Satoki Takizawa, Yoshihiko Yamagata, Kansuke Fujii, Yasuhiro Okuma,
Fuji Electric Co.,Ltd, Japan,
E-mail:takizawa-satoki@fujielectric.co.jp
Abstract
Satoki Takizawa, Yoshihiko Yamagata, Kansuke Fujii, Yasuhiro Okuma,
Fuji Electric Co.,Ltd, Japan,
E-mail:takizawa-satoki@fujielectric.co.jp
A multi-level topology is one of the effective approaches to improve the efficiency of the
power converters. The authors has developed and already presented a medium class
(50kVA) UPS using a 3-level Advanced Neutral-Point–Clamped (A-NPC) topology, which
utilizes RB-IGBTs (Reverse-Blocking IGBT) as a bi-directional switch for clamping of its AC
output to a DC neutral point. The A-NPC topology is simple and enables low conduction
losses of the converters, so that the A-NPC topology is suitable for the UPS which needs
high efficiency. This time, the authors further have developed a large capacity 500kVA UPS
using the A-NPC topology.
This paper described the features of the new 500kVA UPS, adopting the advanced NPC 3level
This paper shows the features of the new 500kVA UPS using the IGBT modules for the ANPC
topology in parallel connection to expand its capacity. In addition, the test results show
that the new 500kVA UPS has a high performance with a high efficiency.
1. Introduction
Recently, new power electronics applications, which are house hold appliances,
automobiles and power conditioners of new energies, have rapidly expanded with advance
of power electronics technology because of their inherent high efficiency and availability. On
the other hand, there is a great demand for reducing energy consumption, which means
carbon dioxide emission, as a countermeasure of global warming in the world today. Thus,
one of the most important missions of power electronics technology is to enable high
efficiency to reduce carbon dioxide emission.
A typical UPS such as for computers and communication networks adopts a double
conversion configuration because of its inherent high reliability. But, the conventional double
conversion type UPS converts whole electric energy twice, so that its efficiency is not so high.
For example, the efficiency of the 3-phase 400 V output without transformer type 500 kVA
UPS is around 95 % in general.
A multi-level topology is one of the effective approaches to improve the efficiency of
converters. Here, a neutral-point-clamped (NPC) converter, which utilizes diodes for
clamping of its AC output to the DC neutral point, is well known as the typical multi-level
converter [1]. But, this NPC converter is not so suitable for low DC-link voltage applications
like the UPS, because the total conduction loss of IGBTs is increased by the series
connection of IGBTs.
The authors has already developed a medium class UPS(50kVA) using another type of a
NPC configuration, named advanced NPC (A-NPC) topology, which utilizes new IGBT
modules including RB-IGBTs (Reverse-Blocking IGBT) [2] as a bi-directional switch for
clamping of its AC output to a DC neutral point [3], [4]. The A-NPC topology is simple and
enables low conduction losses of the converters, so that this topology is suitable for the
high efficiency system.
This paper shows the features of the new 500kVA UPS applying the A-NPC topology by
using IGBT modules which are specialized for this topology.
First, the A-NPC topology is compared with a NPC topology and a conventional 2-level topology
about losses from the UPS. Second, the configuration and specifications of the new
500kVA UPS is described. Third, the experimental results of the UPS are also shown.
2. The characteristics of the A-NPC
Fig. 1.One leg of converters for UPS.
Fig.1 (1), (2), (3) and (4) show the one leg of the converter circuits applied to UPSs,
respectively.Type A is a conventional 2-level circuit where is generally applied to. Type B is
called a 3-level NPC (Neutral-Point–Clamped) converter. The NPC leg consists of four
normal IGBTs (Q3-Q6) as main switches and two clamping diodes (D1, D2). Type C is a 3level
circuit of the A-NPC type and is comprised by main switches(Q7,Q8) with normal IGBTs
and diodes (Q9,Q10) circuits as a bi-directional switch between an AC output point and a
middle potential point of a DC bus.Type D is a 3-level circuit of the A-NPC type that we apply
this time, and only RBIGBTs (Q13,Q14) as a bi-directional switch are comprised between an
AC output point and a middle potential point of a DC bus.
Table 1 shows the results that relatively compared the loss of each circuit method.
As for the switching losses, the 3-level methods almost become half in comparison with
the type A which is a reference.
The conduction loss becomes about double in type B, 3/2 in type C, and rough equal in
type D comparison with the type A.
As for the conduction loss, through a DC bus to an AC output, a current passes two
semiconductor devices in type B, it passes one or two semiconductor devices in type C,
and it passes only one semiconductor device in type D like type A. In addition, the 3-level
methods enable downsizing of filters which are connected to the AC input and output, and
become able to almost do the loss with half.
It can be seen from the Table-1 that the type D is a method to reduce losses most
theoretically.
Table-1. Relation among the losses of typeA through typeD.
3. A summary of the IGBT module for the A-NPC
An outlook and an internal circuit of the new IGBT module for the A-NPC topologies are
shown in Fig.2.
This module contains whole semiconductor devices of a one leg (single phase) for the ANPC
NPC topologies. The dimensions of this module are W:80 mm, D:110mm and H:30 mm. The
main switches Q1 and Q2 consist of the 6th generation IGBTs and FWDs rated 1200 V/300 A.
The bi-directional switch consists of an anti-parallel connection of Q3 and Q4, which are the
RB-IGBTs rated 600 V/300 A. These RB-IGBTs have bi-directional blocking voltage of 600 V.
This new module is designed for low stray inductances between main terminals P, N and
M. Since the stray inductance causes a spike voltage when devices switch, the stray
inductance should be reduced to suppress the spike voltage. The stray inductance of this
module is less than 40 nH. It is equal to a stray inductance in a standard 2-in-1 type IGBT
module.
Fig. 2.Outlook and internal circuit of the IGBT module for A-NPC(4MBI300VG-120R,
Tentative)
4. The new 500kVA UPS
Fig.3 shows an exterior view of the conventional UPS and the newly developed UPS of
500 kVA. The dimensions of the new UPS are W: 1500 mm, D:900 mm and H:1950 mm.
This UPS realizes approximately 33% of volume reduction, and 36% weight reduction (2.5t→
1.6t) in comparison with the conventional UPS.
The rated specifications are shown in Table-2. This UPS satisfies all the specifications as
a general UPS, but also it is an unprecedented characteristic that the output of 500kW
(output power factor equal to 1.0) is possible at the rating power. Therefore, this UPS is
applicable to computer loads which were controlled to unity power factor.
The main circuit configuration of this UPS is shown in Fig.4. The circuit utilizes a
transformer-less design concept and consists of a PWM rectifier circuit, a PWM inverter
circuit, a battery boost chopper circuit and a DC voltage balancer circuit. Both of the PWM
rectifier and the PWM inverter adopt the A-NPC configuration. To make up a large capacity
UPS, new modules shown in Fig.2 are connected in parallel. And the boost chopper is
constituted by the 600V rated 2-level IGBT modules (conventional modules) connected in
series. By adopting this chopper topology, DCLs for the boost chopper were able to achieve
downsizing.
By the above-mentioned constitution, this new UPS was able to realize high efficiency with
downsizing by the transformer-less and the A-NPC topologies.
Fig. 3.Exterior view of the convenntional UPS and the newUPS.
Fig. 4.Main circuit configuration of the new UPS.
5. Test results of the UPS
In this section, several test results of the new 500kVA UPS are shown. Test conditions of
the input and output voltages were 3-phase 415V, 60Hz, and load conditions were 500kW.
5.1. System efficiency
Fig.5 shows the system efficiency curves of the conventional UPS and the new UPS. In
comparison with the conventional UPS, the new UPS has superior efficiency due to the ANPC
topology. Efficiency more than 2 points has improved almost in all load conditions. Also
the high efficiency of more than 97% is obtained around the 60% load condition. In addition,
the efficiency is more than 96.5% at output ratio of more than 30% load conditions.
Thus, this new UPS is most suitable for a system, such as power supplies which need
high efficiency with constant power supplying under low load operation.
5.2. Transient response in power failure and power return
Fig.6 shows the operation during a power failure and a power return. It can be seen that
the output voltage and the output current are maintained constant by the battery power and
the boost chopper operation.
5.3. Parallel off operation between two UPSs
Fig.7 shows the parallel off operation when the two of one UPS quits its operation. There
is no disordered output voltage waveform and output current waveform respectively, and the
both UPS currents are transferred to zero or full load current smoothly from the parallel off
point. From this result, parallel control scheme for UPSs works well.
As for this, this UPS is applicable to a further large capacity system.
6. Conclusions
This paper described the features of the new 500kVA UPS, adopting the advanced NPC 3level
topologies and the specified IGBT modules for this topology. Downsizing of 33% of
ratios, high efficiency of 97% and high performance for the UPS has been achieved.
Therefore, this UPS is suitable for power supply facilities such as for an internet data
center (IDC) required low environmental load and high reliability.
7. References
[1] A. Nabae, I. Takahashi, H. Akagi, “A New Neutral-Point–Clamped PWM Inverter“, IEEE
Trans. on industrial applications, vol. 1A-17, no. 5, pp518-523, 1981.
[2] M. Takei, T. Naito, K, Ueno, “The reverse Blocking IGBT for Matrix Converter with UltraThin Wafer Technology“, in proceedings of ISPSD’03, pp156-159, 2003.
[3] H. Wakimoto, M. Ogino, D. H. Lu*, S. Takizawa, H. Nakazawa M. Yatsu, Y. Takahashi,
“600V Reverse Blocking IGBTs with Low On-state Voltage” PCIM Europe 2011.
[4] M.Yatsu, K.Fujii, S.Takizawa, Y.Yamakata, Y.Okuma,
“A Study of High Efficiency UPS Using Advanced Three-level Topology” PCIM Europe
2010, pp550-555, 2010.
