NPCs

Question 1

What is the problem with small vectors

Answer 1

They have a DOMINANT effect on Vz

Question 2

Explain what happens in commutation for an NPC

Answer 2

One of the 2 switches that was conducting is turned off at the start of 𝛿. So only 1 switch conducts during 𝛿. At the end of 𝛿, the next switch to be turned on is turned on. Now there are 2 switches conducting again.

Question 3

What is the purpose of the added in 𝛿 time

Answer 3

So each switching device only has to withstand Vdc/2

Question 4

What is the value of V_AN during 𝛿

Answer 4

Vdc/2

Question 5

Which 2 switches are involved with clamping

Answer 5

S2 and S3

Question 6

How can you tell when a switching state corresponds to a medium vector

Answer 6

If it is not part of the normal POO sequence, and has an O in it

Question 7

How do you tell if a given small vector will cause an increase or decrease in Vz

Answer 7

If Z is connected (via a load) to P, Vz will charge. If Z is connected to N, Vz will discharge

Question 8

What is the phase difference between ViN and ii if pf=0

Answer 8

ii will LAG ViN by 90 degrees

Question 9

What do you have to include when asked to explain how many times Vref is synthesised per To

Answer 9

• Drawing of a-B frame with 2 thetas on it
• Say that a full cycle happens every To
• Say that N=To/Ts

Question 10

What to do when it says to sketch VAN and gate signals

Answer 10

We always want VAN to be the stepladder waveform, so start with that
Then use the NPC switching states table to work out which states will cause which part of the VAN waveform
Sketch the gate signals according to those states

Question 11

What to include when describing the charge balancing problem

Answer 11

ViN vs ii, and shade in the redundant state parts

Equivalent circuit with the switch going to 1,2,3