A Novel Diode-Clamped Modular Multilevel Converter With Simplified Capacitor Voltage-Balancing Control
Summary (3 min read)
- Attractive for high voltage-level power conversion in renewable power generation applications.
- Experiments based on a laboratory prototype were carried out and the results validated the capacitor balancing performance of the proposed topology.
- Its modularity and scalability enable it to meet any voltage level requirement -.
- From this generalized multilevel converter topology, several other multilevel topologies can be derived including the diode-clamped, capacitor-clamped, cascaded H-bridge, Marx and modular multilevel topologies .
- Experimental validations of the proposed DCM2C are presented in Section IV.
A. Topology of the proposed DCM2C
- The generalized multilevel converter was proposed as a primary multilevel topology and many other multilevel topologies can be derived from it.
- Meanwhile the capacitor voltages are clamped by switches Sc1-Sc12.
- Compared with the MMC, the Marx multilevel converter uses an extra switch in each SM to realize the capacitor voltage balancing without the requirement of voltage sensors and complicated control methods .
- Vj, j=a, b, c, are the modulation signals of the three phase-legs.
A. Analysis of the balancing circuit
- In practical operation, both the clamping diode and switch have the forward on voltage, which should be taken into account, especially when n is high.
- (12) 2) iarm < 0: The negative current flows through the antiparallel diode in the switch and the voltage drop equals the diode forward on voltage uantiD. Fig. 11 (b) shows the voltage distribution diagram in this case.
- Generally the power flowing through the balancing circuit equals half of the power difference between the two SMs.
- Thus the current rating of the clamping diode can be very low, e.g. 10% of that of the main switch.
- High current pulses may appear in the recovery from a serious imbalance.
B. Discussion on DCM2C losses
- The loss of MMC is consumed by the power switches, parasitic resistance and control circuits.
- Since the last two parts account for so small proportion of the total losses that they can be neglected, only the power switch loss is investigated.
- PaD mainly contains the conduction loss PaDcon and recovery loss PaDrec.
- The current flowing through the clamping diode is much lower than the arm current.
C. Device requirements of MMC and DCM2C
- Table II lists the main device requirements of MMC and DCM2C.
- Regardless of the devices which possess the same cost in the two converters, such as the power switches and storage capacitors, the cost difference is mainly related to the clamping diodes, inductors, voltage sensors and capacitor voltage measuring circuit.
- The DCM2C requires more power diodes and inductors than the MMC, but much fewer voltage sensors and measuring circuits.
- Additionally, the current rating of the clamping diode and the inductor is much lower than that of the main switches.
IV. EXPERIMENTAL RESULTS AND DISCUSSION
- A three-phase DCM2C prototype has been developed for experiments, as shown in Fig. 12.
- The control unit is based on DSP (TMS320F28335) and FPGA (EP3C25F324), and as many as 80 optical fibers are used to transmit switching signals, communication and fault signals.
- Furthermore, a Tektronix scope TDS2024 is used to record the experimental data.
- Several comparison experiments were carried out to investigate the features of this topology.
A. Experiment I: Voltage balancing verification and efficiency test
- This experiment aims to validate the effectiveness of balancing-branches of DCM2C.
- When the relays are closed, the balancing-branches are enabled, otherwise disabled.
- The serious unbalanced capacitor voltages were balanced very quickly as shown in Fig. 14.
- Remove the 2kΩ-resistors attached to SMs and: 1) Keep the relay contacts open.
- It can be seen that the DCM2C efficiency is close to that of MMC, and both rise as the output power increases.
B. Experiment II: Balancing process in detail
- This experiment aims to investigate the balancing process with the switch on and off.
- A voltage deviation, as shown in Fig. 16, existed between SM2 and SM3.
- When Sn3 switched on, the balancing current pulse appeared with a voltage deviation decrease.
- When Sn3 switched off, the current dropped to zero.
- The initial deviation was about 9 V and the first current peak was about 11 A. Generally the voltage deviation would not be that large in the steady operation.
C. Experiment III: Test for balancing capability
- An experiment was carried out to test the balancing capability of the DCM2C when the loss difference between the SMs is high.
- In fact the power resistors connected in SM1 and SM6 represent the best and worst situations for balancing performance because of the direction of the diodes.
- The results in the two cases are presented in Fig. 18 (a) and (b) respectively.
- As shown in Fig. 18, the capacitor voltage deviations between the maximum and minimum in (a) and (b) are about 5V and 7V respectively.
- The capacitor voltages can still be well balanced when the power difference between SMs is high.
D. Experiment IV: Influence of the main circuit current
- The output change may affect the capacitor voltage balance, and an experiment of sudden load change has been carried out.
- The converter started with no load and then the load was suddenly increased as shown in Fig. 19 (b). Fig. 19 (f) shows the DC-bus voltage when increasing the load.
- When the load was increased, the capacitor voltage had a drop of about 7 V. Before the load increase the current flowing through the clamping diode was high frequency narrow pulses with amplitude of lower than 1 A (Fig. 19 (d)).
E. Experiment V: Operation with 50Hz and 20Hz
- This experiment aims to validate the effectiveness of the proposed topology when it operates with 50Hz and 20Hz.
- Due to the limited channels of the scope, only some typical signals are sampled and displayed.
- Comparing the results, it can be noted that when the operation frequency is lower, the capacitor voltage ripples become larger.
- The voltage deviation between SM1 and SM6 also becomes bigger, but still within 5V.
- The amplitudes of currents flowing through the clamping diode D1, D3 and D5 have no obvious variation, but the occurrence frequency of the current-bumps becomes lower.
- Low power rating clamping diode and inductors are used to replace the balancing switch in Marx multilevel converter in this paper, and the diode-clamped modular multilevel converter (DCM2C) is proposed.
- The capacitor voltage control of the converter is so simple that the computation burden is almost the same with that of the two-level converter.
- In addition, the current rating of the clamping diodes and inductors is much lower than that of the main switches in the converter.
- The efficiency of DCM2C is only slightly lower than the MMC’s efficiency because the extra losses of the clamping diodes are relatively small.
- This converter can be used in high voltage and high power converting applications such as high voltage direct current transmission, wind power generation, and especially offshore wind power generation.
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Cites methods from "A Novel Diode-Clamped Modular Multi..."
...ancing techniques based on the modified modulation are also introduced in  and , which also suffers from the same drawback by adding more isolated voltage sensors and control...
Cites background from "A Novel Diode-Clamped Modular Multi..."
...The capacitors of each level of a multi-level converter were frequently connected to next or previous level capacitors by using the numerous amount of switch in ....
...Therefore, the high efficiency in balancing can be obtained independently of imbalance states –....
"A Novel Diode-Clamped Modular Multi..." refers background in this paper
...Since 1980, multilevel converters have been developed extensively –....
"A Novel Diode-Clamped Modular Multi..." refers background in this paper
...Furthermore, the capacitor voltage-balancing control is difficult and complicated –....
"A Novel Diode-Clamped Modular Multi..." refers background or methods in this paper
...Its modularity and scalability enable it to meet any voltage level requirement –....
...The most widely accepted voltage-balancing strategy is based on a sorting method ....
...The semiconductor losses in MMC can be potentially reduced to be 1% , so are the losses in DCM2C....
...Many researchers concentrate on developing control and modulation strategies to solve the problem –....