SimPowerSystems

Universal Bridge

Implement a universal power converter with selectable topologies and power electronic devices

Library

Power Electronics

Description

The Universal Bridge block implements a universal three-phase power converter that consists of up to six power switches connected in a bridge configuration. The type of power switch and converter configuration are selectable from the dialog box.

The Universal Bridge block allows simulation of converters using both naturally commutated (or line-commutated) power electronic devices (diodes or thyristors) and forced-commutated devices (GTO, IGBT, MOSFET).

The Universal Bridge block is the basic block for building two-level voltage-sourced converters (VSC).

Diode and Thyristor bridges:

GTO-Diode and IGBT-Diode bridges:

MOSFET-Diode and Ideal Switch bridges:

Note    The device numbering is different if the power electronic devices are naturally commutated or forced-commutated. For a naturally commutated converter, numbering follows the natural order of commutation.

Dialog Box and Parameters

Number of bridge arms

Set to 1 or 2 to get a single-phase converter (two or four switching devices). Set to 3 to get a three-phase converter connected in Graetz bridge configuration (six switching devices).

Snubber resistance Rs

The snubber resistance, in ohms (). Set the Snubber resistance Rs parameter to inf to eliminate the snubbers from the model.

Snubber capacitance Cs

The snubber capacitance, in farads (F). Set the Snubber capacitance Cs parameter to 0 to eliminate the snubbers, or to inf to get a resistive snubber.

In order to avoid numerical oscillations when your system is discretized, you need to specify Rs and Cs snubber values for diode and thyristor bridges. For forced-commutated devices (GTO, IGBT, or MOSFET), the bridge operates satisfactorily with purely resistive snubbers as long as firing pulses are sent to switching devices.

If firing pulses to forced-commutated devices are blocked, only antiparallel diodes operate, and the bridge operates as a diode rectifier. In this condition appropriate values of Rs and Cs must also be used.

When the system is discretized, use the following formulas to compute approximate values of Rs and Cs:

where

These Rs and Cs values are derived from the following two criteria:

• The snubber leakage current at fundamental frequency is less than 0.1% of nominal current when power electronic devices are not conducting.
• The RC time constant of snubbers is higher than two times the sample time Ts.
  These Rs and Cs values that guarantee numerical stability of the discretized bridge can be different from actual values used in a physical circuit.

Power electronic device

Select the type of power electronic device to use in the bridge.

Ron

Internal resistance of the selected device, in ohms ().

Lon

Internal inductance, in henries (H), for the diode or the thyristor device. When the bridge is discretized, the Lon parameter must be set to zero.

Forward voltage Vf

This parameter is available only when the selected Power electronic device is Diodes or Thyristors.

Forward voltage, in volts (V), across the device when it is conducting.

Forward voltages [Device Vf, Diode Vfd]

This parameter is available when the selected Power electronic device is GTO/Diodes or IGBT/Diodes.

Forward voltages, in volts (V), of the forced-commutated devices (GTO, MOSFET, or IGBT) and of the antiparallel diodes.

[Tf (s) Tt (s)]

Fall time Tf and tail time Tt, in seconds (s), for the GTO or the IGBT devices.

Measurements

Select Device voltages to measure the voltages across the six power electronic device terminals.

Select Device currents to measure the currents flowing through the six power electronic devices. If antiparallel diodes are used, the measured current is the total current in the forced-commutated device (GTO, MOSFET, or IGBT) and in the antiparallel diode. A positive current therefore indicates a current flowing in the forced-commutated device and a negative current indicates a current flowing in the diode. If snubber devices are defined, the measured currents are the ones flowing through the power electronic devices only.

Select UAB UBC UCA UDC voltages to measure the terminal voltages (AC and DC) of the Universal Bridge block.

Select All voltages and currents to measure all voltages and currents defined for the Universal Bridge block.

Place a Multimeter block in your model to display the selected measurements during the simulation. In the Available Measurements menu of the Multimeter block, the measurement is identified by a label followed by the block name.

Measurement	Label
Device voltages	Usw1:, Usw2:,Usw3:,Usw4:,Usw5:,Usw6:
Branch current	Isw1:, Isw2:, Isw3:, Isw4:, Isw5:, Isw6:
Terminal voltages	Uab:, Ubc):, Uca:, Udc:

Inputs and Outputs

A B C + -

The three AC connectors and the two DC connectors of the bridge.

g

The gate input, except for the case of a diode bridge. The g input accepts a Simulink vector gating signal containing two, four, or six pulse trains, depending on the number of bridge arms (1, 2, or 3). The gating signals are sent to the power switches according to the number shown in the diagrams above.

Note    The pulse ordering in the vector of the gate signals corresponds to the switch number indicated in the six circuits shown in the Description section. For the diode and thyristor bridges, the pulse ordering corresponds to the natural order of commutation. For all other forced-commutated switches, pulses are sent to upper and lower switches of phases A, B, and C with the following order: [A upper A lower B upper B lower C upper C lower].

Assumptions and Limitations

Universal Bridge blocks can be discretized for use in a discrete time step simulation. In this case, the internal commutation logic of the Universal Bridge takes care of the commutation between the power switches and the diodes in the converter arms.

Note    In a converter built with individual forced-commutated power components (GTOs, MOSFETs, IGBTs), discretization of the model is not available. See the Improving Simulation Performance chapter for more details.

Example

The power_bridges demo illustrates the use of two Universal Bridge blocks in an ac/dc/ac converter consisting of a rectifier feeding an IGBT inverter through a DC link. The inverter is pulse-width modulated (PWM) to produce a three-phase 50 Hz sinusoidal voltage to the load. In this example the inverter chopping frequency is 2000 Hz.

The IGBT inverter is controlled with a PI regulator in order to maintain a 1 p.u. voltage (380 Vrms, 50 Hz) at the load terminals.

A Multimeter block is used to observe commutation of currents between diodes 1 and 3 in the diode bridge and between IGBT/Diodes switches 1 and 2 in the IGBT bridge.

Start simulation. After a transient period of approximately 40 ms, the system reaches a steady state. Observe voltage waveforms at DC bus, inverter output, and load on Scope1. The harmonics generated by the inverter around multiples of 2 kHz are filtered by the LC filter. As expected the peak value of the load voltage is 537 V (380 V RMS).

In steady state the mean value of the modulation index is m = 0.77, and the mean value of the DC voltage is 780 V. The fundamental component of 50 Hz voltage buried in the chopped inverter voltage is therefore

Vab = 780 V * 0.612 * 0.80 = 382 V RMS

Observe diode currents on trace 1 of Scope2, showing commutation from diode 1 to diode 3. Also observe on trace 2 currents in switches 1 and 2 of the IGBT/Diode bridge (upper and lower switches connected to phase A). These two currents are complementary. A positive current indicates a current flowing in the IGBT, whereas a negative current indicates a current flowing in the antiparallel diode.

See Also

Diode, GTO, Ideal Switch, IGBT, MOSFET, Multimeter,Three-Level Bridge, Thyristor

Total Harmonic Distortion

Voltage Measurement