. The basic operation principles of the MMC converter are described in detail [3], [6].
de-rating of the converter.
Keywords-
MMC
Converters,
renewable
energy
sources,
thermal stresses, wind power generation.
I.
[NTRODUCTION
The amount of wind turbines installed worldwide has been growing constantly during the latest decades. As the power capacity of wind turbines has increased up to 10 MW the connection to grid using traditional two-[evel converters requires a large amount of series or parallel connected power devices in order to achieve the required power. The re[iability of such a converter is decreased while the complexity is increased. Multi[evel converters are known to be a promising solution in wind turbine applications due to their ability to work with higher output voltage levels and therefore obtain larger output powers using the power devices available nowadays [1]. Among various multilevel topologies such as Neutral Point Clamped (NPC), Flying Capacitor (FC), and Cascaded H Bridge (CHB) the Modular Multilevel Converter (MMC) is a fairly new topology which has been introduced in 2002 [2]. The advantages of the MMC which make it an attractive solution for wind power generation system are the modular design, redundancy, simple voltage scaling, possibility to connect the converter directly to the grid without a transformer [3 ]. While the application of the MMC converter for High Voltage DC (HVDC) transmission systems is well reported in
978-1-4799-2705-0/14/$31.00 ©2014 IEEE
MODULAR MULTILEVEL CONVERTER FOR CASE STUDY
Fig. 1. Simplified schematic of the MMC for wind turbine application.
548
The 2014 International Power Electronics Conference A.
Selection of the components
5 SNA 1800E170100 from ABB [11] are summarized in Table I.
Considering the operation principles of the MMC converter the IGBT modules and the capacitors of the SMs have to be selected with the rated voltage u SM
=
UDC N
(1)
where UDC is a DC link voltage and N is a number of SM in an arm. A safety margin has to be taking into account in order to have kind of redundancy. The selection of the SM capacitor is a compromise between the capacitor size, cost and the voltage requirements. The capacitance of the SM capacitor can be calculated based on desired voltage ripple factor E: having a value between 0 and 1 as it is presented in [7] c SM
=
2
,1.W
SM
Fig. 2. 2 MW MMC converter used in a wind turbine.
TABLE I
PARAMETERS OF SIMULATED 2 MW MMC CONVERTER
Rated active power, MW
SM
=�.
S
3 k·OJ.n ·N
[
1_
(
)
k'COsrp 2 2
J� 2
l.l
Rated current, A
1674
Arm inductance, IlH
113.7 (0.15 p.u.)
SM capacitance, mF
20.8
Carrier frequency, Hz
1550
Filter inductance,mH
0.15 (0.2 p.u.)
IGBT module
ABB 5SNA 1800EI70100
l.l
IGBT commutated voltage, kV
(3 )
where S is the apparent power of the converter, COn is the output angular frequency, k is the voltage modulation index, N is a number of SM in an arm, and cosrp is power factor (P F). The arm inductors Larm are used in the MMC converter for the suppressing the circulating currents and limiting the fault current during short circuit between the DC link terminals. The analytical expressions for the arm inductance calculation based on these two issues are given in [8]. In this paper the value of the arm inductance is selected to be 0.15 per unit which is a commonly used value for the MMC converter [9]. However, it should be checked that the selected SM capacitance and the arm inductance do not create a resonance as shown in [10]. B.
0.69
DC voltage, kV
where f>.WSM IS the energy change In one SM which IS estimated by ,1.W
1
Rated line-to-line voltage, kV
(2)
·c· U SM 2
2
Number of SMs per arm
IGBT maximum current,kA
1.8
Total number of IGBT modules Total MVA rating of all lGBT modules
12 23.76
The converter is simulated using Matlab/Simulink and PLECS Blockset [12]. Level-shifted pulse-width modulation (PWM) is applied to obtain the desired arm voltage. For simplicity the reference to the modulator is generated using a direct strategy presented in [13 ]. Simulated waveforms are shown in Fig. 3 . The power factor of grid side converter i s i n Fig. 3 selected to be PF= l. 3000 .,------
2000
�
--
13 '0
Low voltage solution for wind power application
1000
� c: os
Converter with minimum amount of SMs in a phase leg is not presented in the literature, however, this solution may be beneficial due to reduced component counts in comparison with the MMC having more SMs. Thus, the first studied converter has a rated power of 2 MW and only 2 SMs in a phase leg. This converter produces 690 V line-to-line voltage and is used to connect a low-voltage generator to the grid (Fig. 2). It can be an alternative to the existing two-level wind power converter based on 690 V grid voltage. The parameters of the converter and ratings of the selected IGBT module
$ c:
