Voltage Dips and Interruptions
Exercises on Sag, Swell and Lost Cycles
Contents
Mains Voltage Swell Simulations
Two major problems are associated with mains voltage swell and sag:
- When voltage swell occurs there is a danger that any voltage surge clamping components may be damaged by current flowing in the vicinity of voltage peaks. Surge protection circuits are designed to absorb the energy present in very short duration impulses.
- When voltage sag occurs, the there is a danger that the voltage regulator in the power supply drops out of regulation.
We shall start by considering voltage swell.
Fig X6.1 shows the circuit we shall use to examine voltage swell.
Figure X6.1 Circuit used for Simulating Voltage Swell
Components D5, D6, D7, D8, and C1 form the bridge rectifier of a SMPS. Resistor R1 represents the SMPS load.
Components R3, R4, R5, R6, D1, D2, D3, and D4 form a bidirectional surge protection circuit. Resistor R2 is a current limiting resistor and resistor R1 is used to monitor the current through zener diode D1.
The mains supply voltage generator is an amplitude modulated voltage source which may be obtained from:
MB/Place/Component/Signal Voltage Sources/AM_Voltage
The AM Voltage Generator setup is:
Carrier Amplitude = 340V
Carrier Frequency = 50Hz
Modulation index = 0- this value of modulation index allows us to test the circuit for nominal mains conditions
(Modulation Frequency) = 2Hz
This setup allows us to modulate the amplitude of the supply voltage, which allows us to simulates both swell and sag.
The transient simulation setup is:
Start time = 0s
Stop time = 0.2s
Max Time Step (TMAX) = 10uS
All other values are left at their default values.
Monitored variables are:
Mains voltage (V9)
Rectifier Input Voltage (V6)
Zener Current (V2) Note that the value of the current monitoring resistor R1 is 1 Ohm, so we can read this voltage directly in amps.
Fig X6.2 (a) shows the simulated waveforms. The zener current (green) is virtually zero.
Figure X6.2 Zero Sag and Swell
Fig X6.2 (b) shows the values of the mains voltage, the rectifier input voltage and the zener current under nominal conditions. The zener current is in the range -20 to + 50uA. The dissipation in the zener diode is small.
Fig X6.3 Shows the simulated voltages when the mains voltage swell and sag is 20%. Note we have referred to the modulation index as the percentage swell and sag, for convenience.
The AM Voltage Generator setup is:
Carrier Amplitude= 340V
Carrier Frequency = 50Hz
Modulation index = 0.2
Modulation Frequency = 2Hz
Figure X6.3 20% Sag and Swell
Above about 100ms the zener diodes are just about conducting. The peak values of zener current may be obtained from Fig X6.3 (b) as about 250mA. Peak zener dissipation is about 42W
Fig X6.4 (a) shows the voltage and current waveforms for a 30% swell and sag.
The AM Voltage Generator setup is:
Carrier Amplitude 340V
Carrier Frequency 50Hz
Modulation index = 0.3
Modulation Frequency = 2Hz
Figure X6.4 30% Sag and Swell
The zener diodes fail after about 150mS. It is not clear if the simulator produces the correct waveforms after diode failure.
Figure X6.5 Zener Diode Current flowing due to Voltage Swell.
Fig X6.5 (b) shows the zener diode current prior to zener failure. The diode current increase as the mains voltage swell is clearly evident. Peak zener current is about 1.5A, prior to diode failure. This corresponds to a peak zener diode dissipation of 200*1.5= 300W
To obtain the graph shown in Fig X6.5 (b):
In the grapher view,
Right click on trace/ trace properties/
Select the Left axis and set the range to -2V +2V
Select the Bottom Axis and set the ranges to 0 to 0.15s
Voltage Sag Simulations
In this section we shall simulate the behaviour of a SMPS rectifier circuit when the mains input voltage is subjected to sag.
The circuit to be simulated is shown in Fig X6.6. The rectifier circuit is the same as that used in the previous section. The surge protection circuit has been removed, to simplify simulation.
The mains voltage input has been set to 162V peak, which corresponds to 115V rms, 60 Hz, because this is when the voltage regulator is most likely to drop out.
Figure X6.6 Simulation Circuit for Voltage Sag
The AM Voltage Generator setup is:
Carrier Amplitude 162V
Carrier Frequency 60Hz
Modulation index = 0.8
Modulation Frequency = 2Hz
We have assumed a regulator drop out voltage of 100V d.c.
The waveforms obtained are shown in Fig X6.7. Note that the rectifier diodes cut off as the mains input voltage falls.
Figure X6.7 Simulation Waveforms for Voltage Sag
The transient setup details used are shown in Fig X6.8
Figure X6.8 Transient Setup Details
Note the use of an expression, V(3)-V(2), to plot the rectifier d.c. output voltage. This is necessary because one end of the mains input signal is used as the circuit reference. the voltages at nodes 3 and 2 have been hidden in the grapher view.
To add an expression, add all the variables to be used in the analysis, in this case V(1), V(2) and V(3).
Then click the add expression button to open the expression window.
Finally type in the expression of interest, in this case, V(3)-V(2), into the expression window
Note that regulator drop out occurs at 100V d.c. some time after the bridge rectifier diodes cease to conduct.
Voltage Regulator Drop Out
This exercise is part of assignment 2.
Updated 13/05/08 RJH