The Distribution Management Systems (DMS) of Electric Power Systems (EPS) with a high penetration of Distributed Energy Resources (DER) will require near-real-time two-way information exchanges with advanced microgrids connected to the distribution system to adequately perform their advanced applications.
It is assumed here that an advanced microgrid, in addition to loads and distributed generation, may include capacitors, voltage and var regulators, remedial action schemes, demand response capabilities, and other controllable variables. It is also assumed that the advanced microgrid is controlled by a Microgrid Energy Management System (µEMS), which is a major actor interfacing with the DMS.
The near-real-time information exchanges between the DMS and the µEMS may include the following:
- Operational conditions for coordinating emergency actions in case of a bulk power system contingency.
Different contingency analyses of the EPS conducted by its EMS and DMS, as well as changes in the microgrid operational conditions, may result in different coordination of the preventive and corrective measures between the EPS and the microgrids. To implement the updated coordination actions in a timely manner, adequately timely information exchange between the DMS and the µEMS is needed.
- Operational conditions for volt/var control in connected mode under normal operating conditions.
In some cases, the DMS and µEMS objectives for volt/var control may be in conflict or may involve a cost for one party to meet the objective of another party. In these cases, timely information exchange between the DMS and the µEMS is needed.
- Updates of real and reactive net and actual load-to-voltage and generation-to-voltage dependencies aggregated at the microgrid Point of Common Coupling (PCC) [1]
The DMS needs to know the load-to-voltage dependencies at the microgrid PCCs under normal and emergency conditions to properly perform its advanced applications [2], such as EPS Situational Awareness, Volt/var/Watt Optimization (VVWO), Contingency Analysis, Service Restoration, Evaluation of VVWO benefits [3], etc. These dependencies are changing due to changes in numerous factors; e.g. the load-generation balance of the microgrids, the voltage at the DER terminals, the composition of sources of reactive power in the microgrids, the modes and settings of DER volt/var control, the voltage ride-through setups, etc. Therefore, the µEMS should perform corresponding analyses of the operations of the microgrid under different voltages at the PCC and inform the DMS about the near-real-time dependencies.
- Updates of capability curves of the microgrid's DERs aggregated at the PCC
The nominal capability curve of a DER is a three-dimensional dependency in the Watt, var, and Volt coordinates. The operational capability of a DER may be additionally limited by the operational voltage and/or current constraints of the distribution grid. All these variables may rapidly change in time. Hence, the aggregated capability curves also change and should be analyzed by the µEMS and submitted to the DMS in a timely manner.
- Updates of the aggregated amount of dispatchable real and reactive loads
The aggregated dispatchable load of a microgrid consists of the capability of the DER to change its real and/or reactive load within its capability curve, voltage impact on the load, available demand response, and other load management means. All these components change in time due to different internal and external factors. The µEMS should analyze the current amount of the dispatchable load and the conditions for its execution and submit the updates to the DMS in a timely manner. With this information, the DMS can effectively perform its advanced applications and load management actions.
- Updates of short-term look-ahead dependencies of the microgrid operational model on external conditions and time
The composite model of the microgrid operations aggregated at the PCC consists of a number of components. Some of them are listed above. All the components of the operational microgrid model may change due to daily time cycles, weather conditions, real-time prices, and other external factors. Some of the external factors can be delivered from the DMS to the µEMS and some can be obtained by the µEMS from external systems and/or from local sensors. The µEMS should assess the expected impact of the forecasted external factors on the components of the microgrid operational model and submit the updates of the model to the DMS in a timely manner.
- Updates on the operational conditions for restorative actions after an emergency situation
A timely information exchange of the current and the possible operating conditions of both the EPS and the microgrid is critical for a successful restoration of services after an emergency situation. The priorities of the restoration process depend on the readiness of the EPS for the restoration of load, on the need for black starts, on the load-generation balance in the microgrid which is ready to be reconnected, and on other factors. The relevant information should be determined accordingly by the DMS and by the µEMS and exchanged with each other.
Some illustrations of the contents of the information exchanges discussed above are presented below.
Some assumed operational conditions for the coordination of emergency actions in case of a bulk power system contingency are presented in Table 1. The examples are limited to one distribution feeder with one or more microgrids connected to the feeder and relate to the Under Frequency Load Shedding (UFLS) schemes of the EPS and the microgrids (µUFLS). The conditions differ in terms of the load-generation balance at the feeder head and at the microgrid PCC (PCC-netP), the relationships between the µUFLS and net power flow at the PCC of the microgrid (µG), and by the setups of the corresponding UFLS schemes.
Table 1. Sample operational conditions that may require near-real-time re-coordination of EPS and microgrid UFLS.
#
Feeder head Load-Gen balance
Micro-grid PCC Load-Gen balance
μUFLS vs PCC-netP
Feeder status
What EPS would prefer
What μG would prefer
1
Load-rich
Load-rich
μUFLS
Feeder is included in the UFLS of EPS and may disconnect
μUFLS is compliant with EPS requirements; μUFLS works before the UFLS of EPS for this feeder; if μG stays connected and the feeder is still connected, DERs ride through
If feeder disconnects, μG disconnects, additional load- shedding means are activated to balance the load with the generation; DERs ride through.
2
Load-rich
Load-rich
μUFLS
Feeder is not included in the UFLS of EPS and stays connected
μUFLS is compliant with EPS requirements; if the μG stays connected, DERs ride through
μUFLS is compliant with EPS requirements; μG stays connected; DERs ride through, if EPS provides frequency within ride-through ranges
3
Load-rich
Load-rich
μUFLS>PCC-netP
Feeder is included in the UFLS of EPS and may disconnect
μUFLS is compliant with EPS requirements; μUFLS works before the UFLS of EPS for this feeder; μG stays connected as long the feeder is connected, DERs ride through
If the μG disconnects before the μUFLS works, less load of the microgrid would be shed; if feeder disconnects before the μUFLS works, μG should also disconnect before μUFLS works, and the μUFLS balances the load with the generation in the island; DERs ride through.
4
Load-rich
Load-rich
μUFLS>PCC-netP
Feeder is not included in the UFLS of EPS and stays connected
μUFLS is compliant with EPS requirements; μG stays connected, DERs ride through
μG disconnects before the μUFLS works; μUFLS balances the load with the generation in the island; DERs ride through (less μG load is shed in island mode).
5
Load-rich
Gen-rich
N/A
Feeder is included in the UFLS of EPS and may disconnect
μUFLS is compliant with EPS requirements; μG stays connected as long the feeder is connected, DERs ride through
μG disconnects from EPS before the μUFLS works; or μG stays connected as long as feeder is connected, but the μUFLS is not activated and DERs ride through, if EPS provides frequency within ride-through ranges
6
Load-rich
Gen-rich
N/A
Feeder is not included in the UFLS of EPS and stays connected
μUFLS is compliant with EPS requirements; if μG stays connected, DERs ride through
μG disconnects from EPS before the μUFLS works; or μG stays connected, but the μUFLS is not activated; DERs ride through, if EPS provides frequency within ride-through ranges
7
Gen-rich
Load-rich
μUFLS
Feeder is not included in the UFLS of EPS and stays connected
μUFLS is compliant with EPS requirements; if μG stays connected, DERs ride through
μUFLS compliant with EPS requirements; μG stays connected; DERs ride through, if EPS provides frequency within ride-through ranges
8
Gen-rich
Load-rich
μUFLS>PCC-netP
Feeder is not included in the UFLS of EPS and stays connected
μUFLS is compliant with EPS requirements; if μG stays connected, DERs ride through
μG disconnects from EPS before the μUFLS works or μG stays connected, if μG agrees to provide additional load-shedding support; DERs ride through, if EPS provides frequency within ride-through ranges
9
Gen-rich
Gen-rich
N/A
Feeder is not included in the UFLS of EPS and stays connected
μUFLS is compliant with EPS requirements; if μG stays connected, DERs ride through
μG disconnects from EPS before the μUFLS works or μG stays connected, but the μUFLS is deactivated, unless the μG agrees to provide additional load-shedding support; DERs ride through, if EPS provides frequency within ride-through ranges
As seen in the table, the relationships between the loads, generation, UFLS of the EPS, and µUFLS may change in near-real time, which means that the coordination of the UFLS setting, points of impacts, and DER protection may also change. To implement the required changes, a timely information exchange between the EPS and microgrids is needed.
Some operational conditions for volt/var control in connected mode under normal operating conditions are illustrated in Table 2.
Table 2. Sample operational conditions that may require near-real-time information exchange between DMS and µEMS.
VVO objective
EPS view
μG view
Comments
Ensure standard voltages at customer service terminals
The EPS operator is concerned about keeping the voltage within standard voltage limits at the EPS customer terminals and within other agreed limits at the μPCC.
The microgrid operator is concerned about keeping the voltage within standard voltage limits at microgrid's customer terminals and about respecting the contractual conditions regarding the voltage tolerance at the μPCC at minimum cost.
If the objectives are in conflict, a condition-specific decision should be made.
Reduce load and/or conserve energy within given voltage limits
The EPS operator is interested in reduction of the aggregated load and/or energy, including the reduction of the intake by the microgrids, or the increase in the injection of power by the microgrid
The microgrid operator may not be interested in the load reduction at the time of the need for load reduction in the EPS.
Depending on what the microgrid should do to reduce the load or energy, there may be a conflict between the EPS and microgrid objectives
Mitigate the adverse impacts of the DER variability
The EPS operator would like the microgrid to participate to full extent in the compensation of the voltage fluctuations in the EPS grid by the microgrid's reactive power control. In addition to compensation of the voltage fluctuations due to DER variability by opposite changes in the reactive power, the DMS may temporarily change the settings of the volt/var controlling devices to reduce their sensitivity to voltage fluctuations.
The microgrid operator is primarily interested in the compensation of the voltage fluctuations within the microgrid,
The DMS actions can lead to violations of the agreed-upon voltage tolerances at the μPCCs. The microgrid may not need to use all microgrid's reactive resources to compensate the voltage fluctuations within the microgrid
Reduce energy losses
The EPS operator wants to reduce losses in the EPS circuits
The microgrid operator wants to reduce the losses within the microgrid
There can be a conflict between these objectives for the EPS and for the μG.
As seen in Table 2, the interests of the EPS operator and the microgrid operators regarding the volt/var control may conflict depending on the changing operational conditions within and outside of the microgrid. To implement mutually acceptable volt/var control actions, a timely information exchange between the EPS and microgrids is needed.
More examples of information exchanges between the EPS and microgrids are presented in Part 2 of the article.
References.
1. Nokhum Markushevich,"New Aspects of IVVO in Active Distribution Networks," Presented at IEEE PES 2012 T and D conference
2. Nokhum Markushevich, "Applications of Advanced Distribution Automation in the Smart Grid Environment," T&D Online Magazine, January-February 2010 issue. Available: http://www.electricenergyonline.com/?page=mag_archives
3. Nokhum Markushevich, Alex Berman, and Ron Nielsen, "Methodologies for Assessment of Actual Field Results of Distribution Voltage and Var Optimization," presented at IEEE PES 2012 T and D conference