A Modular Multilevel Inverter Using Single DC Voltage Source for Static Var Compensators Firman Sasongko 1 and Pekik Argo Dahono 2 School of Electrical Engineering and Informatics, Institute of Technology BandungJl. Ganesha 10, Bandung 40132, Indonesia 1 [email protected]2 [email protected]Abstract—Multilevel inverter has emerged as a new solution ofpower converter for high power applications. Many efforts have been done to obtain the best performance of multilevel inverter to provide the need of power converter for high-power medium- voltage applications. Multilevel inverter using modular-cascaded topology with single dc voltage source is presented in this manuscript. Inverter topology, features and control method will be discussed. Simulation and experimental results for static var compensator application are included to verify the effectiveness of the proposed method. Keywords—Modular multilevel inverter, control system, static var compensator.I.I NTRODUCTIONReactive power compensation has become an indispensable requirement to provide a better power system performance [1], [2]. Var compensator system has three major roles: improving the transient stability, damping the power oscillation, and supporting the grid voltage to prevent voltage instability. In recent years, static var compensators are preferable to theirtraditional counterpart of using rotating synchronous condenser and mechanically switched capacitors or inductors [3] –[6]. Static var compensator provides faster time response to absorb or generate the reactive power. The advances ofpower electronic devices, analytical tools, and micro- computer technologies has create the more sophisticated power converter to be used for static var compensator and other high-power applications. Multilevel system is especially important in high-powerapplications such as Flexible AC Transmission System (FACTS). At present, most of FACTS controllers that have been installed worldwide are using conventional two-level inverter modules that are interconnected by using a special design multipulse transformer [7] –[9]. In order to reduce the switching losses, the inverter switching devices are switched at the fundamental frequency. The transformer is configured in such a way so that certain low-frequency harmonics are eliminated. The output voltage is controlled by adjusting the dc voltage of the inverter with the consequence of slow control response. Thus, a multilevel inverter may become an alternative solution to achieve a simple structure converterwith a fast control response for high -power applications. The concept of multilevel converters has been introduced since 1975. Since then, various multilevel convertertopologies were proposed [10] –[13]. These converters are suitable for high-power medium-voltage applications. The main advantage of multilevel converter is that high output voltage can be obtained without series connection ofswitching devices. Moreover, better output waveforms can be obtained without the need of high switching frequency operation with the associated high switching losses. Several inverter topologies are available today formultilevel output voltage operation [10] –[12]. Diode-clamped multilevel inverter, especially the three-level inverter, also known as neutral-point clamped (NPC) inverter, has found wide application in high-power medium-voltage (MV) drives. This inverter topology has some drawbacks such as additional clamping diodes, complicated PWM switching pattern design, and possible deviation of neutral point voltage. Anotherapparent multilevel topology is the multilevel flying-capacitorinverter which evolved from the two-level inverter by adding dc capacitors to the cascaded switches. However, this invertertopology has some limitations including the need of a large number of dc capacitors with separate pre-charge circuits and complex capacitor voltage balancing control problem. Cascaded H-bridge (CHB) multilevel inverter is one of the popular converter topologies used in high-power medium- voltage applications. It is composed of a multiple units ofsingle-phase H-bridge power cells. The H-bridge cells are normally connected in cascade on their ac side to obtain medium-voltage operation and low harmonic distortion. In practice, the number of power cells in a CHB inverter is mainly determined by its operating voltage and manufacturing cost. The use of identical power cells leads to a modularstructure, which is an effective means for cost reduction. In this inverter topology, however, a number of isolated dc sources for each H-bridge cell are needed. Thus, a complex control method is required to ensure voltage balance in each dc capacitor [13]. In this paper, a new modular multilevel inverter topology based on cascaded H-bridge cells is proposed. Neither
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Modular Multilevel Converter using Single DC Capacitor
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7/28/2019 Modular Multilevel Converter using Single DC Capacitor
A static var compensator can be considered as voltage
source converter which connected in parallel to the power grid
through series inductance as shown in Fig. 5. The line
resistance is usually very small and can be neglected. The
objective of multilevel inverter control system is to ensure dc
voltage and reactive power flow at a desired command. When
the inverter voltage vi
is higher than grid voltage vg, inverter
current will lead the voltage by 90o
(reactive power injection).
On the contrary if the inverter voltage vi is smaller than grid
voltage vg, then inverter current will lag the voltage by 90o
(reactive power absorption). Thus, controlling the inverter
voltage magnitude means controlling the reactive power flow.Although theoretically var compensator does not exchange
active power to the grid, the inverter internal losses will cause
the capacitor voltage to deviate from its nominal value. By
adjusting the phase angle α between inverter and grid voltages,
the active current will flow in/out to keep the dc voltage
constant.
V g
V i jω LC I i
I iV g
V i
jω LC I i I i
V g
V i I i
V L
Leading Reactive Power
Lagging Reactive Power
Charging
Discharging
Power Grid
v i i i v g
Static Var
Compensator
V dc
LC
V iV L
V g
I i
α
α
Fig. 5. Static var compensator model and its operation modes.
The circuit equation for three-phase system as in Fig. 5 can
be written as
(5)
In d – q synchronous reference frame, this equation can be
written as follows:
[] [ ] (6)
where ω is system frequency; the subscript ‗d‘ and ‗q‘ are d-
axis and q-axis voltage/current component respectively.
Because the grid voltage vector is always aligned with d -
axis voltage component vgd, the q-axis component of grid
voltage vgq is always zero. The instantaneous active and
reactive power in d – q synchronous reference frame can be
expressed as [14]
(7)
From (7), the active and reactive power control can directly be determined by active and reactive current provided a
constant grid voltage. Therefore, controlling the reactive
current iiq alone is sufficient to control reactive power to the
grid. Moreover, to keep a constant dc voltage by controllingactive power flow, only the active current iid need to becontrolled. Thus, a fast current controller is desirable in this
method to achieve the system with fast dynamic time response.
A. Static Var Compensator with Proposed Multilevel Inverter
The complete control system and block diagram of the
proposed static var compensator is shown in Fig. 6. There are
two reference values in this system, which are the dc voltage
reference and q-axis current reference which
proportional to reactive power q. The control system will then
produce α* and β * commands, which will control the activeand reactive power respectively. The α*and β * angle can be
obtained from d - and q-axis voltage references as
(
) (8)
(9)
where K is a topology characteristic constant and r is the
transformer ratio. The K value will be unique for each cell
numbers as in (3) with h = 1. For N = 5, K is equal to 7.45,
while for N = 3, K is equal to 4.49.
The inverter output voltage must be synchronized to the
power grid voltage. For this purpose, a phase locked loop
(PLL) circuit is used to obtain the grid voltage angle θ . This
angle will be used for all d – q transformation process.
The dc voltage reference is compared to the actual dc
capacitor voltage which then will be processed by a PI
controller to generate the d -axis current reference . The
actual d - and q-axis currents, which obtained from inverter
currents using d – q transformation, are then compared to the
reference values and the PI current controllers will
compensate the errors. The output of the current controllers isthe desired d -axis and q-axis inverter output voltages. By
using a look up table, the required β and α angle can bedetermined.
B. Decoupled Current Control
The plant block diagram as shown in Fig. 6 implies that the
d - and q-axis currents cannot be controlled independently. To
solve the coupling problem, a feed-forward technique as
shown in current controller block diagram of Fig. 6 is used.
The actual output currents I id and I iq are multiplied by the line
reactance ωLC to produce additional signals to cancel out the
coupling effects. By using this method, the d -axis currents can
7/28/2019 Modular Multilevel Converter using Single DC Capacitor
To further validate the proposed system and its control
strategy as a static var compensator, a prototype of seven-level
modular inverter has been built and carry out the experiment
based on Table II parameters. The control system will be
implemented in dSPACE (DS1104) platform which has
MPC8240 250 MHz core processor with DSP TMS320F240
as slave. The controller can provide a powerful system for
floating point numbers calculation.
For real time evaluation of the control system, a Graphical
User Interface (GUI) will be designed using MATLAB/
Simulink and dSPACE platform as can be seen in Fig. 11. The
previously explained control scheme will be automatically processed and run in DS1104 via PCI card slot. The GUI
platform will provide the input references such as reactive
power and capacitor voltage references, and also shows the
system parameters continuously, e.g. the system voltage,
inverter voltage and current, phase angle, injected reactive
power and dc capacitor voltage.
Fig. 11. Experimental control system.
VI. CONCLUSION
This paper has proposed a modular cascaded multilevelinverter. Inverter topology, switching pattern and control
system for static var compensator have been presented in
detail. The simulation results show that the proposed
multilevel inverter has a fast dynamic response to
inject/absorb reactive power to/from the system. With the proposed control system scheme, the control response can be
adjusted as desired. Moreover, the dc voltage can be
maintained at a constant level under dynamic condition.
In general, the proposed topology has the advantages of its
modularity, equal utilization factors among inverter blocksand simple control procedure. As a single dc capacitor is used,
neither unbalance problem nor complex controllers are existed.
Thus, the proposed modular multilevel inverter provides some
features with which very applicable to low-cost high-power
applications.
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