A precision 10 bit A/D converter reads the current flowing through the bus bar or cable connection at the bottom of the rugged steel enclosure, and stores the waveform in memory. A continuous readout of TRUE RMS current is indicated at all times, and accurate readings of time and current for each operation are displayed as they occur.
After the test, one may view and analyze the waveform of each shot, and this waveform can be saved to disk for future reference. Upon exit from the program, current and time readings are saved to a disk file.
Optional software (TCC) is available which can interface to data¬bases of various recloser types, and can compare readings to manufacturers’ data for immediate GO/NOGO verification of test results. Data can be saved and formatted reports printed.
The Ortmaster has been fully tested on an EIL Model ORT-560 recloser test set, and has been proven to provide much better accuracy and consistency, especially on waveforms with moderate to severe current decay. The exclusive software controlled TRUE RMS computation always gives the correct current reading, and properly reads waveforms with DC offset and distortion. This system was designed and tested by Paul E. Schoen, now president of Electrical Test Instruments. In 1980, he was awarded patent #4,307,345 for a reclos¬er test set.
The Electrical Test Instruments Ortmaster is a self-contained unit consisting of:
- A rugged steel enclosure which may be bolted to the frame of the recloser test set.
- A high-current capacity shunt (ORTM-2) for simple connection to the output of the test set.
- A 120 VAC line cord for connection to a source of power.
- Internal noise filtering and surge protection.
- A high current shunt and protection circuitry for reliability and accuracy when measuring currents with DC components.
- Precision low-noise amplifiers and solid-state range switching.
- A highly accurate 10 bit analog-to-digital converter.
- A high-speed serial interface to a standard serial or USB port.
- Easy-to-use real-time software for accurate current and time measurements.
- A fused, filtered and grounded convenience outlet for connection of PC compatible computer.
LOW VOLTAGE TESTING
To appreciate how the Ortmaster works, it is important to have an understanding of the effects of low-voltage testing of reclosers. When a fault occurs on a power line protected by a recloser, typically 14.4 KV AC, the impedance of the power line circuit is also fairly high, so that fault currents are in the order of 5 to 20 times continuous rated current. A 100 ampere recloser will typically see a fault current of about 1000 am¬peres, which translates to an impedance of about 14 ohms.
When a recloser operates, a plunger is drawn into a coil, which causes an increase in impedance. A 100 ampere, type 4E recloser has an impedance change from 0.072 ohms to 0.227 ohms, as the plunger moves inside the coil. In the example given, at high voltage, this change in impedance has minimal effect on the current. Recloser curves assume current flow to remain constant. The impedance change at high voltage has a negligible effect on timing.
Practical test methods for reclosers are limited by reasona¬ble available power, and safe voltage levels. The power involved in this example is 14.4 MVA, which is far beyond available power. Most recloser test sets are 10 to 50 KVA, which limits power available for testing.
The manufacturer recommends a test voltage of 138 VAC for this recloser, for a test current of 800 amps. If a solid 138 VAC is applied to the recloser, it will begin its operation at 1916 amps, which requires over 200 KVA, but will complete its opera¬tion at only 608 amps. Obviously, this will make it difficult to measure the current unless a true RMS computation of the entire pulse is utilized.
Most recloser test sets incorporate some sort of resistive compensation, which can limit the current change to about 10-20%, but even this amount of decay can affect the current metering.
Several methods of current measurement are utilized by different recloser test sets. Some systems read the current near the beginning of the operation, and others track the current and read it near the end of the operation. However, the recloser operates on the true RMS value of the current over the entire length of the pulse.
For example, assume the recloser is a 100 Amp type L, with a D curve, and current decay of 20%. The expected trip time at 1000 Amps is 0.3163 seconds, but 0.4914 seconds at 800 Amps. The average current is actually about 900 Amps, with an expected trip time of 0.3807 seconds. If the 0.3807 second trip time is com¬pared to the curve at 1000 amps, the time would appear to be about 17% slow. If your test set measures at the end of the operation, reading 800 Amps, the recloser would appear to be about 29% fast. Either reading would appear to be out of toler¬ance.
HOW ORTMASTER WORKS
The Ortmaster records the entire waveform of each current pulse, by taking 2400 samples per second, or about 40 samples per cycle. After each operation has completed, a true-RMS computation is performed, taking into account any distortion, decay, or DC offset, which directly affect recloser operation and timing. Extensive testing has determined that timing will closely match that which will be produced at high voltages and minimal decay under actual operating conditions.
In addition to its unequaled accuracy of current measure¬ment, the ORTMASTER also excels in timing measurement. At 2400 samples per second, a precision of 0.417 mSec is possible. When interfaced with the TCC software, test results can be instantly verified.