Standards Analysis
5.2 iPower Flexibility Interface
48 CHAPTER 5. STANDARDS ANALYSIS
The iPower flexibility interface supports a direct control setup, between the DERs and the aggregator. Direct control refers to a setup where an aggregator dictates the actions of a DER unit for example by telling the unit to produces this much energy and is facilitated by a two-way communication between the aggregator and the DER. The DERs reports their local flexibility to the aggregator and the aggregator controls the DERs based on this information. The flexibility interface, as illustrated in figure 5.3, is situated between the aggregator/virtual power plant and the DERs.
The following is a selection of the characteristics described in the iPower Flexibility Interface:
• Geographical location
The location of a DER in the power grid is important in the case, where the DER supports the grid as an ancillary service.
• Local control or remote control
In the case, where the DER is part of the primary frequency reserve for the power grid, the DER must measure the grid frequency locally and a local control loop, determines if the unit should activate. In the case of the DER being in secondary, tertiary or manual reserves, the DER could be activated using remote signals.
• Combined deliveries
DERs can be either pure consumption units, pure productions units or a combined consumption and productions units. It is therefore important to be able to distinguish between these types of units.
• Active/reactive power
Both the active and reactive power is required to be communicated to the aggregator, in order for the aggregator to properly place bids on the power markets.
• Limitations
Limitations of the DER includes power consumption/production levels and frequency levels.
(how many times, given an interval, is it feasible to have the energy level go up and down.)
• Tracking
The ability to track a given remote power reference or to track a planned power schedule.
• Runtime
Minimum and maximum runtime. E.g., a unit must run for at least 30 min when started.
• Downtime
Minimum and maximum downtime. E.g., when a unit is turned off, it must be turned off for at least 1 hour.
• Cost
The aggregator needs to know the cost associated with the active/reactive power production cost, energy level dependent cost, unit start-up cost, unit shutdown cost for discrete power units: cost depending on the number of power level changes and flexible activation time costs.
(E.g. the later the activation, the higher the cost).
• Contracts
For an aggregator to be able to manage a DER, some contract or agreement exists between the aggregator and the DER. This is communicated through the flexibility interface.
5.2. IPOWER FLEXIBILITY INTERFACE 49 The information model of the flexibility interface is based on these characteristics, and is divided into a number of flexibility blocks, which form the basis of the flexibility interface. In figure 5.4, a graphical overview of the flexibility blocks is presented.
Block 1:
Type [M]
1. DER Name 2. DER Type
Block 2:
Electrical Connection Point [M]
1. Geographical connection point 2. Voltage level at connection point
Block 3:
Status [M]
1. DER Status
2. Remote control error 3. Remote control enabled
●
●
●
Block 4:
Active Power Production [O]
1. Remote control enabled, reference tracking
2. Remote control, shedule tracking 3. Allow remote control, reference tracking
Block 5:
Primary Frequency Control [O]
1. Remote control enabled, continuous primary frequency reserve
2. Remote control enabled, discrete primary frequency reserve
3. Allow control enabled, continuous primary frequency reserve
Block 6:
Flexible Startup Time [O]
1. Remote control enabled, flexible startup time
2. Allow control enabled, flexible startup time
3. Active and reactive power profiles
●
●
●
●
●
●
●
●
●
Block 7:
Energy Storage [O]
1. Storage dynamics
2. Storage dynamics, time varying 3. Stored energy
Block 8:
Log [O]
1. Enable logging 2. Name and data
3. Pointer to attribute that should be logged
Block 9:
Cost [O]
1. Enable costs 2. Cost function
●
●
●
Figure 5.4:Flexibility Interface frame blocks[7]
A flexibility model of a specific DER is constructed, by using the relevant blocks to describe its flexibility. This concept is shown in figure 5.5 and illustrates an information model of a single DER unit, constructed by a collection of blocks comprising the DERs flexibility information model. In the figure, the flexibility information model consists of n blocks, each block being comprised by a number of attributes containing the actual data. The specific flexibility information model for a device is known as a flexibility frame.
Block 1:
Type [M]
1. DER Name 2. DER Type 3. Contract type
Block 2:
Electrical Connection Point [M]
1. Geographical connection point 2. Voltage level
Block n:
Active Power Production [O]
1. Active power control possible 2. Active power schedule possible 3. Active power schedule horizon
●
●
●
●
●
●
●
●
●
●●●
Figure 5.5:Example of a flexibility frame[7]
In a flexibility frame, some blocks are mandatory and some are optional, denoted in the figure with [M] for mandatory and [O] for optional.
50 CHAPTER 5. STANDARDS ANALYSIS
5.2.1 Flexibility Interface in the Information Model
The work with the Flexibility Interface (FI) done in [7] provides a modeling of the FI concept that are compatible with the IEC 61850 data model. The FI can be suitably inserted into the IEC 61850 standard, as an extension to the IEC 61850-7-420 standard sub part.
As the FI data model is based on the IEC 61850 data model, the information model in the prototype is able to handle the FI with no additional modification. The work done in [7] only concerns the modeling of the FI data model and in order to use the FI in the information model and the prototype, the FI needs to equipped with a base reference and the contents in the FI blocks needs to be mapped to reference names that can be used in the data model, such that the data can be referenced. In figure 5.6, is an example of how the FI Block 1 is mapped to the data model.