Larger Distributed Energy Resources (DER) generation plants, such as wind, solar or photovoltaic (PV) plants, are generally connected to the power network at the medium voltage (MV) level.
The challenge is to keep the voltage and frequency stable with a higher degree of DER plants and generate the necessary power to cover the gap for the reduced amount of large and stable generation plants.
To cope with that change, IEC released a new standard, IEC 60255-181:2019, with the focus on frequency protection, specifying relay performances and test methodologies of frequency protection schemes, including Rate of Change of Frequency (ROCOF), which Megger complies with in its solutions.
Protection in DER plants
The protection circuits and functions are very similar to regular substations at MV level. Since it is in an MV environment, the protection functions in PV farms are basic. Typically they are under/over voltage, under/over frequency, ROCOF, vector shift, and unbalance protection functions.
Frequency protection functions
The new IEC standard affects how tests are performed on frequency protection functions, including ROCOF. For example, the standard includes a standardised formula for frequency change with smooth frequency change which better reflects the reality, and specifies test frequencies and report structures.
Self-Powered Relays (SPRs)
SPRs are frequently used since they harvest their energy from the network and don't need a DC supply to operate, keeping the costs down. Even though they normally have basic protection functions, they can be a challenge to test for modern relay testers.
The ultimate toolbox for three-phase protection testing in PV plants, wind farms and substations
Wiring and main circuits
Make sure that all cables are connected to the right object with correct polarity. Easily performed with SVERKER900 by injecting low voltage, different for each phase.
Meters and transducers
With a primary capability of 105 A and 900 V and an integrated Volt and Ampere meter, it is easy to make a simple verification that the meters and transducers work properly.
Controls and signal functions
By applying voltages and currents SCADA signals, control commands, interlocking- and blocking signals can be simulated or detected to verify appropriate behavior.
Relay protection of renewables are normally three phase with relatively basic protection functions but there are test challenges to look out for, i.e., continuous ramp and self-powered relays.
Both current and voltage transformers and their circuits need to be verified to achieve proper protection of the DER plant.