Transient Recovery Voltage Part-2

1. Transient Recovery Voltage Analysis
Part-2 Final
By
Karthikeyan.S

2. Modelling:
• The most common problem found in EMTP case on TRV is to model the stray capacitance for
all equipment in the substation especially for the very large inductive machines like
transformer , reactor,..etc. The reason behind it is even a small value of capacitance can
contribute to the Rate of Rise of Recovery Voltage.
• For Line modelling , either go for constant paramter or frequency dependent model it would
be better. Comparing to IEEE guidelines model , FD model would give more accurate result.
• TRV is fairly a local study which means 30-40 miles radius around the circuit breaker would
be modelled especially for line TRV. Whenever these trasient studies are performed , by
default a travelling wave will be forwarding in the circuit. The network should be modelled
in such a way that the travelling wave would go far enough and don’t have time to come
back and in practice if it have time to come back it means then the remote network also
need to be modelled. This is especially valide for Line TRV.
• When it comes to Transformer (Transformer Limited Fault) or Reactor TRV, it is not as critical
to model far off away the way of the capacitor of the breaker but for line charge it is
definitely important.
Sample Network:
The Below marked line need to be performed TRV studies. Here the total line length is 10KM(Short
Line). It is being modelled as 2 KM one line and in series with an other 8 KM Line. The reason behind
this type of modelling is which is very easy to place a single phase to ground in between them and
then it can isolated with the help of a TRV breaker. For the sake of modelling , both 2KM and 8KM
Line are FD model. When we perform TRV analysis, we typically focus on worst case scenario. They
are
1. Single Phase to Ground Fault( This will give us the worst RRRV . When the breaker trips,
current would be cut and the voltage will rise very quickly due to the first travelling wave at
the point where the fault is applied)
2. Three Phase Fault( This will give us the worst maximum TRV)

3. For TRV studies , a specific breaker is recommended for TRV in EMTP Tools
Network Details:
1. all transmission lines are modelled up to the next discontinuity.
2. Here , the transformers are not modelled with discontinuity why because it's a large inductive
device and transients will not go through it and there is no need to model discontinuity but
before it a simple network equivalent or a network equivalent source can be placed. This
source might can have parameters like source impedance which may be defined either with
short circuit impedance or with a short circuit current and power.
3. Substation is modelled here as node and all capacitances of the substation are lumped as a
single capacitor.

4. 4. The stray capacitance of the Transformer are modelled as below image
For better precision, consider the straight capacitance between winding to winding and from
the primary to the secondary and winding to ground on the other side. This would be even a
best practice.
Fault Specification (3 phase/ single phase to ground/Transformer limited fault):
Terminal Fault (3 phase):

5. A three-phase fault ungrounded is being applied here which is the worst-case scenario would happen
for the sake of the demonstration which would open the TRV breaker. Now the TRV Breaker
parameters are entered. Here the circuit breaker opens at zero crossing for each phase individually.
Each phase will cut the current individually because the zero crossing will not occur at the same time.
The current margin needs to be specified which is called the chopping current (ranges from 0 to
maximum 50). Ex: For a VCB it is 50A
At the TRV Data tab of TRV Breaker properties dialog box, data required for the TRV Envelope need
to be entered. They are
1. Standard
2. Frequency
3. Breaker Class
4. Rated Voltage
5. Rated Short circuit current
6. Type of fault
7. Short circuit current to clear
The short circuit current to clear is the main parameter to define the TRV Envelope. This data can be
obtained from the steady state simulation.

6. Now the time domain simulation is started
The TRV is un successful.
Reason: Prospective TRV is not remain inside the inherent TRV

7. Analysis:
As 3 phase ungrounded fault is the most unlikely happening worst case scenario, modelling the
breaker with fault would increase the current rating of the breaker which in turn not a cost-
effective solution too.
Now the circuit is given a 3 phase to ground fault which is a normally happening scenario mostly
modelled for designing circuit breaker.
Similar methodology is followed for the Single Phase to Ground fault. Here also the TRV is
successful.

8. Transformer limited Fault (IEEE C37.06.01)
Network:
Type of Fault: 3ph-ground fault

9. TRV Data:
if the CB cuts at the maximum rated short circuit current then the curve here which will be used is
what is called the t100 which is the TRV for the maximum current.
if the CB cuts at 60% of this maximum current, then the curve here which will be used is what is
called the t60 and similarly we have the t30 and the t10.
This is a typical type of curve we get for a transformer fault. It is called as a Resonant TRV. It is
almost a pure sinusoidal where the frequency depends on the transformer inductance and the stray
capacitance between the breaker and the transformer.
In medium voltage application it's possible to use a RC Snubber circuit in order to lower this
frequency and the magnitude. The selected RC Snubber will have the resistance which will be equal
to the surge impedance of the cable/line system so typically between 40 and 70 ohm and then we will
put a capacitance of a 0.4 or a few micro farads in order to lower the frequency.
Credits to: EMTP Learner World.