Measuring differential current at high precision and low cost
Function and design
DI sensors are current sensors used for simultaneous monitoring of currents in phases and neutral conductors and for activation of automatic shut-off in the case of hazardous electrical faults. They are always used in cases where direct current and alternating current circuits are directly connected; as the name implies, they can measure both AC and DC current. Unlike operating current sensors, DI sensors do not measure the load current, but only the differential current between the individual current-carrying conductors (in a three-phase system: L1, L2, L3 and N).
The magnetic system enables differences between multiple currents to be monitored simultaneously and measured directly by a single sensor.
VAC’s sensor design with a magnetic probe records the nominal state (absence of magnetic field in the air gap of a closed-loop sensor core) using a high-frequency fluxgate probe. This functional principle has numerous advantages. Individual characteristics of the probe do not impact on measurement precision, enabling optimum temperature independence and long-term stability to be achieved. The switching frequency (approx. 400 kHz) used by the sensor is far higher than the frequency range in use and is therefore generally interference-free. Rapid changes in input current bypass the probe and are transmitted via the transformer.
Differential sensors must fulfil an array of challenging requirements. They must convert differential current of 30 mA at operating currents of up to 50 Arms, be impervious to external interference and have high temperature stability.
The sensors are shielded against external fields, such as the earth’s magnetic field or fields generated in the application device, by a high-quality metallic shield (see fig. 1), also manufactured by VAC.
Fig. 1 Various types: push-through and with 2 and 4 primary copper bars
Chart 1 shows selected active types with some key technical data. For full resolution click here.
Various types and technical specifications
As the differential currents to be monitored only occur in the event of electrical faults and are extremely low (mA), maximum measurement precision is critical.
As Chart 1 shows, VAC differential current sensors are designed for a typical differential current range of 30 to 300 mA, with a measurement range of +/- 850 mA and measurement precision to 1.5% of 100 mA. Offset current is minimized by automatic sensor core demagnetization, which can be triggered by the supply voltage or as required, with a range from 0 to approx. 10 kHz and maximum operating current of 50 Arms. The sensors are designed for mains voltage of up to 600 V and working voltage up to 1,000 V in line with EN50178.
They operate from a unipolar +5 V supply voltage and have a voltage output for direct connection to an A/D converter.
Integrated self-monitoring and testing functions signal faults in the magnetic probe or compensation winding or excessively low supply voltage. An additional test winding is used to monitor the sensor functions using an externally generated test current, in a similar fashion to the test button on a classic residual-current device.
Both active sensors with integrated electronics and passive sensors for connecting basic external electronics are available. Users can also select the required measurement range; the sensors indicate when the range is exceeded, and additional functions of demagnetization and fault indication are available as standard logic levels via separate inputs and outputs on the IC.
Security and key benefits for all applications
As both direct and alternating current faults can occur in customer applications, a DI sensor capable of monitoring both AC and DC is necessary. Type A FI residual current circuit-breakers (RCCBs) are often the sole type installed in private households; however, they are unable to identify and deactivate DC fault currents. Users seeking to install a transformerless solar inverter or charge an electric vehicle from a home power supply would therefore require a costly Type B FI RCCB to guarantee safety in the presence of DC fault current. By using a VAC DI sensor in the residual current monitoring unit (RCMU) integrated into a transformerless solar inverter, customers can save themselves the high costs of installing a Type B residual current circuit breaker, and provide all-current sensitivity and safety for persons and systems at low cost.
The differential current that occurs in solar inverters is the sum of the capacitive leakage current generated systematically by the PV modules and the ohmic residual current generated, for example, by faulty insulation of the PV system. Power analysis calculation in the inverter is used to differentiate between the currents.
If a fault occurs, DI sensors shut down the affected device when a pre-set limit is reached, before defects such as insulation faults can become hazardous to life and limb or electrical defects are sufficient to cause fires. Different applications require different sensor types. In future, charging stations for electric vehicles will require protection under IEC 62752, where shut-off must occur at 6 mA DC to protect the in-house Type A residual current circuit breaker and at 30 mA AC to prevent harm to persons. Solar inverters and frequency converters must shut off at 30 mA (protection of persons) and 300 mA (fire protection).
A versatile range of uses and low prices
Today, DI sensors are used in transformerless solar inverters but are equally useful for frequency converters such as for pumps or electric drives, and in electric vehicle charging stations.
The broad range of possible applications and outstanding characteristics of VAC DI sensors ensure that VAC is the ideal partner for its customers. In fact, standards for solar inverters and electric vehicle charging stations actually contain requirements for DI sensors. To fulfil these requirements, sensors need to be powerful, low-cost and compact. To satisfy new standards including IEC 62752, the automotive industry also requires universal DI sensors for charging electric vehicles to avoid hazardous situations if a vehicle battery (DC) is connected to a home power supply (DC).
All VAC current sensors are optimized for fast, low-cost assembly combined with the highest product and production quality. They have been produced in their millions at the VAC plant in Horna Streda (Slovakian Republic) and the Shenyang plant (China) for many years.
Overview: Comparison with other measurement systems
The advantages of VAC’s DI sensors have already been described. But what other proven current measurement processes are available, and what are the advantages and disadvantages of these methods?
Shunt
A shunt system requires two (single-phase system) or four (three-phase system) identical resistors in each phase and in the neutral conductor, as resistance values must be identical to ensure comparison is correct. While this would be possible in principle, it requires carefully selected high-precision resistors for each measurement system, resulting in high costs.
Hall effect sensor
Open-loop sensors with Hall probes are intrinsically less precise than the related closed-loop sensors, and are thus unsuitable for use as DI sensors. Closed-loop sensors with Hall probes are currently not supplied as DI sensors owing to the lower precision of the Hall probe, particularly with respect to temperature and long-term drift.
For more information on VAC current sensors, visit the company website at www.vacuumschmelze.de
For general information on the topic of residual current detection see the Wikipedia article on “Residual-current device”.
About the author:
Dipl-Ing. (FH) Susanne Ganz, Product Manager Current Sensors at Vacuumschmelze GmbH, Hanau (Germany).
Susanne Ganz holds a graduate in Energy and Automation Technology from Gießen-Friedberg University of Applied Sciences (today Technische Hochschule Mittelhessen – University of Applied Sciences, THM). Before she joined Vacuumschmelze, she held positions at Heraeus Holding GmbH (Head of Production), SAMSON AG (Customer Training), VAC Product Marketing (Product Manager Current Sensors).
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