Testing power supplies using Oscilloscope
Consider Fig. 1 where a voltage source / current source is applied to a load (A box is shown and it is assumed to be a resistance R). If we do not use power electronics for amplitude shift then efficiency apart, we can only step down the amplitude.
Both of these two methods exhibits low efficiency when the ratio between the input and output is low.
Efficiency = (Po/Pin)= (VoIo)/(VinIo). For the current source the (Vo/Vin) is constant therefore, the efficiency= (Io/Iin). Same is the case with the voltage source.
Now, since the output is always less than 1 and that if the network has to step down the input current of 10A to 1A, the efficiency is only 10% as shown in Fig.2. The red part accounts for the power loss.
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| Figure 1 Resistance based dc-dc voltage converter (click to zoom) |
For voltage source the output voltage can be expressed as
Vo= (Vs R)/(Rs+R)
Therefore, the value of series resistance Rs can be calculated as
Rs= R((Vin/Vo)-1)
Therefore, in order to have a real value of Rs the output voltage Vo can never be greater than the input voltage.
Similarly, for the current source network shown , the output current can be expressed as
Io=Iin((Rsh/Rsh )+R)
Therefore, the value of shunt resistance Rshcan be written as
Rsh=R/((Iin/Io)-1)
This shows that in order to have the value of Rsh greater than zero the output current amplitude cannot be greater than the input current.
Both of these two methods exhibits low efficiency when the ratio between the input and output is low.
Efficiency = (Po/Pin)= (VoIo)/(VinIo). For the current source the (Vo/Vin) is constant therefore, the efficiency= (Io/Iin). Same is the case with the voltage source.
Now, since the output is always less than 1 and that if the network has to step down the input current of 10A to 1A, the efficiency is only 10% as shown in Fig.2. The red part accounts for the power loss.
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| Figure 2 Ratio of input and output (Click to zoom) |
A front-end power electronic converter is a term used when a converter is connected to an ac mains/source either using a transformer or without using a transformer.
A back-end power electronic converter is one which is connected to the load. Such converter term is used frequently in a two stage power converter system.
In a two stage PV inverter system the inverter is called back-end power electronic converter whereas, the dc-dc converter connected with the solar PV module is called as front-end converter.
A back-end power electronic converter is one which is connected to the load. Such converter term is used frequently in a two stage power converter system.
In a two stage PV inverter system the inverter is called back-end power electronic converter whereas, the dc-dc converter connected with the solar PV module is called as front-end converter.
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A Maximum power point tracking (MPPT) system is essential for enhancing the efficiency of the PV system. There are various algorithms for the MPPT system that includes the traditional hill climbing MPPT, AI based MPPT and fixed voltage based algorithms. The testing of these algorithms is important to evaluate their performance under various kinds of environmental conditions. In the research community following are the test conditions applied on the MPPT system for its evaluation.
- Standard testing condition. The MPPT system is provided a standard testing condition of a PV module with 1000 W/m2 irradiance and temperature of 25 C.
- Step change in irradiance: To simulate the behavior of MPPT under dynamic weather condition the environmental parameters are changed in step mode. Usually irradiance is changed because in reality the change in temperature is slow. So the irradiance is changed from say 1000 W/m2 to 500 W/m2 and then to 200 W/m2 . This step change evaluates the system response under abrupt changes. It is pertinent to mention that in this testing condition not only the step change is applied in decreasing fashion but can also be applied in increasing mode i.e at starting from 500 W/m2 to 1000 W/m2 with step values of 100 or 50 or whatever. The step change increment and decrements are two different things and the algorithms particularly the hill climbing behaves differently. Therefore, the MPPT must be tested for both increasing irradiance as well as for decreasing irradiance.
Condition 2 for the testing of four different MPPT methods - The environmental conditions are changed gradually in ramp function. This testing condition simulates the normal weather condition with sunlight increasing linearly from sunrise to noon and then stays constant for few hours and then begins to descend in the same manner.
- Another weather condition can be simulated by merging point 2 and 3 to have a normal day scenario with abrupt changes in between. This simulates the birds or some building shade on the PV panel.
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| Condition 4 |