28 SMART GRID www.ebg-resistors.com
Issue 5 2017 Power Electronics Europe www.power-mag.com
Smart Sensors for GridModernization Grid modernization initiatives are gaining momentum around the globe. Upgraded energy grids enablethe integration of conventional energy sources with renewables and energy storage, as well as creatingresiliency in the face of cyber-attacks and climate challenges. Low-inductance thick-film resistors play avital role as sensing elements in these applications. R. Ratzi, Miba Energy Holding Austria; A. Kleinand T. Zimmerman, EBG Austria and USA
Modernizing the grid is an opportunityfor many players across the value chain. Amajor growth area is in metering andmonitoring smart grid applications, whichin the long run smooth out the powerflowing to and from the grid, and cut wasteby enabling optimal generation. If we knowhow much power is being produced, thenless power needs to be produced. Todayprecise measurements can be taken at thepoint of transmission and distribution, butto accomplish this, smart grid sensors areneeded.
Huge opportunities for sensorsThere are a multitude of metering /monitoring points that are being upgradedwith smart grid optimization, from point ofuse to transmission and distribution, bothusing the same types of technology just ondifferent scales. In regards to transmissionand distribution, in the past, veryrudimentary tools were used to test andmonitor the grid. Flashing lights, forexample, were used on Fault CurrentIndicators, repair personnel had to bedispatched in multiple areas to find onefault by visual confirmation. Then theindustry moved to wireless data locators.
These types of locators would give aposition of the fault, in return wouldreduce the time and efforts in locating thefault thus a faster repair. Today precisemeasurements can be taken at the pointof transmission and distribution, but toaccomplish this, smart grid sensors areneeded.
Market researcher IHS tracked SmartGrid Sensors as a stand-alone category inthe Distribution Automation (DA) marketand predicted that by 2021, demand fornew Smart Grid Sensor technology willgrow 1,200 %, making it one of the fastestgrowing segments in DA. It will surpass itsearliest predecessor, non-communicatingFault Current Indicator (FCI) sensortechnology.
Another important application of voltagesensors are intelligent transformer
substations. There are used in the mediumvoltage secondary distribution grid. This isa key application which allows the grid tocounterbalance voltage variations causede.g. by local PV systems or fast chargingstations for electrical vehicles.
The voltage divider With this opportunity comes a challenge.The grid typically operates at a highvoltage, but measurement equipment suchas sensors should not exposed to this highvoltage as it could pose both a reliabilityand a safety issue. So a voltage divider isused to reduce the voltage to a muchlower level. If, for example, the grid is at5,000 volts, you might use a 100:1 divider,so the sensor only sees 50 V maximum.
Voltage dividers can be made withcapacitors (a capacitive divider) or withresistors. It is in the voltage dividers thataccuracy becomes important. If a capacitorhas a tolerance of +/- 2 %, and it used asa component in the divider, at the highend that 2 % error could give 51 V, or atthe other end 49 V. Either way, with a 2 %error like this the grid voltage is between4900 and 5100 V, because the divider isno more accurate than this. A second andeven more important factor is thermalstability. Over a typical temperature rangeof 0 to 50C, for example, a capacitor maydrift by 0,5 %. On a cold day, the voltageis going to measure a different value thanon a hot day. This is not because the gridvoltage is any different, but because of theinaccuracy of the sensor due to thermaldrifts.
Viewing it mathematically, if a resistorhas a thermal coefficient of 50 parts permillion (ppm) over a 50 K range that is2500 ppm is an accuracy of2500/1000000 = 0,25 %. This makes thesensor with a resistive divider is twice asaccurate over temperature as a capacitivedivider. The more accurate the real-timedata is the more efficient and cost effectivethe grid can become. This is a prerequisitefor functionalities like intelligent power grid
with active load management or intelligenttransformer substations, which areintegrated in an overall smart grid powersystem management by remote signalingand control.
Thick-film resistors EBG Resistors is a manufacturer of low-inductance thick-film resistors that can beused for high voltages, up to 90 kV on asingle resistor. Higher voltage loads arepossible if the resistor is potted. The EBGresistor product lines consist of anextensive variety of metal oxide productsmade with an exclusive METOXFILMformulation (see Figures 1 and 2).
METOXFILM or Thick Film resistors arethe main choice for the high voltage needsof smart grid sensors, which will be usedto monitor and measure transmission linesthat operate at voltages from 20 kV to 230kV (medium to high voltage grid) andeven higher. The other main types ofresistor technologies, such as wire wound,carbon film, bulk, etc., are not meant tohandle such high continuous voltages. EBGresistors have been used for years with themove to digital and controllable powermonitoring, where long-term stability andlow-to-no drift is a goal. The resistors usedfor metering applications are typically flatsand smaller cylindrical with tight tolerancesand tight temperature coefficients. Typicallyin these point-of-use metering locations,such as industrial and commercial smartmeters, the resistors have the same job assensing, as voltage dividers, dividing thevoltage to a usable and manageable levelfor all of the other internal circuitry.
In sensing applications, EBGs resistorsare used in sensors applied directly to thevoltage transmission lines, where itsnecessary for them to be able towithstand high voltage loads and maintaintight tolerances and low drift under load.In the past, these sensing applicationshave been performed with a capacitiveload. However, with more precisemeasurements required by power
www.ebg-resistors.com SMART GRID 29
www.power-mag.com Issue 5 2017 Power Electronics Europe
generation companies, the capacitive loaddivider can no longer be used andresistive loads are now required.
EBG can customize its high voltageresistors for exacting TCR and stabilityrequirements over wide operatingtemperature range. The company canproduce them with tolerances as low as+/-0.1 % and down to 15 ppm on flats -even lower (down to < 5 ppm) oncylindrical parts. Ongoing development aim
at even lower thermal coefficients intemperature ranges starting from -10 C to85 C. In addition, EBG can alsocustomize mechanical dimensions of flatsand cylindrical resistors as well asproducing with custom terminals andterminations with multiple types ofprotective coatings to fit the application.Therefore, these resistors are made toorder and typically highly customized.
Some of these applications are in other
areas that are linked to the Grid, such as;E-Mobility in fast charging stations and EVbusses, green power generation in Solarand wind, the medical market, (CT scans,MRIs and X-ray machines, to name a few).Theyre also used in motor drives, welders,power supplies, and instrumentation,within the industrial market where the EBGpower resistors absorb high energy pulseloads and protects the machinery fromfailing due to unexpected voltage spikes.
Figure 2: Ultra-stable SHP high-voltage resistorsFigure 1: MTX type high-voltage resistor (divider)
/// THE SMART CHOICE FOR SMART GRID SENSORS
Highest precision over a wide temperature range (overall stability