Volume 41 January 2007
High-Performance Multichannel Power-Line Monitoring with Simultaneous-Sampling ADCs
By Colm Slattery [email@example.com]
Substations can be classified into two categories, according to voltage level: high-voltage includes 500-kV, 330-kV, and some 220-kV substations, while 220-kV terminal substations, 110-kV, and 35‑kV substations are considered medium- or low-voltage. High-voltage (transmission) substations are large outdoor sites. Low-voltage (distribution) substations are indoor systems located in urban areas to handle high load density.
Improved signal processing technologies make it possible to achieve better than 0.1% accuracy in next-generation systems, as compared to present systemsí typical 0.5% accuracy levels—an improvement mainly achieved with the use of high-performance simultaneous-sampling ADCs (analog-to-digital converters); they provide the resolution and performance that will be needed for future systems.
Figure 1. Waveforms in a typical 3-phase system.
The ADC takes 32 sets of simultaneous samples of three CT and three PT outputs and stores the results in RAM. The system then calculates a DFT on all six outputs, and presents the results in real- and imaginary format, (A + jB). Magnitude and phase information for each transformer can be calculated as follows:
With A + jB and C + jD as the real and imaginary terms for CT1 and PT1, the magnitudes (Mi) and phases (Pi) are:
The power through the PT1/CT1 pair is:
Similar calculations for the power through PT2/CT2 and PT3/CT3 give ‹2 and ‹3. The total average power in the system is calculated by summing the three power terms:
This method uses a DFT and the above calculations to determine the system power at a single frequency. Performing a fast Fourier transform (FFT) instead of a DFT provides data on harmonics and other higher frequency components; this can allow calculation of additional information, such as system losses or the effects of unwanted noise.
Accurate measurement of these small signals requires a high-resolution ADC with excellent signal-to-noise (S/N). The multichannel ADCs that are used must also be capable of simultaneous sampling. Currently available systems have 14-bit capability—the 4-channel14-bit quad ADC, 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. To make accurate 3-phase current- and voltage measurements, the ADC should be capable of sampling six channels simultaneously, and it must have excellent SNR to measure small signals. Where many ADCs are used in one system, low power dissipation is also important.
An example of a device that meets all of these requirements is the, which includes six low-power 16-bit, 250-kSPS successive-approximation ADCs in a single package. Shown in Figure 2, the AD7656 is fabricated in the industrial CMOS ( ģ) process, which combines high-voltage devices with submicron CMOS and complementary bipolar technologies. iCMOS makes possible a wide range of high-performance analog ICs that are capable of high-voltage operation. Unlike analog ICs using conventional CMOS processes, iCMOS components can readily accept bipolar input signals, providing increased performance and dramatically reducing power consumption and package size.
2. The AD7656 has six simultaneous-sampling ADCs,
With 86.6-dB SNR, as shown in Figure 3, the AD7656 provides the performance needed to measure small ac outputs from transformers. Its 250-kSPS update rate is helpful in simplifying designs that need fast data acquisition in order to do real-time FFT post-processing. It is capable of directly accepting ±5-V and ±10-V outputs from the transformer without gain- or level shifting—and consumes a maximum of only 150 mW per device. This is an important consideration when a board must house many ADC channels. Because some systems require as many as 128 channels (as many as 22 six-channel ADCs) on one board, power dissipation can be a critical specification.
3. Peak-to-peak noise is a critical specification in power-line
Beyond the ADC
Figure 4. Power-line monitoring system.
ADC Reference Consideration
Generally, a low-drift reference is also important for reducing reference sensitivity to temperature. A simple calculation can help in understanding the importance of drift and in deciding whether to go with the internal reference. A 16-bit ADC with 10‑V full-scale input has a resolution of 152 µV. The drift specification of the AD7656ís internal reference is 25 ppm/įC maximum (6 ppm/°C typical). Over a 50°C temperature range, the reference could drift as much as 1250 ppm; or about 12.5 mV. In applications where drift is important, an external low-drift reference, such as the (1 ppm/įC), would be a better choice. A 1 ppm/°C reference will drift by only 0.5 mV over a 50°C temperature range.
The noise generated by the driver amplifier must be kept as low as possible to preserve the SNR and transition noise performance of the ADC. A low-noise amplifier is also useful for measuring small ac signals. The amplifierís total offset error, including drift, over the full temperature range should be less than the required resolution. The/ / family of amplifiers combines excellent noise performance (8.5 nV/rtHz) with low offset drift. For example, the OP1177 op amp specifies 60-µV maximum offset and 0.7-µV/°C maximum offset drift. Over a 50°C operating range, the maximum offset drift is 35 µV, so the total error due to offset and offset drift will be less than 95 µV, or 0.0625 LSB.
For power-line monitoring applications, power considerations can be important, especially when up to 128 channels may be measured on one board. The OP1177 family typically consumes a supply current of less than 400 µA per amplifier.
The following table compares some recommended amplifiers for power-line monitoring applications.
ADC Power Supply
RS-232 is often used to connect multiple systems, so isolation between each system and the bus is critical. Digital isolators do not support the RS-232 standard, so they cannot be used between the transceiver and the cable; instead they are used between the transceiver and the local system. Combining an ADuM1402 iCoupler digital isolator, anRS-232 transceiver, and an isolated power supply eliminates ground loops and provides effective protection against surge damage.
For systems using the RS-485 protocol, thesingle-chip isolated RS-485 transceiver is available (Figure 5). It can support data rates up to 20 Mbps and has a 2.5-kV isolation rating.
Figure 5. ADM2486 is a cost- and space-saving isolated RS-485 transceiver.
Thisprocessor—a highly integrated system-on-a-chip—includes a CAN 2.0B controller, a TWI controller, two UART ports, an SPI port, two serial ports (SPORTs), nine general-purpose 32-bit timers (eight with PWM capability), a real-time clock, a watchdog timer, and a parallel peripheral interface (PPI). These peripherals provide the flexibility needed to communicate across multiple parts and interfaces in the system.
Blackfin processors such as theand include an IEEE-compliant 802.3 10/100 Ethernet MAC (Media Access Controller). This is now a standard requirement for many power-line monitoring systems.
The power supply lines to the ADC should use the largest possible traces to provide low impedance paths and reduce the effect of glitches on the power supply lines. Good connections should be made between the AD7656 supply pins and the power tracks on the board; this should involve the use of single- or multiple vias for each supply pin. Good decoupling is also important to lower the supply impedance presented to the AD7656 and to reduce the magnitude of the supply spikes. Paralleled decoupling capacitors, typically 100 nF and 10 µF, should be placed on all of the power supply pins, close to—or ideally right up against—these pins and their corresponding ground pins.
Higher system performance
can be achieved by using high-performance ADCs, such as
the AD7656. With six channels and
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