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Modern power plant control offers auxiliary services beyond the grid

By Dr. Stijn Stevens, CTO at skytron energy GmbH

Introduction: Solar power systems have moved from being a mere curiosity to a professional and substantial element in the electrical power supply chain, participating in grid regulation and energy trade. The emergence of monitoring systems initiated the customer's awareness and advanced the know-how of PV systems, as well as the quality of PV module and inverter manufacturers.

As PV systems matured and their acceptance has grown, additional services such as voltage support, power factor correction, and self-consumption were offered. Again, monitoring systems watched over quality and contract fulfilment, offering the operator tools to analyze, report, and maintain the power plant and preserve optimal operation, securing the investor's assets.

Recently, due to rapidly evolving price degradation, battery solutions are becoming an attractive investment for private investors, utilities, independent system operators (ISO), and transmission system operators (TSO). A battery solution can be used in conjunction with a PV power plant. The addition of this component to the system, in terms of its unknown future be-havior; maturity of the different technologies; the complexity; the difficulty of designing battery systems with an appropriate (dis-)charge profile; the unpredictability of weather profiles; and the implications of future legislation on the high upfront investment leave investors insecure. This announces a new era for controlling modern power plants.

Modern power plant control: Modern power plant controls (PPC) entail a symbiosis between the classical controller and the monitoring system. With the incorporation of the upcoming batteries, the PPC will additionally have to support frequency regulation (FR), non-/spinning reserve, voltage support, self-consumption, power backup, or black start. Some of these services require a faster control rate (100ms) as is necessary for the PV plant. The battery system itself comprises multiple battery systems, sometimes with different characteristics due to time-shifted investments. The system is bound by internal constraints to provide maximum power and/or energy. This leads to the necessity of individual control of, and communication with, each battery system separately. The overlaying of multiple objectives increases the complexity of the controller, as the controller software consists of one solution to solve all cases.

A battery project is designed to fulfill one or more objectives. These are contractually fixed; therefore, the design comprises enough battery power and capacity over the plant's lifespan. Both large-scale battery applications and hybrid power plant design tools are in the beginning of their learning curve, inevitably leading to inaccurate battery capacity and system fault tolerance planning. This is where the monitoring system supports the asset owner and indirectly contributes to the maturing process of battery systems in general.

The monitoring system offers posteriori, real-time, and priori analyses of the power plant including battery systems, and it forms a legal foundation to prove compliance with the operating terms and conditions.

A posteriori analysis comprises PV, battery, and plant-specific characteristics. Some characteristics for the battery are number-of-cycles-per-day diagram, DC voltage and current window maps, round-trip efficiency tracking, capacity tracking, state-of-charge operation diagram, heat profiles of the battery system, number, cause, and duration of outages, state-of-health diagram, and number, date, and type of battery checks that are performed. Additionally, multiple equivalent systems are compared with one another to identify extraordinary behavior. Important plant characteristics are how efficiently the battery capacity is used and business-case-specific targets. These key performance indices are reported automatically to the asset owner.

The real-time analysis keeps track of instantaneous health of the system, failure notification and identification, and service partner notification. Additionally, direct control of the complete power system enables the operator to (de-)activate parts or the complete system.

A priori analysis empowers the asset owner to optimize current and future investments with respect to real or changing use cases. The analysis incorporates historic plant data, e.g. weather data, conversion efficiencies of inverters, degradation of PV modules, capacity degradation of battery, battery module and inverter outages, and plant prediction models. The asset owner gets reported whether the plant is on target and/or can deliver extra services to increase revenue.

The monitoring system in battery plants is indispensable, as it largely reduces the asset owner's insecurities.

Just like monitoring systems changed PV systems in their early stage, so will they, yet again, drive modern power plant control as an indispensable element in this new battery era.

Conclusion: Designing hybrid plants for a specific business case is a challenging job and deemed to be prone to inaccurate design, leading to suboptimal use of resources. Monitoring systems empower the operator/owner to analyze performance, check investments, and forecast investment opportunities with respect to plant upgrade, plant maintenance, and changing business cases.

 


September/October 2017