Case Study - Transient Analysis of a Power System using EMTP-RV Software

UNIVERSITY OF SOUTHERN CALIFORNIA

Professor’s Name- Syed Ahmed

Subject- EE524 (Transients in Power System)

Name – Satwik Jain

USC ID- 1854593672

Email Id- satwikja@usc.edu

MSinElectricalEngineering(ElectricPower)

Spring 16

08Fall

/ Akshay Anand Nerurkar

/nerurkar@usc.edu

/8759392138

INTRODUCTION Transient disturbances in power systems may damage key equipment, potentially having a great impact

on system reliability. These transients may be introduced during normal switching operations,

interruption of short circuits, lightning strikes, or due to equipment failure. Phasor analysis or other

simplified analysis methods are usually inadequate due to system frequency dependencies and

nonlinearities. Therefore, time-domain computer models are typically developed as a means of predicting

the severity of the transient occurrences. The simulations are typically performed using simulation

software such as the Electromagnetic Transient Program (EMTP). In this work, a royalty-free version of

EMTP is used.

Some of the cases are discussed below:

CapacitorBankSwitching:During the switching of shunt capacitor banks, high magnitude and high frequency transients can occur.

In earlier years, shunt capacitor banks have been more commonly installed at distribution and lower sub

transmission levels. However, there has been a recent proliferation of new capacitor banks at transmission

levels. Since larger higher-voltage capacitor banks have more stored energy and there is a higher system

X/R ratio and less damping, this has raised concerns at utilities (like Northern States Power Company)

regarding the vulnerability of their equipment.

Figure: Capacitor Bank Switching Main Design

When SW6 is closed at 15 ms and SW5 is closed at 20 ms. Following results were observed:

Figure: Transient Voltage across SW6

Figure: Transient voltages across SW5

The peak of the transients goes up to approx. 48k i.e. 1.5 times of the Rated voltage.

Figure: Inrush Current across CB near Generator

We can observe from above figure, the inrush current from the generator due to switching is almost 3 kA

that is really high to damage the circuit. Energization inrush is a transient occurring when the first (or

only) bank at the bus is energized. The transient is characterized by a surge of current having a high

magnitude and a frequency as high as several hundred Hertz. There is also a transient overvoltage on the

bus, caused by the surge of inrush current coming from the system source.

Figure: Current through SW5 (towards 2nd capacitor)

Back-to-BackSwitching:Energizing the second bank C5 when the first bank C4 is already energized (already closed at steady state) is called back-to- back switching, and is simulated by closing switch SW11 when C4 is already operating in steady state. The resulting inrush to C5 is a high-frequency transient which primarily involves the series combination of C4, L8, and C5, driven by the voltage V(0) on C4 at the instant SW11 is closed. Just as with energization of a single bank, worst case occurs when the bus voltage is at its positive or negative peak value. This transient is typically in the kHz range.

Figure: Main design showing Back-to-Back Switching of capacitor bank

The following graphs were observed:

Figure: Current and Voltage observed across Main Circuit breaker (SW7) respectively

Figure: Inrush Current through Circuit breaker of C4 (SW12)

Figure: Back-to-Back Inrush current through circuit breaker SW11 when it is energized

TransformerSwitchingTransient: Transformer switching is the most commonly performed operation in any power delivery system and

most of the times this operation can be performed without any undesirable consequences. However, given

the right combination of system parameters, switching can result in a violent interaction between the

circuit breaker and the transformer.

This type of failures can best be explained by understanding as to how the switching induced transients

are produced and how the transformer windings react to them. Switching transients can be produced by

the breaker contact re-ignitions and / or by current chopping.

Figure: Main Design for Transformer Switching (during High Inductive Load)

Figure: High Inrush current through Circuit breaker 1

Figure: High Inrush current through Circuit breaker 2

This high magnitude of Inrush current may damage the windings of transformer. It may also cause

blasting of the transformer due to Insulation damage.

Figure : Voltage observed across Circuit breaker 1

Figure : Voltage observed across Circuit breaker 2

Controlled Switching can help to reduce the peaks (or eliminate) these high Inrush currents.

TransientRecoveryVoltages:

Figure: Main design to analysis Transient Recovery Voltages

In the above circuit, Switch 2 is closed at 10ms and opened at 20 ms, creating a ground fault for 10ms

period. The main Circuit breaker (SW1) is closed at steady state but is opened at 15ms.

We observe a High Transient Recovery across the Circuit breaker (SW1) during interruption of fault

current. This can be seen from below figure:

Figure: Transient Recovery voltages across SW1 during interruption of fault current

Figure: Fault current through SW1 during interruption

As we can observe from the above figure, the fault current interrupted by the Main circuit breaker (SW1)

is around 2.7 kA i.e. approx. 4.5 times of the rated current.

APPENDIX

Transient Recovery Voltages: A transient recovery voltage (or TRV) for high-voltage Circuit Breakers is the voltage that appears across the terminals after current interruption. It is a critical parameter of fault interruption by a high-voltage circuit breaker; its characteristics (amplitude, rate of rise) can lead either to a successful current interruption or to a failure (called re-ignition or restrike).

Circuit Parameters: R1 = 0.5 Ohms; L5 = 3 mH; R4 = 0.001 Ohms; L6 = 12 mH

C4 = 40.1 uF (18 MVAR); C5 = 22.3 uF (10 MVAR) C6 = 601 uF

Dist. Transformer: 34.5/115 kV; R5 = 0.001 Ohms; L7 = 12 mH

Circuit Parameters: Generators: 15kV; Transformers: 15/345 kV; Transmission Lines: 10 miles, R’ = 0.2687, L’ = 2.981, C’

= 36.933; C1= 1 uF; C2= 1 uF