Operation of a Battery Energy Storage System
Publication Date: 2015-Nov-17
The IP.com Prior Art Database
A method of operating a BESS with at least two parallel converter units connecting respectively a first and a second string of series connected battery cells to an AC bus wherein during a cycle the first string operates in a charging mode and the second string operates in a discharging mode and wherein operating modes are shifted at the end of the cycle comprises the steps of establishing or defining a battery degradation objective function including and penalizing at least one of a cycle degradation based on a DoD prediction of the present cycle a cycle end status based on a SoC prediction at the end of the present cycle and a heating cost based on a integrated cell current during the present cycle providing a short term forecast of the BESS ouput power P t and evaluating for a plurality of tentative end points of the present cycle the battery degradation function for each cell string of the BESS the BESS operating according to the forecast BESS output power and determining the end of the present cycle such as to minimize the battery degradation function
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Background of the invention
Battery technology is critical for utility-scale applications where continuous power is needed in both directions by charging and discharging. Examples include power system frequency control, ramp rate control for wind power generation, and solar PV output smoothing during cloudy days. During such operations, the battery energy storage system (BESS) responds to a variation of a certain power system parameter, e.g. system frequency, in order to keep this parameter within a range defined by system operation rules/regulations.
An analysis of a typical frequency control signal profile from a real electric power system shows that battery operation is dominated by shallow energy cycles (Figure 1; the control signal is a PJM Frequency regulation signal: http://www.pjm.com/markets-and-operations/ancillary-services/mkt-based-regulation/fast-response- regulation-signal.aspx.).
Period energy 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Figure 1. Example of shallow energy cycles for a battery with 15 min discharge capacity following control signal
The layout of a BESS can vary, but a MW-scale system for utility applications usually consists of multiple modules connected in parallel. Each module includes a power convertion unit and a string of batteries and is controlled by a master controller. A practical example of such solution by Ecoult is shown in Figure 2.
Figure 2. Example of BESS architecture proposed by Ecoult
This battery system is used for a "secondary" frequency regulation called Area Control Error regulation in the PJM control area and acts according to a control signal called RegD sent every 2 seconds by a transmission
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system operator to qualified and contracted (via a market bidding process) suppliers of frequency regulation power.
Example of the control signal - active power in/out (for a BESS with rated power of ±3 MW) is shown in Figure
3. A BESS is able follow the control signal very precisely and therefore receives a higher performance score compared to classic fossil power units with mechanical power train due to their limited ramp rates.
Figure 3. Accuracy of BESS responce
The BESS state of charge (SoC) changes in response to the PJM control signal. SoC goes down when the BESS is discharging and up when it is recharging. The master controller operates all four battery strings in a similar direction i.e. charge or discharge. Figure 4 shows a SoC variation of four parallel strings. We can observe that SoC of all strings is almost identical at any given time meaning that during operation all four strings are exposed to a very large number of shallow energy cycles.
Figure 4. BESS state of charge variation
According to our investigation on battery degradation mecha...