Lecture 28 360 chapter 9_ power electronics inverters

04/12/23 360 Chapter 9 Power Electronics 1

EEE 360

Energy Conversion and Transport

George G. Karady & Keith Holbert

Chapter 9Introduction to Motor Control and Power

Electronics


Lecture 28


9.5.1 Voltage Source Inverter with Pulse Width

Modulation


9.5.1 Voltage Source Inverter

• The amplitude of the harmonics can be reduced by using the pulse width modulation (PWM) technique.

• The basic concept of the PWM method is the division of the on-time into several on and off periods with varying duration.

• The rms value of the ac voltage is controlled by the on-time of the switches,.



• The most frequently used PWM technique is sinusoidal pulse width modulation.

• This approach requires a bridge converter with IGBT or MOSFET switches shunted by an anti-parallel connected diode.

• The diode allows current flow in the opposite direction when the switch is open.



• These freewheeling diodes prevent inductive current interruption

• This provides protection against transient overvoltage, which may cause reverse breakdown of the IGBT and MOSFET switches.

• The typical circuit diagram is shown in Figure 9.44.



RloadVdc

Vac

IdcLload

S1

S4

S3

S2

Figure 9.44 Single-phase voltage source converter.



• During the positive cycle, S1 and S2 are switched by the high frequency pulse train shown in Figure 9.45.

• During the negative cycle, the pulse train switches S3 and S4.

• The load inductance integrates the generated pulse train and produces a sinusoidal voltage (Vac) and current wave, as shown in Figure 9.45.



• The width of each pulse is varied in proportion to the amplitude of a sine wave. A typical PWM waveform is shown in Figure 9.45.

• The switches in this converter are controlled by gate pulses.

• The gate signal contains several pulses distributed along the half-cycle.



Figure 9.45 Gate pulse input signal, and ac voltage and current outputs of a pulse width modulation (PWM) converter.

Time

0s 10ms 20ms 30ms 40ms 50msI(V5)*10 V(L1:1,VOUT-) V(V5:+,V5:-)

-400

0

400

PWM OutputVoltage

LoadCurrent

LoadVoltage

Time

0s 10ms 20ms 30ms 40ms 50msI(V5)*10 V(L1:1,VOUT-) V(V5:+,V5:-)

-400

0

400


9.5.1 Voltage Source Inverter• The control circuit produces the gate pulse

train by generation of a triangular carrier wave and a sinusoidal reference signal.

• The two signals are compared, and when the carrier wave is larger than the reference signal, the gate signal is positive.


9.5.1 Voltage Source Inverter• When the carrier wave is smaller than the

reference signal, the gate signal is zero. • This results in a gate pulse with variable width. • Figure 9.46

– (a) shows the carrier wave and reference sine wave; – (b) depicts the resulting gate signal with variable

width pulses. It has to be noted that several other methods are used for generation of PWM signals



-1.0V

Time

0s 5ms 10ms 15ms 20ms 25msV(PWM_TRI1.E1:IN+) V(PWM_TRI1.Vtri:+)

0V

1.0V

Reference signalCarrier wave

(a) Triangular carrier wave and sinusoidal reference signal


9.5.1 Voltage Source InverterGate pulse with variable width

Time

0s 5ms 10ms 15ms 20ms 25msV(PWM_TRI1:s)

-1.0V

0V

1.0V

(b) Variable-width gate pulse signal

Figure 9.46 Pulse width modulation (PWM) signals.


9.5.1 Voltage Source Inverter• The frequency of the reference sine wave

determines the frequency of the generated ac voltage.

• The amplitude of the ac voltage can be regulated by the variation of the reference signal amplitude.


9.5.1 Voltage Source Inverter• The amplitude of the fundamental component

of the ac voltage is:

• The modulation index is the ratio of the peak-to-peak ac voltage (2Vac) to the dc voltage.

dcdccarrier

controlac VmV

V

VV


9.5.1 Voltage Source InverterFreewheeling diode

• The inverter interrupts the current several times each cycle.

• The interruption of an inductive current would

generate unacceptably high overvoltage. • This overvoltage generation is eliminated by

providing freewheeling diodes connected in parallel with the switches.



Freewheeling diode

• When the

switches open, the current, if inductive, is diverted to the diodes, as shown in Figure 9.47

RloadVdc

Vac

IdcLload

S1

S4

S3

S2

Current when switches S1 and S2 closed and S3 and S4 open

Current when switches S1 and S2 open and S3 and S4 open

Figure 9.47 Freewheeling diode operation.



• The diagram shows the current path when switches S1 and S2 are closed, and switches S3 and S4 are open.

• When switches S1 and S2 open (now all switches are open), the current diverts through the diodes of switches S3 and S4.

• This current diversion prevents the interruption of inductive current.

RloadVdc

Vac

IdcLload

S1

S4

S3

S2

Current when switches S1 and S2 closed and S3 and S4 open

Current when switches S1 and S2 open and S3 and S4 open

Figure 9.47 Freewheeling diode operation.


9.5.2 Line Commutated Inverter



• The current source inverter operation requires both ac and dc sources

• In addition to an inductance that maintains the dc current constant or at least assures continuous dc current.

• The inverter operation requires a delay angle between 90° and 180°.



• Figure 9.48 shows bridge circuit generated waveforms for 90° and 170° delay angles.– (a) shows that the delay

angle between the ac voltage

– The square-shaped ac current is 90°

– The average dc voltage is zero

– Because the magnitude and duration of the positive and negative segments of the dc voltage are identical.

(a) Delay angle of 90°

0 30 60 90 120 150 180 210 240 270 300 330 360200

160

120

80

40

0

40

80

120

160

200

Vac t( )

V

Vdc t V

Iac t A

tdeg



• Figure 9.48 shows bridge circuit generated waveforms for 90° and 170° delay angles.– (b) shows that the delay

angle between the ac voltage and the square-shaped ac current is 170°,

– The average dc voltage is negative

– Because the duration of the positive part of the dc voltage is almost non-existent (i.e., 10° out of 180°).

. This implies that the power flows from the dc to the ac side

•(b) Delay angle of 170°

0 30 60 90 120 150 180 210 240 270 300 330 360200

160

120

80

40

0

40

80

120

160

200

Vac t( )

V

Vdc t V

Iac t A

tdeg



• The practical use of the line-commutated inverter requires the regulation of the dc voltage to maintain constant dc current.

• Figure 9.49(a) shows the bridge converter circuit that produces the voltages shown in Figure 9.48.

• Using the Thévenin equivalent, the converter can be replaced by a dc source and impedance, as shown in Figure 9.49(b).

• The average dc current in this equivalent circuit is:

R

VVI dc_sourcedc_inv

dc


9.5.2 Line Commutated Inverter• The practical use of the line-commutated inverter requires the

regulation of the dc voltage to maintain constant dc current. • Figure 9.49(a) shows the bridge converter circuit that

produces the voltages shown in Figure 9.48.

Vdc_inv

Idc

Vdc_source

Vac

ACTh4

Th1

Th2

Th3

VacVdc_inv

Vdc_source

Idc

RVoltagedifference

Voltagedifference

A B

Figure 9.49 Single-phase line-commutated inverter B) Equivalent circuit

A) Circuit diagram



Vdc_inv

Idc

Vdc_source

Voltagedifference

R

Figure 9.50 Voltage difference during inversion.


Vdc_inv

Idc

Vdc_source

Voltagedifference

R

Vdc_inv

Idc

Vdc_source

Voltagedifference

R

• The converter delay angle and the dc source voltage must be controlled simultaneously to maintain a constant voltage difference and dc current.

• In a practical circuit, the required voltage difference is generally small

• Consequently, both the inverter-produced voltage and the source voltage must be negative, as shown in Figure 9.50.



• The dc current can be maintained constant by keeping the voltage difference constant.

• Consequently, if the delay angle increases, Vdc_inv is reduced;

• The maintenance of constant dc current and voltage difference requires the appropriate reduction of the Vdc_source.

Vdc_inv

Idc

Vdc_source

Voltagedifference

R




Vdc_inv

Idc

Vdc_source

Voltagedifference

R



Vdc_inv

Idc

Vdc_source

Voltagedifference

R

Vdc_inv

Idc

Vdc_source

Voltagedifference

R

• If the firing angle increases beyond 90°, – Vdc_inv becomes

negative; – The maintenance of

constant dc current and voltage difference requires the change of Vdc_source polarity.

• In this case, the power flows from the dc to the ac circuit,



Vdc_inv

Idc

Vdc_source

Voltagedifference

R



Vdc_inv

Idc

Vdc_source

Voltagedifference

R

Vdc_inv

Idc

Vdc_source

Voltagedifference

R

,

• The current and voltage have

– the same direction in the dc source (generator)

– opposite directions in the inverter (load) as demonstrated


9.5.3 High Voltage DC transmission



• High-voltage dc lines are used to transport large amounts of energy over a long distance.

• A representative application is the Pacific DC Intertie, which interconnects the Los Angeles area with Oregon.

• The voltage of the DC Intertie is ±500 kV and the maximum energy transport is 3100 MW.

• More than one hundred dc transmission systems

operate around the world, one of the oldest and most famous is the cable interconnection between England and France.



ACfilter

ACfilter

DCfilter

DCfilter

DC Line (+)

DC Line (-)Converter

Transformer Inductance Inductance

Converter

Transformer

Figure 9.51 Concept of high-voltage dc transmission



• Figure 9.51 show a simplified connection diagram for a high-voltage dc (HVDC) system.

• The major elements are two converter stations interconnected by a dc transmission line.

• The converter station can operate in both inverter and rectifier modes, which permits energy transfer in both directions.

• Each converter station contains two converters.



• The two converters are connected in series at the dc side. The series connection node (middle point) is grounded.

• One of the converters generates the positive, and the other produces the negative dc voltage.

• The harmonics are filtered at both the ac and dc sides.



• Each converter is typically supplied by a transformer. The HVDC system uses two different transformer types at each converter station. – One of the converters is supplied by a wye-wye

transformer – The other is connected to a wye-delta transformer.

• This produces a 30° phase shift between the dc voltage outputs of the two converters.

• The phase shift produces a smoother dc output voltage.



• Figure 9.52 shows the converters in a HVDC station.

• Each converter contains six high voltage valves, with several hundred thyristors connected in series.

• The valves are shielded by rounded aluminum

electrodes as shown in the Fig 9.52



Figure 9.52 Valve hall of a DC converter station.

Engineering

Lecture 28 360 chapter 9_ power electronics inverters