From the June 2017 issue
The Strongest Link in the Chain
Inverters, solar energy, storage systems, and grid connections
Renewable energy—and solar energy in particular—has a problem. Because it is subject to the vagaries of sunshine and wind for power generation, it is often not available when it is needed most. For those nighttime hours when the sun is not available and cloudy days when sunlight is minimal, provisions have to be made to compensate for the lack of energy production.
This can be done either by tying the solar energy power system into an existing electrical grid that provides a steady and continuous baseline of electrical power, or excess energy created during a period of bright sunshine that can be stored for later use. A chain has to be forged, linking the sun to power generation (concentrated solar heat, photovoltaic cells, or Stirling engines) to intermediaries (energy storage systems or base line grid) to users.
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It’s the connection between the power generation system and the intermediaries that is critical. Rooftop photovoltaic solar energy systems generate direct current (DC) electricity. Appliances, households, businesses, and industry use alternating current (AC). The device that converts DC produced by photovoltaic cell arrays into useable AC is called an inverter. These devices need to be the strongest link in the renewable energy supply and storage chain in order for the system to work properly. In the case of solar energy storage system, inverters take the DC generated by the batteries that have been charged by the solar cells and converts it to AC. In the case of grid ties and islanded distributed energy systems, inverters convert the current to AC for sale back to the grid.
But this is not just a matter of “plug and play.” There are multiple issues associated with grid and battery inverters. Connectivity to the grid requires an inverter whose AC output matches the frequency and voltage of the grid receiving its electrical power. Significant dangers and even potential damage to the local grid can result if the grid experiences a power outage and the renewable energy source does not safely isolate (aka “island”) itself from the downed grid. To ensure that this does not happen the inverters need to be able to read the grid and cut off its AC output as needed. If the distributed energy source also recharges electrical storage batteries, inverters will be needed to convert the DC power discharged by the battery into useable AC. This operation combined with the ability to detect interruptions in grid power supply is essential for those systems that utilize batteries as emergency backup power sources. Which raises the question as to why inverters connecting distributed energy source, electrical storage batteries, and the primary power grid are necessary in the first place.
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Solar Energy Production, Energy Storage Requirements, and Selling the Electricity Back to the Grid
Getting back to the need for matching solar power production with demand, it should be noted that demand is something of a moving target as it can vary considerably over the course of a normal workday. A graph of available solar energy supply more or less traces the arc of the Sun across the sky in the form of a parabola. The terminating ends of the parabola match the time of sunrise and sunset for each day when solar power generating potential is zero. It peaks at noon each day at the top of the parabolic arc before it descends again to nightfall. The height and width of this curve will vary with the seasons and even varies slightly from day to day. Climate and weather changes, especially cloud formation and cloud cover, can greatly degrade the availability of solar supply.
Electrical power demand, on the other hand, remains roughly constant from one work day to the next. Except for small alterations in lighting demand that come with seasonal variations for nighttime duration, and with the heating and variable cooling requirements that change with the season, electrical power demand is mostly a function of local economic activity and population density. Economically, energy demand from agriculture and industry are mostly constant with small seasonal variations. However, additional power demands made by residential occupancy and commercial activities can vary hourly during a work day, making total demand fluctuate somewhat. As a result, total demand peaks around 2 p.m. in the afternoon to coincide with commercial activity and then peaks again around 7 p.m. in the evening to coincide with residential use. This five-hour period from 2 p.m. to 7 p.m. is usually referred to as the “extended peak” period. Demand then declines until the early morning hours but never completely falls to zero.
The graphing of the daily and local solar energy curve with the work day demand curve provides an illustration of how much storage is required for non-supply hours. The peak of the supply curve usually occurs around noon, prior to the extended peak of demand. This excess production of solar energy is what needs to be stored for later use in the early morning and early evening (when potential demand exceeds potential supply) and for late night hours when supply is not available at all. Or this excess power production can be sold back to the grid to reduce the production loads of the central power generation system. To do either requires inverters.
Credit: iStock/MATJAZ SLANIC
How Inverters Work and How Their Performance is Evaluated
Whether directly generated from the solar cell array or discharged from a battery that stored energy from the array, DC gets transformed into AC by way of an inverter. An inverter is like a switch that can pulse to DC and reverse its flow 50 to 60 times per second, depending on the frequency of the AC output. It is essentially a switching unit connected to an electricity transformer. A transformer transforms AC current from low voltage to high voltage via primary and secondary coils of wire wrapped around a jointly shared iron core. Inverters also use electromagnetic switches that switch back and forth the direction of the current.
To smooth out the square waves created by this sudden binary switching back and forth, inverters utilize inductors and capacitors to make the output current rise and fall more gradually so as to match the smooth sine wave of AC current. The square or rectangular wave forms are produced by low-quality inverters. Better quality inverters produce trapezoidal wave AC (quasi-sine waves). These medium-quality inverters utilize electronic components such as thyristors and diodes to achieve this smoothing, and are relatively inexpensive. Additionally, pulse width modulators (PWI), digital power controllers, are used to create desired AC sine wave. Intermediate trapezoidal waves actually transmit more power and can be subject to overheating. The best quality inverters produce true sine wave AC (pure sine waves). These more expensive inverters use donut-shaped (toroid) transformer cores.
Connecting inverters in parallel results in higher power, while connecting them in series yields higher voltage. An inverter’s operating power coincides with its voltage. For example a 100-watt inverter will function at 12 to 48 volts. As a rule of thumb, inverter efficiency increases with power output. At low current, inverter efficiency can fall below 50% but exceed 90% at high power yields. Since most inverters are designed for high-current conditions, typical efficacies for inverters connecting DC generated by solar photovoltaic arrays is normally around 95%.
To measure these outputs and provide a means of rating inverter performance, Sandia National Laboratories has developed a testing program which has been adopted and standardized by the California Energy Commission (CEC). It is now the primary means of rating inverters for use in PV to grid power systems. Under this protocol, inverter efficiency is measured at six levels relative to the inverter’s rated AC power output (10%, 20%, 30%, 50%, 75%, and 100%). Each is then assigned a weighting factor (0.04, 0.05, 0.12, 0.21, 0.53, and 0.5, respectively). This is repeated for three DC power levels. Multiplying each power level by its weighted factor, and then adding up the results for each of the six power levels gives the inverter’s weighted average efficiency. This value is used for planning and designing of renewable DC to grid AC power systems.
Isolated, independent, off-grid systems utilize stand alone inverters that transform the DC generated by storage batteries located onsite into AC. The batteries are charged daily by DC energy generated by the solar cells. By contrast, in solar power systems that interact with the local utility, grid-tied inverters send power directly from the solar array as AC with the exact voltage and frequency utilized by the local power grid. In short, grid-tied inverters must be synchronized to interface with a utility line. Hybrid, or bimodal/bi-directional, inverters give the operator the flexibility to either operate as a standalone power source or as a grid-tied system. With such a system, an owner does not lose power along with the rest of the grid if there is a power outage, but also has the flexibility to sell electrical current back to the utility, unlike the stand alone operator. In doing so they must avoid the potentially dangerous situation of islanding.
Coupled with energy storage systems like this one from Tesla, SolarEdge inverters offer energy independence.
Anti-Islanding, Intentional Islanding, and Smart Islanding Inverters
One of the most important issues in grid inverter interface design is the issue of islanding. This refers to what happens to the local recipients of power from a renewable energy source when electric power from the local grid fails. Under these conditions, unless prevented from doing so, the local distributed energy source will continue to provide electrical power for its customers. In the case of a solar panel array, it will continue to provide power so long as the sun is shining sufficiently. It, in effect, creates a local “island” of electrical power.
While this is good for the solar energy system’s customer, who continues to have the lights on while surrounded by neighbors left in the dark, it can be extremely dangerous for utility line workers trying to restore power who don’t realize that the power line is still charged. In general, this can complicate the power restoration operation. This issue will only become more serious as solar energy and other renewables obtain a larger share of the electrical power market and are integrated into exiting grid systems. If not addressed, islanding can threaten the stability of entire grid systems.
The inverters’ response to this situation can take the form of anti-islanding, intentional islanding, or smart islanding. Anti-islanding involves the deliberate stopping of power production from a grid-tied renewable power system whenever a general blackout occurs in the grid it is attached to. When configured for intentional islanding, the local distributed energy system ceases to supply the local grid while continuing to provide power to its primary customers. In effect, the solar panel array serves more as a backup power system to its building.
But how does the local renewable energy system “know” that the larger grid has failed? It’s not a simple thing to positively identify the loss of power in a grid without recording a significant number of false positives. To a human, the loss of power is obvious—the lights won’t go on. To the system however, it is difficult to tell the difference between normal fluctuations in grid service and voltage and actual grid failure. This is not a case in which the inverter can wait for a significant down time period to ensure that the grid is off. The whole point of islanding is to ensure instant cutting off of the renewable energy source from the rest of the grid.
The opposite problem, frequent cut off of power supply to the larger grid, would result in lost revenue to the distributed energy system owner loss of power supply from the main grid.
A situation in which an inverter cannot immediately detect the loss of power from the main grid is referred to as a non-detect zone (NDZ). Research continues on methods to reduce the opportunity for an NDZ to occur. But most current technology relies on passively detecting and reading losses in voltage from the failed grid. Which brings us the research and development of “smart inverters.” This research is being led by the department of Energy (DOE) and its Solar Energy Grid Integration System (SEGIS) program. The smart inverters combined with balance of system (BOS) elements developed by SEGIS are essential to allow expansion of grid-tied PV distributed energy sources.
AC Coupling and Battery Storage
When distributed energy systems are not tied to the grid, they rely upon battery storage of excess energy to provide balanced power supply throughout the day. In grid-tied systems, these batteries can also be charged by current from the grid. Both processes recharge the batteries by reversing the current backwards through the battery. When called upon to produce energy, batteries produce DC which must pass though a battery inverter to be transformed into useable AC. Like islanding, AC coupling occurs when the main power grid goes down. However, instead of shutting down the renewable energy system or cutting it off from the main grid, AC coupling starts the energy flowing for the battery. It is what triggers the operation of an electric storage battery as a backup power supply.
The biggest concern in recharging storage batteries is over-charging. The main function of battery inverters is to closely and precisely regulate the AC voltage of the battery’s discharge no matter what the battery’s current charge and voltage levels are. Adjustments are continuous to ensure that the battery’s resultant AC current maintains a standard 240-volt AC output. The batteries themselves can vary from 40 volts for traditional lead-acid batteries to over 400 volts for contemporary lithium-ion batteries.
The difficulty lies in the need for the excess power generated by the solar array to charge the battery without causing the AC output of the battery’s inverter to significantly fluctuate. The connection itself is especially complicated in grid-tied systems with the AC power output from the solar arrays inverter running parallel to the AC discharge from the battery and the excess solar energy not sent to the grid being used to recharge the battery during daylight hours. So in addition to potentially causing fluctuations in the battery discharge voltage, the simultaneous recharge of the battery needs to be closely regulated to prevent over-charging.
To prevent over-charging, a circuit breaker is often used. This circuit breaker is located between the grid-tied inverter and the battery inverter. If the battery accumulates excessive current or voltage, the circuit breaker trips and prevents further charging. This system is effective and safely prevents over-charging but can cause excessive and repetitive interruptions in grid-tied inverter operations. To avoid this, some systems utilize frequency slews. A slew occurs when the rate at which the delivered power a distributed energy source rises or falls during a given time period. Battery inverters that utilize this mechanism deliberately create a slew condition when the sense excessive voltage or current, dropping the input frequency and off lining the grid-tied inverter.
Engineering Design Basics of Grid-Tied Solar Power System Inverters
No two distributed energy systems are the same. Massive utility and commercial solar arrays can provide orders of magnitude more energy than residential rooftop systems. Local climate and latitude will affect energy production rates, while anticipated utilization will drive storage requirements. Each installation has to be almost custom-designed for its particular location and application in order to develop a proper “distributed architecture” for the system. There are multiple factors that need to be considered for the final design of the system and the type of inverter required to achieve optimized grid tie in:
- Location of the solar array. These include latitude, climate and weather, local shade, roof top orientation, angle, and available deployment area. The effective size of the solar array is a function of its available flat area and its angle of orientation to the sun.
- Operational characteristics of the grid-tied inverters, as defined by their continuous AC power output. “Continuous” is defined by the National Electrical Code as maximum output for more than three hours. So despite the potentially intermittent or distributed energy system output, all inverters associated with a grid-tied system should be rated for continuous operation.
- Operational characteristics of the solar array modules, including operating specs and rated power output. The characteristics of the grid-tied inverter will determine the solar array’s maximum power output. So the inverter’s output must match the supply of power from the solar array. If the generated power exceeds the inverter’s output capacity, the waste energy will be expressed as waste heat. This waste heat could potentially pose a safety issue and will certainly degrade the inverter’s operation and reduce its operating lifetime. Since solar cells usually operate at about 80% of rated capacity, most manufacturers allow a solar array power rating of up to 125% of the inverter’s continuous power rating. This rule of thumb may not apply to cold but sunny climates that can result in above normal solar array output (keeping solar cells cold either in cold or floating on a body of water can increase their overall efficiency by effectively dealing with waste heat).
In any case, the cost of a grid-tied inverter or battery inverter will probably be much lower than the total cost of the completed solar energy system. As a potentially small line item in the overall budget, there is no reason why designers and installers can’t go for the high-quality inverter for inclusion into the system design.
Chilicon Power Inversion Systems
Chilicon is a manufacturer of high-quality inverters. Their grid-tied products include advanced features such as AC coupling, demand response, output optimization, voltage and power output control, and a self-power supply mode. Their CP250E and CP-100 Cortex Gateway inverters include power factor control (varying from minus 80% to positive 80%), volt-watt control which allows the inverters to automatically back-off production in order to limit grid voltage rise, and current transformer (CT) controlled production modes controlling micro-inverter array energy production.
The CP-100 Gateway is capable of monitoring any Chilicon Power microinverter, assisting with the array setup and creation of the installation for Cloud monitoring purposes. And, it can do a lot more on the home automation front by monitoring home power consumption and generation. Thanks to its wireless connectivity, it is also able to serve as a basic home alarm security controller.
The Chilicon Power CP-100 Cortex Gateway is able to read information from up to 10 CT clamps via a 908MHz wireless link. This information is then “actionable” in real time with the micro inverters via broad case power line communication. The Cortex then continually monitors the current entering and leaving the battery inverter. If the difference between these two currents exceeds a programmable threshold, for example 5 Amps, then micro inverter production is reduced. If the current is less than the threshold, then micro-inverter production is increased.
SolarEdge’s StorEdge is an on-grid, DC-coupled storage solution that manages and monitors both solar generation from PV arrays and electrical energy storage in batteries. This solution is designed for residential use and allows homeowners to optimize self-consumption, while its backup solution provides backup power in the event of grid interruption to power pre-selected loads. Simultaneously, the StorEdge solution optimizes self-consumption and can meet requirements, such as limitations to exporting power back to the grid, offering demand response and peak shaving, and performing time-of-use shifting to reduce electric bills. By combining SolarEdge’s breakthrough PV inverter technology with leading battery storage systems, such as the Tesla Powerwall Home Battery, the StorEdge solution maximizes energy independence from the grid. The system can be configured so that the PV and battery powers important loads such as the refrigerator, TV, lights, and AC outlets. The StorEdge system can be programmed to operate according to different charge and discharge profiles, also referred to as Time of Use (TOU) arbitrage. By increasing energy consumption when electric demand from the grid is low (off-peak tariffs) and lowering consumption when demand is high (peak tariffs), household electricity bills can be reduced.
The StorEdge solution includes power optimizers to increase rooftop energy harvest and requires no additional conversions from AC to DC and back to AC. Its simple design and installation allows for outdoor installation for flexibility in locating the battery, and needs no special wires, just the same photovoltaic cables utilized by the array. It is set up to allow for remote access to the software that operates the inverter and battery. This allows system owners to continuously monitor battery status, PV production, and self-consumption data, battery energy levels, determine remaining hours of backup power, and allows remote troubleshooting. To ensure safe operation the PV array and battery voltage are reduced to a safe voltage upon AC shut down when not in backup mode.