unidirectional or bidirectional, depending on the nature of the relationship. Operational flows involve all the relevant information needed to coordinate market and network operational functions. Figure 1 is a simple coordination skeleton diagram that illustrates TDC tiers. Additionally, three reference conceptual TDC coordination models illustrate a spectrum of potential coordination architectural models (Figure 2). The various potential TDC coordination models under discussion globally fall within this spectrum. Transmission System Operator Utility Bulk Generation Transmission System IPP Generation Bulk Energy Storage Distribution Distribution Operations Customers IPP DER Distribution System Customer DERs/Microgrid Customer Usage Distribution Storage/Microgrids Customer DERs/Microgrid Aggregator figure 1. An example of a coordination skeleton diagram. The customer usage (unresponsive load) is shown even though it is not actually coordinated, except for the demand response (part of DERs). IPP: independent power producer. Total TSO Hybrid TSO Total DSO The TSO optimizes the entire power system into the distribution system, including the dispatch coordination of all DER services and schedules. The TSO optimizes the bulk power system, including dispatch of all wholesale DER services, but has no visibility into the distribution system. The TSO optimizes the entire power system. The TSO sees a single aggregate or virtual resource at each T-D interface, managed by a DSO. The DSO is responsible for the reliable distribution network operations and providing distribution network visibility to the TSO. The DSO optimizes the distribution system, including dispatch of all distribution DER services, and coordinates with the TSO on all DER dispatch. The DSO is responsible for the physical coordination and aggregation of all DER services into a single resource at the T-D interface and wholesale market. The customer/aggregator coordinates with the TSO; no operational interface with the DSO. The customer/aggregator coordinates with both the TSO and DSO. The customer/aggregator coordinates with the DSO; no operational interface with the TSO. figure 2. The spectrum of conceptual models of TDC coordination. september/october 2019 ieee power & energy magazine 31