【正文】 easing the switching frequency. In this way, the harmonic performance of the inverter is improved, also obtaining better efficiency and reliability respect to the conventional twolevel inverter topology. The connection to the utility grid is made through a stepup transformer and a low pass filter in order to reduce the perturbation on the distribution system. As the VSI needs a fixed DClink in order to allow a decoupled control of both active and reactive powers exchange with the electric grid, an interface in the DC side of the VSI is required. For this purpose, an intermediate DC/DC converter in a boost topology is used, linking the output of the fullwave rectifier bridge to the DC bus of the inverter. . DC/DC converter The standard unidirectional topology of the DC/DC boost (aka stepup) converter or chopper consist of a switchingmode device containing basically two semiconductor switches (a rectifier diode and a power transistor with its corresponding antiparallel diode) and two energy storage devices (an inductor and a smoothing capacitor) for producing an output DC voltage at a level higher than its input DC voltage [7]. The steadystate voltage and current relations of the boost converter operating in continuous conduction mode are given by Eq. (5). ； beingthe chopper input current (inductor current),the output voltage (DClink voltage), the boost converter input voltage, the chopper output current and D the duty cycle of the DC/DC converter. . Voltage source inverter The threephase threelevel voltage source inverter proposed corresponds to a DC/AC switching power inverter operated through sinusoidal pulse width modulation (PWM) techniques [8]. As the highfrequency harmonics produced by the inverter as a result of the PWM control techniques employed are mostly filtered by the sinusoidal line filter, the VSI can be seen as an ideal sinusoidal voltage source. This ideal inverter is shuntconnected to the network at the point of mon coupling (PCC) through an equivalent inductance Ls, accounting for the leakage of the stepup coupling transformer and an equivalent series resistance Rs, representing the transformers winding resistance and VSI semiconductors conduction losses [9]. The magnetizing inductance of the step up transformer can also be taken into consideration through a mutual equivalent inductance M. In the DC side of the inverter, losses are accounted by and the equivalent capacitance of both DC bus capacitors through . Under the assumption that the system has no zero sequence ponents, all currents and voltages can be uniquely transformed into the synchronousrotating dq reference frame, by applying Park’s transformation. Thus, the new coordinate system is defined with the daxis always coincident with the instantaneous voltage vector（=，）. As a result, the daxis current ponent contributes to the instantaneous active power and the qaxis current ponent represents the instantaneous reactive power. The dynamics equations governing the instantaneous values of the threephase output voltages in the AC side of the VSI and the current exchanged with the utility grid can be derived in the d—q reference frame as follows [8,9]: where, : Laplace variable, defined for t 0. :synchronous angular speed of the grid voltage at the fundamental frequency. m: modulation index of the VSI ， : average switching factors of the VSI in the d—q frame, andthe phaseshift of the VSI output voltage from the reference position. Fig. 5–Proposed multilevel control Scheme for the threephase gridconnected MHPP.3. Proposed control strategy The proposed control of the threephase gridconnected MHPP consists of an external, middle and internal level, as depicted in Fig. 5.. External level control The external level control (left side of Fig. 5) is responsible for determining the active and reactive power exchange between the MHPP and the utility grid, through an active power control mode (APCM) and a voltage control mode (VCM), respectively. The VCM is designed to control the voltage at the PCC of the VSI, through the modulation of the reactive ponent of the output current (fundamental quadrature ponent, ). To this aim, the magnitude of the voltage vector at the PCC () ispared to a voltage reference. An error signal is produced and then fed to a proportional–integral (PI) controller with a regulation droop Rd. The main purpose of a gridconnected MHPP is to transfer the maximum hydro power into the electric system. In this way, the APCM aims at matching the active power to be injected into the electric grid with the maximum instant power generated by the hydraulic turbine generator. The generated output power signalis then converted to a direct current reference () for the middle level control. Maximum power point tracking means that the MHPP is always supposed to be operated at maximum output voltage/ current rating. From Eqs. (3) and (4), the optimal rotational speed u opt of the hydraulic turbine rotor for a given water flow rate Q can be used to obtain the maximum turbine efficiencyand then the maximum mechanical output power of the turbine. Unfortunately, measuring the water flowing per second in the rotor of the hydraulic turbine is difficult。nPrats MA, et al. Power electronic systems for the grid integration of renewable energy sources: a survey. IEEE Transactions on Industrial Electronics June2006。34(2):409–15.[4] Fang H, Chen L, Dlakavu N, Shen Z. Basic modeling and simulation tool for analysis of hydraulic transients in hydroelectric power plants. IEEE Transactions on Energy Conversion Sept 2008。 參考文獻(xiàn)[1] Willis HL, Scott WG. Distributed power generation – planning and evaluation. 1st ed. Marcel Dekker, ISBN 0824703367。當(dāng)VCM被激活時，有一股300 var的無功功率被迅速的注入到了電網(wǎng)中去 ，它旨在調(diào)節(jié)公共耦合點(diǎn)（PCC）處的電壓。在這種情況下，與有最大功率跟蹤器參與控制的情況相比較，輸入到電網(wǎng)中的有功功率要平均少30%左右。正如觀察到的那樣，采用擾動觀察法 (Pamp。4. 計算機(jī)數(shù)字模擬仿真