Energy Management Control to meet MTRs in PV plants
Authors: Víctor Andrés Díaz, José Ramón Gordillo, Carlos Infante and Manuel Lagares | Position: R+D Department
General control scheme
The increasing inclusion of renewable energy plants into National Electric Grids has driven regulatory agencies to the establishment of a set of requirements in order to ensure that these plants will help to achieve the whole grid stability. These sets of requirements to be met by the plants are named Minimum Technical Requirements (MTRs), and can be categorised as:
- Voltage Requirements
- Frequency Requirements
- Reactive Power Requirements
Depending on the country, MTRs must take into account the additional devices which shall be needed to interconnect a PV plant to the electrical grid together with PV inverters. Among others, the use of energy storage elements as well as elements of reactive power compensation may be needed.
Each set of requirements defines the engineering design of the different elements of a power plant. These elements have to be developed in accordance with the needs of active and reactive power injection; in addition, they have to define communications architecture between the different elements in order to ensure suitable response times. Main devices to meet Minimum Technical Requirements (MTRs) are:
PPC: Power Plant Controller. The PPC is the main control system responsible of generating control references in order to manage the power flow at the point of interconnection (POI). This device performs different types of control of both active and reactive power and also commands the devices of the plant.
The PPC communicates with other devices through two independent network interfaces, control and supervision, according to the needs of the plant.
STATCOM: Static Synchronous Compensator. Its function is to inject capacitive, inductive or reactive power in order to meet voltage, power factor or reactive power references according to the PPC control.
Capacitors Bank. A group of several identical capacitors interconnected in parallel or in series. They act as a source of capacitive reactive power.
PPS: Plug and Play Storage System. It works in energy hybrid generation systems to inject or absorb active power. The Plug and Play Integrated Storage Systems designed by GPTech focuses on strategic markets in the most demanding grid environments: Chile, Brazil, USA, etc.
- Plug & Play Containerized Solution, fully equipped and tested for direct connection to MV.
- Battery Power Converter System based on the GPTech conversion technology.
- Developed in conjunction with leading battery manufacturer’s brands.
- Setting charging and discharging times to suit the needs of the plant and improve the life cycle of the storage cells.
GPTech is an experienced partner in the development of hybrid plants. The company has already developed controllers for various energy combinations, such as wind diesel power generators, grid-tie wind PV battery generators and grid isolated PV battery storage systems.
GPTech Power Plant Controller (PPC) is designed to fulfill MTRs in many different countries such as Puerto Rico, US or South Africa, where GPTech PPC has been successfully installed in a 37,8 MW CPV facility, following the MTRs set by the Main National Utility ESKOM.
This PV plant implements the following controls:
- Voltage Control
- Reactive Power Reference Control
- Power Factor Control
- Ramp Rate Control
- Active Power Curtailment Control
Voltage, Power Factor and Reactive Reference Control System. A constant voltage control is required. PPC must have a closed loop control VRS, equivalent to the Automatic Voltage Regulator in conventional generators. The VRS controller regulation is based on closed loop actions with parallel reactive droop compensation. The VRS set-point must also be adjustable by the operator.
In the example above mentioned in South Africa, constant power factor and fixed reactive power control are required. The PPC gives priority to each control, depending on the plant operator commands.
Generally, voltage control is carried out by the PV inverter reactive capability. In other power plants, VAR compensator devices, such STATCOMs or capacitors banks, provide a source or a sink of reactive power if necessary.
PV inverters and MTRs devices will receive a continuous external reactive reference. If capacitors banks are included, the PPC reactive control shall be able to manage these references and integrate them in output references and the command for the connection of the capacitors. Capacitors banks will be connected following a defined sequence, so they will give a step reactive response depending on their capacity.
Ramp Rate and Curtailment Control. Active Power Curtailment is the limit for the amount of active power that the facility is allowed to generate. This instruction may be issued manually or automatically via remote control facility.
Ramp Rate Control is required to smoothly transition from one power output level to another. The PV facility shall be able to control the rate of change of power output during some circumstances, including:
- Rate of increase/decrease of active power due to irradiance
- Rate of increase of power when an external curtailment of power output is released
- Rate of decrease in power when an external curtailment limit is engaged
This ramp rate may apply both to the increase and decrease of power output and is independent of meteorological conditions.
Frequency Response/Regulation. Generally, the PV facility shall provide an immediate real power primary frequency response, proportional to frequency deviations from scheduled frequency, similar to governor response. The PPC has several possible modes of frequency control divided in two categories. The first category refers to a plant without storage systems where PPC can only deal with over frequency. The second category is designed for plants with storage systems where they will face both under frequency as over frequency.
In order to meet MTRs, increasing power when frequency deviation is negative or achieving ramp control may be necessary depending on the country. In this case, the PV Plant needs to add an external Plug and Play Storage System. The following figure shows the PV Plant behavior with Plug and Play Storage considering an irradiance perturbation.
Considering the control algorithms studied before, PPS allows the system being ready to feed or store a specific amount of power at each moment. At this point, the state of charge (SOC) management becomes critical. The SOC management control will make the SOC value to be within a target band whenever possible. The way to achieve this objective is by adding a ∆P to the reference power that comes from the PPC ramp and frequency control algorithm. So PPC control strategy for SOC management must avoid the following situations:
- If PPS reaches to SOC maximum, the storage system is only available for feed power.
- If PPS reaches to SOC minimum, the storage system is only available for storage energy.
Otherwise, the real power control becomes inefficient and the whole system energy production will not meet the optimum point. This type of control is typically demanded in countries with a weak electrical system, as, for example, Puerto Rico. In weak systems, the PV facility needs to be able to respond to any perturbation as long as possible. For that reason, PREPA, the regulatory agency in Puerto Rico, has one of the most restricts minimum technical requirements.
For those cases, GPTech has developed its PPC and PPS specifically to accomplish with weak systems. These solutions can be integrated with any number of devices from different manufacturers and, also, implements the most common communication protocols, what makes them ready for any energy challenge.
