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A B S T R A C T
- Power^ Electronics^ Systems^ Business^ Group,^ Fuji^ Electric Co., Ltd.
1. Introduction
In recent years, data centers have increasingly been using electricity, which has created a demand for improved efficiency of uninterruptible power systems (UPSs). Fuji Electric has met this demand with the “UPS7000HX Series(1)^ ” with 400-V output. The Series has achieved high efficiency by employing a non- insulated system and three-level conversion. To further improve the efficiency of an entire data center, one method is to use a three-phase, four-wire UPS instead of a three-phase, three-wire type. It is capable of feeding directly to a server without a power distribution unit (PDU) transformer, saving space and cost while achieving high efficiency. This paper presents a way to utilize three-phase, four-wire UPSs and the “UPS7000HX-T4,” a three- phase, four-wire UPS offered by Fuji Electric. It also describes how to build a three-phase, four-wire system by using an insulated three-phase, three-wire UPS with an inverter transformer.
2. Comparison Between Three-Phase, Four-Wire
and Three-Phase, Three-Wire Power Supply
Systems
Three-phase, four-wire UPSs are widely in use out- side Japan but not many examples of use are found in Japan. Three-phase, three-wire power supply systems, which are the mainstream in Japan, have benefits such as being able to handle server load harmonics and suppressing short-circuit currents due to the PDU transformer provided. This is assumed to be one rea- son why three-phase, four-wire power supply systems
YASUMOTO, Koji *^ WANG, Fangfang *^ TAKIZAWA, Ayumu *
High-Efficient Power Supply Systems Utilizing
3-Phase 4-Wire Uninterruptible Power Systems
In recent years, data centers have increasingly been using electricity, and their power supply systems need to improve efficiency. Fuji Electric has offered power supply systems using 400-V, 3-phase, 4-wire uninterruptible power systems, as well as 3-phase, 4-wire power supply systems using conventional 200-V, 3-phase, 3-wire power supply systems. Both types eliminate the need for a PDU transformer, achieving high efficiency. Using a current-lim- iting circuit-breaker enables short-circuit protective coordination. Furthermore, theIgr method allows users to reliably monitor insulation and reduces the effect of harmonics on theK-factor and transformer equivalent capacitance more than standard power supplies.
are not employed often. Table 1 shows a comparison between three-phase, four-wire and three-phase, three-wire power supply systems; and Fig. 1, the main circuit wiring diagrams of the two power supply systems. The three-phase, three-wire system is provided with a PDU transformer for converting the voltage into single-phase 230 V to be supplied to a server load. Meanwhile, the three-phase, four-wire system uses a four-wire UPS with the secondary voltage of the in- put transformer being 400 V. It can feed single-phase 230 V to a server load between the neutral and power supply phases, eliminating the need for a PDU trans-
Table 1 Comparison of three-phase, four-wire power supply system with three-phase, three-wire type Items Three-phase, system^ four-wire Three-phase system^ three-wire
Efficiency (^) No PDUGood transformer
Fair PDU transformer provided
Space (^) No PDUGood transformer
Fair PDU transformer provided
Cost (^) No PDUGood transformer
Fair PDU transformer provided Short-circuit current
Good Reduced with current- limiting circuit breaker
Good Reduced with PDU transformer
Insulation monitoring
Good Provided for input transformer
Fair Provided for input transformer and PDU transformer
Input transformer
Fair Measures for harmonic current required
Good Harmonic current dealt with by PDU transformer Third and triplen harmonics
Fair Triplen harmonic super- imposition
Good Dealt with by PDU transformer
High-Efficient Power Supply Systems Utilizing 3-Phase 4-Wire Uninterruptible Power Systems 59
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former. Therefore, in order to suppress an increase in short circuit current, a current limiting breaker is pro- vided to perform protection coordination. In three-phase, four-wire system, the third har- monic and triplen harmonic currents are superimposed on the neutral phase. In some cases, the current ca- pacity of the neutral phase is made larger than the power phase. However, the current capacity of the neutral phase can be made equal to that of the power supply phase as described below, and this allows the system to reduce the introduction costs. In addition, ground fault can be monitored for a three-phase, four- wire UPS using an insulation monitoring device, im- proving system reliability.
3. Outline of Three-Phase, Four-Wire Power
Supply Systems
3.1 Power supply system based on the “UPS7000HX-T4” (three-phase, four-wire) The UPS7000HX-T4 (three-phase, four-wire type) is a large-capacity UPS with a 400-V output developed for data centers. Figure 1(a) shows the main circuit wiring diagram. Figure 2 shows the external appear- ance of the UPS7000HX-T4; and Table 2, the specifica- tions. UPS7000HX-T4 provides a capacity of 500 kVA, alternating current (AC) input, bypass output and AC output at voltages of 380 V, 400 V and 415 V. The main circuit consists of an input filter circuit, recti- fier, inverter and output filter circuit. The UPS has
achieved high efficiency by employing a non-insulated system and three-level PWM control. While not shown in Fig. 1, a battery circuit is connected to the direct current (DC) intermediate part between the rectifier and inverter. The neutral phase of the star connection in the input transformer is grounded with Class B. The AC and direct feed inputs are drawn in with a three-phase, three-wire system and the neutral phase is connected to point M between the two capacitors in the DC cir- cuit. No switch is provided between the neutral line of the transformer and the UPS so as to prevent the neu- tral phase from being cut off.
Rectifier
P
N
N
M (^) Inverter
(a) Three-phase four-wire power supply system
Server load
MC
1 230 V
MC
PDU transformer 420/400 V
6,600/400 V (^) MC
ACSW
Rectifier
P
M (^) Inverter
(b) Three-phase three-wire power supply system
Server load
MC
1 230V
MC
6,600/420 V (^) MC
ACSW
φ
φ
Fig.1 Main circuit wiring diagrams of three-phase, four-wire and three-phase, three-wire power supply systems
Fig.2 “UPS7000HX-T4”
High-Efficient Power Supply Systems Utilizing 3-Phase 4-Wire Uninterruptible Power Systems 61
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used for the UPS output and on the server load side. Short-circuit protective coordination is required be- tween the MCCB on the load side and branch MCCB, and a current-limiting circuit breaker is used for the MCCB on the server load side. Figure 6 shows the cutoff characteristics of Fuji Electric’s “BM Series” current-limiting circuit breaker with a 32 AT. The horizontal axis represents the short-circuit current ob- tained by calculation; and the vertical axis, the peak value of the current that flows at the time of cutoff resulting from operating the MCCB. The current- limiting circuit breaker of 32 AT can limit the short- circuit current of 6.7 kA to 2.0 kA (with a 2.8 kA peak). For the MCCB on the upper side, one with a character- istic of not starting instantaneous operation at 2.0 kA is selected for short-circuit protective coordination. The three-phase short-circuit current at point F directly under the UPS in the eight-unit parallel re- dundant system can reaches very high current 91.5 kA, approximately six times that of the single-unit system.
close to a server load, through a 2P MCCB. Figure 5 shows an equation for calculating short circuit current for a three-phase short circuit, two-phase short circuit and short circuit between the neutral and power sup- ply phases. The ratio of each short-circuit current to that of a three-phase short circuit as the reference is as follows: (a) The two-phase short-circuit current is √¯3/ times the three-phase short-circuit current. (b) The short-circuit current between the neutral
and power supply phases is (Z 1 + Z 2 )/(Z 1 + 2 Z 2 )
times the three-phase short-circuit current.
Z 1 is the total of impedances per phase on the
power supply side including the transformer, and Z 2 is
the cable impedance of the secondary side of the trans- former and what follows. The short-circuit current calculation conditions are as follows: a short-circuit current on the power sup- ply side of 20 kA; input transformer 550 kVA with a percentage impedance of 5%; high-voltage cable CVT 60 sq of length 30 m; four low-voltage cable CV, 250 sq, 2 core of length 60 m; and low-voltage cable from the main line to the server load CV 5.5 sq of length 1 m. Table 3 shows the results of calculation. The three-phase short-circuit current at point F directly under the UPS in the single-unit system is 15.1 kA (at 400 V) and the short-circuit current be- tween the neutral and power supply phases at point F2 directly under the MCCB on the load side is 6.7 kA (at 230 V). MCCBs with 800 AT and 32 AT are mainly
Virtual neutral line (^) V
Short circuit
(a) Three-phase short circuit (b) Two-phase short circuit (c) Short circuit between neutral and power supply phases
Z (^) 1 Z (^) 2 I (^) S
Z 1 Z (^2) Z 1 Z (^2)
3
V
Z (^) 1 Z (^) 2
Z (^) 1 Z (^) 2 Z (^) 1 Z (^) 2 Z (^) 2
3
V
Short circuit Short circuit
Z (^) 1 Z (^) 2 I (^) S
Z (^) 1 Z (^) (^2) I (^) S Z (^) 1 Z (^) 2
I S3 =
Z 1 +Z 2
V
Z 1 +Z 2
IS2 = V
I
V
S1 =^
Z 1 + 2+ Z 2
Fig.5 Short-circuit current calculation equation for three-phase short circuit, two-phase short circuit and short circuit between neutral and power supply phases
Table 3 Short-circuit current value comparison between single-unit and 8-unit-in-parallel redundant systems
System Point F
Point F (Calculated value)
Point F (Cutoff current by current-limiting circuit breaker) Single unit 15.1 kA 6.7 kA 2.0 kA 8 units in parallel 91.5 kA^ 10.6 kA^ 2.8 kA
BM3RSB, HB I (^) p (Ue = 230 V/240 V) BM3RHB
0.16 A
0.25 A
0.4 A
0.63 A
1 A
1.6 A
2.5 A
4 A 6.3 A
10 A
13 A
16 A 20 A
25 A 32 A
10.6 kA (RMS value)
4.0-kA peak 2.8 kA (RMS value)
0.1 1 10 100 I (^) sc (rms)(kA)
10
1
I^ p
(kA)
Fig.6 Cutoff characteristic of “BM Series” current-limiting circuit breakers
62 FUJI ELECTRIC REVIEW^ vol.65 no.1^2019
rent transformer (ZCT) to detect a leak current.
(b) The I or method uses the system voltage as the
reference and detects a current by insulation
resistance. The I or and the current I oc, which
flows through ground capacitance, are separated and extracted by computing.
(c) The I gr method superimposes an electric current
from a power supply with different frequency from the commercial power supply on the cur- rent between each power supply and the ground and monitors insulation by measuring active current. A superimposed voltage of 0.5 V and frequency of 20 Hz, for example, are used for de- tecting active current of the same phase as that of this power supply.
Of these, the I gr method has features such as no ef-
fect of capacitance, the ability to detect insulation dete- rioration of the ground and neutral phases, detection of three-wire balance insulation deterioration, and insus- ceptibility to an imbalance of the ground capacitance, and it provides a reliable insulation monitoring.
With the I gr method, an I gr power supply can be
inserted into the neutral phase of the secondary side of the star connection in the input transformer of a non- insulated UPS to monitor the insulation of the power system including the UPS main body. With some in- sulation devices, however, the ground capacitance is limited, and the ground capacitance of the grounding capacitor in the UPS, cable and the load side must be calculated to consider whether or not to adopt this method.
6. Dealing with Harmonic Currents
Harmonic currents can affect the UPS input trans- former capacity and current capacity of the neutral phase. The input transformer, to which a harmonic load is directly connected when the UPS is switched to the bypass, must be selected in view of the capacity required when a harmonic current flows. Therefore, the transformer equivalent capacitance based on the limits for harmonic current emissions of JIS C 61000-
3-2 and the K-factor value specified in the UL standard
must be calculated. The third harmonic current and any integer multiple of the three have the same phase because there is no difference in the phase angle, and a current flows in the neutral phase is three times as large as that in each power supply phase. When the neutral phase current can exceed the rated current of the MCCB, the current capacity of only the neutral phase must be made large. Accordingly, we see if the current capacity can be made the same level as the power supply phase. JIS C 61000-3-2 specifies the limits for harmonic current emissions for devices with an input current per phase of up to 20 A. For the harmonic current limits, devices can be grouped into four classes: A, B, C and D. The limits for the largest allowable harmonic cur-
In the parallel bus part, an appropriate measure must be taken to prevent short-circuit accidents. Never- theless, as shown in Fig. 6, the short-circuit current between the neutral and power supply phases at F directly under the MCCB on the load side is 10.6 kA but is limited by the current-limiting circuit breaker to 2.8 kA (with a 4.0 kA peak). Short-circuit protective coordination can be easily achieved by using a current-limiting circuit breaker ei- ther in the single-unit or parallel system.
5. Ground Fault Protection of Low-Voltage
Circuit
To protect the UPS and the server load from
ground fault, the most reliable method is the I gr
method using an insulation monitoring device.
5.1 Ground-fault current of three-phase, four-wire UPS With the input transformer provided on the pri- mary side of the UPS, the contact-preventing plate and neutral phase of the secondary winding are grounded with Class B. The load voltage on the server side is 230 V and grounded with Class D. To the closed-loop circuit during a ground fault, the Class B grounding resistor of the neutral phase and Class D grounding re- sistor of the load on the server side are thus connected in series. Figure 7 shows how the ground-fault current flows and the calculation equation. In the main circuit of the non-insulated UPS, the insulated gate bipolar transistors (IGBTs) are provided with free wheeling diodes in anti-parallel configuration. Therefore, even if an insulation failure occurs on the load side, a closed- loop circuit is formed through the free wheeling diode when one IGBT is turned on, allowing the ground-fault current to be detected.
5.2 Insulation monitoring of UPS system Insulation monitoring devices of UPS systems may be one of three types depending on the method used:
I o, I or or I gr method.
(a) The I o method uses a zero-phase-sequence cur-
Class B
Z (^) B
Class D
Z (^) D
3
V
I (^) g I
V
g =^
Z B +Z D
Fig.7 Ground-fault current equivalent circuit and calculation equation
