Electrical physicists from Czech Technical University have provided additional evidence that new current sensors introduce errors when assessing current through iron conductors. It’s crucial to correct this flaw in the new sensors so that operators of the electrical grid can correctly respond to threats to the system. The researchers show how a difference in a conductor’s magnetic permeability, the degree of material’s magnetization response in a magnetic field, affects the precision of new sensors.
With the addition of new renewable energy sources and smart homes that demand more information, the electrical grid is becoming more complex.
“If you have [a] grid at the edge of capacity, you have to be careful to monitor all the transients (power surges)," said Pavel Ripka, author of the paper.
Surges are overloads or failure to the system, which can be caused by something as simple as a broken power line or more dramatic events like lightning strikes or geomagnetic storms.
“Every day you get a lot of these small events (surges) within a big power grid, and sometimes it is difficult to interpret them," Ripka said. "If it is something really serious, you should switch off parts of the grid to prevent catastrophic damage, but if it's a short transient which will finish fast there is no need to disconnect the grid. It's a risky business to distinguish between these events, because if you underestimate the danger then parts of the distribution installations can be damaged causing serious blackouts. But if you overestimate and disconnect, it is a problem because connecting these grids back together is quite complicated."
To address the increasing complexity of the grid and power outage threats, there has been an increase in the use of ground current sensors in the past few years. New, yokeless current sensors are popular because of their low cost and compact size. The sensors are good for assessing currents in nonmagnetic conductors, like copper and aluminum. Ground conductors are usually iron due to mechanical strength -- which has a high magnetic permeability.
Using the new sensors to measure ground currents when iron is present is kind of like using a thermometer to assess if the heating needs to be switched on, not taking into account where the thermometer is placed. The thermometer’s reading can be different if it is placed next to a door or a window. In the same way, this study shows that not taking into account the magnetic permeability of a conductor distorts the accuracy of a reading with a yokeless sensor.
Ripka and his team matched experimental measurements with theoretical simulations to highlight the difference in yokeless sensor readings between nonmagnetic and magnetic conductors.
"We can show how to design (yokeless) current sensors so that they are not so susceptible to this type of error," Ripka said. "[This study is] just a small reminder to make [engineers] design sensors safely."
To prove this point further, Ripka’s group is starting to take long-term readings at power stations, comparing results to commercial uncalibrated sensors. In the future, Ripka envisions cooperating with geophysicists to correlate ground currents and geomagnetic activity to better understand how these currents are distributed within the earth and predict future disruptions to the grid.
A paper on this research was published in AIP Advances.