Installation Guide

12 Introduction
processor processes the signals in real time. If you connect secondary current
transformers (CTs), the measurement processor can calculate V
RMS
and monitor
power quality events (such as sag, swell, line frequency, harmonic content, and
energy).
The voltage measurement circuitry can be calibrated at the factory to provide
increased accuracy. With calibration it is possible to achieve better than 2%
accuracy for V
RMS
readings.
The secondary voltage sensor is available as a factory option.
Primary and Secondary Current Sensing
You can connect the ECN main processor system card to up to three external
current transformers or Rogowski coils. Because the type of sensor technology
determines the signal conditioning circuitry needed, the analog conditioning
circuit is located on an adaptor board that is fitted to the ECN processor system
card. Thus, a change in this circuit does not require a new processor system card,
but only the replacement of the conditioning circuit.
The external sensors provide isolated voltages proportional to the current
waveform to the ECN processor system card through connectors mounted in the
ECN’s external connection panel. The sensor inputs can be connected to either
the primary or secondary lines of the transformer.
The typical supported current sensors provide either the standard 0.333 V
AC
full
scale output or di/dt output. In the case of the 0.333 V standard, the signal
conditioning circuit consists of three differential amplifiers. In the case of the
Rogowski coils, it consists of three voltage integrators.
The 0.333 V standard is supported by multiple vendors with clamp-on current
transformers covering primary currents from 200 A to 2 kA. The di/dt sensors or
Rogowski sensors are more flexible than CTs and are available in various forms
that can be wrapped around awkwardly shaped bus bars.
The signals from the conditioning circuit are fed to the measurement processor so
that I
RMS
can be calculated in real time. If the current sensors are on the
secondary of the transformer, then I
RMS
and V
RMS
measurements can be combined
to calculate total power, reactive energy, and active energy.
The current measurement circuitry can be calibrated at the factory to provide
increased accuracy. With calibration it is possible to achieve better than 2%
accuracy for I
RMS
readings.
Real-Time Clock
The ECN measurement processor provides real-time clock function for the ECN
unit. The real-time clock is accurate to at least +/- 6 ppm (6 µs drift). This tight
accuracy is achieved by calibrating the clock during manufacturing.
The ECN measurement processor adjusts the real-time clock, as needed, based on
internal temperature measurements to maintain clock accuracy over the
operating temperature range of the ECN. The COS system software can also
adjust the real-time clock in the field, as needed.
When mains power is removed from ECN, the measurement processor switches
to battery backup power to maintain the real-time clock. In this mode, the