User manual MATLAB SIMPOWERSYSTEMS 5

[. . . ] SimPowerSystemsTM 5 User's Guide Hydro-Québec How to Contact The MathWorks Web Newsgroup www. mathworks. com/contact_TS. html Technical Support www. mathworks. com comp. soft-sys. matlab suggest@mathworks. com bugs@mathworks. com doc@mathworks. com service@mathworks. com info@mathworks. com Product enhancement suggestions Bug reports Documentation error reports Order status, license renewals, passcodes Sales, pricing, and general information 508-647-7000 (Phone) 508-647-7001 (Fax) The MathWorks, Inc. 3 Apple Hill Drive Natick, MA 01760-2098 For contact information about worldwide offices, see the MathWorks Web site. SimPowerSystemsTM User's Guide © COPYRIGHT 1998­2010 by Hydro-Québec and The MathWorks, Inc. The software described in this document is furnished under a license agreement. The software may be used or copied only under the terms of the license agreement. [. . . ] In the AC1, AC2, AC3, AC4, and AC6 models, a braking resistor in 4-39 4 Systems with Electric Drives series with a chopper ensures the braking of the motor-load system. This braking scheme is called dynamic braking. It is placed in parallel with the DC bus in order to prevent its voltage from increasing when the motor decelerates. With dynamic braking, the kinetic energy of the motor-load system is converted into heat dissipated in the braking resistor. Modulation Techniques The VSI inverters used in the AC drive models of the library are based on two types of modulation, hysteresis modulation and space vector pulse width modulation (PWM). The hysteresis modulation is a feedback current control method where the motor current tracks the reference current within a hysteresis band. The following figure shows the operation principle of the hysteresis modulation. The controller generates the sinusoidal reference current of desired magnitude and frequency that is compared with the actual motor line current. If the current exceeds the upper limit of the hysteresis band, the upper switch of the inverter arm is turned off and the lower switch is turned on. As a result, the current starts to decay. If the current crosses the lower limit of the hysteresis band, the lower switch of the inverter arm is turned off and the upper switch is turned on. As a result, the current gets back into the hysteresis band. Hence, the actual current is forced to track the reference current within the hysteresis band. 4-40 Simulating an AC Motor Drive Operation Principle of Hysteresis Modulation The following figure shows the hysteresis current control modulation scheme, consisting of three hysteresis comparators, one for each phase. This type of closed-loop PWM is used in AC3 and AC5 models. 4-41 4 Systems with Electric Drives Typical Hysteresis Current Controller The space vector modulation technique differs from the hysteresis modulation in that there are not separate comparators used for each of the three phases. Instead, a reference voltage space vector VS is produced as a whole, sampled at a fixed frequency, and then constructed through adequate timing of adjacent nonzero inverter voltage space vectors V1 to V6 and the zero voltage space vectors V0, V7. A simplified diagram of a VSI inverter is shown below. In this diagram, the conduction state of the three legs of the inverter is represented by three logic variables, SA, SB, and SC. A logical 1 means that the upper switch is conducting and logical 0 means that the lower switch is conducting. 4-42 Simulating an AC Motor Drive Simplified Diagram of a VSI PWM Inverter The switching of SA, SB, SC results in eight states for the inverter. The switching states and the corresponding phase to neutral voltages are summarized in Inverter Space Voltage Vectors on page 4-44. The six active vectors are an angle of 60 degrees apart and describe a hexagon boundary. For the location of the VS vector shown in Inverter Space Vector Voltages on page 4-44, as an example, the way to generate the inverter output is to use the adjacent vectors V1 and V2 on a part-time basis to satisfy the average output demand. The voltage VS can be resolved as where Va and Vb are the components of VS along V1 and V2, respectively. Considering the period Tc during which the average output should match the command, we can write the time durations of the two states 1 and 2 and the zero voltage state as 4-43 4 Systems with Electric Drives Inverter Space Voltage Vectors Space Voltage Vector State 0 1 2 3 4 5 6 7 SA 1 1 1 0 0 0 1 0 SB 1 0 1 1 1 0 0 0 SC 1 0 0 0 1 1 1 0 Inverter Operation Freewheeling Active Active Active Active Active Active Freewheeling Inverter Space Vector Voltages 4-44 Simulating an AC Motor Drive Open-Loop Volts/Hertz Control The AC machine stator flux is equal to the stator voltage to frequency ratio since where therefore Since the motor is fed with a variable AC source voltage and frequency, it is important to maintain the volts/Hz constant in the constant torque region if magnetic saturation is to be avoided. [. . . ] Over that time frame the turbine speed increases from 0. 8 pu to 1. 21 pu. Initially, the pitch angle of the turbine blades is zero degree and the turbine operating point follows the red curve of the turbine power characteristics up to point D. Then the pitch angle is increased from 0 deg to 0. 76 deg to limit the mechanical power. Observe also the voltage and the generated reactive power. [. . . ]