05-09-2016, 02:27 PM
A Unified Phase-Shift Modulation for Optimized Synchronization of Parallel Resonant Inverters in High Frequency Power System
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Introduction
The high frequency alternating current (ac) (HFAC) power distribution system (PDS) proposed by NASA initially has been investigated for space station and aerospace decades ago [1]. Recently, it has already become a preferred alternative to the traditional dc distribution system in computer and telecom [2], electric vehicle [3], and renewable energy microgrid [4].The merits of HFAC can be generally summa rized by: 1) flexible conversion of different voltage grades; 2) effective electrical isolation using compact high frequency transformers; 3) significant saving in component count and size; 4) improvement in the system dynamic response; 5) reduction or elimination of acoustic noise; and 6) more safety with increasing frequency. A typical structure diagram of HFAC PDS as shown in Fig. 1(a) is formed by the source and the load side. The source side is constructed by a number of front-end inverters in parallel connection to deliver power from the input line to the transmission track, and the load side is constituted by point-of-use converters to absorb the power from the transmission track [5]. High performance resonant inverter serves as the HFAC source side to provide HF output, flexible power expansion capability, and better load performance. However, the control of the resonant inverter is complicated in HFAC circumstance because of the following factors: 1) Input voltage varies over a wide scope; 2) HF power expansion is difficult; 3) soft switching is required to operate in a wide range; 4) output voltage with low total harmonic distortion (THD) is necessary for precise load; and 5) unknown distributed load and multiple load interactions lead to more dynamic and nonlinear characteristics. Therefore, great efforts from topology, modulation, and control strategy have been done to solve these issues in HFAC power source.
From the view of topology, a variety of high frequency resonant inverters are presented in [6]. High frequency ac–ac can also be found in matrix power converters [7]. Most of them can be divided into the switching network and resonant tank, in which the switching network converts the dc voltage to a quasi-square waveform and the resonant tank designed for the harmonic filter also provides zero voltage switching (ZVS) conditions. The single-stage phase-shift modulation (PSM) resonant inverter constructed by full bridge and inductor capacitor-inductor-capacitor (LCLC) resonant tank is the best candidate of HF power source because of outstanding comprehensive abilities. The full bridge provides the large power grade with less switch stress. It is easy to implement ZVS at fixed frequency in PSM. The resonant tank formed by four energy storage components provides the sinusoidal output with less THD and more flexible ZVS configuration [8], [9]. Thus, the rational HF power source as shown in Fig. 1(b) is a parallel system constructed by a single-stage resonant inverter. The connection impedance(Zc=Rc+jXc)is deliberately placed to prevent the circulating current [10], and it also is non negligible because of the connection cable.
Another very important approach to solve the aforementioned high frequency issues is the appropriate strategy of modulation and control. The advanced magnitude controller is proposed in [11] and [12] to regulate the resonant converter. However, most of them are complicated and costly.A frequency-based controller titled by one cycle robust controller was easy to be implemented by discrete devices and can effectively resist the perturbations from input line, load, and components [13], but the exclusive magnitude control is far from enough for load sharing in parallel system. Because of the prominent discrepancy of phase angle, the unavoidable circulation current and the unwanted power losses are severe in parallel system. The current sharing control (CSC) algorithm accomplishes high frequency load sharing using average magnitude and phasor transformer [13]. However, the phasor transformer-based CSC is complicated, and the average magnitude-based CSC needs an unperturbed phase angle. A symmetrical PSM [14], [15] is proposed to maintain the phase angle unperturbed in magnitude modulation. However, it only provides a rough current sharing because it fails to consider output discrepancy from component tolerance in connection impedance and series–parallel resonant tank (Ls,Lp,Cs,Cp). In order to perfectly accomplish current sharing in parallel system, independent phase control is indispensable to operate with magnitude control together. An independent small-signal based phase controller is proposed for the two-stage resonant inverter [16], but it uses separated power circuit to regulate the magnitude and phase. If the regulations of phase and magnitude can be integrated together by a unified modulation, optimized synchronization and advanced CSC can be achieved in the parallel single-stage resonant inverter.
Power factor (PF) is the cosine of the angular difference between voltage and current. It is = cos (Vs^Is). It can varycalculated as PF = cos between zero and one depending on the type of load. If the supply voltage and current are in-phase with each other, then the power factor of the circuit (cosϕ) is unity. The power electronic switching devices introduce distortion into the system. As a result, the power factor gets lowered. The diode bridge rectifier with capacitive filter is used as the fundamental block of many power electronics converters. Due to its non-linear nature, non-sinusoidal current is drawn from the utility and harmonics are injected into the utility lines. The injected current has lower order of harmonics and causes voltage distortion and poor power factor at input AC mains. This causes slow varying ripples at DC output load resulting in lower efficiency and larger size of AC and DC filters [2]. These converters are required to operate with high switching frequencies due to demand for small filter size and high power density. High-switching frequency operation results in higher switching losses, increased electromagnetic interference (EMI), noise and reduced converter efficiency [3]. To overcome these drawbacks, the switches of buck-boost converter are operated with zero voltage and zero current switching. High-switching frequency with SS provides low switching stress and losses, high-power density, less volume and lowered ratings for the components, high reliability and efficiency. To improve the efficiency, a large number of soft switching technique including resonant circuits have been proposed [4]-[7]. But these converters increase the number of switches and stages in power conversion circuit thus complicating the sequence of switching operation, excessive voltage and current stresses, and also narrower line and load ranges.
This paper describes a single stage AC-DC converter with high power factor. For high power application power handling capacity is increased so full bridge resonant converter is adopted which is combined with two Buck-boost type PFC circuits. Two active power switches act as a PFC circuits Therefore, power handing capacity increased. A high power factor at the input line is achieved by operating the PFCs at discontinuous conduction mode. The output voltage is regulated by controlling the ON/OFF time of switches present in buck-boost converter. The higher order harmonics are eliminated by using low pass filter, which reduce the size of filter and increases the power factor. Here soft switching can be obtained by using a new partial resonant converter. The higher order harmonics are eliminated by using low pass filter, which reduce the size of filter and increases the power factor. Here soft switching can be obtained by using a full bridge resonant converter. The proposed system has the advantage of less components and less switching losses.
. LITERATURE REVIEW
2.1 AC/DC converter topologies for the space station
A new class of AC/DC converter topologies (Type-1 converters) is described, suitable for use in an advanced single-phase sine-wave voltage, high-frequency power distribution system, of the type that was proposed for a 20 kHz Space Station primary electrical power distribution system. The converter comprises a transformer, a resonant network, a current controller, a diode rectifier, and an output filter. The input AC voltage source is converted into a sinusoidal current source using the resonant network. The output of this current source is rectified by the diode rectifier and is controlled by the current controller. The controlled rectified current is then filtered by the output filter to obtain a constant voltage across the load. Three distinct converter topologies, Type-1A, Type-1B, and Type 1-C, are described, and their performance characteristics are presented. All three types have a close-to-unity rated power factor (greater than 0.98), low total harmonic distortion in input current (less than 5%), and high conversion efficiency (greater than 96%)
2.2 A power factor corrected AC-AC inverter topology using a unified controller for high frequency power distribution architecture
This paper presents an AC-AC inverter for a high frequency power distribution architecture. The inverter includes a high frequency resonant inverter and a buck-boost power factor correction stage. A unified controller controls both the resonant inverter and power factor correction stages. Unlike other single stage power factor corrected inverter topologies, the proposed inverter system has reduced DC bus voltage stress for the universal input line voltage. The proposed inverter is found attractive in low power applications
2.3 High frequency ac vs. dc distribution system for next generation hybrid electric vehicle
The paper proposes and then demonstrates the viability of high frequency AC (HFAC) for propulsion power distribution system in the next generation advanced electric/hybrid vehicle. In justifying this viability, the HFAC system has been studied thoroughly and compared with the traditional DC distribution system with regard to cost, weight, performance and other capabilities after performing component sizing calculation of both the systems. The superior features of HFAC system are highlighted, and its possible drawbacks an mentioned. The DC, resonant link DC and HFAC (single-phase and polyphase) distribution systems have been discussed. The single phase HFAC system is shows to be superior to the others. A preliminary modelling, control strategy development and computer simulation study validate the feasibility of HFAC for propulsion power distribution system
2.4 “A low frequency ac to high frequency ac inverter with build-in power factor correction and soft-switching
This paper describes a single stage AC- DC converter with high power factor. The diode- capacitor type of rectifier cause low power factor because of its nonlinearity. PFC serves to smooth out power drawn and regulates the output voltage. High power factor at the input is assured by operating the buck-boost converter at discontinuous conduction mode of operation. With same operation on both cycle and detailed designed circuit parameter, zero- voltage switching on all the active switches of the converter can be retained to achieve good efficiency. This gives soft switching condition which increases the efficiency of the system and reduces the switching power losses. The buck boost converter and the filter circuit are used to re-shape the input current waveform so as to be in phase with input voltage waveform. The design, analysis, simulation and hardware realization of the AC-DC converter with soft switching. This mode begins at when turning off the MOSFETs (M2 and M3), since the load current ir is negative at the switching off time. The diodes (D9 and D12) are forced to free wheel ir. The drain to sources voltage (Vds2 and Vds3) of M2 and M3 are combined to -0.7 v. The voltage across the resonant circuit is equal to dc-link voltage Vdc3 and Vdc4.After some time gating signal are given to MOSFETs (M1 and M4) but there are still in off condition. The voltage in the reactive component L1 is equal to the line voltage. The inductor current Ip1 increases linearly from zero. Then M1 is turned on at zero voltage.
Since the circuit operates equally, the operation of the negative half cycle of the line voltage are equal to positive half cycle, except for inductor and power factor correction circuit .Hence the circuit is analyzed for positive half cycle only. The circuit operation divided into seven modes of operation with respect to conducting switches. Each modes are explained below
2.4 A review of distributed power systems. Part II. High frequency ac distributed power systems
The present development state in high frequency (HF) AC distributed power systems (DPS) is reviewed. First, background and motivations of developing HF AC-DPS are addressed. Two types of basic HF AC-DPSs based on sine wave and square/PWM (pulse width modulated) wave bus are described, and the system level design considerations are discussed. Further, the issues and challenges in this research area are identified. These issues include high electromagnetic interference (EMI) level, difficulty to back up power, non redundant system structure and limited post-regulation capability, etc. Finally, a viable HF AC-DPS is proposed, which is expected to yield effective EMI trade-off and system redundancy
2.5 Automated optimal design of input filters for direct ac/ac matrix converters
This paper presents a novel method to design the input filter for a direct ac/ac matrix converter using genetic algorithms (GA) optimization. The input filter for a matrix converter is a very important and critical part of the conversion structure and careful design is necessary to ensure high input power quality, compactness, and stability. The GA will optimize structure and parameters of the input filter as a function of different factors such as energy storage, weight, and volume. The effectiveness of this design method is demonstrated through a wide range of simulations using Saber and experimental results on a laboratory prototype. The same methodology could also be adapted and applied to any converter configuration such as, for example, traditional voltage source converters
3.SYSTEM STUDY
3.1 Existing system
The single-stage phase-shift modulation (PSM) resonant inverter constructed by full bridge and inductor capacitor-inductor-capacitor (LCLC) resonant tank is the best candidate of HF power source because of outstanding comprehensive abilities. The full bridge provides the large power grade with less switch stress. It is easy to implement ZVS at fixed frequency in PSM. The resonant tank formed by four energy storage components provides the sinusoidal output with less THD and more flexible ZVS configuration [8], [9]. Thus, the rational HF power source as shown in Fig. 1(b) is a parallel system constructed by a single-stage resonant inverter. The connection impedance(Zc=Rc+jXc)is deliberately placed to prevent the circulating current [10], and it also is non negligible magnitude and phase. If the regulations of phase and magnitude can be integrated together by a unified modulation, optimized synchronization and advanced CSC can be achieved in the parallel single-stage resonant inverter.
3.2 proposed system
This paper proposes a novel PSM to achieve the optimized synchronization. The rest of this paper is organized as follows. The ac phase angle analysis of the single-stage resonant inverter is presented in Section II to testify the significance of phase control. After the introduction of the existing PSM, a unified PSM is proposed in Section III. Complying with the requirements of ZVS and THD, the modulation scope of magnitude and phase is illustrated in Section IV. The performance evaluation, including simulation and experiment, is presented in Section V, followed by concluding remarks.
3.2.1 AC PHASE ANALYSIS
The ac phase analysis is accomplished to analyze the relations of phase and component parameters. The equivalent schematic of the resonant inverter is simplified to Fig. 2. The input voltage of the resonant tank is an ideal quasi-square
Wave form Va obtained by full bridge chopping. Xe and Re are the equivalent reactance and resistance from the secondary side, both of which form equivalent impedance Ze.Vp andVs are the voltages of the primary and secondary sides. Nis the turns ratio of the transformer. The connection reactance and resistance (Xc,Rc) are small enough as compared with other circuit parameters. Zs andZpare the series and parallel impedances of the resonant tank.Ro is the load resistance. The parasite resistances are neglected in impedance equivalent circuit.
The Fourier decomposition of Va is
Where Vin is the input voltage, δis the pulsewidth of phaseshift magnitude,fs is the switching frequency, and va1is the first harmonic of va. If only the fundamental component is available caused by the ideal filter performance of the LCLC resonant tank, the voltage over the parallel resonant branch or over the equivalent load is
Vp1=Va1((Zp1//(Ze1)/(Zs1+Zp1//Ze1)), whereZs1,Zp1,
And Ze1 are the fundamental impedances of Zs, Zp, and Ze,