You've hit on a classic and often challenging problem in electrical system coordination: achieving selectivity when devices have identical or nearly identical ratings, especially in a system with parallel generators and multiple large loads. Here's a breakdown of the problem, the principles, and potential solutions, drawing on your experience with TCC curves and relative fault currents.

Understanding the Problem Identical LTPU Settings

When all your 4000A ACB and 6000AF breakers have the same Long-Time Pickup (LTPU) setting, they'll all try to trip at roughly the same overload current level. This eliminates the intended hierarchy of a coordination scheme.

Overlapping Overload Region

As you've noted, regardless of other settings (like Instantaneous or Short-Time), the overload region of the TCC curves will overlap significantly. An overload on one load could trip the generator main, unnecessarily shutting down a significant portion of the system.

Parallel Generators

Parallel generators introduce complexity. They share the load, but their fault contribution also adds up on the common bus. This makes coordination calculations more involved.

High Fault Current Rating

Using fuses to achieve the 200kA rating complicates coordination. Fuses have very specific time-current characteristics, and coordinating with them requires careful selection of the fuse rating and the ACB settings.

Coordination Principles and Approaches1. Relative Fault Current and TCC Curve Shifting (Your Previous Experience)



Concept

The fault current seen at the generator breakers is indeed lower than the fault current seen at the main distribution board. This is due to the impedance of the generator windings, step-up transformers (if any), and connecting cables.

Shifting the Curve

By understanding the impedance between the generators and the main board, you can calculate the "relative" fault current reduction at the generator breakers. This allows you to effectively shift the generator breaker TCC curves to the right (higher current, longer time) compared to the downstream breakers.

Example

Let's say you calculate that a fault on the main board would result in only 75% of that fault current flowing through a particular generator breaker. This means you can effectively treat the generator breaker as though it's "seeing" a lower fault current than it actually is.

Implementation

You would then use this adjusted current value when evaluating the TCC curve for the generator breaker. You may be able to effectively increase the LTPU setting for the generator breaker, which allows it to carry a higher overload condition for longer before tripping. This allows downstream devices to clear the overload first.

Important Note

You need to carefully and accurately calculate these impedance values and current reductions. Incorrect calculations can lead to mis-coordination and potentially damaging faults.2.

Adjusting Time Delay Settings (Long-Time Delay - LTD)

This is the primary method when LTPU is constrained. Even if the LTPU settings are close, you can introduce a longer time delay (LTD) on the generator main breakers. This gives the downstream load breakers time to trip on overloads before the generator breakers do.

Key Consideration

The LTD setting must be long enough to allow downstream devices to clear faults without causing excessive damage or voltage dips on the system. This often requires careful analysis and compromise.3.

Short-Time Pickup (STPU) and Short-Time Delay (STD) Coordination

While overload is the main problem, consider the short-time region as well. You can set the STPU and STD of the generator main breakers to coordinate with the downstream load breakers. Ensure that the load breakers trip faster for short-time faults closer to the load.

Grading

Set STPU values higher as you move upstream towards the generator mains.

Note

Fuses don't typically have adjustable STPU/STD, so this coordination is mainly between the ACBs.4.

Instantaneous Trip (IT) Coordination

Set the instantaneous trip settings (IT) of the generator main breakers to be above the maximum fault current that can be seen at the load breakers during a fault close to the load breakers. This ensures that downstream faults are cleared by the load breakers' instantaneous trip before the generator breakers have a chance to operate.5.

Zone Selective Interlocking (ZSI)

If your ACBs support ZSI, this is a highly effective method. ZSI allows downstream breakers to "restrain" upstream breakers from tripping instantaneously for faults in the downstream zone.

How it works

When a downstream breaker detects a fault, it sends a signal to the upstream breaker. The upstream breaker delays its instantaneous trip (or disables it altogether) allowing the downstream breaker to clear the fault first.

Benefits

Improves selectivity and reduces unnecessary tripping of upstream breakers.

Consideration

ZSI requires dedicated wiring between the breakers.6.

Differential Relaying (Optional for Large Systems)

For very critical and large systems, differential relays can be used on the generators and bus. These relays detect faults based on current balance and provide extremely fast and selective tripping.

Drawback

More complex and expensive.7.

Fuse Coordination

Fuse coordination is tricky. You'll need to carefully select the fuse size and type to coordinate with the ACB curves.

Consult Fuse Manufacturer Data

Use the fuse manufacturer's time-current curves to ensure that the fuse will clear faults faster than the ACB for the appropriate fault current levels.

Total Clearing Time vs. Pre-Arcing Time

Coordinate with the total clearing time of the downstream fuse and the pre-arcing time of the upstream fuse.

Practical Steps1. Detailed System Study

This is essential. You need a comprehensive short circuit and coordination study performed by a qualified electrical engineer.

Impedance Data

Accurate impedance data for all components (generators, transformers, cables, breakers) is critical.

Fault Current Calculations

Perform fault current calculations at various points in the system (generator terminals, main bus, load terminals).

TCC Curve Plotting

Plot TCC curves for all protective devices (ACBs, fuses) on the same graph to visualize coordination.2.

Generator Characteristics

Obtain accurate data for your generators: Subtransient Reactance (X"d) Transient Reactance (X'd) Synchronous Reactance (Xd) Negative Sequence Reactance (X2) Zero Sequence Reactance (X0)3.

Adjustable Settings

Maximize the use of adjustable settings on the ACBs: LTPU (Long-Time Pickup) - Use sparingly and only when justified by impedance calculations. LTD (Long-Time Delay) - Primarily used to coordinate overloads STPU (Short-Time Pickup) STD (Short-Time Delay) IT (Instantaneous Trip)4.

Coordination Software

Use power system analysis software (e.g., ETAP, SKM, EasyPower) to model the system and perform coordination studies. This software makes it much easier to analyze TCC curves and adjust settings.5.

Documentation

Document all coordination settings and assumptions in a clear and organized manner.

Example Scenario (Illustrative)

Let's say you have a 4000A generator main breaker and a 4000A load breaker. Your short-circuit study shows: Fault current at the main bus (downstream of the generator breaker): 60kA Fault current at the generator breaker location: 45kA (due to generator impedance)You might:1. Set the generator breaker LTPU close to its nominal rating.
2. Set a longer LTD on the generator breaker compared to the load breaker.3. Set the load breaker's STPU and STD to values to coordinate with the fuse and below the generator breaker settings.
4. Set the generator breaker's IT above the fault current that can be seen at the load terminals.5. Potentially use ZSI to further improve coordination.

Important Considerations Voltage Dip

During overloads, excessive time delays can lead to voltage dips that can affect sensitive equipment.

Equipment Damage

Prolonged overloads can damage equipment. Coordination settings must balance selectivity with equipment protection.*

Maintenance: Ensure that the protective devices are regularly tested and maintained to ensure proper operation.By carefully considering these principles and following a systematic approach, you can improve the selectivity of your electrical system and minimize the risk of unnecessary outages. Remember, a professional power system study is highly recommended for this type of complex system. Flag for review