Designing an instrumentation system for high current measurement requires careful consideration of the trade-offs associated with each type of sensing device. The purpose of this application note is to help engineers understand the sensing choices available and the corresponding trade-offs with each technology.
Electricity is the movement of negatively charged electrons in a conductor from a region of high electron density to a region of low electron density. The difference in electric potential between these regions is known as voltage (measured in volts). This provides the electromotive force to move electrons.
The rate of charge flow carried by these electrons per second is what is known as electric current (measure in amperes), while the opposition to this current flow is known as a resistance (measured in Ohms).
Electric current is by convention said to flow from a region of high electric potential to a region of low electric potential, opposite to the direction of electron flow.
As shown in Figure 1, current measurement is a series measurement of electron flow. There are two ways to make this measurement with a current sensor:
This application note focuses on indirect technologies as they apply to the measurement of electrical power. Listed devices target the measurement of inputs and outputs of electromechanical systems such as grid-tied inverters, variable-speed motor drives, motors, chargers, generators, appliances, and transformers. The required bandwidth of such systems is typically below 1MHz. For higher bandwidths, please refer to the high-bandwidth oscilloscope probes on the Yokogawa Test&Measurement website.
The use of a current clamp or current transformer greatly simplifies measuring high currents (>50A) where physical constraints (e.g., conductor sizes, insertion losses, safety) make a direct measurement through the precision internal shunt of a power analyzer, DMM, or external shunt into a data acquisition instrument impractical. This convenience comes with a cost, and system designers must educate themselves on these trade-offs in order to make the best practical engineering decision.
Key specifications for each device must be considered when selecting a current measurement device.
Once all of the specifications are established, the sensor technology can be identified.
The most important specification to consider is the purpose of the measurement, as this often determines the rest of the specifications.
For example, if a high current measurement is needed for benchmarking power, energy consumption, or efficiency, then the accuracy specification will dictate the appropriate technology. Likewise, if a high current measurement is needed for understanding general current consumption, waveform shape, or event capture, then a different technology would likely be appropriate.
Indirect current measurement relies upon sensing the magnetic field generated by a current-carrying conductor. Sensing this field is accomplished through a variety of technologies such as AC current transformers, Hall-effect sensors, Rogowski coils, and fluxgate sensors. Each one of these technologies has associated trade-offs that must be considered within the system design specifications.
General selection guidelines by technology and application include:
All of these technologies can have measurement errors attributed to factors such as linearity, offset, temperature, or noise.
To compensate for linearity errors, operate the sensing technology in a “zero flux” condition, where the magnetic field being measured by the sensor is essentially zero. This is accomplished through a compensation winding inside of the current transformer that generates an equal-yet-opposing magnetic field to that of the primary field. This winding is driven in a closed-loop circuit formed by the sensor (fluxgate or Hall) and associated amplification circuitry as shown in Figure 8. This allows the sensor to essentially operate around a zero sensing condition (a single point), minimizing any gain errors. Offset errors can further be eliminated by applying a zero offset or a “nulling function” in the sensing instrument that the current transformer is connected to, such as a data acquisition, oscilloscope, or power analyzer.
Figure 8. Closed-loop Hall-effect zero flux CT.
Current sensing technology and associated electronics have limited bandwidth. However, an advantage of the closedloop zero flux configuration is that at higher frequencies, the compensation winding acts as an AC current transformer. This significantly extends the bandwidth and reduces the response time of the transducer. Essentially, the zero flux (closed-loop) current transformer design incorporates multiple current sensing techniques (e.g., AC current transformer, Hall-effect, fluxgate). As a result, zero flux designs are capable of measuring AC, DC, and complex waveforms of any shape.
Yet another advantage of the closed-loop design is that the output signal is current-based. This provides a more robust signal in high noise environments. It is also the preferred signal for power analyzers, which can directly measure current with high accuracy. When dealing with any current transformer, the output is considered a constant current source. This means as current flows through the primary conductor, the secondary must never be left as an open circuit. An open circuit essentially produces infinite resistance, by Ohm’s law V = I*R, and results in a very high voltage that damages the current transformer and presents a significant safety hazard. Design systems with caution so that the current transformer does not disconnect when performing a measurement.
After selecting the appropriate current sensing product, the integration of the device must be carefully engineered, thoroughly evaluating all output types, accuracy, measurement range, and interconnections.
A manufacturer of UPS systems is engineering an instrumentation system to monitor and make measurements on six phases of AC voltage, current, power, and one DC phase (480Vrms, 75Arms). Power and harmonic monitoring is important. However, capturing distorted wave shapes during changeover events is critical. The instrumentation system is data acquisition-based and requires the ability to easily move to different installations.
Data acquisition systems generally consist of voltage signals and are typically not highly accurate when making power measurements, so it makes little sense to select a highly accurate current device. In this case, the engineer has selected a Hall-effect-based clamp-style sensor, which is valid for any type of wave shape (AC/DC).
A manufacturer of inverter-based motor drives is engineering an instrumentation system for benchmarking power measurements on three phases of PWM-based AC voltage, current, power, and one DC input phase (800Vrms, 1100Arms). Waveforms are of interest. However, capturing the most accurate power measurements is most critical, as the efficiencies of new inverter designs are over 90%. The instrumentation system is power analyzer-based and must provide highly accurate power, energy, and harmonic measurements.
In this instance, the engineer has selected a fluxgate-based zero flux current transformer. The fluxgate is valid for any type of wave shape (AC/DC) and provides the highest accuracy solution for PWM application requiring high bandwidths. Power analyzers are highly accurate devices, and the most precise current measurement is required to keep errors to a minimum. A low-accuracy clamp or fixed-frequency current measurement device would not be a good selection.
When it is necessary to lengthen the cables of the IT, IN, or CT series current transformers, the voltage drop in power supply lines and total burden resistance seen by the transformer needs to be considered.
The specifications for the 5A module state the internal resistance is 100 mΩhm. The IN 2000-S specifications show the maximum burden resistance at 3000Apk is approximately 1 Ω @ 25C, with a transformation ratio of 2000:1 (NS), and an overhead current consumption of about 200mA (IC).
Recommendations on wire types and shielding for the power supply cables are detailed in the IST power supply user manual. For long runs, twist the signal wires from the power supply to the power analyzer to reduce the influence of noise, though bear in mind this increases the total length of the wires.
Power analyzer uncertainty is a percentage of reading plus a percentage of range error. Current transformers are a percentage of reading plus offset. The following shows methods for estimating the total uncertainty in a power analyzer and current transformer system.
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