Instrumenting DC Power Systems with the Highland Technology V180 and AT180 Electrical Measurement Modules

Instrumenting DC Power Systems with the AT180 and the V180

Although the Highland AT180 and V180 products are most commonly used to measure AC power systems, they are capable of measuring DC or mixed AC/DC power systems. This application note discusses some aspects of using the Series 180 products for DC power measurement.

GENERAL THEORY

The basic measurement algorithms used by the Series 180 modules is discussed in Theory of Operation, application note AT180-AN2. The algorithm used is effective for all waveforms, including DC inputs. Given a DC line voltage input and a DC line current input (each discussed below) the Series 180 modules will report accurate voltage, current, power, and KWH values. Because the modules report true RMS voltage and current, the voltage and current values will be reported as the absolute value of the applied inputs (that is, the + sign value is lost). If this is undesirable, a modified version of the x180 firmware is available which preserves the DC sign. In all cases, true bipolar power and KWH values are reported.

VOLTAGE INPUTS

The standard C740 4-wire potential transducer may be used for DC as well as AC signal acquisition. One C740 could be used to acquire a single DC voltage (for example, +48 volts and ground) and simultaneously acquire a single-phase AC signal (for example, 120 Volts and its neutral). In most cases, however, it is preferable-and safer-to use two separate C740s, one for AC pickup and the other for the DC circuits.

The standard C740 may be used with AC or DC inputs up to 480 volts RMS. In low-voltage DC systems, the high division ratio of the standard C740 may not be ideal for voltage acquisition. Two optional versions of the C740 are available, the C740-48 (usable at up to 70 volts DC) and the C740-120 (usable at up to 175 volts DC). Either of these units may be used to improve measurement resolution and accuracy as appropriate. When the Highland PARSET program is

used to set up a Series 180 module, an appropriate "PT RATIO" value may be entered to compensate for the division RATIOS of the C740-48 or C740-120 versions. The PT RATIO values should be 0.1 for the -48 unit, and 0.25 for the -120 version.

DC CURRENT SENSORS

The standard C701-series current transformers WILL NOT work with DC current signals. Three DC current sensor types are compatible with the series 180 energy measurement modules:

1. Hall-effect DC current transformers. These devices behave like regular AC current transformers, but measure AC or DC currents properly and require an external power supply. Suitable units are manufactured by FW Bell, and LEM USA. Since the Series 180 modules accept CT inputs scaled for 0.250 or 0.3333 Volts full scale, the Hall sensors must be similarly calibrated, or an external voltage divider must be used to scale their outputs appropriately.

2. Very high accuracy "Zero Flux" DC current transformers (DCCTs) are manufactured by Holec, Dynapower, and Danfysic; with an appropriate load resistor, they can be scaled to produce the required 0.25 or 0.3333 volt full-scale signal.

3. Conventional resistive current shunts may be used with integrated isolation amplifiers to sense DC current flow and to scale and isolate the shunt signal. IT IS GENERALLY NOT SAFE TO CONNECT A RESISTIVE SHUNT DIRECTLY TO THE INPUT OF A SERIES 180 MODULE.

One virtue of the Series 180 modules is their very wide dynamic range and very small "zero power" offset error. This performance is achieved by using autozero and correllation techniques; however, these techniques are inherently ineffective at DC, since ANY zero-current offset in a DC current sensor must be assumed to be a legitimate current flow and thus must be measured as such. Hall-effect current sensors may exhibit significant DC offsets (affected by temperature changes, the earth's magnetic field, or time) of as much as 1 percent of full scale. Zero-flux transducers are quite expensive, but display zero and linearity errors in the parts-per-million range. A good current shunt/isolation amplifier combination can have zero and linearity errors in the 0.1 per cent range.

Please consult Highland Technology if we can be of assistance in applying the Series 180 modules to DC power systems.





	Instrumenting DC Power Systems with the AT180 and the V180 Although the Highland AT180 and V180 products are most commonly used to measure AC power systems, they are capable of measuring DC or mixed AC/DC power systems. This application note discusses some aspects of using the Series 180 products for DC power measurement. GENERAL THEORY The basic measurement algorithms used by the Series 180 modules is discussed in Theory of Operation, application note AT180-AN2. The algorithm used is effective for all waveforms, including DC inputs. Given a DC line voltage input and a DC line current input (each discussed below) the Series 180 modules will report accurate voltage, current, power, and KWH values. Because the modules report true RMS voltage and current, the voltage and current values will be reported as the absolute value of the applied inputs (that is, the + sign value is lost). If this is undesirable, a modified version of the x180 firmware is available which preserves the DC sign. In all cases, true bipolar power and KWH values are reported. VOLTAGE INPUTS The standard C740 4-wire potential transducer may be used for DC as well as AC signal acquisition. One C740 could be used to acquire a single DC voltage (for example, +48 volts and ground) and simultaneously acquire a single-phase AC signal (for example, 120 Volts and its neutral). In most cases, however, it is preferable-and safer-to use two separate C740s, one for AC pickup and the other for the DC circuits. The standard C740 may be used with AC or DC inputs up to 480 volts RMS. In low-voltage DC systems, the high division ratio of the standard C740 may not be ideal for voltage acquisition. Two optional versions of the C740 are available, the C740-48 (usable at up to 70 volts DC) and the C740-120 (usable at up to 175 volts DC). Either of these units may be used to improve measurement resolution and accuracy as appropriate. When the Highland PARSET program is used to set up a Series 180 module, an appropriate "PT RATIO" value may be entered to compensate for the division RATIOS of the C740-48 or C740-120 versions. The PT RATIO values should be 0.1 for the -48 unit, and 0.25 for the -120 version. DC CURRENT SENSORS The standard C701-series current transformers WILL NOT work with DC current signals. Three DC current sensor types are compatible with the series 180 energy measurement modules: 1. Hall-effect DC current transformers. These devices behave like regular AC current transformers, but measure AC or DC currents properly and require an external power supply. Suitable units are manufactured by FW Bell, and LEM USA. Since the Series 180 modules accept CT inputs scaled for 0.250 or 0.3333 Volts full scale, the Hall sensors must be similarly calibrated, or an external voltage divider must be used to scale their outputs appropriately. 2. Very high accuracy "Zero Flux" DC current transformers (DCCTs) are manufactured by Holec, Dynapower, and Danfysic; with an appropriate load resistor, they can be scaled to produce the required 0.25 or 0.3333 volt full-scale signal. 3. Conventional resistive current shunts may be used with integrated isolation amplifiers to sense DC current flow and to scale and isolate the shunt signal. IT IS GENERALLY NOT SAFE TO CONNECT A RESISTIVE SHUNT DIRECTLY TO THE INPUT OF A SERIES 180 MODULE. One virtue of the Series 180 modules is their very wide dynamic range and very small "zero power" offset error. This performance is achieved by using autozero and correllation techniques; however, these techniques are inherently ineffective at DC, since ANY zero-current offset in a DC current sensor must be assumed to be a legitimate current flow and thus must be measured as such. Hall-effect current sensors may exhibit significant DC offsets (affected by temperature changes, the earth's magnetic field, or time) of as much as 1 percent of full scale. Zero-flux transducers are quite expensive, but display zero and linearity errors in the parts-per-million range. A good current shunt/isolation amplifier combination can have zero and linearity errors in the 0.1 per cent range. Please consult Highland Technology if we can be of assistance in applying the Series 180 modules to DC power systems.