Figure 1 shows waveforms in a typical 3-phase measurement system. Each power phase is represented by a current transformer (CT) and a voltage transformer (PT). The average power in the system at any instant is calculated by rapidly taking a number of samples of the output of each transformer, performing a discrete Fourier transform (DFT) on the sampled data, and performing the necessary multiplications and summations.This determines system power at one frequency. Performing a fast Fourier transform (FFT) instead of a DFT provides data on harmonics and other higher frequency components, which allows calculation of additional information, such as system losses or effects of unwanted noise.

System requirements

A substation may contain hundreds of transformers. The measured voltages and currents are scaled so the ±5V or ±10V full-scale output range of the transformers represents a range much greater than the full-scale power output capability of the power line. Typical transformer outputs lie in a ±20mV range. Larger signals occur rarely, and usually imply a system fault. Accurate measurement of these small signals requires a high-resolution ADC with excellent signal-to-noise (SNR). For 3-phase current- and voltage measurements, the ADCs must also be capable of simultaneously sampling six channels. Currently available systems have 14-bit capability-the 4-channel AD7865 14-bit quad ADC from Analog Devices, for example, accepts true bipolar signals and provides 80-dB SNR. However, there is an increasing need for higher-performance multichannel ADCs, with 16-bit resolution at sampling rates of 10 kSPS.

The power-line measurement system

A complete power-line measurement system is shown in Figure 2. As well as the ADC, many other factors must be considered when designing a high-performance system, including voltage reference, input amplifiers, isolation and communications.Whether to use the ADC's internal reference or an external reference depends on the system requirements. When multiple ADCs are used on a single board, an external reference works best, as a common reference can eliminate part-to-part reference variations. Generally, a low-drift reference is also important for reducing reference sensitivity to temperature. For the amplifier, the key requirements are low noise and low offset. Noise must be kept low to preserve the SNR and transition noise performance of the ADC. A low-noise amplifier is also useful for measuring small AC signals. The amplifier's total offset error, including drift, over the full temperature range should be less than the required resolution. Both analogue and digital power supplies are required for ADCs. Most systems have a 5V digital supply, but many do not have a 5V analogue supply. Since using the same supply for both analogue and digital circuitry could couple unwanted noise into the system, it is generally a practice to be avoided. Power consumption can be critical, especially when up to 128 channels (as many as 22 six-channel ADCs) may be measured on one board.The systems in a substation require communication with a remote main system controller, typically with electrical isolation. Optocoupler solutions, with their LEDs and photodiodes, are now often being supplanted by iCoupler digital isolators, which use chip-scale microtransformers. iCoupler devices have two- to four-times faster data rates than commonly used high-speed optocouplers-and require as little as 1/50 the power. Finally, digital signal processing (DSP) is required to perform complex calculations.

Practical design considerations

Special consideration should be given to the ADC's location and surroundings when designing the PCB. Analog and digital circuitry should be separated and confined to certain areas of the board, and at least one ground plane should be used. Avoid running digital lines under the ADC because they couple noise onto the die. The analogue ground plane should be allowed to run under the ADC to avoid noise coupling. Clocks and other high-speed switching signals should be shielded with digital ground to avoid radiating noise, and should never run near analogue signal paths. Crossover of digital and analogue signals should be avoided. Traces on different but close layers of the board should run at right angles to reduce feedthrough. The ADC power supply lines should use the largest possible traces to provide low impedance paths and reduce the effect of power supply glitches. Good connections should be made between the ADC's supply pins and the power tracks on the board, using single- or multiple vias for each supply pin. Good decoupling is also important to lower the supply impedance presented to the ADC and to reduce the magnitude of the supply spikes. Paralleled decoupling capacitors, typically 100 nF and 10 �F, should be placed on all of the power supply pins, close to-or ideally right up against-these pins and their corresponding ground pins.

Conclusion

Increasing worldwide power demands are driving an increase in the number of power lines and power-line substations. As more and more automated monitoring- and fault-detection systems are required, the trend will be towards systems with a large number of channels. With multiple ADCs on each board, efficient use of board area and power dissipation become critical as system designers try to reduce cost while increasing performance.