Design of protection schemes in networks with future electric energy delivery

Networks with future electric energy delivery

Future Renewable Electric Energy delivery and Management (FREEDM) system was developed as a smart grid system with a motivation to include renewable sources into the existing power grid. FREEDM system is a culmination of high bandwidth digital communication, power electronics and digital communication.

It replaces conventional transformer of 60 Hz with solid state transformer (SST) incorporating bidirectional power flow. Solid state transformer is a package of cascaded rectifier, dual active bridge converter and inverter. It has an input of 7.2 kV AC and output of 120 V AC single phase, 208 V AC (3 phase) and 400 V DC.

All the DC loads, distributed generation and energy storage devices are connected to the DC link and AC loads are connected to the AC output.

To demonstrate the advancement done in FREEDM project, a 1 MW green energy smart hub is under development. The proposed FREEDM loop allows consumers to plug and play energy sources or storage devices from anywhere on the loop.

To provide effective power flow, power distribution and fault detection; intelligent energy management (IEM) and intelligent fault detection (IFD) control schemes are incorporated in the FREEDM system.

Following Figure 1 shows the schematic layout of the FREEDM system and Figure 2 shows the single line diagram of the FREEDM system. The FREEDM loop is connected to grid and in the case of grid failure, the loop can operate independently.

Objectives of the research

The power flow in the FREEDM system is bi-directional due to the presence of distributed generation (DG) and the conventional protection methodologies must be modified accordingly to detect fault conditions and prevent false tripping. The solid state transformers (SST) have self-protection, which shuts them down during faults when the voltage goes below a certain threshold voltage and it can be adjusted.

In the case of a loop system during faults, the voltage of the entire loop plummets to zero or typically a low value. In such a scenario all the SST’s connected to the loop will shut down due to its self-protection.

Hence, it is important to isolate the faulted part of the network from the rest of the system to prevent the shutdown for SST’s. Pilot differential protection was developed as a solution to the above mentioned problem. It was able to detect faults within quarter of a cycle but it suffered from the problem of communication.

Ethernet cable was used as the communication medium to transfer the sampled signals from the current transformers to the central processor. The central processor analyzes the sampled current signals, generates the trip depending up on the system conditions and sends the trip signal back to the breaker.

A protection method is developed that could protect looped systems with multiple sources without using any sort of communication. Directional relays with time inverse over current characteristics are coordinated affectively to detect and sectionalize the fault location without affecting the healthy part of the system.

This serves as a reliable back-up protection system when the communication system fails.

A new pilot protection method is developed using the commercial SEL relays which uses the direction of fault currents to locate the fault. The communication is done using fiber-optic cables and the only data needed to be transferred between the relays is the fault location which is transferred in the form of digital bits.

Simulation, hardware implementation and real time digital system (RTDS) validation is explained in the thesis. A protection method is suggested monitoring the sychrophasor measurements, voltage of the system and current during faults.