Distributed Energy Resource (DER) are defined as energy resources comprised of generation and/or storage and/or controllable load which is connected at the low or medium voltage distribution level. The term “DER” may indicate a single DER unit, but can also be a collection of DER units. This collection may also be called a DER plant or a DER facility. DER may be hierarchical, such that a DER plant may consist of multiple areas or buildings, each with multiple DER units.
In addition DER plants may act as microgrids. These microgrids may be permanently disconnected from the main grid, but in most cases are normally connected, but are designed to disconnect and operate autonomously if necessary.
DER Energy Management Systems (DERMS) are needed to manage DER. A DERMS may be used by a utility to manage DER within their territory, or a facility DERMS may operate the DER within a plant or facility, and/or may be used by an aggregator to manage the DER that they are responsible for.
Brief History of DER Functions
Until recently, Distributed Energy Resources (DER), if they existed at all, were just considered as negative load. If they were small enough (residential rooftop PV systems), they were essentially ignored by utilities, except to test that they did not export power during outages. No communications were expected, with the exception of metering so that energy flows in both directions might be measured. In some cases, customers were paid for any exported energy (net metering) or were otherwise compensated (bills for load were reduced but not made negative by any exported energy). In other cases, just the monthly net load was metered. Utilities generally felt that these small DER were a nuisance but not a safety or reliability threat.
Over time, the solar vendors that installed even the smaller PV systems started to add proprietary communications just for monitoring purposes, usually because they were benefiting financially from their installations and needed to know if these systems had developed any problems.
However, as more and larger DER (usually just PV systems) were installed, utilities began to worry that these uncontrolled and uncontrollable systems might impact their reliability by overloading circuits that were not designed to handle reverse flows of power and that the variability of solar generation could cause voltage or frequency problems. They started to push back on DER vendors by not allowing installations that might cause local voltage problems or potential reverse power flows at substations. Since the utilities only had 5-minute or 15-minute metering access (at most), they had inadequate visibility into their own power systems and were very concerned about what sizes of DERs and how many DERs they could or should allow to interconnect for safety and reliability reasons.
At the same time, pressure for meeting renewable goals was building and DER vendors were anxious to sell and install as much DER as they could. During sometimes rancorous meetings, utilities would vent on the dangers of DERs to the grid. But each time they described a problem, the vendors would push back, saying, “Our smart inverters can solve that problem if you would let us”. Soon the meetings turned more positive, and the utilities started saying, “Oh? You can? How? OK! Let’s do it!”. The devil was still in the details, but there was a growing vision of how increasing numbers of DER could be interconnected with the grid by defining the DER functional “grid codes” for their smart inverters.
Need for Communications – Enter IEC 61850 Data Models
In the US, a new DER standard was developed, IEEE 1547-2018, which defined both the functional requirements for the DER grid codes as well as the communication “interoperability” requirements. In Europe, similar requirements were also developed, eventually becoming EN 50549 standards. It became increasingly clear that these interoperable communication requirements necessitated the development of a standard for the information to be exchanged among the hundreds of utilities and millions of DER. IEC 61850-7-420 was the obvious choice for such a data object standard, and so, intense work on Edition 2 was started in 2017 and will published in early 2021.
The DER functions in IEC 61850-7-420 includes the following mandatory grid codes as well as market-based functions, although some are not complete, pending better definitions of the functional requirements.
DER Mandatory Grid Codes
Different jurisdictions have determined certain DER capabilities to be mandatory according to specific grid codes, including European network codes (EN 50549), IEEE 1547:2018 for North America, and the California Rule 21 grid codes. These grid codes include:
Disconnect / Connect: The DER is commanded to connect or to disconnect from the grid
Cease to Energize / Return to Service: “Cease to energize” is different from the Disconnect command in that it only requires that the DER not export active power and only export minimal reactive
power to the grid. This implies that the DER may continue to generate active power so long as there is adequate load within the facility to prevent any active power export.
High/Low Voltage Ride-Through: The DER “rides through” temporary voltage fluctuations by following the utility-specified voltage ride-through parameters to avoid tripping off unnecessarily.
High/Low Frequency Ride-Through: The DER “rides through” temporary fluctuations in frequency by following the utility-specified frequency ride-through parameters to avoid tripping off unnecessarily.
Dynamic Reactive Current Support: The DER reacts against rapid voltage changes (spikes and sags) to provide dynamic system stabilization, dV/dt.
Frequency-Watt Droop: The DER decreases (increases) active power as the grid frequency increases (decreases) from nominal frequency, according to the droop curve. This function focuses on returning the frequency to its nominal value.
Frequency-Watt Emergency (Frequency Sensitivity): The DER responds to frequency excursions beyond the normal frequency range by rapidly changing its production or consumption rate.
Volt-Watt Control: The DER responds to changes in the voltage at the Referenced ECP by changing its production or consumption rate.
Fixed (Constant) Power Factor: The DER’s power factor is set to a fixed value.
Fixed (Constant) Reactive Power: The DER provides a fixed amount of reactive power.
Volt-VAr Control: The DER responds to changes in voltage at the Referenced ECP by supplying or absorbing reactive power, according to a defined volt-VAr curve, in order to maintain the desired voltage level.
Watt-VAr Control: The DER responds to changes in active power at the Referenced ECP by changing its reactive power, according to a defined watt-VAr curve, in order to maintain the desired
Watt-PF Control: The DER responds to changes in power at the Referenced ECP by changing its power factor.
Limit Active Power : The DER limits the production and/or consumption of active power, based on export or import of active power at the Referenced ECP.
Set Active Power: The DER produces or consumes active power at the set level.
Low Frequency-Watt Fast Load Shedding for Demand Side Management: Loads are disconnected in stages when needed in response to low frequency situations.
Low Voltage-Watt Load Shedding for Demand Side Management: Loads are disconnected when needed in response to low voltage situations.
Monitoring key status, alarm, and measurement values: The DER provides nameplate, configuration, status, measurements, and other requested data.
Scheduling of Power Settings and Functions: The DER follows the schedule which consists of a start time and date, time offset from the start, and a duration for each power setting or function enable/disable.
Market-based DER Functions
Market-based DER functions are typically invoked for financial incentives or for contractual reasons. Some of the most important functions include:
Reduce (increase) Energy Demand based on Price: The DER reduces energy demand by either reducing load or by increasing generation (or vice versa).
Peak Power Limiting: The DER limits the load at the Referenced ECP after it exceeds a threshold target power level.
Load Following: The DER counteracts the load by a percentage at the Referenced ECP, after it starts to exceed a threshold target power level.
Generation Following: The consumption and/or production of the DER counteracts generation power at the Referenced ECP.
Dynamic Active Power Smoothing: The DER produces or absorbs active power in order to smooth the changes in the power level at the Referenced ECP.
Rate of Change of Power – dW/dt: Frequency-active power Primary Control operational function. The DER changes its watt output or input to provide frequency support to maintain frequency within normal limits.
Automatic Generation Control (AGC): The DER responds to raise and lower power level requests to provide frequency regulation support.
Operating Reserve (Spinning Reserve): The DER provides operating reserve for use by the grid if requested.
Synthetic or Artificial Inertia Frequency-Active Power: The DER responds to the rate of change of frequency (ROCOF) by changing its watt output or input to minimize spikes and sags.
Coordinated Charge/Discharge Management: The DER determines when and how fast to charge or discharge energy storage systems (including electric vehicles), so long as it meets its target state of charge level obligation by the specified time.
Frequency-Active Power Smoothing: The DER responds to changes in frequency at the Referenced ECP by changing its consumption or production rate based on frequency deviations from nominal, as a means for countering those frequency deviations Df/dt.
Power Factor Limiting (Correcting): The DER supplies or absorbs reactive power to hold the power factor at the Referenced ECP within the PF limit.
Delta Power Control: Decrease active power output to ensure there remains the necessary “spinning reserve” amount that was bid into the market.
Power Ramp Rate Control: The power increase and decrease is limited by specified maximum ramp rates.
Dynamic Volt-Watt: Dynamically absorb or produce additional watts in proportion to the instantaneous difference from a moving average of the measured voltage.
Microgrid Islanding Control: The microgrid plans for and carries out intentional Islanding as well as unintentional islanding.
Black Start Capability: The DER is capable of providing black start capabilities within its facility or microgrid.
Backup Power: The DER provides immediate backup power to a facility.