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Who Benefits from Voltage/var Control in Active Distribution Networks?

Who Benefits from Voltage/var Control in Active Distribution Networks?

Nokhum Markushevich

Smart Grid Operations Consulting

Currently, the dominant way of controlling voltage in distribution is by changing the transformation ratio of substation transformers with under-load tap changers (LTC) and/or of the line voltage regulators. When the high penetration of Distributed Energy Resources (DER) with reactive power capabilities becomes a reality, a significant additional resource for volt/var control in distribution will become available.   The decisive difference here is that the DERs control voltage by changing their reactive power injections.  In order to reduce voltage at the customer terminals, DERs reduce their injection of reactive power, sometimes going into absorption of reactive power.  In many cases, the voltage reduction results in reduction of the customer real and reactive loads. However, the reduction of injection of reactive power by the DERs means an increase of reactive power flow from the balancing source to the consumers of kvars. The increase of the reactive power flow may result in an increase in real and reactive power losses, if the reduction of the natural real and reactive load does not override the effect of reduced injections of the kvars. If the effect of voltage reduction on load reduction exceeds the increase in losses, then the voltage reduction results in overall generation reduction, if not, the generation goes up, and there is no overall reduction of generation and no system-wide energy conservation.

The controllers of the smart inverters with the reactive power capabilities may operate in different Volt/var control (VVC) modes and with different settings [1]. 

If the mode of VVC is set to provide maximum reactive power, the lagging reactive power flow in distribution and/or in transmission may reduce, or the leading power flow may increase.  In this case, the voltage at the customer terminals may be higher; hence, the load may be greater, while the losses may be smaller.  To compensate fully or partially for the voltage increase at the customer terminals, the Distribution System Operator (DSO) or the Distribution Management System (DMS) may reduce the voltage at the supplying bus (if there are no other constraints). In this case, coordinated actions of the DER operators and the DSO/DMS are needed.

In the active distribution networks with high penetration of DER with reactive power capabilities, the change of the net reactive load at the transmission buses may be much more significant than in the current distribution systems. 

As follows from the above suppositions, the different modes of DER VVC result in different operating condition in different power system domains, such as customer, distribution, generation and transmission domains.  The operational objectives of the different stakeholders involved in power system operations may be different under different circumstances.  For instance, the customers are interested in reduction of their bills. Ultimately, the customers pay for the direct electric service provided and the energy losses incurred due to these services. However, the loss adjustment rule applied to the customer bills is different in different utilities and, typically, is adjusted much latter than the actual losses occurred. In any event, the variable portion of the regular customer bills are based on the energy consumption and, sometimes, on the peak demand, as well as on the load power factor. Therefore, some customers are interested in just energy conservation, other in energy conservation and peak demand reduction, and some are interested in a higher power factors. Some customers may be interested in reduction of the future bills, i.e., in minimizing the sum of their energy consumption and energy losses attributable to them.

The distribution utility and aggregators may be also interested in load management, energy conservation, loss reduction, and power factor improvement at particular physical or virtual buses, depending on the existing incentives and/or agreement with other stakeholders.  The distribution utility may be especially interested in loading (amperes) reduction in particular circuit elements.

The operators of the bulk generators may be interested in reduction of the need in peak generation, in meeting their obligation in providing the operating reserve and other ancillary services, in reduction of the reactive power demand, which may increase the real power capability

The operations of the active distribution system will have a more significant effect on the operations of the transmission systems, affecting power flow in transmission circuits, transmission losses, power factors and voltages at the transmission busses, bulk generation of the real and reactive power, and ultimately the Locational Marginal Prices (LMP). Hence, the Transmission System Operator may be interested in volt/var support provided by the distribution utility and/or aggregators, in unloading of transmission circuit element, in transmission loss reduction, etc.

It means that the transmission system operator (TSO) and the corresponding Energy Management Systems (EMS) need to take into account the aggregated at the transmission buses effects and the capabilities of the active distribution network. For instance, the TSO/EMS should know in advance how operational parameters at the transmission buses would change with the change of the bus voltages under the current modes of the VVC in distribution.  If under the current mode of VVC in distribution, the operational parameters in the transmission system approach undesirable levels, another possible way of VVC in distribution may be requested by the TSO.

As follows from above, different VVC objectives of different stakeholders may be in conflict. To resolve these conflicts, situation-specific agreements and timely exchange of relevant information between the stakeholders should be provided. The corresponding control systems should be updated to adapt the new requirements in practical consistency with the actual complexity of the smart grid operations.  

As it was suggested in [2]-[6], such exchange of information between the DSO/DMS and TSO/EMS can be provided through the Transmission Bus Load Model (TBLM) and through information exchanges between the advanced microgrids and DSO/DMS. Some of the complexities of such information support of the power system operations in the smart grid environment are described in [2]-[16].

 Note: More details on this subject are available in [17].

References.

1.      Common Functions for Smart Inverters, Version 3. Available: file:///C:/Data/SGOC/Education/Book/New%20papers/CVR/000000003002002233%20(1).pdf

2.      Nokhum Markushevich,”New Aspects of IVVO in Active Distribution Networks,” Presented at IEEE PES 2012 T and D conference

3.      Nokhum Markushevich, Cross-cutting Aspects of Smart Distribution Grid Applications, Presented at IEEE PES GM 2011, Detroit

4.      Transmission Bus Load Model – the Bridge for Cross-Cutting Information Exchange between Distribution and Transmission Domains, Available: http://collaborate.nist.gov/twiki-sggrid/pub/SmartGrid/TnD/SGOC_presentation_to_SGIP_03-30_-2011a.pdf

5.      Development of Transmission Bus Load Model (TBLM) Use cases for DMS support of information exchange between DMS and EMS Version 14, 2013. Available: http://collaborate.nist.gov/twiki-sggrid/pub/SmartGrid/TnD/TBLMUseCase_V14-03-13-13-posted.pdf

6.      Development of Transmission Bus Load Model (TBLM). Use cases for DMS support of information exchange between DMS and EMS. Version 5g, 2016. Available: https://members.sgip.org/kws/groups/sgip-drgs-b/document?document_id=9053https://members.sgip.org/kws/groups/sgip-drgs-b/document?document_id=9053 (SGIP members)

7.      Use Cases on Information Support of Interactions between Advanced Microgrids and Electric Distribution Systems. Available: https://members.sgip.org/kws/groups/sgip-drgs-c/documents (SGIP memebers)

8.      Information Support for Coordination of EPS and Microgrid Load Shedding Schemes. Available:   http://smartgrid.epri.com/Repository/Repository.aspx/

9.      Coordination of Volt/var control in Connected Mode under Normal Operating Conditions. Available:   http://smartgrid.epri.com/Repository/Repository.aspx/

10.    Update aggregated at PCC real and reactive load-to-voltage dependencies under normal operating conditions.  Available:   http://smartgrid.epri.com/Repository/Repository.aspx/

11.    Updates of capability curves of the microgrid’s reactive power sources. Available:   http://smartgrid.epri.com/Repository/Repository.aspx/

12.    Updating information on microgrid dispatchable load. Available:   http://smartgrid.epri.com/Repository/Repository.aspx/

13.    Updates of the information on overlaps of different load management means within microgrids.  Available:   http://smartgrid.epri.com/Repository/Repository.aspx/

14.    Updating dependencies of the microgrid operational model on external conditions. Available:   http://smartgrid.epri.com/Repository/Repository.aspx/

15.    Update aggregated at PCC real and reactive load-to-frequency and load-to-voltage dependencies in the emergency ranges. Available:   http://smartgrid.epri.com/Repository/Repository.aspx/

16.    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  (SGIP members)

17.    Nokhum Markushevich, “Voltage/var Optimization in Active Distribution Networks” https://www.scribd.com/document/376758184/Voltage-var-Optimization-in-Active-Distribution-Networks

 

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