Which current sensor for power measurement?

Overview
Whether it's power supply management, electricity usage control or monitoring the general state of a system's "electric grid" large or small, gauging energy conservation and doing it cost-effectively looms heavily in more applications.

Most designers equate power measurements with power supply management. It's tied in with power generation and distribution, but also industry professionals monitoring their power quality and power factor to control rate tariffs imposed by their utilities, especially when operating low-power-factor loads.

Energy sub-metering is also gaining importance among facility and plant managers as it allows tracking and allocating energy costs. It also allows managers to perform advanced analysis of electricity consumption for best system efficiency. Intelligent sizing of power supplies depends on knowing the peak consumption and planning for sufficient safety margin to minimize system failures. Energy sub-metering is also required to understand and manage commercial and industrial layouts and identify efficiency issues caused by defective appliances and inefficient facilities usage (e.g., inappropriate lighting, heating, or air conditioning).

Another related major area, condition monitoring, requires systems to anticipate and react to failures that can impinge upon vital services. Here again, electrical power measurement is much more efficient that tranducer based systems using temperature, pressure, or vibration sensors. What's the best current-sensor solution for your task? Here's how to go about finding it.

Selection criteria
Whatever the application, pay careful attention to the following parameters in designing your power monitoring system:

1. Accuracy: Class 1 power meters require current sensors with much better than 1 percent accuracy. The sensor is generally expensive and utilizes costly manufacturing processes. An alternative to using expensive sensor elements is to calibrate the power meter for each single element. Taking into account the specific characteristics of each sensor allows you to use it in its most precise operating mode and to mitigate the variations from one sensor to another.

2. Drift: A sensor's drift defines its ability to sustain a given reading over time independent from the initial system calibration. Some variations in sensor characteristics may be caused by changes in ambient humidity and temperature as well as component aging. Clearly the designer's task is to select a sensor with low-drift consistent with the most cost-effective solution

3. Linearity: We can define linearity as the sensor's ability to retain its characteristics over its full operating range. A high linearity of the analog sensing part is essential to provide accurate measurement over a wide range of primary currents, and especially at low current levels. Several technologies offer good performance over only a limited measuring range, thus limiting their application to rather high or low currents.

4. Phase shift: The accuracy of the true active power or energy calculation is related not only to the accuracy and linearity of the AC current and voltage sensor in terms of amplitude, but also the phase shift that may occur between the measurement of these correlated values. The phase shift should of course be as low as possible.

5. Integration: The sensor's current transformers are essentially self powered, and do not require wiring other than a 2-wire output connection to the main power monitor unit. Many of them provide calibrated standard outputs, allowing easy integration in the power monitoring system. The typical 1- and 5-amp or 333 mV outputs are compatible with most standard power meters on the market.

High-accuracy power meters require specific calibration with sensors that are no longer interchangeable. As a consequence, these systems often deliver low-current outputs that are safer than traditional 1- and 5-amp signals. Sensors with current outputs are almost insensitive to interference and are preferable to voltage outputs when long cables are required to connect the sensors to the power meter.

6. Price: The cost of the sensors come into play especially when three accurate current sensors are required for 3-phase power measurement. When considering overall price, also consider installation and maintenance costs. Although more expensive, reliable and easy to install and replace, split-core sensors bring greater cost reduction to the system as a whole.

The transformers
Solid-core current transformers
Power measurement systems generally implement contactless current sensors rather than shunts, because the latter cause power losses as well as installation and safety issues. Traditional solid-core current sensors are based on the principle of a transformer, i.e. primary and secondary windings magnetically linked by a core. The measured current induces a magnetic field in the core which generates a current in the secondary windings proportionate to the primary current divided by the number of turns of the secondary windings. These basic current transformers are designed to measure sinusoidal alternating currents in the typical 50/60 Hz range. This well known technology is very affordable due to the use of common materials and processes.

Solid-core current transformers offer a cost-effective and accurate solution for designing power meters dedicated to new equipment and buildings. They are not suitable, however, for the numerous applications involving power monitoring of existing machines and facilities, where it would be necessary to shut down power and disconnect cables before retrofitting the solid-core sensors in all the places where they might be used. Installing power-metering systems is generally not possible, prohibitively expensive and even dangerous if it requires a service interruption, even for a short while (e.g. stopping a production line, a telecom or datacenter power supply, or nuclear plant equipment).

Split-core current transformers
Contactless self-powered split core current transformers simply snap over a conductor, without the need to screw or weld on complex brackets. Thus installation and maintenance is straightforward. They can be installed in electrical control panels, thus avoiding complex wiring, to remotely monitor devices that sometimes operate in inaccessible or harsh environments. Split-core transformers can be retrofitted into a live installation without disturbing it, which often make them the best choice for engineers designing power meters.

But these advantages have a price. The split-core current transformer is more expensive and less accurate than the solid-core transformer. Thus the specific application is important.

Split-core current transformers are generally based on the same principle as solid-core transformers. In split-core, however, the magnetic core comprises two parts and thus can be separated. The part's inaccuracy mainly comes from the imperfect contacts between the two parts; and because the secondary windings are not uniformly distributed around the magnetic core (around one of the two parts only). The price and performance of these transformers generally are a function of these physical and mechanical characteristics. Very flat contact surfaces are required as well as sufficient pressure between the two parts of the core. The part's case generally features flexible sections or materials and/or hinges to provide sufficient compression as well as a reliable opening mechanism.

FeSi split-core current transformers
FeSi has been widely used in split-core current transformers, mainly due to its affordable price. The performance offered by this material is generally poor, however. It has poor linearity (especially at low currents) and a large phase shift. This restricts its use to low-cost current transformers for measuring rather high currents, where power monitoring doesn't require high accuracy. Often, system engineers need only a rough estimation of the power consumption rather than an analysis of exact consumption. In this application, the system user can draw operating time profiles. In these cases the large phase shift is not a big issue. A typical application is branch current monitoring in panel boards allowing a system to detect when some circuits may come to overload and generate an alarm or balance loads.

Another disadvantage of FeSi current transformers is that they are large and heavy. They're thus not very appropriate for environments with limited space.

FeNi split-core current transformers
FeNi has been a better material for split-core current transformers for a long time. It offers good performance but at high price. And it's a good alternative to FeSi when accuracy and phase shift are important, or when the transformers need to measure small currents.

Apart from the price, FeNi current transformers have some limitations. As with FeSi current transformers, they take up considerable space within industrial facilities and panel boards. They also suffer from poor linearity and drift, mainly due to the air gaps induced by the split-core architecture.