DC Link Requirements for Grid Tie Inverters</h1> <article> <h1>DC Link Requirements for Grid Tie Inverters</h1> <p>Dave Zendzian — Tue, 24 Jan 2012 17:11:00 GMT</p> <p style="text-align: left;">A common question we field regarding Grid-Tie inverters goes something like "Can I interface to a 480V grid with a 680V DC Link?". To answer this question, lets consider a typical Grid Tie Inverter or Active Front End application as illustrated in the figure below.</p> <p></p> <p style="text-align: left;">Generally, our application code uses center-aligned, space vector pulse width modulation (SVM) techniques for maximum DC Link voltage utilization. Using SVM, the switch-mode AC voltage output of the inverter, before the filter components, is limited by the dead-time of the power stage and any minimum pulse requirements of the IGBT drivers. For example, 3usec of dead-time when switching at 10kHz, results in a maximum achievable duty-cyle of 97%. Depending on theswitching frequency, these hardware limitations will set the maximum, peak, line to line output voltage to ~95% to 97% of the DC Link. Note however, that this is the point at which the duty-cycle is clamped, and the controls become non-linear. In practice, you will want to allow several percent duty-cycle for control headroom in order to provide the controller linear operating range. As such, 92-94% is a more practical design value. </p> <p style="text-align: left;">The previous discussion addresses the maximum obtainable voltage at the output of the IGBTs. Considering a GTI or AFE converter can source current into the grid, one also has to allow for voltage drops across the filter components at maximum output current. Let's consider a typical 100kW Grid Tie Inverter interfacing to a 480V AC line. At 10% high line, the maximum peak line voltage is 480*1.1*SQRT(2) = 747Vpk. Assumming 30Vpk is dropped across the filter components, the required minimum DC Link voltage is given by (747+30)/0.92 = 845V.</p> </article> <article> <h1>Common Mode Filtering Considerations for Grid Tie Inverters</h1> <p>John O'Connor — Thu, 12 Jan 2012 19:40:00 GMT</p> <p>In non-isolated, grid-tie inverter applications, it is common practice to connect the neutral point of the 3-phase AC grid to earth ground. Unfortunately, this creates a problem for DC link common mode filtering. Typical control methods utilize space vector modulation to control the power stage, which introduces a common mode voltage in order to maximize the DC link voltage utilization. When you connect a capacitor between DC link and neutral, the space vector common mode voltage is impressed across it. Unless the capacitor is very small, it will adversely effect operation, often leading to control loop instabilities. In addition, this capacitor will tend to resonate with the grid connect filter, causing further behavioral problems.</p> <p>As such, implementing a common mode filter for these converters requires a slightly different approach in order for the system to operate correctly. First, if you are trying to control EMI, you should be able to get away with fairly small capacitors. Very low ESL is generally much more important than capacitance value. Use a number of small caps (in the range of 0.01uF) in parallel to drive ESL to an insignificant value. Even though you grid filter is inductive, the large components tend to have a lot of parasitic capacitance. Inserting a common mode inductance by passing the three AC lines through one or more large ferrite toroids will be much more effective at mitigating high frequency noise than the large inductors, and will reduce the EMI capacitor peak currents, and the resulting resonant frequency. Keep in mind that you are still going to see mid-frequency oscillations with this configuration, but the higher frequency EMI (several MHZ+) will be much lower.</p> <p>You may also want to include a similar common mode inductor on the source side to allow common mode filtering of the supply lines. You may actually find that it’s OK for the DC link to have a moderate common mode voltage with respect to earth ground as long as you are able to attenuate most of the common mode signal from the AC and source lines.</p> </article> <article> <h1>Effects of Cable Length on SCR Drive Waveforms</h1> <p>Dave Zendzian — Thu, 06 Oct 2011 13:36:00 GMT</p> <p>A general rule of thumb when driving SCRs is to use as short a cable as possible in order to provide the most efficient turn-on characteristics. As cable length increases, two important parameters are degraded: inductance and series resistance. </p> <p>In order to minimize inductance, gate drive cables should be constructed in a manner which tightly couples, preferably with a twisted pair, the gate and cathode connections. However, regradless of the construction technique, increased cable length results in increased inductance, which in turn slows the rise time, di/dt, of the gate drive current pulses. The second concern when lengthening cables is the assocated increase in series resistance which reduces the peak current, I<sub>GM</sub>, delivered to the SCR gate. </p> <p>The magnitude of the gate drive current I<sub>GM</sub>, as well as it's di/dt rise time, directly effect key SCR performance characteristics, including:</p> <ul> <li>Turn-on Delay</li> <li>Turn-on fall time of teh anode voltage</li> <li>Turn-on switching losses</li> <li>di/dt of the anode current</li> </ul> <p>High I<sub>GM</sub>, typically >2Amps, and rise times of > 2Amp/sec help ensure reliable and efficient triggering of SCR power devices. We recently tested OZSCR1000 and OZSCR1100 gate drive performance using a variety of cables and cable lengths. The following figure illustrates the gate drive current pulse waveform when driven with a 13.5"cable, (500mA/div, 2usec/div). The second figure illustrates the current pulse waveform when driven with a 150" cable, (500mA/div, 2usec/div). From these scope plots, the effects of both inductance and resistance are obvious by the increased rise time and reduced current level in the second plot. Please reference Oztek <a href="http://www.oztekcorp.com/Default.aspx?app=LeadgenDownload&shortpath=white+papers%2fAN-0001+Effects+of+Cable+Length+on+SCR+Drive+Waveform.pdf" title="Application note AN-0001" target="_self">Application note AN-0001</a> for more detailed test results.</p> <p></p> <p></p> <p> </p> <br /> </article> <article> <h1>Grid Energy Storage Is a Growth Industry to Watch</h1> <p>John O'Connor — Wed, 21 Sep 2011 00:56:00 GMT</p> The market for grid energy storage is robust and growing. According to a study published by <a href="http://www.greentechmedia.com/research/report/grid-scale-energy-storage-technologies-and-forecasts-through-2015" title="GTM Research" target="_blank">GTM Research</a>, the U.S. market for utility-scale grid storage is expected to rise from an annual value of about a quarter of a billion dollars in 2009 to about $2.5 billion by 2015. <br /><br />The potential exists for continued rapid growth in this market in the United States and worldwide. Demand will be filled by companies pursuing a variety of technologies to fill the needs of the power grid.<br /><br /><strong>What is Grid Energy Storage?</strong><br /><br />In an ideal energy world, demand for electric power would be matched exactly by electric power supplied, with no need for backup power sources or storage anywhere in the grid. But, in the real world, demand and supply are often mismatched. <br /><br />Demand for electric power varies by time of day and changes with every season. Power supply varies based on many factors, including plant downtime, changes in fuel supply and prices, and the intermittent nature of renewable energy sources such as wind and solar.<br /> <br />The imbalance between demand and supply is evened out by various technologies that store energy and make it available to the grid as it is needed. With the rise of the "<a href="http://www.oztekcorp.com/blog/bid/61296/What-is-Smart-Grid-Technology" title="smart grid" target="_self">smart grid</a>" and the increasing importance of renewable energy sources, the demand for grid energy storage and the complex technologies that supply it is large and growing.<br /><br /><strong>Sources of Grid Energy Storage</strong><br /><br />There are numerous technologies already available or being developed for supplying reliable and economic grid storage. Batteries and pumped hydro systems are among the most mature grid storage technologies. <br /><br />Batteries can store excess electricity from wind and solar plants for distribution to the grid when it is dark or the wind stops blowing. One strategy on the drawing board is to use electric cars as sources of grid energy while they are plugged in at night for recharging. <br /><br />Pumped hydro systems use excess generating capacity during off-peak hours to pump water up hill for later release through turbines to generate electricity during peak demand periods.<br /><br />Other technologies at varying stages of research and commercialization include flywheels, compressed air storage, super-conducting magnets and ultra-capacitors.<br /><br /><strong>Interfacing with the Grid</strong><br /><br />With each of these energy storage technologies comes the need to be able to store and return the energy to and from the grid as needed. <a href="http://www.oztekcorp.com/products/inverters/" title="Grid Tie Inverters" target="_self">Grid Tie Inverters</a> (GTI) or <a href="http://www.oztekcorp.com/products/inverters/grid-tie-inverter-afe-controler/" title="Active Front End" target="_self">Active Front End</a> (AFE) power converters are typically used to provide this function. Advanced digital control solutions such as the <a href="http://www.oztekcorp.com/products/inverters/grid-tie-inverter-afe-controler/" title="OZGTI3000" target="_self">OZGTI3000</a> can be used to implement custom OEM solutions for this power conversion application.<br /><br />Each grid storage technology has costs and benefits. The grid of the future will use a combination of several of them for maintaining the crucial balance between supply and demand. <br /><br /> <p>Photo by <a href="http://www.flickr.com/photos/kevinrigdon/5632580211/sizes/m/in/photostream/" target="_blank">kevin rigdon</a>.</p> </article> <article> <h1>What is Smart Grid Technology?</h1> <p>Dave Zendzian — Tue, 26 Jul 2011 18:09:00 GMT</p> <p>Demand for electricity is expected to continue growing at historical rates for the foreseeable future. While utility companies strive to meet that demand by financing and constructing new power-generating plants, fueled by coal, atomic energy, wind, sunlight, and water, they’re also scrambling to create a “smart grid.”</p> <p>The essence of the smart grid technology is using information technologies to better manage the generation, flow, and most important, the consumption of electricity over the course of each day and each season. By monitoring and managing the flow of current at a much finer scale than is possible today - metering its usage down to the level of individual office buildings and homes and even specific light sockets and appliances - it should be possible, experts say, to better match supply and demand on a minute-by-minute basis and thus conserve electricity and save everyone significant money.</p> <p>As it is, the nation’s hundreds of electrical power-generating facilities pour their electricity into a vast network of long-distance and local power lines - the electrical grid. Much like water in a set of tanks connected by pipes, this electricity sloshes around within this grid and the grid’s managers do their best to move current to wherever demand is highest - to the time zone where it’s morning, for instance, and millions of people are switching on lights, showering, shaving, cooking breakfast, and starting their clothes washers. </p> <p>Unfortunately, all the grid’s managers can do is increase supply as best they can. They have no control over demand. They cannot, for instance, charge Mrs. Jones more for running her washing machine at breakfast time rather than in the middle of the night, when overall demand for electricity is particularly low. Instead, the power companies always have to be ready to supply power to every Mrs. Jones at any moment of the day. This has meant constantly building more and bigger generating stations. </p> <p>Smart grid technology would change this scenario by enabling market signals to be collected, distributed, and interpreted in more or less real-time. As demand rose in a particular area - due to unexpectedly hot weather, for instance - residential and office air-conditioners might be remotely adjusted by the power company to run a bit warmer in return for a break in the price of electricity. </p> <p>Ultimately, by providing more control over the consumption of electricity, the use of smart grid technology could save the U.S. billions of dollars.</p> <p>Photo by <a href="http://www.flickr.com/photos/goingwimax/4052976069/sizes/s/in/photostream/" target="_blank">Going Wimax</a>.</p> </article> <article> <h1>Wind Power Projection Dependent On Improved Energy Storage Technology</h1> <p>John O'Connor — Wed, 20 Jul 2011 18:59:00 GMT</p> Long believed by many to be one of the "purer" sources of alternative energy, any wind power projection has noted the rapid growth in the industry during recent years. The World Wind Energy Association calculates that electricity from wind accounted for 2.5% of worldwide consumption in 2010 at 430 TWh, and that this output doubled what was available only three years earlier. Many European nations, such as Spain and Denmark have begun intensive investments in wind energy and Germany is expected to be a major player following the forecasted shutdown of the nation's nuclear industry. Outside of Europe, China has aggressively pushed wind energy in Asia and currently has the world's largest installed wind power capacity at almost 42,000 MW. <br /><span></span><br />Though many continue to advocate for wind energy as an alternative to a dependence on fossil fuels, numerous challenges still lie in the way of wind's widespread acceptance as an energy source. Since wind is fortunately quite benign, especially when compared to the negative potential of nuclear power, safety hazards do not amount to much. However, due to its fickle nature, wind faces an substantial economic challenge wherever it is installed. <br /><br />Even though there are a number of ideal "wind farm" sites worldwide, wind is always somewhat intermittent and possibly unreliable. There is no guarantee that strong winds will blow during times of peak energy consumption, and since storage capacity (e.g. batteries) are very expensive and relatively primitive energy storage is a challenge. This forces turbine operators to dump excess energy into the power grid, often at times when the price for electricity is very low. This in turn makes the income from wind power unreliable and discourages investment in the industry, even though the technology has been proven and is now produced on a rapidly expanding scale. However, wind power projections estimate yearly growth worldwide to be 28% annually (World Wind Energy Association), and this coupled with improvements in storage technology could make wind a vital component of the global communities future energy solution. </article> <article> <h1>Alternative Energy Distribution Methods</h1> <p>Dave Zendzian — Tue, 12 Jul 2011 15:11:00 GMT</p> <p>The term alternative energy generally describes renewable sources such as solar, wind, and hydroelectric power. Hydroelectric energy accounts for the majority of U.S. renewable energy, at 70 percent, though the use of solar, wind and other renewable energy is continuing to grow. Methods of alternative energy distribution vary, and here I’ll provide a brief description of the three most prevalent methods. It’s important to keep in mind that digitally controlled power systems will help prevent wasted energy, no matter which distribution system is used. </p> <p><br /><strong>Large, Centralized Facilities </strong></p> <div>Currently, most of the alternative energy generated in industrial countries comes from large, centralized facilities. These plants can produce high volumes of electricity and transmit it long distances. “Over the last 20 years, the cost of electricity from utility-scale wind systems has dropped by more than 80%," according to the American Wind Energy Association. Large-scale solar power plants, such as the Long Island Solar Farm currently under construction, will be able to produce 32 megawatts of power when complete—enough to power 4,500 homes. And leading the way is hydroelectric power, a non-polluting energy source that makes up about 19% the world's energy production with the largest plants in China, Brazil, Canada and the United States.</div> <p><strong>Localized Alternative Energy </strong></p> <div>Keeping alternative energy distribution local is another option that's had successful results. In Germany, 80% of their solar energy comes from rooftop solar panels. Advances in available technology have made DIY solar setups more accessible to the general public, as have tax incentives for businesses and individuals. Small wind turbines also provide a more localized distribution and have low maintenance and pollution. Some organizations, such as Slippery Rock University in Pennsylvania, utilize a combination of alternative energy distribution systems that include wind and solar power. </div> <p><br /><strong>Microgrids </strong></p> <div>A microgrid normally connects to a centralized grid (macrogrid), but is a smaller, localized grouping of electricity generation. It is able to disconnect from the macrogrid and function autonomously, and usually is interconnected at low voltage. Fuel cells, wind, and solar energy can all be run off of a microgrid. Because it can isolate from the larger network, microgrids offer reliable power. </div> <p>Photo by <a href="http://www.flickr.com/photos/freshandeasy/3480673788/sizes/s/in/photostream/" target="_blank">Fresh & Easy Neighborhood Market</a>.</p> </article> <article> <h1>Utility-Scale Flywheel Energy Storage & Grid Tie Inverters</h1> <p>John O'Connor — Tue, 07 Jun 2011 19:20:00 GMT</p> <p>A few days ago <a href="http://phx.corporate-ir.net/phoenix.zhtml?c=123367&p=irol-newsArticle&ID=1569408&highlight">Beacon Power</a> announced that their installation of the world’s first 20MW Flywheel plant is currently operating at 18MW, and is expected to be running at full, 20MW capacity before the end of the month. We’re pretty excited about this at Oztek, as we provide the controls for both the <a href="http://www.oztekcorp.com/products/inverters/grid-tie-inverter-afe-controler/">grid tie inverter</a> that interfaces with the grid, and the <a href="http://www.oztekcorp.com/products/motor-controls/ozmtr3000-motor-controller">sensorless permanent magnet motor drive </a> used for the flywheel in Beacon's system. This program posed many design challenges, and it’s always extremely satisfying to see things finally come together, particularly on something as large and complex as this. </p> <p></p> <p style="text-align: center;"><strong>World's Largest Flywheel Plant in Stephentown, NY</strong></p> <p>This exciting news was picked up by quite a few sites, and as with any emerging technology, response seems to be as much confusion as it is appreciation and interest in what it offers. The Beacon system is not a power generating plant. Rather, it is a frequency regulation plant that stabilizes the grid by providing power during periods of high demand, and storing energy during periods of low demand. The need for this stems from the fact that power plants cannot rapidly throttle power output to respond to sudden changes in demand. Elaborate systems have been developed over the years to deal with this that, by their very nature, require a net surplus of power generating and distribution capacity to be built in. However, these systems still place demands on power generation plants that cause additional wear and tear, and greater production of CO<sub>2</sub> and other emissions than if the plants were allowed to operate under more steady state conditions. The Beacon system is clean and very efficient, wasting only a small percentage of the energy it processes, and it’s able to respond to large demand changes in seconds.</p> <p>For those interested in more detail, here’s an interesting article <a href="http://seekingalpha.com/article/107832-alternative-energy-storage-why-frequency-regulation-is-important">Why Frequency Regulation is Important</a>, as well as a more comprehensive white paper on the subject <a href="http://www.ornl.gov/~webworks/cppr/y2001/rpt/122302.pdf">Frequency Regulation Basics - Oak Ridge NL</a> .</p> </article> <article> <h1>MODBUS SCR Controller</h1> <p>Dave Zendzian — Mon, 23 May 2011 17:49:00 GMT</p> <p>Conventional SCR controllers typically use simple analog and discrete digital signals to control and supervise operation of the product, whether it be an SCR controller for heaters, an SCR controlled rectifier, or an SCR motor control. However, as customers expect more and more out of our products, this simple user interface limits our ability to provide value add in the products we design.</p> <p>The OZSCR1000 <a title="SCR Controller" href="http://www.oztekcorp.com/products/scr-controllers/" target="_self">SCR Controller</a> overcomes this limitation by providing a MODBUS serial interface in addition to the conventional control interfaces. While the MODBUS interface provides a simple means to configure the product, it also opens the door to advanced features. Using MODBUS, the user can:</p> <ul> <li>Implement sophisticated architectures employing multiple control boards</li> <li>Monitor system performance using fault and instrumentation data</li> <li> Provide improved maintenance & trouble shooting using detailed fault and performance data</li> </ul> <p>MODBUS is one of the most popular industrial communication protocols in use today. It's simple, inexpensive, and easy to use. MODBUS was developed in 1979 by Modicon (now Schneider Electric) for use in communicating with multiple devices over a single pair of twisted wires. While the original implementation operated over an RS-232 hardware layer, today's MODBUS devices run over virtually all communications media, including wireless, fiber, ethernet, etc. Today, MODBUS-IDA (<a title="www.MODBUS.org" href="http://www.modbus.org/" target="_self">www.MODBUS.org</a>), the largest organized group of MODBUS users and vendors, continues to support the MODBUS protocol worldwide.</p> <p>The <a title="OZSCR1000 " href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">OZSCR1000 </a>implements the MODBUS RTU protocol over an RS-485, multi-drop, serial interface, allowing many devices to operate on a single pair of twisted wire. The following figure illustrates a typical system with a PLC serving as the MODBUS master controlling several slave devices:</p> <p></p> <br /> <div class="mcePaste" id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"></div> </article> <article> <h1>Power Factor and Grid Tie Inverters</h1> <p>John O'Connor — Fri, 29 Apr 2011 19:35:00 GMT</p> <p>One reason engineers specify active rectifiers (a.k.a. active front-ends) for their systems is that they can operate with near unity power factor. Being nearly the same system (see my recent post "<a title="Active Front-end or Grid-tie Inverter?" href="http://www.oztekcorp.com/blog/bid/38348/Active-Front-end-or-Grid-tie-Inverter" target="_self">Active Front-end or Grid Tie Inverter?</a>"), grid tie inverters share this same beneficial characteristic. However, this does not mean that an active rectifier or grid tie inverter <em>must</em> operate with near unity power factor, and in fact, we can use this to our advantage in certain applications. This also implies that we may not want to specify power factor as a means of quantifying how well the system minimizes AC line current harmonics.</p> <p>Let’s back up for a moment and first consider what we mean by <em>Power Factor</em>. The purest definition is that power factor is the ratio of measured power to apparent power; that is PF = W/VA. However, you may be more familiar with a couple of other equations that address two very different classes of applications. PF = VA*COS(phi), where phi is the phase angle between voltage and current, is valid for linear loads that draw sinusoidal current and have negligible harmonic distortion. This equation is well know by those working with AC induction motors, and until the last 20 or so years, is the one most used by electrical engineers. </p> <p></p> <p>Typical inductive (AC motor) current and voltage</p> <p>More recently, PF = 1/SQRT(1+THD<sup>2</sup>), has come into common use, as it addresses nonlinear rectifier loads used in a wide range of electronic equipment. This equation ignores phase shift, however, the error this introduces is small in typical cases where THD dominates.</p> <p></p> <p>Typical 3-phase current and voltage with bridge rectifier and LC filter</p> <p>So how does this relate to power factor and <a title="grid-tie inverters" href="http://www.oztekcorp.com/products/inverters/ozgti3000-grid-tie-afe-controller/" target="_self">grid tie inverters</a>? Consider first how the grid tie inverter’s power stage and modulator are configured. Most circuits (<a title="OZGTI3000" href="http://www.oztekcorp.com/products/inverters/ozgti3000-grid-tie-afe-controller/" target="_blank">OZGTI3000</a> and others used by Oztek) are designed to inherently produce sinusoidal voltages. This means that even without control loop intervention, to a first order, the inverter emulates a linear system. We can then vary the AC voltage amplitude and phase angle in relation to the line voltage to produce currents with any phase relationship to the AC line that we want. In fact, it may be obvious at this point that the resulting currents are simply a result of the differential voltage impressed across the grid interface filter impedance. </p> <p>With a properly designed filter, the differential voltage is relatively small in comparison the AC line voltage, making the precision afforded by closed loop control necessary for a practical implementation. Closed loop control also helps compensate for non-ideal power stage behavior, and we determine how well this is all working by measuring THD. Phase angle can actually be set to any practical angle. If you want to move real power, it has to be in the vicinity of zero degrees, but it can be leading or lagging. This can be used to advantage in larger installations to offset the lagging power factor produced by induction motor loads, or to compensate for transformer leakage and distribution inductances. </p> <p>Typical end-systems usually require compliance with more complex distortion specifications like IEEE 519, which specify current harmonic limits in absolute terms, place limits and both individual harmonics and the total, and limit even harmonics to much lower levels than odd harmonics. In specifying grid tie inverters and <a title="active front-ends" href="http://www.oztekcorp.com/products/inverters/ozgti3000-grid-tie-afe-controller/" target="_self">active front-ends</a>, you should therefore be primarily concerned with how well current harmonic distortion is controlled and whether current phase angle is readily adjustable, not power factor.</p> </article> <article> <h1>SCR Driver Board Firing Pulses</h1> <p>Jay Goodell — Mon, 18 Apr 2011 19:52:00 GMT</p> <p>The newly introduced OZSCR1000 <a title="SCR firing board" href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">SCR firing board</a> is capable of controlling a wide range of commercially available SCRs. SCRs operate similarly to diodes, with current flowing in only one direction through the device. In both SCRs and diodes, current cannot pass through the device until the anode is at a higher potential than the cathode, or what is referred to as forward bias. SCRs add additional control by delaying the flow of current until a control signal is also provided to enable a forward biased device. Current will continue to flow in an <a title="SCR" href="http://www.oztekcorp.com/products/scr-controllers/" target="_self">SCR</a> until the device is reverse biased. Once an SCR turns off, it will not conduct current again until it has forward bias applied and the control signal is applied.</p> <p>The signal required to control an SCR is a current applied between the SCR's gate terminal and cathode. Since the cathode connection of an SCR is often connected to the output of a power system, the voltage at this terminal can vary by hundreds or thousands of volts with respect to other circuitry in the system. This often necessitates high voltage isolation between the SCR signals and other circuitry. The Oztek SCR driver board is designed for continuous operation with voltages up to 1000Vac. Signal transformers provide a reliable and cost effective way to isolate the control signals.</p> <p>A wide variety of loads can be connected to SCR based power stages. These loads can vary from purely resistive to purely inductive, with unlimited combinations in between. A resistive load will draw current directly in phase with the applied voltage. The current in an inductive load can lag the voltage by up to 90°. In a 60Hz system, this variation causes a window of longer than 4mS when the control signal must be present to guarantee the SCR will turn on. A large transformer would be required to sustain a control signal of this duration.</p> <p>To keep the size of the SCR driver board small and to allow several SCR firing circuits to be included on a single board, smaller pulse transformers are used. Instead of providing a single long firing pulse to turn the SCR on, several shorter pulses are output in rapid succession. When viewed on an oscilloscope this pulse train resembles a "picket fence."</p> <p></p> <p>A larger magnitude pulse is produced at the beginning of each firing sequence. This larger pulse helps to ensure a complete turn on of the SCR, minimizing stress on the device and reducing losses. The smaller sustaining pulses are sufficient to ensure the SCR stays in full conduction under light or discontinuous loads. Due to phase lag in systems with inductive loading, it is possible for the initial large magnitude pulse to occur while the SCR is still reverse biased. If this occurs, the sustaining pulses will turn on the SCR immediately after the device becomes forward biased. Under these conditions the losses in the SCR are still minimized because the current at turn on is just beginning to ramp up.</p> <p>The OZSCR1000 <a title="SCR firing and control board" href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">SCR firing and control board</a> provides several user configurable parameters to control all aspects of the firing pulses. Their duration, quantity, and frequency can all be tuned to optimize system performance. The default configuration should be adequate in many systems as most commercially available devices have been designed to operate with similar control signals.</p> </article> <article> <h1>Controlling Power with SCR: Phase Angle vs Zero Crossing Mode</h1> <p>Dave Zendzian — Wed, 30 Mar 2011 22:31:00 GMT</p> <p>When controlling power with SCR based designs, engineers are faced with the decision of what type of control mode to use; zero-crossing or phase angle control. In order to make the right decision the design engineer must understand the advantages and disadvantages of both control modes as well as the attributes of his or her load.</p> <p>When operating in SCR phase angle control mode, the user can directly set the point on the AC voltage waveform at which the SCR's will be switched on which in turn varies the power.<span> </span>By advancing or delaying the turn-on point or "phase angle", the user can regulate the power delivered to the load, as illustrated in the figure below. Phase angle control of SCR's has the advantage of being able to control to very fine resolution while it's disadvantages include higher harmonic distortion and RFI when compared to zero-crossing techniques. It is best suited for fast responding loads such as lamps, as well as transformer coupled or inductive loads.</p> <p class="MsoBodyText" style="margin-right: 0in;"></p> <p class="MsoBodyText" style="margin-right: 0in;">Zero-crossing control mode (also called fast cycling, integral cycle, or burst firing) operates by turning the SCR's on only when the instantaneous value of the sinusoidal voltage is zero, as illustrated in the figure below. It's advantages include very low harmonics and RFI as well as higher reliability because the SCR is turned on at zero voltage and current.</p> <p class="MsoBodyText" style="margin-right: 0in;"></p> <p>Zero crossing control mode is ideally suited for SCR temperature controllers designed for purely resistive loads that can accommodate fast, full power on/off cycling.</p> <p>In general, almost all applications of zero crossing control mode involve SCR controller for heaters, including dryers, kilns, ovens, and environmental chambers.</p> <p class="MsoBodyText" style="margin-right: 0in;">Oztek's <a title="OZSCR1000" href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">OZSCR1000</a> Digital SCR Driver Board can be easily configured for either phase angle or zero-crossing control. Both phase angle as well as zero-crossing cycle count can be controlled via an analog interface or directly using the Modbus interface.</p> <div class="mcePaste" id="_mcePaste" style="position: absolute; left: -10000px; top: 86px; width: 1px; height: 1px; overflow: hidden;"></div> </article> <article> <h1>SCR Control Goes Digital</h1> <p>Dave Zendzian — Fri, 25 Feb 2011 17:13:00 GMT</p> <p>SCR Control circuits and products have been around for decades. An industry standard SCR controller typically consists of an SCR firing circuit, a phase locked loop (PLL) or line synchronization circuit, and a means of controlling the firing angle of the SCR device, typically a 0-5V or 4-20mA analog input. While there are currently several industry standard SCR control products available that provide these basic analog control functions, Oztek's OZSCR1000 is described as a <a title="digitally controlled SCR firing and control board" href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">digitally controlled SCR firing and control board</a>. So what can a "Digital" SCR control product possibly bring to the party?</p> <p></p> <p>Digital control means different things to different people. The <a title="OZSCR1000" href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">OZSCR1000</a> provides several facets of digital SCR control, including digital line synchronization, control law implementation, and product configuration. At it's heart, an SCR controller must provide reliable synchronization to the input line voltage. On the <a title="OZSCR1000" href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">OZSCR1000</a>, line voltage inputs are low pass filtered and immediately converted into digital signals by A/D converters. From that point foward the phase detector and PLL algorithm are implemented digitally, using a combination of microprocessor and high speed logic resources. This approach ensures reliable, jitter free operation when compared to similar analog circuit techniques.</p> <p>Digital control law implementation implies that the closed-loop feedback control algorithms are also implemented within the microcontroller, as opposed to analog circuitry such as op-amps, etc. The <a title="OZSCR1000 SCR control functions" href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">OZSCR1000 SCR control functions</a> are implemented as Proportional plus Integral (PI) algorithms with built in anti-windup protection. The digital nature of the controllers inherently eliminate the effects of temperature, component tolerances, and large signal analog effects. However, since they are user configurable via the serial interface, they also simplify and speed-up initial system design. When developing and testing their controller, the engineer no longer needs to use his or her soldering iron to change their control loop compensation, they simply reconfigure the digital loop gains. </p> <p>While the <a title="OZSCR1000" href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">OZSCR1000</a> can be configured to accept standard analog voltage or 4-20mA current command inputs, the addition of the Mod-bus based serial port makes it possible to implement a complete digital control approach. When properly configured, the serial interface can be used to send either open loop phase commands or closed loop voltage or current commands.</p> <p>Digital product configuration can save the system manufacturer significant time and money. The <a title="OZSCR1000" href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">OZSCR1000</a> is 100% software configurable over a standard serial port. For example, the controller can be configured for open loop control or closed loop control; voltage, current, or serial interface command; phase angle or zero crossing control, all by the simple push of a button. There is no longer a need to swap components to "tune" or configure a circuit, no longer a need to stock multiple variants of the same board to handle diferent end product configurations, and no longer a need to mix and match different boards to realize your control configuration. All of the circuitry is provided on the <a title="OZSCR1000" href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">OZSCR1000</a>, it simply requires a one-time configuration step using the Oztek supplied configuration tool.</p> </article> <article> <h1>Active Front-end or Grid Tie Inverter?</h1> <p>John O'Connor — Thu, 27 Jan 2011 22:58:00 GMT</p> <p>We have the habit here at Oztek of using the term "active front-end" (AFE) and grid tie inverter (GTI) interchangeably. On more than one occasion discussing our <a title="grid-tie inverter controller" href="http://www.oztekcorp.com/products/inverters/ozgti3000-grid-tie-afe-controller/" target="_blank">grid tie inverter controller</a>, I’ve caught myself somewhere mid-conversation inadvertently switch to using the term active front-end. It's of no concern internally, but quite frequently its cause for confusion when speaking with customers. Earlier this week I received a call from a customer that was pretty certain an AFE would not work in his GTI application. This got me thinking, "What <em>exactly</em> is the difference between an active front-end and a grid tie inverter?" </p> <p>At Oztek, we loosely define AFEs as inverters that develop a regulated DC link, and GTI’s as inverters that transfer power from a DC link to an AC line, but this is really only a generalization. I see the real difference being the application, not the inverter itself. The application may dictate agency approval requirements or current harmonic requirements. It may dictate how power is allowed to flow, or how the system is to respond under certain fault conditions, but even with these and other application differences, nowhere will you find a clear, differentiating factor in the inverter itself. </p> <p>Both GTIs and AFEs (also known as active rectifiers) actively convert between AC and DC power. Common topologies are fully capable of transferring power in both directions. Limiting power flow to one direction, as is typically required for grid tie inverter applications, is ensured through control, and is not an inherent inverter characteristic. Both GTIs and AFEs must present a high impedance to the AC line, since the voltage is determined by other sources. This is the key difference from voltage source inverters, which are designed to have a low source impedance for driving AC loads. While not the only implementation method, we employ current loop control to provide a suitably high AC impedance at the line frequency. A voltage source inverter, on the other hand, will employ voltage feedback. This affects not only the control loop, but also the output filter design.</p> <p>A simple Active Rectifier definition is “a non-isolated AC-DC converter with two key benefits over passive rectifier systems; output voltage regulation, and AC input harmonic reduction.” The term “Active Front-end” describes the same thing, while eliminating the term “rectifier”, which I think incorrectly implies a unidirectional converter. Rather than “output voltage”, I like to use the term “DC link voltage”, since as we’ve noted, the converter is inherently bi-directional, and thus the DC side can be the output or input. You can see where this is going. Call it what you like, an active front-end or active rectifier has the ability to transfer power from the DC link to the AC line, which is essentilly the same function that a grid tie inverter performs. </p> <p>It seems then, that DC link regulation is really the key differentiating factor. Grid tie inverters don’t need to regulate their DC link. Rather, their function is to simply transfer power from the DC link to the AC line, but is that always the case? Feeding a grid tie inverter directly from a solar panel output may be a typical configuration, but it's not the only one. Often better system efficiency can be achieved by regulating the source with a DC-DC converter (think of the variability in a solar panel or wind turbine output), allowing the inverter to operate under much more tightly defined conditions. A common implementation is to configure the DC-DC converter to regulate its output voltage. This still has the grid tie inverter operating from a voltage source, but this is not always the best option. </p> <p>We must also consider maximum power point tracking (MPPT) for many systems, and with a two-stage system, this is often best implimented in the DC-DC converter. In the purest sense, this leads to a DC-DC converter with a varying current output, not output voltage, but we can’t leave the DC link voltage uncontrolled. One solution is to add an additional voltage regulation loop, but by itself, this interferes with MPPT. An alternative solution is to put DC link voltage regulation back into the grid tie inverter, turning it into an AFE from the nomenclature point of view. In effect, this results in the grid tie inverter outputting as much power to the AC line as the DC-DC converter will support – exactly the goal of the system. And the difference between a GTI and an AFE in this case? Almost nothing.</p> </article> <article> <h1>Wireless Electric Vehicle Charging at CES 2011</h1> <p>Jon Young — Fri, 07 Jan 2011 14:34:00 GMT</p> <p>This week Las Vegas is hosting the <a title="2011 International Consumer Electronics Show" href="http://www.cesweb.org/" target="_blank">2011 International Consumer Electronics Show</a>, a trade show where hundreds of consumer oriented companies annouce their new products. While I'm not there, I am paying close attention to all of the news coverage.</p> <p>One interesting product that has popped up is from <a title="Fulton Innovation's eCoupled" href="http://ecoupled.com/" target="_blank">Fulton Innovation's eCoupled</a> product line. Their demo in Las Vegas includes wirelessly charging a Tesla Roadster. I believe that wireless charging of Electric Vehicles will greatly increase their adoption in the US. If I can pull into my garage every day, and let the car charge on its own, I'd be much more likely to make the switch to Electric Vehicles. Nothing to plug in - just park and walk away.</p> <p>Electric vehicles require power conversion systems for charging the batteries and converting stored energy in the battery to drive the motor. Oztek <a title="DC/DC products" href="http://www.oztekcorp.com/products/oem-solutions/dc-dc-converters/" target="_self">DC/DC products</a> and <a title="Active Front Ends" href="http://www.oztekcorp.com/products/inverters/ozgti3000-grid-tie-afe-controller/" target="_self">Active Front Ends</a> have been used for battery charging systems. While our <a title="Motor Drive" href="http://www.oztekcorp.com/products/oem-solutions/motor-drives/" target="_self">Motor Drive</a> units have been used for a range of Electric Vehicle applications.</p> <p>In my eyes, battery charging is the biggest block in wide spread adoption of Electric Vehicles. With a conventional automobile, consumers are accostemed to a 5 minute fill up at the gas station. Ecoupled envisions charging "stations" as just a parking space. No need for dedicated charging operations. No more stopping to "fill up". </p> <p>[Read the eCoupled Press Release <a title="Here" href="http://ecoupled.com/content/fulton-innovation-raising-bar-wireless-power-new-applications-truly-wireless-world-2011-ces" target="_blank">Here</a>. ]</p> <p>[Original story from <a title="engagdget" href="http://www.engadget.com/2011/01/06/fulton-innovation-blows-our-minds-with-ecoupled-wireless-tesla/" target="_blank">engagdget</a>.]</p> </article> <article> <h1>Renewable Energy Tax Incentives Extended</h1> <p>Jon Young — Thu, 23 Dec 2010 15:08:00 GMT</p> <p>President Obama signed the Tax Relief, Unemployment Insurance Reauthorization, and Job Creation Act of 2010 on Friday, December 17th, 2010. This will extend the Section 1603 Treasury Grant Program.</p> <p><a title="Altamont Pass Wind Turbines by footloosiety, on Flickr" href="http://www.flickr.com/photos/footloosiety/2758316525/"></a></p> <em>[Image licensed under Flickr Creative Commons and provided by user <a title="footloosiety" href="http://www.flickr.com/photos/footloosiety/" target="_blank">footloosiety</a>]</em> <p>The Grant Program was created as part of the American Recovery and Reinvestment Act of 2009. The program can cover up to 30% of the cost of Alternative Energy projects, such as wind or solar power installations. The program is credited with creating thousands of jobs and generating billions of dollars of investments.</p> <p>The extension allows firms to apply for the grant for projects that are started or put in service in 2011. The previous grants only covered projects started in 2009 or 2010. Section 1603 is not related to the Home Owner's tax credit for energy efficieny improvements (Section 25C).</p> <p>Oztek's <a title="OZDSP2000" href="http://www.oztekcorp.com/products/dsp-control-boards/ozdsp2000-fixed-point-inverter-controller/" target="_self">OZDSP2000</a> and <a title="OZDSP3000" href="http://www.oztekcorp.com/products/dsp-control-boards/ozdsp3000-floating-point-inverter-controller/" target="_self">OZDSP3000</a> are used by various customers in both the Wind and Solar industries. Oztek has also provided an OEM solution for a <a title="Solar DC/DC Boost Converter" href="http://www.oztekcorp.com/products/oem-solutions/dc-dc-converters/" target="_self">Solar DC/DC Boost Converter</a>.</p> <p>The extension of 1603 should lead to more project launches in 2011.</p> </article> <article> <h1>Hurling Pumpkins at the World Championship Punkin Chunkin</h1> <p>Jon Young — Thu, 11 Nov 2010 14:24:00 GMT</p> <param name="allowFullScreen" value="true" /> <param name="allowscriptaccess" value="always" /> <param name="src" value="http://www.youtube.com/v/sJUloNUwlRQ?fs=1&hl=en_US&rel=0&hd=1" /> <param name="allowfullscreen" value="true" /> </embed> </object> <div></div> <div> <p>Oztek recently sponsored the Launch-Ness Monster at the <a title="World Championship Punkin Chunkin" href="http://www.punkinchunkin.com/" target="_blank">World Championship Punkin Chunkin</a> in Bridgeville, Delaware. Launch-Ness is captained by Jay Goodell, a Design Engineer at Oztek. I myself helped build the Monster.</p> </div> <div></div> <div></div> <div></div> <div> <p>Punkin Chunkin is a compeition where the goal is to throw a pumpkin further than everyone else. Launch-Ness monster competed in the trebuchet division, finishing 7th with throws of 1097.0 and 1265.86. The video above is of the third shot, which was disqualified due to machine malfunction. The sling that holds the pumpkin seperated from the machine, flying forward into the field.</p> <p>Launch-Ness Monster will be featured on TV in the <a title="Science Channel's Punkin Chunkin Special" href="http://science.discovery.com/tv/punkin-chunkin/" target="_blank">Science Channel's Punkin Chunkin Special</a>. The special will air Thanksgiving Night (November 25th), at 8 PM.</p> </div> <div></div> <div></div> <div></div> <div></div> <div></div> <div> <p>Over on the<a title=" Launch-Ness Monster blog" href="http://www.launchnessmonster.com/" target="_blank"> Launch-Ness Monster blog</a>, Jay is already laying out plans for improving the machine for the 2011 "Chunk". Check out the blog to see what our Engineers do in their free time!</p> </div> <div></div> </article> <article> <h1>Porting FreeMODBUS for the SCR1000 Control Board</h1> <p>Jon Young — Thu, 28 Oct 2010 14:43:00 GMT</p> <p>For our <a title="SCR1000 " href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">SCR1000 </a>board, we needed to implement a Modbus stack for communicating with the board over RS-485. We have previously ported the <a title="FreeMODBUS" href="http://freemodbus.berlios.de/" target="_blank">FreeMODBUS</a> stack to the Texas Instruments 28335 for use on the <a title="OZDSP3000 " href="http://www.oztekcorp.com/products/dsp-control-boards/ozdsp3000/" target="_self">OZDSP3000 </a>board, so again we selected FreeMODBUS for use on the Stellaris part that controls the <a title="SCR1000" href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">SCR1000</a>. </p> <p>The <a title="SCR1000 " href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">SCR1000 </a>only requires the use of the RTU mode of Modbus, so the port should have been simple. However, the interrupts differ from that of the 28335, so I needed to make some simple modifications. For receive data, I had to modify the ISR handler to remove all of the data from the UART RX FIFO, instead of just a single byte:</p> <pre style="padding-left: 30px;">if ( status & (UART_INT_RX | UART_INT_RT) )<br />{</pre> <pre style="padding-left: 30px;"> while ( MAP_UARTCHarsAvail (UART0_BASE))</pre> <pre style="padding-left: 30px;"> pxMBFrameCBByteReceived ();</pre> <pre style="padding-left: 30px;"> .....</pre> <p>That got the receives working in short order. Next up was sending a response. The TX interrupt sources are limited to different "full" levels of the TRX FIFO. The setup of the FreeMODBUS library assumes an empty interrupt. Luckily, we are using Rev C3 silicon of the Stellaris part, which provides access to the ETO - end of transmission interrupt. Enabling this interrupt, which disables the FIFO interrupts, and modifying <em>eMBRTUSend()</em> solved the problem:</p> <pre style="padding-left: 30px;">/* Activate the transmitter */</pre> <pre style="padding-left: 30px;">eSndState = STATE_TX_XMIT;</pre> <pre style="padding-left: 30px;">vMBPortSerialEnable ( FALSE, TRUE );</pre> <pre style="padding-left: 30px;">pxMBFrameCBTransmiterEmpty (); //This was added by Oztek</pre> <p>This will kick of the transfer of the first byte, and then the library is free to continue operating as expected. The EOT interrupt will call the ISR handler, which in turn calls <em>pxMBFrameCBTransmitterEmpty()</em> until all of the bytes have been sent.</p> <p>We use the calls to <em>vMBPortSerialEnable(,)</em> to handle setting the direction of the enable bit for the RS485 transceiver on the <a title="SCR1000" href="http://www.oztekcorp.com/products/scr-controllers/ozscr1000/" target="_self">SCR1000</a>.</p> </article> </main></body></html>