Synchronous Frame Hysteresis Controller for Three Phase UPS Based on Z-Source Inverter M.Lakshmana Kumar1
K.Chitra2
PG Scholar, Bannari Amman Institute of Technology lakshmanakumar2010@gmail.com
Assistant Professor Bannari Amman Institute of Technology E-mail:chitrak@bitsathy.ac.in
1
2
Abstract-This paper presents the novel synchronous frame hysteresis controller for three phase Uninterruptible Power Supply (UPS)based on Z-Source Inverter (ZSI). The proposedsynchronous frame approach minimizes the hysteresis control time delay to onesixthof the fundamental period such that the dynamic response is significantly improved. The hysteresis band currentcontroller determines the switching signals, and the algorithm based onanextension of Synchronous Reference Frame(SRF) Theory (d-q-0) is used to determine thesuitable current reference signals. A filter is applied to further enhance the accuracy of steadystate tracking.Low total harmonic distortion has been achieved under heavilydistorted nonlinear load and unbalanced load.The ZSI provides agreat alternative with lower cost, higher reliability and higher efficiency. Index Terms- CSI, DQ algorithm,Hysteresis Controller,Uninterruptible power supply, VSI, ZSI,
1. INTRODUCTION Uninterruptable power supply (UPS) is used to supply very high-quality,continuous, and disturbance-free ac power to critical loads such as medical equipment,home appliances, computers, and communication systems[15]. A good UPS system requires not onlyexcellent steady-state performance in terms of voltage regulation, switching losses and Total HarmonicDistortions (THD) but also a fast transientresponse during load step change.UPS that differ from an emergency power system or standby generator in that it will provide instantaneous protection from input power interruptions by supplying the energy stored in the batteries. UPS is an electrical apparatus that providesemergency power to a load when the main power fails. Some highperformancefeedback control techniques such asmultiloopstate feedback control,dead-beat control and predictive control have been proposed and investigated“[7,8]”. Highqualityoutput voltage and fast dynamic responsehave been demonstrated with thesemethods. The controllerbandwidth is limited due todigital implementation.Selective harmonic compensation employs the idea that eachorder of harmonicdistortions can be compensated individually.This technique is targeted at excellentsteady-state outputand it is implementable either in the stationary frame or insynchronous rotating frames[14]. ____________________________________ M.Lakshmanakumar is currently pursuing Master of Engineering in Bannari Amman Institute of Technology, India,Ph:9578228161, E-mail:lakshmanakumar2010@gmail.com K.Chitra is currently working as Assistant Professor in Bannari Amman Institute of technology,India,Ph:9976719928, E-mail: chitrak@bitsathy.ac.in Dr.A.Jeevanandham is currently working as Professor in Bannari Amman Institute of technology,India,Ph:9843688895 E-mail: jeeva932@gmail.com
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Fig.1 Block Diagram of the Controller.
2. HYSTERESIS CONTROLLER Hysteresis controllers utilize some type of hysteresis in the comparison of the line currents to the current references[2].The hysteresis control schemeprovides excellent dynamic performance, because it acts quickly.Due to the advances of power electronics and inverter topology, the currentcontrolledvoltage-sourcepulsewidth modulated(PWM)inverterisusuallypreferredfor quick response and accurate control[3]. These power devices may be applicable in ac motors, active filters, and Uninterruptible Power Supply (UPS). Current controlled PWM inverter offers substantial advantages ineliminating stator dynamics in high performance AC drives[5].One version of hysteresis control, uses three independent controllers, one foreach phase.The control for one inverter leg is shown in Fig.2[6].When the line currentbecomes greater (less) than the current reference by the hysteresis band,theinverter leg is switched in the negative (positive) direction, which providesaninstantaneous current limit if the neutral is connected to the dc bus midpoint.
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Therefore, the hysteresis band specifies the maximum current ripple as summing neither controller nor inverter delays[1].The inverter switching frequency will vary over a fundamental inverter periodsince the current ripple is specified by the hysteresis band. In a system without a neutral connection, the actual current error can reachdouble the hysteresis band assuming the three-phase current reference sums tozero[10].
Fig.4 Block Diagram of Synchronous Frame DQ Extraction
Fig.2 Circuit for Hysteresis Current Controller 3. SYNCHRONOUS FRAME CONTROL ALGORITHM In the SRF control, the load current signals are transformed into the conventional rotating frame d-q. If theta is the transformation angle, the transformation is defined by: cos(⍬) xd xq = −sin(⍬) x0 1 2
cos(⍬ −
2π
−sin(⍬ − 1 2
)
3 2π 3
)
cos(⍬ −
4π
)
xa −sin (⍬ − ) xb 3 x0 1 3 4π
Unbalanced and sinusoidal mains voltage conditions withangle θ is a uniformlyincreasing function of time.This transformation angle is sensitive to voltageharmonicsand unbalance; therefore dθ/dt may not beconstant over a mains period[9]. With thistransformation the direct current component is Ia iα Ib (2) = C iβ Ic id cos ⍬ iq = − sin ⍬ where ⍬ = tan−1
sin ⍬ cos ⍬
iα iβ (3)
vβ vα
(1)
2
Fig.3 shows the basic configuration of synchronous reference frame [13]. In the SRF is atime varying anglethat represents the angular position of the referenceframe which isrotating at constant speed insynchronism with the three phase ac voltages. In the SRF is a time varying angle that represents the angular position of the reference framewhich is rotating at constant speed in synchronism with the three phase ac voltages [16]. In this case thespeedof the reference frame is practically constant, that is, the method behaves as if the reference frame moment of inertia is infinite.
id iq =
1 v 2α +v 2β
vα −vβ
v β iα v α iβ
Icomp ,a i Icomp ,b = [c]T α iβ Icomp ,c
(4)
(5)
The fundamentalcurrents of the d-q components are now dc values. Theharmonics appear like ripple. Harmonic isolationof the d-q transformed signal isachieved byremoving the dc offset [11].
4. SIMULATION AND RESULTS The Fig.5 shows the simulation of Synchronous frame hysteresis controlled Z-Source inverter for UPS applications. The input DC voltage of 100V is given to the three phase inverter through the impedance network. The current from the inverter is given to the DQ extraction frame in order to increase the dynamic response of the system and to reduce the switching frequency of the hysteresis current controller.
Fig.3 Principle of Synchronous Frame Theory This is accomplished using high pass filters (HPF). Inspite of a high pass filter, allow low pass filter is used to obtain the reference source current in d-q coordinates[4].
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Fig.7 Simulation of Hysteresis Controller
The simulation diagram of the Hysteresis Current Controller is shown in Fig 7.The output current of DQ extraction and the actual current is given as input to thecontroller. The error signal is passed through the hysteresis band and pulses aregenerated. Fig.5 Simulation Diagram of Proposed Method
4.1 OUTPUT WAVEFORMS
Thepulses generated from the hysteresis current controller are given to the three phaseinverter.
Fig.8 Current waveform after DQ Extraction
Fig.6 Simulation Diagram of DQ Extraction The simulation diagram of DQ extraction frame is shown in the Fig 6. Theload voltage is compared with the reference voltage and the error signal is amplifiedwith the PI controller. The output of the PI controller is given to the Low pass filter inthe DQ transformation block. The output current waveform shows the dynamicresponse of the system.
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Fig.9 Reference Current Given To the Hysteresis Controller
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Fig.13 Output Current Waveform
4.2 THD ANALYSIS OF THE SYSTEM Fig.10 Actual Current Waveform for Hysteresis Controller
Fig.13 THD analysis of the system
Fig.11 Pulse Generated From the Hysteresis Controller
The output waveforms of the Synchronous frame hysteresis controlled Z-sourceinverter at various stages are shown. The output current from the DQ extraction showsthe dynamic response of the system. The pulses generated from the hysteresiscontroller are shown in Fig 11. THD analysis of the system is carried out whichresults in 17.11%.
5. CONCLUSION
Fig.12 Output Voltage Waveform
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A new synchronous frame hysteresis control Strategy has been developed forthree phase UPS inverters. By designing the Hysteresis control to compensate theharmonics in three different synchronous rotating frames, all the stationary frameharmonic distortions are reduced considerably. Therefore, the voltage regulationrequirement can be satisfied under general load conditions (linear and non-linear loads). But this controlstructure also hassome drawbacks, such as the destructionof the inverter if its highest switchingfrequency is not limitedand the interdependence of the load phases. In order to limitthis frequency in a single-phase case and extend this result to the three-phase case througha transformation (were dq transformation is employed)avoiding thecomplexity.Hysteresis current controller results high dynamic response,easy toimplementand eliminates the complex calculations of the existing methods. The Totalharmonic distortion of the proposed method achieved is 17.11%.
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