Recap: Our Single Phase Transmission System
As a re-cap, our transmission system circuit diagram and MATLAB Simulink model are introduced below.
The circuit illustrated in the figure below represents an equivalent power system feeding a 300 km transmission line. The line feeds a static load at its receiving end and is also provided with voltage compensation by means of a shunt reactor. A circuit breaker allows us to switch the circuit in or out of service, i.e., to energise or de-energise the line.
Figure: Simplified Transmission System Sketch
Recap: Our Simulink Model
The Simulink model contains a generator (with equivalent circuit parameters), a transmission line PI section, a load, a shunt reactor and a circuit breaker controlled by an instantaneous protection system (a protection system with a single time-invariant threshold value). Several measurement blocks are also included in the model, which allow us to view the results of our simulation.
The simulation utilises a variable step solver (continuous Powergui block).
Figure: Simulink model
Introduction to Differential Protection System
In this exercise, students shall model a differential protection system covering the transmission line.
In a differential protection scheme, the current is measured at each end of the zone of protection and the relay branch measures the sum of these currents.
When operating normally or when subject to faults that are external to the zone of protection:
I1 – I2 = 0 : The relay does not trip the CBs.
When subject to faults within the zone of protection:
I1 – I2 ≠ 0 : The relay trips both CBs.
A summary of how differential protection is used to protect feeders and busbars in an electrical system is presented in the following 9 min YouTube video:
Coursework Exercise - Modelling
Save and make a copy of your previous model.
- Disconnect the previously modelled instantaneous protection system.
- Model the differential protection scheme illustrated on the previous slide to provide unit protection to the transmission line. The scheme should be designed based on the following notes:
- Two circuit breakers are required; one at each end of the Transmission Line.
- The CTs should have a ratio of 120/5
- The threshold value for the over current relay should be calibrated such that normal losses in the transmission line do not cause spurious operation.
- Circuit breaker operating time: 80 ms.
Coursework Exercise - Reporting
- Configure the model to log the RMS current and voltage at each end of the transmission line and pass this data to the Simulation Data Inspector for export.
- Run at least three simulations to demonstrate that the differential protection scheme operates correctly in response to each of the following test scenarios:
- Normal operating conditions (no fault): The protection system does not trip the CBs.
- Faults external to the zone of protection: The protection system does not trip the CBs.
- Faults internal to the zone of protection: The protection system trips both CBs to isolate the line.
- Export the data to Microsoft Excel; including all variables [I1_RMS, I2_RMS, V1_RMS and V2_RMS] under each test scenario.
- Using the template provided, write a short academic paper detailing the findings of your analysis.
The paper length should not be greater than 2500 words. Note that the purpose of the paper is to describe the modelled protection system and present evidence of its functionality under each operating scenario.
The template document - P4 Coursework - Report Template provides further guidance for students. All guidance should be read in full prior to undertaking the modelling task to avoid the need for re-work.