A discrimination study, also known as selectivity analysis, ensures that in a low-voltage electrical system, only the faulted section is disconnected when a fault occurs. This prevents unnecessary power loss and ensures the rest of the system remains operational. Discrimination is essential for system protection and safety, reducing downtime, equipment damage, and operational disruptions. Conducting a discrimination study involves analyzing the coordination between protective devices, such as circuit breakers and fuses, to ensure proper functioning.
In this guide, we’ll go step-by-step through the process of performing a discrimination study for a low-voltage installation and generating a report on any changes needed to optimize the system.
- Understand the Electrical System Design
Before starting the study, it’s important to gather detailed information about the low-voltage installation. You need to understand how the electrical system is laid out, the components it uses, and the operational requirements.
Gather the following information:
- Single-line diagrams of the installation.
- Electrical load details, including load types (motors, lighting, HVAC, etc.) and load ratings.
- Protective device specifications, including current ratings, trip curves, and response times of circuit breakers, relays, and fuses.
- System voltage levels and configurations (TN, TT, IT systems).
- Fault current calculations for different points in the network.
By reviewing the design, you can determine the protection zones, the fault current levels, and where discrimination is most important (e.g., for critical loads or expensive equipment).
- Identify Protective Devices in the System
List all protective devices, including:
- Circuit breakers (moulded case circuit breakers, miniature circuit breakers, air circuit breakers).
- Fuses.
- Protective relays.
Make sure to record the settings for each device, including:
- Current rating.
- Time delay settings.
- Trip curves (I-t characteristics).
This is crucial for evaluating how the devices interact under fault conditions.
- Analyze Current Discrimination Settings
Evaluate the current settings of the protective devices:
- Check the coordination between devices at different levels of the system hierarchy (upstream and downstream devices).
- Use software tools such as ETAP, DIGSILENT PowerFactory, or Ecodial to simulate faults and observe the system’s response. These tools can graphically display time-current curves (TCC) and help in analyzing whether the settings are correct.
- Compare trip curves: The curves for devices in series should be such that downstream devices (closer to the fault) trip before upstream devices to maintain selectivity.
Common Fault Scenarios:
- Overload: Devices should trip after a delay, allowing for temporary overcurrents, like motor startup, but not prolonged overloading.
- Short circuit: The device nearest to the fault should react almost immediately to clear the fault.
If multiple devices trip for a single fault, or if an upstream device trips instead of the downstream device, the system lacks proper discrimination.
- Simulate Fault Scenarios
Run fault simulations at different points in the system, particularly at:
- Main incoming supply.
- Sub-distribution boards.
- Final circuits.
For each fault scenario, record which device trips and how long it takes to respond. Faults can be:
- Phase-to-phase faults.
- Phase-to-ground faults.
Using the simulation tools, you can adjust device settings, test different configurations, and determine the most effective protection coordination.
- Evaluate the Discrimination Performance
Assess the performance of the system based on the following:
- Does the closest protective device trip?
- Is the fault cleared within an acceptable time?
- Are critical loads left unaffected by the fault?
If a fault at a downstream level causes upstream devices to trip, this is a sign that changes need to be made to improve discrimination.
- Identify Changes Needed
Once you’ve evaluated the discrimination performance, you will likely need to recommend changes to optimize the system. Common recommendations include:
- Adjusting trip settings of circuit breakers (current or time delay).
- Replacing or upgrading devices to those with better discrimination capabilities.
- Changing fuse types or ratings to match the discrimination requirements of upstream and downstream devices.
- Adding intermediate protection devices to improve coordination.
- Generate the Discrimination Study Report
Your report should detail all aspects of the study, including analysis, recommendations, and necessary changes. A well-structured report should contain the following sections:
- Executive Summary
Provide a brief overview of the system’s current discrimination performance and the key changes that are required to ensure proper protection coordination.
- Introduction
- Objective of the study (ensure proper fault clearance with minimal power disruption).
- Brief description of the electrical installation.
- System Overview
- Provide a description of the low-voltage system.
- Include single-line diagrams and detailed information on protective devices.
- Methodology
- Explain the approach used to assess the discrimination, including simulation software and methods of analyzing fault scenarios.
- Describe the fault types simulated (e.g., overload, short circuit) and the assumptions made (e.g., fault current levels).
- Analysis and Findings
- Present the current settings of the protective devices.
- Provide time-current curve comparisons and highlight areas where discrimination failed.
- Include simulation data showing the performance of protective devices under different fault conditions.
- Recommendations
- List the changes required to improve discrimination, such as:
- Adjusting time-delay settings or current thresholds.
- Replacing certain devices with models offering better discrimination capabilities.
- Introducing selective coordination devices if necessary.
- Conclusion
Summarize the key changes that will enhance system reliability and protection. Reinforce the importance of implementing these changes to avoid unnecessary shutdowns or equipment damage.
- Appendices
- Time-current curves (TCC) of the existing setup.
- Revised TCCs based on the proposed changes.
- System diagrams.
- Implementation and Re-Testing
After generating the report and making the necessary changes, the final step is to implement the recommended changes. Once changes are made:
- Re-test the system to ensure that discrimination has been improved.
- Conduct another round of simulations to confirm that faults are isolated effectively, with only the relevant protective devices operating.
Conducting a discrimination study for a low-voltage installation is critical for ensuring that faults are handled efficiently and safely. By following the steps outlined above, you can ensure that protective devices in the system are coordinated properly and that only the faulted section of the system is disconnected when needed. The final report should clearly explain your findings and recommend changes that will improve the overall protection system.
