Giving Grid Sensors a Boost

Thursday, November 9, 2017 @ 08:11 AM gHale


New current sensors introduce errors when assessing current through iron conductors so it is important to correct the flaw in new sensors so grid operators can correctly respond to threats to the system.

There is 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, said electrical physicists from Czech Technical University. Finding the issue is one thing, they also are offering recommendations for improving sensor accuracy.

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With the addition of new renewable energy sources and smart homes demanding 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, an author of a paper on the subject.

Surges are overloads or failures to the system, which can end up 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,” he said. 

To address the increasing complexity of the grid and power outage threats, there has been an increase in use of ground current sensors. New yokeless current sensors are popular because of their low cost and compact size. These sensors are good for assessing currents in nonmagnetic conductors such as copper and aluminum. However, ground conductors are usually iron due to its mechanical strength, and iron has a high magnetic permeability. 

Using these new sensors to measure ground currents when iron is present is a bit like using a thermometer to assess if the heating needs to be switched on, not taking into account where exactly the thermometer is placed. Near a door or window, the thermometer’s reading can be affected differently than elsewhere. In the same way, this study has shown not taking the magnetic permeability of a conductor into account 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 further prove the point, 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 even predict future disruptions to the grid. 



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