 Journal of Power and Energy Engineering, 2014, 2, 334-339 Published Online April 2014 in SciRes. http://www.scirp.org/journal/jpee http://dx.doi.org/10.4236/jpee.2014.24044 How to cite this paper: Jia, J., Zhao, J., Liu, D.C., Cheng, C., Wang, J. and Qi, L.G. (2014) The Study of Optimal Structure and Value of Dump Resistance in Direct-Drive Permanent Magnet Synchronous Generators. Journal of Power and Energy Engineering, 2, 334-339. http://dx.doi.org/10.4236/jpee.2014.24044 The Study of Optimal Structure and Value of Dump Resistance in Direct-Drive Permanent Magnet Synchronous Generators Jun Jia, Jie Zhao, Dichen Liu, Chen Cheng, Jun Wang, Linge Qi School of Electrical Engineering, Wuhan University, Wuhan, China Email: jiajuntec@163.com Received February 2014 Abstract This paper studied the direct-drive permanent magnet synchronous machine (permanent magnet synchronous generator, PMSG) Chopper optimal topology and resistance value. Compared the dif- ferent Chopper circuit low voltage ride-through capability in the same grid fault conditions in si- mulation. This paper computes the dump resistance ceiling according to the power electronic de- vices and over-current capability. Obtaining the dump resistance low limit according to the tem- perature resistance allows, and calculating the optimal value by drop voltage in the DC-Bus during the fault. The feasibility of the proposed algorithm is verified by simulation results. Keywords Permanent Magnetic Synchronous Generator (PMSG); Optimal Resistance; Chopper 1. Introduction Direct-drive wind turbines is a kind of generator driven directly by the wind, also known as gearless wind gene- rators, using multi-pole generator motor and impeller driven direct connection, the traditional gear box parts has been eliminated. Because the gearboxes are easily to overload and failure in currently megawatt wind turbines, the direct drive wind generators without gearbox, has high efficiency at low wind speed, low noise, high life expectancy, reduced crew volume, lower operation and maintenance costs, and many other advantages [1,2]. Direct drive variable speed constant frequency (DDVSCF) wind power system shows in Figure 1 [3], the wind wheel synchronous generator connected directly without raising speed gearbox. The wind energy trans- forms into AC which frequency, amplitude, phase are all change with time and then convert into DC by conver- ter, and then converted by the inverter to a three-phase alternating current of constant amplitude and constant frequency to the grid. The active and reactive power of system is controlled by the power electronic converters to tracking the maximum power point and the maximum efficient use of wind energy [4-7]. The generator in Direct drive wind power generation system (DDWEGS) are two main types: rotor electric field concentrated winding rotor permanent magnet synchronous generator and excitation permanent magnet synchronous generators. Concentrated winding rotor electric field type synchronous generator (DDWEGS) needs to provide excitation current to rotor which needs slip rings and brushes. The failure rate of slip rings and  J. Jia et al. Figure 1. The diagram of PMSG wind energy generation system. brushes are high, and require periodic replacement. The permanent magnet rotor with permanent magnet excita- tion for DDWEGS material to build the rotor field, eliminating the need for slip rings, brushes and gear boxes and other devices without regular maintenance, it has compact system architecture, high machine reliability and operating efficiency. Direct drive wind power generation system using permanent magnet synchronous generator, without excita- tion control, It has wide range of motor operating speed and high motor power density, smaller size. Some direct-drive that combined wind wheel with external rotor, eliminating the hub, rotor blades mounted directly outside, further simplifying the structure for reducing the weight . 2. Chopper Circuit Hardware Connections When the system is functioning properly, the ChopperIGBT shuts down, and Chopper circuit does not access system. When a fall happens to the system voltage, DC input power exceeds output power. That will make the DC voltage increase, and threaten the safety of capacitor and inverter. At this time Chopper circuit conducts the IGBT, and put it into dump resistance. The excess energy is consumed by the dump resistance, so that the DC bus voltage can be stable [8]. Today there are two main structures of the Chopper circuit which can increase the dumping load in DC side. The dump resistances in Figure 2(a) connect directly to the DC side by IGBT while in Figure 2(b) dump resistances connect to the DC-side by Buck circuit. In Figure 2(a) the dump resistance connects directly to the high voltage DC bus, so the requirement of load voltage tolerance is high. When in Fig- ure 2(b) dump resistance passes Buck step-down circuit, it can make the requirement of the withstanding ability in resistance for the voltage lower, but the circuit adds the inductance component. When dump resistance cut, generating extra energy consumed by the dumping the resistance, so we need to ensure reliable heat dissipation; this paper mainly focuses on comparing two scenarios in Figure 2 [3,9]. 3. Dumping Direct-Drive Permanent-Magnet Synchronous Motor Resistor Value In the process of circuit failure, since the power imbalance caused by the external network power voltage drop, DC bus capacitor voltage rises, at this time by putting Chopper circuit can consume excess active in order to maintain the DC voltage. When converter losses are ignored, the difference between the active and of the output of rectifi- er and inverter is [10] (6) After input dump resistance, DC voltage shall be maintained in the voltage within the range of values allowed, so dump resistance should read as follows: (7) is the maximum voltage allowed by DC bus. Considering the Depth of voltage dips, Network-side converter overload, the maximum allowable voltage DC and other factors, the Theoretical upper limit for dump  J. Jia et al. AC DC AC DC Multipole PMSG AC DC AC DC Generator-side converter Grid-side converter (a) (b) Grid Multipole PMSG Generator-side converter Grid-side converter Grid Figure 2. Chopper circuit scheme. resistance is: 2 3(1 ) dcmax dmax Ng g U RPu i αβ =−+ (8) In the formula: is unit rated activity; is the ratio of fault voltage and rated voltage; is rated vol- tage famous value; is network-side converter rated current known values; is the proportions of exceeds its rating when goes through the network side converter current. Dump resistance may burn when through the large current; therefore we must ensure that the resistor temper- ature rise is less than the maximum temperature rise. The former is equal to the cooling temperature resistance plus the relative cooling of the temperature rise . By heat balance equation, (9) In the formula: is the dump resistance power consumption, w; ( ) is the specific heat; ( ) is the cooling medium volume, The unit of is Dump resistance cooling is the heat exchange of forced Convection and thermal conduct at the same time, according to the formula for convective heat transfer, (10) ( ) is the convective heat transfer between the two interfaces temperature difference, that is the resis- tance temperature of the surrounding cooling medium; is the overall heat transfer in terms of dump resis- tance power consumption. Make , is conversion Factor; ( ) is heat transfer coef- ficient; ( ) is heat transfer area, and it is the entire surface area of dump resistance, Therefore, the dump resistance temperature-rise is: 12 1 max max T T TpT qc A θ δ ∆ =∆+∆=+≤∆ (11) According the formula (6), we can get the maximum allowed power of dump resistance is , according to , the dump resistance limit is: (12) According to resistance range from (8) to (12 ), when there is a serious failure in the grid, the direct drive unit is not off-grid.  J. Jia et al. 4. The Equivalent Circuit Model of Chopper and Its Simulation We use a hybrid power system electromagnetic transient simulation program DigSILENT/Power Factory to build the model, and simulate the transient process PMSG wind turbines in the case of a three-phase short circuit. Make comparison of equivalent model of transient stability of wind turbines in different chopper circuit state [5,11-14]. The capacity of PMSG in the DigSILENT model in this paper is 1.5 WM, and the inverter capacity is 2.5 MVA, which is 166.6% of PMSG capacity. Rated voltage of DC side is 5.4 KV, rated voltage of PMSG stator side is 3.3 KV, network-side converter outlet voltage 3.31 KV, through the transformer and the power line to 220 KV. During normal operation, the output is active PMSG 1.5 WM, reactive power is 0.113 MVar. 4.1. The Impact of Dump Resistance Value of Chopper to the Direct Drive Wind Power Generation System By formula (8) calculated in the model ranges in dump resistance to between and . We take 5, 10, 15, 20 four experiments to simulation results are as follows in Figure 3. It can be seen that the DC voltage has the most stable voltage when dumping the resistance value at around , thus it can achieve the best effect of the low voltage ride through (LVRT). 4.2. The Impact of Chopper with Buck Circuit to the Direct Drive Wind Power Generation System This section experiments, set a three-phase short circuit grid fault at PCC as 1 s, last 0.1 s. When DC voltage reaches 5670 V (1.05 pu), the Chopper circuit takes action. The dump resistance is 15 ohm, Chopper circuit connection using Figure 2(b) in connection with the inductor. Simulation case is shown in Figure 4. It can be seen that, due to the effect of inductive, the peak voltage of the DC bus during fault is reduced. However, it would have a bad impact on DC-bus when the inductance is too large . 5. Summarize This paper studies the direct-drive permanent magnet synchronous machine in low voltage ride through. Espe- cially Chopper circuit topologies and unloading resistance value, the main conclusions are as follows: Figure 3. The DC-link wave of direct-drive wind energy generation system with different re- sistances in fault. 2 .01.580 1.1600.7400.320-0.100 [s] 1.11 1.06 1.01 0.96 0.91 0.86 DC Bus: Voltage, Magnitude in p.u. DC Bus: Voltage, Magnitude in p.u. DC Bus: Voltage, Magnitude in p.u. DC Bus: Voltage, Magnitude in p.u.  J. Jia et al. Figure 4. The DC-link wave of direct-drive wind energy generation system with different reactance in fault. 1) The difference of dump resistance, inductance and the way connected to Chopper circuit has different ef- fects on low voltage ride through capability. 2) Due to the buffering effect of the inductor, Chopper circuit with Buck Chopper circuit has more excellent results of low voltage ride through than that with only resistance. Accordingly, the use of direct-drive wind power generation system with dumping Buck circuit or dual resistance is more significant to the improvement of low voltage ride through capability. References [1] Hansen, A.D. and Hansen, L.H. 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