Feature

Use simultaneous-sampling ADCs to monitor three-phase ac line power

Multichannel simultaneous-sampling analog-to-digital converters trim your costs while simplifying power-monitoring systems.

By Martin Mason, Maxim Integrated Products -- EDN, 8/18/2009

Advanced power-line monitoring systems combine power-supply monitoring, load-balancing, protection, and metering functions. This allows the power utilities to efficiently deliver grid power while helping consumers control costs. An advanced power-line monitoring system can perform predictive maintenance while detecting and responding to fault conditions. It will allow dynamic load balancing, yielding energy conservation, while monitoring and controlling power-delivery quality and protecting the equipment.

To implement these power-line systems you need to monitor the voltage and current on multiple phases with analog-to-digital converters (ADCs). However, you must synchronize these converters in order be able to meet the stringent standards requirements and to accurately measure power factor. By synchronizing the conversions, you ensure that they sample the three phases and neutral at the same moment. Synchronizing individual converters can be tricky, so various vendors offer simultaneous-sampling ADCs in a single package. If a highly integrated solution is needed, you could also integrate the simultaneous-sampling converters in a custom ASIC.

Varying international standards on the precision of energy measurement complicate the development and widespread adoption of advanced power-line monitoring systems. Real-time power-delivery monitoring, fault detection, fault protection, and dynamic load balancing require stringent accuracy. As an example, the European Union (EU) standard IEC62053 Class 0.2, increasingly used as a common standard worldwide, requires meter precision to be 0.2% of nominal current and voltage. For power-factor measurement accuracy, your sample time phase matching should be 0.1% or better.

These international and local standards also dictate the sample rate you need. These applications typically require accurate simultaneous multichannel measurement over a wide dynamic range of up to 90 dB with a sample rate of 16 kilosamples per second (ksps) or higher. This provides for analysis of multiple harmonics of the ac supply as well as detection of high-speed fault conditions, such as spikes and brownouts.

You should also look at factors such as effective input impedance (Z_IN), signal phase adjustment, and small physical package size when selecting the ADCs for a power-grid monitoring application. Lately, designers are turning toward simultaneous-sampling, multichannel, high-performance ADCs for their power-line monitoring or multichannel supervisory control and data acquisition (SCADA) systems.

A typical power-grid monitoring application

Power companies distribute three-phase power using a wye connection. The term "wye" refers to the arrangement of three transformer windings that join at a common point, the junction of the Y. The line voltages are offset in phase from each other by 120°, one-third of a cycle. If loads on each of the three phases are equal, the system is balanced and no current flows through the neutral line. A fourth wire neutral connects to the junction of the wye. It will accommodate imbalanced loads across the line connections. In a typical power-grid monitoring scheme, you measure each phase's power measurements with a current transformer (CT) and a voltage transformer (Figure 1). This is called a potential transformer (PT) in power-distribution nomenclature. The complete system comprises four such pairs, one pair for each of the three phases plus a neutral pair. The ADCs simultaneously measure the three phases and neutral voltages and currents. You can determine the active, reactive, apparent-energy, and power-factor parameters by performing digital processing on the sampled data. You could then adjust line loads dynamically to correct for power factor, thereby increasing power efficiency. By executing a fast Fourier transform (FFT) on the sampled data, you can do frequency and harmonic distortion metering, as well as highlight information such as system losses and the effects of unwanted noise.

Power-monitoring system requirements

To accommodate standards requirements, your power-monitoring equipment must measure instantaneous current and voltage values with sample rates up to 60-Hz × 256 samples, or greater than 15,360 samples per second (sps).

You can calculate the ADC's dynamic range for a voltage measurement from the maximum and nominal voltages you monitor and from the required accuracy for power measurements. For example, if a design must measure a 1.5-kV (1500V) temporary overvoltage (under a fault condition) with a nominal 220V voltage measurement and a 0.2% specification accuracy requirement, then the total dynamic range of the voltage-measurement subsystem will need to be:

20log ((1500/220) × 2000) = 83 dB

In all the calculations, the assumed required design accuracy is 0.05%, which is better than the standard's 0.2% accuracy requirements. You should use this design margin to ensure compliance to the standard.

Current-sensing requirements also affect ADC specifications. If the design requirements for power monitoring are the typically 10A nominal and 100A maximum, and Class 0.2 (0.2 %) accurate, then the total dynamic range of the current measurement subsystem will need to be:

20log ((100/10) × 2000) = 86 dB

A 16-bit-resolution ADC will achieve this 86-dB dynamic range. To ensure accurate current and voltage measurements, the ADC must be capable of sampling up to eight channels simultaneously (four voltage and four current). You should provide for the ability to correct the current and voltage transformer-induced phase shift for systems that are trying to measure and correct power factor.

Local standards and international requirements

The measurement characteristics of delivered energy must comply with local standards or international requirements. Standards such as the EU standards EN 50160, IEC62053, and IEC61850 dictate both the minimum accuracy and the sample rate needed for a modern multichannel ADC system used in power-system monitoring and metering. Many countries around the globe have adopted versions of the EU standards, so these standards serve as a good example of what measurement requirements the system must meet (Table 1).

Another EU standard, IEC62053, mandates the precision of energy-metering equipment. It defines four classes of meters: Class 2, Class 1, Class 0.5, and Class 0.2. For accurate power-factor measurement, you should phase match to 0.1% or better. For the harmonic voltage, the EN 50160 mandates measurement up to the 25th-order harmonic of 50/60-Hz voltages. For various nonlinear loads such as inductive motors and switching power-supplies drives, your measurements must be done to the 127th-order harmonic of 50/60-Hz voltages. Emerging standards such as IEC61850 recommend the recording of power-system transient events with 256 samples per ac cycle or higher.

ADC alternatives

Several ADCs are able to meet the rigorous standards of these power-grid monitoring applications. A majority of these are six-channel, 16-bit simultaneous-sampling ADCs with sample rates up to 250 ksps. Analog Devices and Linear Technology both offer six low-power 250-ksps successive-approximation register (SAR) ADCs in a single package, the 16-bit AD7656 ADC and the 14-bit LTC2351-14 SAR ADC. Maxim offers the MAX11046 high-precision data converter, which provides eight low-power, 16-bit, simultaneous-sampling, 250-ksps SAR ADCs in a single package while achieving greater than 90-dB signal-to-noise ratio.

Input impedance, Z_IN, is dictated by the input capacitance and sampling frequency:

Z_IN = 1/(C_IN × F_SAMPLE)

where F_SAMPLE is the sampling frequency and a typical C_IN is 15 pF. If the ADC has a high Z_IN, like the MAX11046 and AD7656, you can directly interface with voltage and current measurement transformers. This eliminates external precision instrumentation amplifiers, or buffers, saving system cost, board area, and power (Figure 2).

As the transformer converts a high voltage to a lower voltage, a phase shift occurs in the output. To deal with this, designers may adjust the phase in software, or they can realign the signals inside the ADC. When you de-skew the voltage and current signals, it allows for true and accurate measurement of the power factor in the wye configuration. Traditionally, you would do signal-phase adjustment digitally, as a postprocessing step performed on the ADC data. The AD7656 and MAX11046 ADC converters handle phase adjustment in this manner. You will incur a continuous software overhead with these types of ADCs.

Some ADCs offer input phase adjustments of 0 to 333 μs, with the delay independently settable per channel in 1.33-μs steps. This eliminates the software overhead. The 24-bit, four-channel MAX11040 sigma-delta ADC provides this capability. Each channel includes an adjustable sampling phase that permits internal compensation for phase shift due to external transformers or filters at the inputs. An active-low SYNC input allows periodic alignment of the conversion timing for up to eight devices with a remote timing source. IC package size matters in many power-grid monitoring applications. The MAX11040 sigma-delta ADC uses 15.9 mm² per channel, less than 50% the area of the ADS1274.

In addition to keeping the systems small, designers must also be concerned about preventing system failures due to overloads or other line disturbances. For example, the some ADC devices have built-in overvoltage protection through the use of clamp diodes and an internal logic circuit that sets a fault bit if high voltage is detected. Other ADCs use external diode protection, increasing board area.

Conclusion

Rising worldwide power demands are driving rapid investment in the power-delivery infrastructure, or "smart grid." You can efficiently monitor, deliver, and control grid power by integrating power-supply monitoring, dynamic load-balancing, protection, and metering functions in your advanced power-line systems. Complicating the development and widespread adoption of these systems are the varying standards and requirements regarding the precision and frequency of energy measurement. Stringent specifications such as the EN50160, IEC62053, and IEC61850 standards dictate both the minimum accuracy and sample rate needed for a modern, multichannel ADC system.

Today's simultaneous-sampling ADCs are a natural choice for designs that need to provide high performance while reducing total system cost and minimizing board area. In addition to sample rates and standard requirements, other factors, such as effective input impedance (Z_IN), signal phase adjustment, and small physical package size, are playing a critical role in your ADC selection for a power-grid monitoring application.