The Challenge:

To analyze the waveform captured during the Sudden Short Circuit test of a 210 MW Generator shorted at 50 % of the rated voltage and to calculate the decaying Time Delay and the Reactance.

The Solution:

Create a time effective system using Lab VIEW and reduce the complexity involved in calculation and expected analysis of the signal.

Introduction:

Sudden Short Circuit Test is performed on a Generator to find the dynamic behavior of the same, and to show that the mechanical design of the machine is adequate to withstand the stresses due to the short circuits and related abnormal conditions like lightning, thunder etc. The Output waveform captured across the stator winding of the generator during this test consists of three distinct states

• Sub-Transient state
• Transient State
• Steady State.

System:

The process of Sudden Short circuit is achieved by shorting the output of the three phases of the Generator stator winding to Ground when the generator is rotating at 3000 RPM and at a particular rated voltage of the stator winding. Short circuit is applied by a switch (generally an air breaker) that closes all the phases at almost exactly the same instant.

Measurement:

As shown in the Fig.1. (Block Diagram) the three phase R, Y and B of the generator are connected to the shunt resistor. The air breaker circuit is used to short these output to ground. The shunt resistor gives an output in millivolts proportional to the short-circuited current. The NI 5B40 series was used to achieve isolation, noise rejection, and amplification. The waveform is captured by the acquisition software executing on the PXI 8145 RT target through the 6120 Data Acquisition Card. The signal sampled at a rate of 6000 samples/sec is acquired for duration of three seconds. The National Instrument’s S Series card helped us to samples the entire three phase signals separately.

Utility of configuring the system for desired scaling values for obtaining the final results has been provided. The user just have to set the pre trigger and post trigger duration and the actual trigger level of the current input signal, data is acquired when the current input signal crosses the set trigger level. The acquisition will stop after the specified post trigger duration. Once the signal has been acquired the captured data is written to a file which can be retrieved as and when it is required.

The Fig.2. (Waveform) shows the plot of the Current vs. Time for a period of 0.5 seconds of all the three (U, V and W) Phases as captured by the National Instrument’s 6120.

Analysis:

The analysis sudden Short Circuit waveform involves the following

Modular programming was chosen to solve each step. Once the captured data was read from the file the array operation VI’s were used to find the Peak and Valleys from the waveform. The fitting of the curve were done using General Polynomial fit and the Polynomial Evaluation VI’s. Formulae’s for calculating the Normalized current and the Time Delay and Reactance for the Sub Transient and Transient Component were done as specified in the IEEE standard.

The Sub-Transient component of the signal as given in the IEEE standard lies in the first 30 to 40 cycles the Transient component lies in the 40 to 120 cycles and the remaining waveform consists of the steady state part of the signal.

National Instruments Lab VIEW VI’s helped us in reducing the programming complexity and the time involved for Mathematical calculations for the mentioned parameters. Especially, calculation of the Polynomial coefficients for the required order and algorithm.

Conclusion:

We were able to achieve accurate results for the analysis performed on the captured waveform compared to the system present earlier where the user first had to capture the waveform on an oscilloscope, store the captured waveform in a floppy, read the data print the same and enter the values manually in an DOS based utility which would fit the polynomial and then calculate the Time Delay and reactance’s using the traditional calculator. However, now all these were done within no time.