Feature Power Electronics
istribution grids have changed little over the past century. However, for the last several decades, electric-power use has increased. The International Energy Agency (IEC) estimates worldwide electricity demand rose from 5.1PWh to 17.9PWh (1015Wh) between 1973 and 2010.1
Challenges for the Grid D
David Andeen at Maxim Integrated investigates how the current trend towards smart monitoring is improving distribution-grid reliability
Upgrading the distribution infra- structure is expensive. According to the Edison Electric Institute (EEI), new overhead distribution lines cost from $86,700 to $1,000,000 (USD) per mile.2 Underground distribution lines cost more than three times as much. Utilities, naturally, are eager to squeeze as much use from existing distribution grids as possible. Customers pay for real power - the instantaneous product of the voltage and current waveforms. Non-resistive loads create current waveforms not in phase with the voltage waveform. Power factor is a measure of real power, a fraction of the sum of real and reactive power. Power factors as high as 0.9 accounts for losses of up to 11 percent in grid utilisation (Figure 1). Electronic devices present nonlinear loads, and they are primarily responsi- ble for electricity consumption growth. Nonlinear loads generate current-waveform harmonics, which degrade power factor and grid effi- ciency. Additionally, distributed gen- eration requires the distribution grid become a dynamic two-way system. Customer-based photovoltaic and small-scale wind generation pushes power back onto the distribution grid when power generation exceeds the customer’s immediate use. With the distribution grid’s historic operating model, utilities struggle to detect and respond to a variety of grid conditions, such as voltages at the grid edge from distributed generators. Electric-power utilities are under pressure from customers, industry organisations, and regulatory agencies
to minimise reliability events. Powerline monitors throughout the distribution grid can operate in concert with automated switchgear to quickly identify anomalous operating condi- tions, route around affected areas, and expedite problem resolution.4 Monitoring also provides valuable data from the distribution grid’s edge.
Figure 1:
A power factor less than 1.0 forces the utility to provide current in excess of the useful power it delivers. (Data from JAS Technical Media 2013)
In distribution systems, utilities drive three phases, each offset 120° from the others. Under balanced load conditions, 100 percent of the current flows through the phase connections. Current flowing through the neutral signifies an imbal- ance. An imbalance could indicate, for example, an emerging defect in a sec- ondary transformer’s insulation system. This enables a utility to repair the trans- former on a schedule, saving the costs of an emergency response and the cus- tomers’ inconvenience.
“Analogue and mixed-signal IC makers such as Maxim Integrated provide multi- channel ADCs well suited to these measurements”
The European Union’s IEC 62053 stan- dard for Class 0.2 equipment, typical for power monitors, calls for measure-
ment error ≤0.2 percent of the nominal voltage and current. Power-factor measurement requires phase matching
voltage and current samples to ≤ 0.1 percent. These specifications require closely matched digitisers. Analogue and mixed-signal IC makers such as Maxim Integrated provide multichannel ADCs well suited to these measurements. Features often allow groupings of up to 32 channels of simultaneous sampling.
Powerline monitoring at the secondary- transformer level is projected to exceed the traditional power-transformer market by 2015 (Figure 2).5
In powerline monitors, voltage and current transformers provide scaled rep- resentations of each phases’s voltage and current, respectively. The monitor’s ana- logue front-end (AFE) electronics buffer and measure the resulting signals.
Figure 2: Secondary- transformer monitoring is on track for rapid growth. Data and chart used with permission from GTM Research
Data feeds into a microcontroller or digital signal processor, which provides accurate calculation of power and power factor. Systems can locate capacitor banks to correct power factor and thereby improve overall grid utili- sation. Protection devices can use the power data for fast-acting automated breakers. Such systems can also use the same data stream to close the breaker, minimising the duration of the downstream power interruption. Maxim Integrated
www.maximintegrated.com Enter 206
David Andeen is Reference Design Manager at Maxim Integrated
1. Key World Energy Statistics 2012 OECD/IEA, 2012,
http://www.iea.org/publications/freepublications/publication/kwes.pdf, restated in PWh units by JAS Technical Media 2. Hall, Kenneth L., Out of Sight, Out of Mind 2012, An Updated Study on the Undergrounding Of Overhead Power Lines, Edison Electric Institute, January 2013 3. Data for Table 1 were generated from direct measurements. Data source is JAS Technical Media ©2013 4. Smart-Grid Technologies Can Reduce Power Outages, but Legislative Action Is Needed, National Electrical Manufacturers Association, July 2012,
http://www.nema.org/News/Pages/Smart-Grid-Technologies-Can-Reduce-Power-Outages,
-but-Legislative-Action-Is-Needed.aspx 5. Kellison, Ben, Transformer Monitoring Markets, 2013-2020: Technologies, Forecasts, and Leading Venders, greentechgrid, April 2013,
http://www.greentechmedia.com/articles/read/gtm-research-qa-transformer-monitoring-markets-part-i
20 DECEMBER/JANUARY 2014 Electronics
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