Patent Application
20200052574
The present invention relates to Design of a polygon-connected autotransformer based 72-pulse ac-dc converter, which can be used for harmonic reduction in different applications such as adjustable speed AC or DC motor drives.
As a practical technique, vector-controlled (VC) strategy is implemented in induction motor drives (VCIMDs), serving various applications. These drives utilize voltage source inverters which are fed from conventional six-pulse diode bridge rectifiers. The most important drawback of these rectifiers is their poor power quality, i.e. current harmonics injection into ac grid. The circulation of current harmonics into the source impedance yields in voltage harmonics at the point of common coupling (PCC) and consequently undesired supply voltage conditions for nearby costumers.
The value of current harmonic components, which are injected into the grid by nonlinear loads, should be controlled within the standard limits. The most prominent standards in this field are IEEE standard 519 and the International Electro-technical Commission (IEC) standard.
For VCIMDs, one effective solution to eliminate harmonics is the application of the multi-pulse AC-DC converters. According to the recent investigations, these converters are based on either phase multiplication, phase shifting, pulse doubling or a combined solution (have been reported in U.S. patents such as U.S. Pat. No. 7,274,280, etc). Application of multi-pulse technique (up to 18-pulse) in AC-DC converters are reported in U.S. Pat. No. 7,375,996 where line current THD of more than 5% is experienced under different load conditions.
The polygon-connected autotransformer based 30-pulse (U.S. Pat. No. 7,719,858) was designed for AC-DC power converter. The DC link voltage in this topology is higher than that of a 6-pulse diode bridge rectifier, thus making the scheme non-applicable for retrofit applications.
The T-connected autotransformer based 40-pulse converter has been designed in (U.S. Pat. No. 8,982,595) for Direct torque controlled induction motor drive (DTCIMD) which a current THD between 2.55% to 3.79% from full-load to light-load (20% of full-load), respectively.
In some applications, it is necessary to take strict power quality measures; therefore, it would be essential to apply the converters with higher number of pulses. For instance, in some applications, harmonics are distinguished as signatures by sonar, and unintentionally are capacitively coupled to ship hull resulting in induced hull currents that make the systems such as degaussing equipment malfunction. In this situation, the operation of harmonic generating loads should be limited, in order to have a THD equal or less than 3%.
In order to overcome the above mentioned problems for the THD of the input currents in this invention, the design of a polygon-connected autotransformer based 36-pulse AC-DC converter is proposed. In the proposed structure, two nine-leg diode-bridge rectifiers are paralleled via an Inter-Phase Transformer (IPT) resulting in a 36-pulse output voltage. In order to double the number of pulses up to 72, a tapped IPT with two additional diodes are added to the rectifier outputs. The proposed converters are modeled and simulated using MATLAB software to study its behavior and specifically analyze the power quality indices. Finally, a low-rating laboratory prototype of the proposed 72-pulse converter is constructed in order to verify the simulation results and examine the effectiveness of the proposed topology.
It is well known that a 12-pulse rectified voltage can be made with two paralleled six-pulse three-phase (three-leg) diode-bridge rectifiers. The phase shift between the two supplying voltages should be 30 degrees. Similarly, in order to implement a 36-pulse ac-dc converter, two paralleled 18-pulse bridge rectifiers (two nine-leg rectifiers) are required.
In this case, two sets of nine-phase voltages with a phase difference of 40 degrees between the voltages of each group and 10 degrees difference between the same voltages of two groups are needed. For this purpose, a polygon-connected autotransformer is designed to produce the nine phase voltages. The phasor diagram of the proposed polygon-connected autotransformer with two sets of 9-phase voltages and the required angular displacement is illustrated in
The aforementioned two voltage sets as Va1, Va2, Va3, Va4, Va5, Va6, Va7, Va8, Va9 and Vb1, Vb2, Vb3, Vb4, Vb5, Vb6, Vb7, Vb8, Vb9 are fed to rectifiers I and II, respectively. The similar voltages of two groups, i.e., Va1 and Vb1, are displaced by 10 degrees. Va1 and Vb1 have a phase shift of +5 and −5 degrees from the input voltage of phase A (Va), respectively. The nine-phase voltages can be made from ac grid phase and line voltages using fractions of primary winding turns.
This is illustrated in
V
A
=V
s<0°, VB=Vs<−120°, VC=Vs120°. (1)
where, Vs is the source phase voltage, VA, VB, and VC are three-phase primary winding voltages.
The two sets of nine-phase voltages with their phase shifts are given as follows:
V
a1
=V
s<+5°, Va2=Vs<−35°, Va3=Vs<−75°,
V
a4
=V
s<−115°, Va5=Vs<−155°, Va6=Vs<−195°,
V
a7
=V
s<−235°, Va8=Vs<−275°, Va9=Vs<−315°. (2)
V
b1
=V
s<−5°, Vb2=Vs<−45°, Vb3=Vs<−85°,
V
b4
=V
s<−125°, Vb5=Vs<−165°, Vb6=Vs<−205°,
V
b7
=V
s<−245°, Vb8=Vs<−285°, Vb9=Vs<−325°. (3)
Using the connection arrangement of the polygon-connected autotransformer shown in
V
a1
=V
A
+K
1
V
CA
−K
2
V
BC
V
a2
=V
b1
−K
3
V
AB
+K
4
V
BC
V
a3
=V
b2
−K
7
V
AB
V
a4
=V
B
+K
1
V
AB
−K
2
V
CA
V
a5
=V
b4
−K
3
V
BC
+K
4
V
CA
V
a6
=V
b5
−K
7
V
BC
V
a7
=V
C
+K
1
V
BC
−K
2
V
AB
V
a8
=V
b7
−K
3
V
CA
+K
4
V
AB
V
a9
=V
b8
−K
7
V
CA (4)
V
b1
=V
A
−K
1
V
AB
+K
2
V
BC
V
b2
=V
a2
−K
5
V
AB
+K
6
V
BC
V
b3
=V
a3
+K
6
V
CA
−K
5
V
AB
V
b4
=V
B
−K
1
V
BC
+K
2
V
CA
V
b5
=V
a5
−K
5
V
BC
+K
6
V
CA
V
b6
=V
a6
+K
6
V
AB
−K
5
V
BC
V
b7
=V
C
−K
1
V
CA
+K
2
V
AB
V
b8
=V
a8
−K
5
V
CA
+K
6
V
AB
V
b9
=V
a9
+K
6
V
BC
−K
5
V
CA (5)
where, the line voltages are given as follows:
V
AB=√{square root over (3)}VA<30°, VBC=√{square root over (3)}VB<30°, VCA=√{square root over (3)}VC<30°. (6)
Constants k1-k7 are calculated based on (2)-(6) to determine the required windings turn numbers and achieve the desired phase shift for two voltage sets, as follows:
K
1=0.00254, K2=0.04904, K3=0.11802,
K
4=0.22183, K5=0.0747, K6=0.039747, K7=0.29886. (7)
The schematic diagram of the proposed 36-pulse ac-dc converter is shown in
Using instance, with the autotransformer of the proposed 36-pulse converter, the rectified output voltage is 17% higher than that of a six-pulse rectifier. For retrofit applications, the design procedure should be modified so that the dc-link voltage becomes equal to that of a six-pulse rectifier. This will be accomplished via modifications in the tapping positions of the windings as shown in
|VS|=0.8328|VA| (8)
V
a1
=V
A
−K
1
V
CA
+K
2
V
BC
V
a2
=V
A
−K
3
V
AB
+K
4
V
BC
V
a3
=V
B
+K
5
V
AB
−K
6
V
CA
V
a4
=V
B
+K
1
V
AB
+K
2
V
CA
V
a5
=V
B
−K
3
V
BC
+K
4
V
CA
V
a6
=V
C
+K
5
V
BC
−K
6
V
AB
V
a7
=V
C
+K
1
V
BC
+K
2
V
AB
V
a8
=V
C
−K
3
V
CA
+K
4
V
AB
V
a9
=V
A
+K
5
V
CA
−K
6
V
BC (9)
V
b1
=V
A
−K
1
V
AB
−K
2
V
BC
V
b2
=V
A
−K
5
V
AB
+K
6
V
BC
V
b3
=V
B
+K
3
V
AB
−K
4
V
CA
V
b4
=V
B
−K
1
V
BC
−K
2
V
CA
V
b5
=V
B
−K
5
V
BC
+K
6
V
CA
V
b6
=V
C
+K
3
V
BC
−K
64VAB
V
b7
=V
C
−K
1
V
CA
−K
2
V
AB
V
b8
=V
C
−K
5
V
CA
+K
6
V
AB
V
b9
=V
A
+K
3
V
CA
−K
4
V
BC (10)
Accordingly, the values of constants k1-k7 are recalculated for retrofit applications as follows:
K
1=0.1136, K2=0.01489, K3=0.21188,
K
4=0.16985, K5=0.27408, K6=0.20295, K7=0.24888. (11)
A tapped IPT, as shown in
The Vm is an alternating voltage with both positive and negative half cycles. Hence, D1 conducts when Vm is positive and, on the other hand, D2 conducts when Vm is negative. The MMF equivalence between the windings, when D1 is on can be given as follows:
i
dc1
N
A
=i
dc2
N
B (12)
where, NA and NB are number of turns, as shown in
i
dc1
+i
dc2
=i
dc (13)
Using (12) and (13), the output current of two rectifiers are calculated as follows:
i
dc1=(0.5+Kt)idc
i
dc2=(0.5−Kt)idc (14)
In the above equation, Kt=(NB−0.5Nt)/Nt, where Nt=NA+NB. The same equation can be written, when Vm is in its negative half cycle. Therefore, according to MMF equation, the magnitude of the output currents changes which, results in pulse multiplication in the supply current. It is proved that Kt should be equal to 0.2457 to eliminate the harmonic currents up to the 37th order which can be applied in this application too.
In paralleled-rectifiers, two converters cannot be directly paralleled, as the output voltages are phase-shifted, and unwanted conduction of diodes is probable. Therefore, a ZSBT is required to ensure the independent operation of two paralleled rectifiers. In the proposed 72-pulse converter, the voltage frequency of ZSBT is nine times of the supply frequency and it shows high impedance at zero sequence (and its multiples) harmonic currents and therefore prevents their power flow. Furthermore, the high ripple frequency of the ZSBT voltage makes it small and light. An overall schematic of the proposed 72-pulse ac-dc converter is illustrated in
The designed configuration is simulated using Matlab/Simulink software and power system block set (PSB) toolbox. In these simulations, a three-phase, 460 V, 60 Hz network is utilized as the supply for the 36 and 72-pulse converters via the designed polygon-connected autotransformer, modeled by three multi-winding transformers. The multi-winding transformer block is also used for modeling the ZSBT and tapped IPT.
At the converter output (dc link), a series inductance (L) and a parallel capacitor (C) are connected to feed the IGBT-based Voltage Source Inverter (VSI). The VSI drives a squirrel cage induction motor employing vector-controlled strategy. The simulated induction motor is a 50 hp (37.3 kW), 4-pole, and Y-connected.
Three-phase squirrel cage induction motor—50 hp (37.3 kW), three phase, four pole, Y-connected, 460 V, 60 Hz. Rs=0.0148Ω; Rr=0.0092Ω; Xls=1.14Ω; Xlr=1.14Ω, XLm=3.94Ω, J=3.1 Kg·m2.
Controller parameters: PI controller Kp=300; Ki=2000.
DC link parameters: Ld=2 mH; Cd=3200 μF.
Source impedance: Zs=j0.1884Ω(=3%).
The simulation results are depicted in
The diode D1 conducts when the voltage across the tapped IPT is positive and, vice versa the D2 is on, when the voltage across it is in its negative half-cycle. This conduction sequence of the diodes is the basis of the pulse doubling technique. The current waveforms of the pulse doubling diodes are shown in
The input current waveforms and its harmonics spectrum of the 6-pulse, 36-pulse, and 72-pulse converters extracted and shown in
The current THD of the proposed 36-pulse converter is reduced to 2.21% and 3.64% for full load and light load conditions as shown in
In addition to the supply current THD, other power quality indices such as supply voltage THD, displacement power factor (DPF), distortion factor (DF), and power factor (PF) are also calculated under different loading conditions and listed in Table I. It can be seen that these indices are significantly improved. Moreover, the grid power factor for the 72-pulse topology, reaches unity PF from light to full load conditions.
The apparent power ratings of the polygon-connected autotransformer, tapped IPT, and ZSBT for 72-pulse configuration are calculated using the following equation:
S=0.5ΣVwinding Iwinding (15)
where, Vwinding is the rms voltage across the autotransformer, ZSBT and tapped IPT windings and Iwinding indicates the full load current of the same winding.
These rms values are obtained from simulations with 10 hp (7.5 kW) load as tabulated in Table II. The calculated ratings are 3265.36 VA, 33.85 VA and 8.21 VA for autotransformer (TAN, TBC, and TCA), ZSBT and tapped IPR, respectively, which are 43.77%, 0.45% and 0.11% of the load power rating (7.5 kW), respectively. It means that the required magnetic ratings of the proposed topology is about 44.33% of the load rating while the current THD of less than 3% is achieved. This rating is less than many other topologies of ac-dc converters. It can be seen that a total of 57.26% of magnetic rating is needed to achieve THDi<5% in a 40-pulse ac-dc converter (U.S. Pat. No. 0218982 ?????).
In order to verify the design approach and demonstrate the applicability of the proposed topology, a laboratory prototype of conventional 6-pulse and the proposed 36-pulse and proposed 72-pulse converters are constructed. Several tests have been carried out using an equivalent resistive load under light load to demonstrate the worst case harmonic conditions. The input line current waveforms and their harmonics spectrum for 6-pulse, 36-pulse, and 72-pulse converters are determined using a HWT-1000 harmonic analyzer. It can be seen that there is a good agreement between theoretical and experimental results.
In this invention, a polygon-connected autotransformer was designed and modeled to make a 36-pulse ac-dc converter consisting of two paralleled 18-pulse nine-phase rectifiers. For retrofit applications, the proposed design procedure was modified. A zero-sequence-blocking transformer was added to ensure the independent operation of paralleled rectifiers and a tapped inter-phase transformer was used to double the number of pulses, resulting in decreasing the size and volume of the transformers as well as improvement of the power quality indices at the PCC.
In addition, a laboratory prototype was constructed to show the applicability of the proposed topology. The simulation and experimental results demonstrate the proper operation of the proposed configuration and its good agreement with the limits set by IEEE-519. In summary, the power quality improvement of the supply current/voltage and reduced ratings of the transformers, and consequently reduced cost of the converter, are the major benefits of the proposed 72-pulse ac-dc converter.
