Reactive Power Support of Distribution and Transmission Systems by Active Distribution Networks
The high penetration of Distributed Energy Resources (DER) with smart inverters  will provide significant resources of reactive power that will benefit the distribution and transmission domains. The efficiency of these resources for both domains can be maximized when the most of the smart inverters are able to communicate with each other and utility systems .
The reactive power resources available from the DERs will be used by the distribution system operator (DSO) to meet the objectives of distribution operations, e.g., for conservation voltage reduction (CVR) , for mitigating overvoltage caused by real power injection by DERs , for mitigating voltage fluctuations due to DER intermittency , etc. At the same time, the transmission system operator (TSO) may request these resources to be used for transmission operations objective, e.g., for optimal power flow (OPF) or security constrained dispatch, etc.
To assess the efficiency of utilizing the DER’s reactive power, both the DSO and the TSO should know the availability of these resources and the conditions of utilizing them.
The technical availability of the DER’s reactive power is defined by their nominal and operational capabilities . The nominal capability of an inverter-based DER is limited by its rated AC current. It means that the nominally available kvars of the DER are dependent on the kW and on the voltage at the DER terminals. The voltages at different nodes along the distribution circuits are different. They depend on the overall operating conditions of the circuits and on the operations of the DER itself. Therefore, the nominally available kvars from DERs located at different nodes are different even if the DERs are identical.
However, the nominal kvar capabilities of the DERs are not always available. If the usage of the nominal capabilities results in either voltage, or current violations in the subject circuits, the reactive power (and, sometimes, the real power) of the DERs should be limited. We call the kvar capabilities of the DERs limited by the operational limits “operational capabilities” . The operational capabilities may be significantly different from the nominal ones.
The DSO’s plans for the usage of the available reactive power in distribution and the TSO plans for it may be different. For instance, the DSO’s objective may be the Conservation Voltage Reduction. Under some circumstances, it may mean low injections or even absorptions of the reactive power by the DERs. If the TSO’s objective is minimization of reactive power demand by the distribution system, it may require utilizing the full operational reactive power capability of the DERs. In some cases, these objectives can be reconciled. For instance, if the CVR-required voltage can be provided by lower voltages at the buses feeding the distribution circuit while the DER’s reactive power is on the operational limit, then both objectives are met.
However, if the feeding bus voltage cannot be lowered, the increase in the reactive power generation will result in higher voltages in distribution, i.e., in higher real and reactive loads and lower power factors for some customer-side loads, which contradicts the CVR objective. These “side effects” of approaching the operational limits of the reactive power are the distribution-side “costs” of meeting the TSO’s request.
The transmission-side benefits of such operating conditions are the additional benefits of the OPF and/or more secure operations of the transmission system.
The comparison of the cost and benefits can be the basis for the TSO and DSO decision-making process in case of conflicting objectives.
When it comes to the dispatchable reactive power that can be provided by the distribution system, the net values attributed to the transmission buses should be considered. This value differs from the available reactive power support by the DERs and other sources of reactive power. It also includes the changes of the natural reactive power due to voltage changes, the changes of reactive power losses.
Hence, power flow models of the initial and final situations should be used to determine the net dispatchable load and the consequences of approaching the operational capabilities. These models should be adequate to the relevant operating conditions, i.e., they should correspond to the timing of the subject situation and should reflect the reactions of the active elements to the changing conditions.
The primary input information about the operations of the reactive power sources can be collected from the DER and microgrid controllers  and from other sources of reactive power, if available, as well as from corresponding Data Management Systems and Model Processors of the Distribution Management Systems (DMS) . The corresponding power flow models can be initiated by the request from the TSO. This request should include the timing and the extent of the requested reactive power support. Then, the power flow models and their analyses can be executed by a special DMS application . It should be noted that the parameter limiting the operational capability or the requested dispatchable reactive power could be met under different distributions of the reactive power among the different sources of reactive power. Hence, to optimally meet the requested dispatchable load, an optimization procedure should be used by the DMS. The constraints for this optimization are the voltages at the service points and the currents in any segment of the subject circuit. The challenge here is the adequate modeling of the voltages in the service points, which are mostly located at the meter connection terminals. It means that the power flow model should include adequate equivalents of the secondaries. Some recommendations for such equivalents and the assessment of their accuracy are presented in  and .
The exchange of information between the DSO and TSO can be accomplished through the Transmission Bus Load Model (TBLM), as suggested in .
More details on the subject of this article can be found in .
- Distributed Energy Resources with smart inverters will become significant resources of reactive power for distribution and transmission domains.
- The dispatchable components of these resources are highly dependent on the operating conditions of the distribution system and may significantly change in near-real time.
- In order to securely and optimally coordinate the operations of the distribution and transmission domains, the DMS should include means that can provide timely updates of the dynamic Transmission Bus Load Model.
- In order to provide the required dispatchable reactive power in an optimal way, the DMS should be able to optimize the distribution of the reactive power among its resources to reach the requested support with the minimum cost.
- The information about available reactive power should include the associated “cost” of providing it.
- The transmission EMS applications should be updated to utilize the information included in the smart TBLM.
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