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Multichannel Sampling Keys Accurate Power Line Monitoring

Accurate power-line monitoring has emerged as a criTIcal requirement not only centrally within the power grid itself but also at its edges. For grid-TIed energy-harvesTIng power-generaTIon systems and even for end equipment, ongoing analysis of power quality characteristics helps ensure the health of the grid and attached systems. At the heart of power-monitoring systems, analog-digital converters (ADCs) play a key role in measuring line voltage and current at a level of accuracy appropriate to each application. Engineers can meet a range of requirements—from utility-grade systems with accuracy needed for revenue purposes to basic power monitors for detecting line faults—using high-performance ADCs and associated components from manufacturers including Analog Devices, Copal Electronics, Intersil, Linear Technology, Maxim Integrated, Microchip Technology, ON Semiconductor, Pulse Electronics, and Texas Instruments.

At the most basic level, power-monitoring systems rely on measurement of instantaneous current and voltage using current transformers (CTs) and voltage transformers (termed PT, for potential transformers) output converted by high-speed ADCs (Figure 1). In turn, a processor uses the instantaneous current and voltage measurements to calculate key characteristics including active power, reactive power, apparent power, and power factor—and even more complex calculations such as harmonics, which can result in equipment damage.

Accurate sampling and characteristic analysis of power line monitoring

Figure 1: A typical power-monitoring system samples line current and voltage, using high-speed analog-digital converters (ADCs) to provide high-resolution data to the host processor for calculation of power parameters. (Courtesy of Texas Instruments)

Performing these power calculations depends foremost on accurately measured voltage and current. Standards such as ANSI C12.20 for North America and IEC 62053 dictate specific accuracy levels such as Class 0.2, which requires   ± 0.2 percent accuracy. In practice, power-measurement systems are typically designed to surpass standard accuracy specifications—relying on high-sample rates and high-resolution converters to capture high-speed transients and to ensure reliable calculation of more complex characteristics such as multiple harmonics.

At the same time, measurement of these power characteristics requires tightly synchronized, high-resolution measurement of voltage and current across all three phases and neutral. Synchronized sampling allows extremely accurate measurement of phase angle between voltage and current on each line. Poor synchronization between voltage and current measurements can introduce artifacts, reduce overall accuracy, and significantly compromise more complex power calculations.

Simultaneous sampling

To reduce the complexity of high-accuracy multichannel measurement, designers can turn to highly integrated devices that combine high-resolution ADCs with analog front ends (AFEs) designed to optimize impedance matching, signal dynamic range, offset, and other factors that can erode performance and accuracy of data conversion. Manufacturers typically build these high-performance simultaneous sampling ADCs around successive-approximation register (SAR) converters. SAR ADCs offer high accuracy without the cycle latency associated with the oversampling used in delta-sigma ADCs to achieve high stability and conversion resolution.

For the most demanding applications, semiconductor manufacturers integrate a complex AFE and dedicated SAR ADC for each channel. For highly accurate power monitoring, eight-channel ADCs such as the Maxim Integrated MAX11046 and Texas Instruments ADS8568 allow simultaneous sampling of all three phases and neutral (Figure 2) . With a dedicated AFE and ADC for each channel, these devices are able to achieve very high data rates—250 ksps per channel for the MAX11046 and 510 ksps for the ADS8568 (parallel-output interface).

Accurate sampling and characteristic analysis of power line monitoring

Figure 2: For three-phase power monitoring, simultaneous sampling ADCs provide separate AFEs and ADCs for each of the eight channels required for highly accurate power calculations. (Courtesy of Maxim Integrated)

For some applications, designers can simply tie the unbuffered output of CTs and PTs directly into the inputs of ADCs including the Maxim Integrated MAX11046 and TI ADS8568 as suggested in Figure 2. Devices such as the MAX11046 provide high-input input impedance and self-protecting input clamps to support this type of simple configuration. To achieve maximum throughput, however, ADCs typically require op amp drivers between the transducers and ADC. In fact, evaluation boards from both Maxim Integrated and Texas Instruments include such drivers. On its MAXREFDES30 evaluation board for the MAX11046, Maxim Integrated includes its MAX44252 op amp drivers.   On the TI ADS8688EVM-PDK evaluation board for the ADS8568, Texas Instruments includes its OPA2209 op amp drivers.

Less-stringent requirements

For power monitoring applications with less stringent accuracy requirements, designers can turn to devices with more modest performance attributes. The simpler requirements in a power monitor that only needs to detect faults in the power line can take advantage of lower resolution ADCs. The Texas Instruments LMP92064 current/voltage monitor IC uses this approach to offer a single-line power-monitoring IC that requires few additional components (Figure 3). The device integrates a precision current sense amplifier to measure a load current across a shunt resistor and a buffered voltage channel to measure the voltage supply of the load. Nevertheless, the ability to sample current and voltage channels simultaneously by the independent 125-kSps, 12-bit ADCs supports accurate power calculations.

Accurate sampling and characteristic analysis of power line monitoring

Figure 3: Power-monitor ICs such as the Texas Instruments LMP92064 use a pair of 12-bit ADCs to support power calculations for applications with less-demanding accuracy requirements. (Courtesy of Texas Instruments)

For even simpler monitoring requirements, designers can often use a single ADC with an external multiplexer to monitor voltage and current on each line. In fact, 8:1 multiplexers such as the Intersil DG408, Maxim Integrated MAX4581, and ON Semiconductor MC74HC4051A can support three-phase applications that require more basic power monitoring.

Designers can find ADCs with integrated multiplexers designed to support a broad range of channels, data throughput, and conversion resolution. For three-phase power monitoring, devices such as Analog Devices AD7699, Linear Technology LTC2372, and Texas Instruments ADS8688 integrate an eight-channel multiplexer with ADC and dedicated analog front end for each channel. For example, each input channel of the TI ADS8688 includes over-voltage protection, a programmable gain amplifier (PGA), low-pass filter, and driver feeding the shared multiplexer to the single ADC.

Conclusion

Ensuring grid power quality requires highly accurate measurement of voltage and current on each phase. For grid-tied energy-harvesting systems and equipment, power monitoring similarly requires high-resolution, simultaneous sampling of line voltage and current using ADCs providing a dedicated converter for each channel. Although basic fault monitoring builds on the same principles as high-accuracy measurement applications, designers can use lower-resolution ADCs or multiplexed devices that offer high-resolution multichannel measurement at lower throughput rates. Designers can find ready availability of ADCs designed to meet this broad range of requirements for channel support, bit resolution, and sample rate.

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Multi channel sampling precise key power line monitoring

Accurate power line monitoring has become a key requirement not only in the power grid itself, but also at its edge. For grid connected energy collection and generation systems, even terminal equipment, continuous power quality characteristic analysis is helpful to ensure the health of power grid and connected system. At the heart of the power monitoring system, analog-to-digital converter (ADC) plays a key role in measuring the accuracy of line voltage and current, which is suitable for the level of each application. Engineers can meet various needs from manufacturers including analog devices, obstetric Bao Electronics, INTERSIL and linglilt – for the purpose of revenue required from utility level system accuracy, basic power monitor is used to detect line faults, using high-performance ADC and related components, Maxim integration, microchip technology, ansenmey semiconductor, electronic pulse and Texas Instruments.

At the most basic level, the power monitoring system relies on instantaneous current and voltage. The output of current transformer (CT) and voltage transformer measurement (called Pt, used for voltage transformer) is converted by high-speed ADC (Figure 1). In turn, a processor uses instantaneous current and voltage measurements to calculate key characteristics, including active power, reactive power, apparent power and power factor – even more complex calculations, such as harmonics, which can lead to equipment damage.

Image of typical power monitoring system of Texas Instruments

Accurate sampling and characteristic analysis of power line monitoring

Figure 1: the sample line current and voltage of a typical power monitoring system uses a high-speed analog-to-digital converter (ADC) to provide high-resolution data for the main processor of power parameter calculation. (provided by Texas Instruments)

Performing these power calculations depends on accurately measuring voltage and current on the front. As specified in ANSI c12.20 North America and IEC 62053 standards, such as class 0.2, this requires an accuracy of ± 0.2%. In practice, power measurement systems are usually designed to exceed standard accuracy specifications – relying on high sampling rate and high-resolution converters to capture high-speed transients and ensure more complex characteristics, such as multi harmonic reliable calculation.

At the same time, the measurement of these power characteristics requires closely synchronous, high-resolution measurement of voltage and current in all three stages and neutral. Synchronous sampling allows very accurate measurement of the voltage and current and the phase angle between each row. The lack of synchronization between voltage and current measurements can introduce artifacts, reduce the overall accuracy and significantly compromise more complex power calculations.

Simultaneous sampling

In order to reduce the complexity of high-precision multi-channel measurement, designers can turn to combining high-resolution ADC with highly integrated devices designed to optimize impedance matching analog front end (AFE), signal dynamic range, offset, and other factors that can erode performance and the accuracy of data conversion. Manufacturers typically focus on successive approximation register (SAR) converters for these high-performance synchronous sampling ADCs. SAR ADC provides high accuracy without Δ-Σ The analog-to-digital converter is used to achieve high stability and conversion resolution of oversampling related periodic delay.

For the most demanding applications, semiconductor manufacturers integrate complex AFE and dedicated SAR ADCs for each channel. For highly accurate power monitoring, 8-Channel ADCs such as Maxim’s MAX11046 integration and Texas Instruments ads8568 allow synchronous sampling and neutrality in all three stages (Figure 2). For each channel dedicated AFE and ADC, these devices can achieve very high data rate – 250 ksps per channel for MAX11046 and 510 ksps for ads8568 (parallel output interface).

Maxim’s integrated three-phase power supply monitoring image

Accurate sampling and characteristic analysis of power line monitoring

Figure 2: three phase power monitoring, simultaneous sampling ADC provides independent AFE and ADC, and each eight channel requires highly accurate power calculation. (provided by Maxim integration)

For some applications, designers can simply tie the buffered outputs of CT and Pt directly into the ADC, including the maxim integrated MAX11046 and Ti ads8568 inputs, as shown in Figure 2. Devices such as MAX11046 provide high input impedance and self-protection input clamping to support this type of simple configuration. In order to achieve maximum throughput, however, ADC usually requires the driver of the operational amplifier between the transducer and ADC. In fact, both from Maxim integration and Texas Instruments evaluation board include such drivers. Its maxrefdes30 evaluation board is MAX11046, and Meixin integration includes the driver of its max4452 operational amplifier. The Ti ads8688evm-pdk evaluation board is ads8568, and Texas Instruments includes its opa2209 operational amplifier driver.

Less stringent requirements

For power monitoring applications with less stringent accuracy requirements, designers can turn to devices with milder performance attributes. The simpler requirement is that the power monitor only needs to detect faults, which can take advantage of the ADC with lower resolution on the power line. Texas Instruments lmp92064 current / Voltage Monitoring IC uses this method to provide a single line power monitoring IC, which requires few additional components (Figure 3). The device integrates a precision current detection amplifier to measure the shunt resistance and a buffer voltage channel to measure the load current of the load of the voltage power supply. Nevertheless, with the ability to simultaneously sample current and voltage channels independently of 125 ksps, the 12 bit ADC supports accurate power calculation.

Diagram of Texas Instruments lmp92064

Accurate sampling and characteristic analysis of power line monitoring

Figure 3: power monitor IC, such as Texas Instruments lmp92064, uses a pair of 12 bit ADCs to support power calculation and less demanding accuracy requirements. (provided by Texas Instruments)

For simpler monitoring requirements, designers can often use a single ADC with an external multiplexer to monitor voltage and current on each line. In fact, 8:1 multiplexers such as INTERSIL’s dg408, Maxim integrated max4581 and ansenmey semiconductor mc74hc4051a can support three-phase applications requiring more basic power monitoring.

Designers can find ADC designs that support a wide range of channels, integrate multiplexers, data throughput, and conversion resolution. For three-phase power monitoring, devices such as ADI ad7699, Lingte ltc2372 and Texas Instruments ads8688 integrate an eight channel multiplexer with ADC and a dedicated analog front end for each channel. For example, each input channel of TI’s ads8688 includes overvoltage protection, programmable gain amplifier (PGA), low-pass filter, and drive feed sharing multiplexing to a single ADC.

conclusion

Ensure the high-precision measurement of voltage and current per phase required by the power quality of the power grid. For grid parallel energy acquisition systems and equipment, power monitoring also requires high resolution, simultaneous sampling of line voltage and ADC currently used to provide a dedicated converter for each channel. Although the basic fault monitoring is based on the same principle and high-precision measurement applications, designers can use lower resolution ADC or equipment that provides high-resolution multi-channel measurement and multiplexing at lower throughput rate. Designers can find ready availability designed to meet these broad channel support, bit resolution and sampling rate ADC requirements.

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