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Implement three-phase dual-converter DC drive with circulating current

The high-level schematic shown below is built from six main blocks. The DC motor, the two three-phase full converters, and the bridge firing unit are provided with the SimPowerSystems™ library. More details on these blocks are available in the reference pages for each block. The two other blocks are specific to the Electric Drives library. These blocks are the speed controller and the current controller. They allow speed or torque regulation. A "regulation switch" block allows you to toggle from one type of regulation to the other. During torque regulation the speed controller is disabled. It is possible to use a simplified version of the drive containing an average-value model of the three-phase dual-converter and allowing faster simulation.

The speed regulator shown below uses a PI controller. The controller outputs the armature current reference (in pu) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed. During torque regulation, the speed controller is disabled.

The controller takes the speed reference (in rpm) and the rotor speed of the DC machine as inputs. The speed reference change rate will follow user-defined acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system. The speed measurement is filtered by a first-order low-pass filter.

The current reference output is limited between symmetrical lower and upper limits defined by the user.

The armature current regulator shown below is based on a second PI controller. The regulator controls the armature current by computing the appropriate thyristor firing angles of the two full converters. This generates the converter output voltages needed to obtain the desired armature current and thus the desired electromagnetic torque.

The controller takes the current reference (in pu) and the armature current flowing through the motor as inputs. The current reference is either provided by the speed controller during speed regulation or computed from the torque reference provided by the user during torque regulation. This is managed by the "regulation switch" block.

The armature current input is filtered by a first-order low-pass filter. An arccosine function is used to linearize the control system. The firing angle can vary between 0 and 180 degrees. You can limit the lower and upper limits to intermediate values.

Both converters operate simultaneously, and the two firing angles are controlled so that the sum of their values stays equal to 180 degrees. This produces opposite average voltages at the converter DC output terminals and thus identical average voltages at the DC motor armature, the converters being connected in antiparallel. One converter is working in rectifier mode while the other is in inverter mode.

The simplified converter is shown in the following figure.

**Average-Value Three-Phase Rectifier with
Circulating Current**

It is composed of two controlled current sources on the AC side and one controlled voltage and current source on the DC side. The AC current sources allow the representation of the fundamental three-phase current behaviors according to the following equations

with *α* being the firing angle value, *α*_{0} the
phase angle of phase A, *f * the AC frequency and
*I _{d}* the rectified output
current value. The DC voltage source represents the average voltage
value of the rectified voltage waveform according to the next equation:

with *V*_{rms} being the
input phase-to-phase RMS voltage value and L being the source inductance
value. The controlled current source represents the circulation current
due to the instantaneous voltage difference between both converter
outputs of the dual-converter. This current is equal to

with *L*_{circ,tot} being
the total circulation inductance of the circulation current loop.

The bridge firing unit converts the firing angles, provided by the current controller, to two series of six pulses applied respectively to the thyristor gates of each converter. The bridge firing unit block contains a band-pass filter on voltage measurement to remove voltage harmonics. Two Discrete Synchronized 6-Pulse Generator blocks generate the pulses. Refer to the Synchronized 6-Pulse Generator for more information on this block. When using the average-value converter, the bridge firing unit simply outputs the firing angle value needed by the converter.

The machine is separately excited with a constant DC field voltage source. There is thus no field voltage control. By default, the field current is set to its steady-state value when a simulation is started.

The armature voltage is provided by two three-phase antiparallel-connected converters controlled by two PI regulators. The circulating current produced by the instantaneous voltage difference at the terminal of both converters is limited by inductors connected between these terminals. No smoothing inductance is placed in series with the armature circuit, the armature current oscillations being quite small due to the three-phase voltage source.

The average-value converter represents the average behavior of a three-phase rectifier for continuous armature current in a dual-converter topology. This model is thus not suitable for simulating DC drives under discontinuous armature current conditions. The converter outputs a continuous voltage value equal to the average-value of the real-life rectified voltage. The armature voltage, armature current and electromagnetic torque ripples are thus not represented. The input currents have the frequency and amplitude of the fundamental current component of the real-life input currents.

The model is discrete. Good simulation results have been obtained with a 10 µs time step. The control system (speed and current controllers) samples data following a user-defined sample time in order to simulate a digital controller device. Keep in mind that this sampling time has to be a multiple of the simulation time step.

The average-value converter allows the use of bigger simulation time steps, since it does not generate small time constants (due to the RC snubbers) inherent to the detailed converter. For a controller sampling time of 100 µs good simulation results have been obtained for a simulation time step of 100 µs. This time step cannot be higher than the controller time step.

The **DC Machine** tab displays the parameters
of the DC Machinr block of the powerlib library.

**Output bus mode**Select how the output variables are organized. If you select

**Multiple output buses**, the block has three separate output buses for motor, converter, and controller variables. If you select**Single output bus**, all variables output on a single bus.**Model detail level**Select between the detailed and the average-value inverter.

**Mechanical input**Select between the load torque, the motor speed and the mechanical rotational port as mechanical input. If you select and apply a load torque, the output is the motor speed according to the following differential equation that describes the mechanical system dynamics:

This mechanical system is included in the motor model.

If you select the motor speed as mechanical input, then you get the electromagnetic torque as output, allowing you to represent externally the mechanical system dynamics. The internal mechanical system is not used with this mechanical input selection and the inertia and viscous friction parameters are not displayed.

**Field DC Source**The DC motor field voltage value (V).

**Circulating Current Inductors**The four circulating current inductors inductance value (H).

The Converter 1 and Converter 2 sections of the Converter tab display the parameters of the Universal Bridge block of the powerlib library. Refer to the Universal Bridge for more information on the Universal Bridge block parameters.

**Phase-to-Phase RMS Voltage**Phase-to-phase rms voltage of the three-phase voltage source connected to the A,B,C terminals of the drive (V). This parameter is not used when using the detailed rectifier.

**Frequency**Frequency of the three-phase voltage source connected to the A,B,C terminals of the drive (Hz). This parameter is not used when using the detailed rectifier.

**Source Inductance**Source inductance of the three-phase voltage source connected to the A,B,C terminals of the drive (H). This parameter is not used when using the detailed rectifier.

**Phase Angle of Phase A**Phase angle of phase A of the three-phase voltage source connected to the A,B,C terminals of the drive (deg). This parameter is not used when using the detailed rectifier.

**Schematic Button**When you press this button, a diagram illustrating the speed and current controllers schematics appears.

**Regulation Type**This pop-up menu allows you to choose between speed and torque regulation.

**Sampling Time**The controller (speed and current) sampling time (s). The sampling time has to be a multiple of the simulation time step.

**Nominal Speed**The nominal speed value of the DC motor (rpm). This value is used to convert motor speed from rpm to pu (per unit).

**Initial Speed Reference**The initial speed reference value (rpm). This value allows the user to start a simulation with a speed reference other than 0 rpm.

**Low-Pass Filter Cutoff Frequency**Cutoff frequency of the low-pass filter used to filter the motor speed measurement (Hz).

**Proportional Gain**The proportional gain of the PI speed controller.

**Integral Gain**The integral gain of the PI speed controller.

**Acceleration**The maximum change of speed allowed during motor acceleration (rpm/s). Too great a value can cause armature over-current.

**Deceleration**The maximum change of speed allowed during motor deceleration (rpm/s). Too great a value can cause armature over-current.

**Power and Voltage nominal values**The DC motor nominal power (VA) and voltage (V) values. The nominal power and voltage values are used to convert armature current from amperes to pu (per unit).

**Proportional Gain**The proportional gain of the PI current controller.

**Integral Gain**The integral gain of the PI current controller.

**Low-Pass Filter Cutoff Frequency**Cutoff frequency of the low-pass filter used to filter the armature current measurement (Hz).

**Symmetrical Reference Limit**Symmetrical current reference (pu) limit around 0 pu. 1.5 pu is a common value.

**Alpha Min**Minimum firing angle value (deg). 20 degrees is a common value.

**Alpha Max**Maximum firing angle value (deg). 160 degrees is a common value.

**Frequency of Synchronization Voltages**Frequency of the synchronization voltages used by the discrete synchronized 6-pulse generator block (Hz). This frequency is equal to the line frequency of the three-phase power line. This parameter is not used when using the average-value converter.

**Pulse Width**The width of the pulses applied to the thyristor gates (deg.). This parameter is not used when using the average-value converter.

`SP`The speed or torque set point. The speed set point can be a step function, but the speed change rate will follow the acceleration / deceleration ramps. If the load torque and the speed have opposite signs, the accelerating torque will be the sum of the electromagnetic and load torques.

`Tm`or`Wm`The mechanical input: load torque (Tm) or motor speed (Wm).

`A, B, C`The three-phase electric connections. The voltage must be adequate for the motor size.

`Wm`or`Te`The mechanical output: motor speed (Wm) or electromagnetic torque (Te).

When the **Output bus mode** parameter is set
to **Multiple output buses**, the block has
the following three output buses:

`Motor`The motor measurement vector. This vector is composed of two elements:

The armature voltage

The DC motor measurement vector (containing the speed, armature current, field current, and electromagnetic torque values). Note that the speed signal is converted from rad/s to rpm before output.

`Conv`The three-phase converter measurement vector. It includes:

The output voltage of converter 1

The output voltage of converter 2

The output current of converter 1

The output current of converter 2

Note that all current and voltage values of the detailed bridges can be visualized with the Multimeter block.

`Ctrl`The controller measurement vector. This vector contains:

The armature current reference

The firing angle computed by the current controller

The speed or torque error (difference between the speed reference ramp and actual speed or between the torque reference and actual torque)

The speed reference ramp or torque reference

When the **Output bus mode** parameter is set
to **Single output bus**, the block groups
the Motor, Conv, and Ctrl outputs into a single bus output.

The library contains a 5 hp and a 200 hp drive parameter set. The specifications of these two drives are shown in the following table.

**5 HP and 200 HP Drive Specifications **

5 HP Drive | 200 HP Drive | ||
---|---|---|---|

Drive Input Voltage | |||

Amplitude | 230 V | 380 V | |

Frequency | 60 Hz | 50 Hz | |

| |||

Power | 5 hp | 200 hp | |

Speed | 1750 rpm | 1184 rpm | |

Voltage | 240 V | 440 V |

The `dc4_example` example illustrates the three-phase
dual-converter drive used with the 200 hp drive parameter set during
torque regulation. A 5 hp parameter set is also available in the library.

The converters are fed by a 380 V AC 50 Hz voltage source.

The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.

The initial torque reference is set to 0 N.m and the armature current is null. No electromagnetic torque is produced and the motor stays still.

At t = 0.2 s, the torque reference jumps to 600 N.m. This causes the armature current to rise to about 180 A. The armature current is supplied by converter 1, and the total current in this converter is the sum of load current and circulating current. Converter 2 simply carries the circulating current. Notice that the armature current follows the reference current quite accurately, with fast response time and small overshooting. Observe also that the firing angles are symmetrical around 90 degrees and that the converter average output DC voltages are equal but of opposite signs.

The electromagnetic torque produced by the armature current flow causes the motor to accelerate. The speed rises and starts to stabilize around t = 4 s at about 560 rpm, the sum of the load and viscous friction torques beginning to equalize the electromagnetic torque.

At t = 4 s, the torque reference is set to 0 N.m and the load torque causes the motor to decelerate. Notice that the four reactors keep the current oscillations quite small.

At t = 8 s, the torque reference is set to −300 N.m. The armature current jumps down to −90 A and is now delivered by converter 2 while converter 1 only handles the circulating current. Converter 2 is now working in rectifier mode and converter 1 in inverter mode.

The negative electromagnetic torque produced allows the motor to accelerate in the negative speed plane.

At t = 12 s, speed starts to stabilize around −290 rpm.

**DC4 Example Waveforms (Blue/Green : Detailed
Converter, Red : Average- Value Converter)**

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