Inverters Dr John Fletcher

Basics Inverters are DC to AC converters We can use inverters to generate A dc supply Single-phase AC supply Three-phase AC supply from a single dc source. The basic building block is the inverter ‘leg’. An inverter leg is shown. Vdc is the input, Vout the output.

Inverter switching T1 and T2 are NEVER turned on together. Why? T1 and T2 are switched using PWM in a complementary manner (T2 ON, T1 OFF) Vout is then a switched waveform, just like the basic step-down converter earlier.

Pulse-width Modulation T1,on T2,on

Current Paths Current path if T2 ON, or T1 and T2 OFF Two switches with freewheel diodes provides uni-directional voltage and bi-directional current control. Only when T1 is ON is energy supplied from the source. When T2 ON, a zero voltage loop is applied. With positive current flow → Current path if T2 ON, or T1 and T2 OFF Current path if T1 ON

Current Paths When T1 is ON (or T1 and T2 OFF) energy has to be absorbed by the source. When T2 ON, a zero voltage loop is applied. With negative current flow → Current path if T2 ON Current path if T1 ON, or T1 and T2 OFF

Bridge Leg V-I graph The basic bridge leg can operate in two quadrants of the VI graph. I V

Average Output Voltage A single inverter leg produces an average output voltage: Define a duty cycle or modulation index Hence m must be between 0 and 1. We can make m vary in time therefore we can produce any voltage and any frequency we desire (within the bounds fixed by the switching frequency and Vdc).

Switching Frequency Switch frequency (1/Ts) of the pulse-width modulated (PWM) signal is usually chosen as high as possible to reduce current ripple in the load. Max switching frequency is limited by losses and the ability to manage those device losses (remember lecture 2?) In low power circuits, switching frequency can be as high as ~1 MHz High power circuits (say >500kW) may use frequencies of 1kHz or less.

Single-phase H-bridge Two inverter legs connected in parallel.

Single-phase H-bridge From previous discussion on inverter legs So the average output voltage applied to the load For a sinusoidal output (ma-mb) must vary sinusoidally.

Single-phase H-bridge The modulation indices of both inverter legs vary sinusoidally in time with a modulation depth, m (0<m<0.5) and an offset. If we apply (2) and (3) to (1) we get That is, the modulation depth, m, sets the magnitude of the ac output voltage and ωt sets the frequency. Notice that the dc offset in the modulation indices is co-phasal and does not appear in the output voltage. We can control the magnitude and frequency.

H-Bridge V-I graph The H-bridge can operate in all four quadrants of the VI graph. It can generate both polarities of voltage and control both polarities of current. I V

PWM Generation Ts t1 t2 (t1-t2)/2 Ts/2 m1 m2 Vload Leg 1 output Leg 2 output Carrier waveform Modulation indices of each leg are compared with a triangular carrier waveform. Intersects define the turn-on and turn-off instant of each bridge leg. With this scheme load sees two output voltage pulses per switching cycle. Harmonic spectrum of the applied voltage has components around multiples of the switching frequency.

Three-phase inverters Now if:

Three-phase inverters Inserting modulation indices into (1-3) gives: Three-phase output voltages The circuit is a pulse-width modulated voltage source inverter (VSI).

Grid-connect – basic system “Fault Response of Grid-Connected Inverter Dominated Networks”

System components PLL: Determines the phase angle of the positive sequence fundamental component of the grid voltage, Vg,abc. Lc: Interfacing inductance. Used to control Io,abc. Lf, Cf: Low-pass filter which generates sine wave voltage Vo,abc from switched output Vi,abc. abc/dq blocks: Reference frame transformations from stationary abc to rotating dq and vice-versa. Uses angle output from PLL.

Current Controller Current controller: Adjusts Vi,abc in order to meer iL,abc,ref.

Power controller Power controller: generates iL,dq,ref command to generate P* and Q*.

Inverter Response to Faults Example: Voltage sag to 1/3 pu Inverter attempts to increase output current to maintain P and Q. (Would naturally reach 3 pu in these circumstances.) At time td, current reaches a threshold at which it decides there is a fault. Immediately steps current reference to 2 pu in order to support the network with fault current.