You're asking about the Motor Protection Unit (MPU) and Time-Current Characteristic (TCC) curves for Medium Voltage (MV) breakers, specifically 13.8 kV. Here's a breakdown of how to determine them

Understanding MPU and TCC
MPU (Motor Protection Unit)

This is an electronic relay or microprocessor-based protection system integrated with the MV breaker. It provides protection features such as: Overcurrent (Instantaneous, Time-Delay) Overload Undercurrent Voltage Protection (Overvoltage, Undervoltage) Frequency Protection (Overfrequency, Underfrequency) Ground Fault Phase Imbalance Thermal Overload (if protecting a motor)

TCC (Time-Current Characteristic) Curve

This curve shows the relationship between the magnitude of fault current and the time it takes for the breaker to trip. It defines how quickly the breaker will respond to different levels of overcurrent. TCC curves are used for coordinating the breaker with other protective devices (e.g., fuses, other breakers, relays) in the system to ensure selective tripping.

How to Determine MPU Settings and TCC Curves for MV Breakers1. Manufacturer's Data



MPU Manuals

MV breaker manufacturers DO provide detailed manuals for the MPUs they use. These manuals are your primary source of information. Look for manuals specific to the MPU model used in your breaker. These manuals will cover:

Available Protection Functions

What protection features are built into the MPU (e.g., 50/51, 50G/51G, 27, 59, etc.).

Setting Ranges

The adjustable ranges for each protection function (e.g., overcurrent pickup range, time dial settings).

TCC Curves

The MPU manual will contain families of TCC curves for the overcurrent protection functions (50/51, 50G/51G). These curves will be defined by parameters you set in the MPU.

Programming and Configuration

Instructions on how to program and configure the MPU settings.

Communication Protocols

Information on communication protocols (e.g., Modbus, DNP3) for remote monitoring and control.

Breaker Datasheets

The breaker datasheet itself will often provide basic information on the MPU type installed and refer you to the specific MPU manual.

Contact the Manufacturer

If you can't find the specific manual you need, contact the breaker manufacturer (e.g., Siemens, ABB, Eaton, Schneider Electric). They can provide the correct manual or technical support.2.

Software Tools



Coordination Software

Software such as EasyPower, SKM PowerTools, ETAP, and DigSilent PowerFactory are essential for MV protection coordination studies. These programs have libraries of breaker models (including MV breakers) with their associated TCC curves. You can select your specific breaker model, input its settings, and then the software will generate the TCC curve based on the MPU's characteristics. These tools also allow you to simulate faults and analyze the coordination between different protective devices.

Manufacturer's Software

Some manufacturers provide their own software tools specifically designed for setting and testing their protection relays and MPUs. This software often has built-in curve drawing and coordination features.

Relay Test Sets

Devices like Omicron CMC series and Doble F6150e are used for testing and commissioning protective relays and MV breakers. They can inject simulated fault currents and verify the breaker's trip time according to the programmed TCC curve.3.

Steps to Determine MPU Settings and TCC



Load Study/Fault Study

Perform a load flow and short-circuit study of your system. This is crucial to determine the maximum load current, the available fault current at the breaker location, and the coordination requirements.

Protection Philosophy

Define your protection philosophy. What are you trying to protect (e.g., motor, transformer, bus)? What level of selectivity do you need?

Select Protection Functions

Based on your protection philosophy, select the appropriate protection functions in the MPU (e.g., 50/51 for overcurrent, 50G/51G for ground fault, 27/59 for voltage protection).

Set Pickup Values

Determine the pickup value for each protection function. This is the current or voltage level at which the protection function will initiate. For example, the overcurrent pickup (50/51) should be set above the maximum load current, but below the minimum fault current.

Set Time Delay

Determine the appropriate time delay for each protection function. This is the amount of time the function will wait before tripping the breaker. Time delays are crucial for coordination.

Generate TCC Curve

Using the MPU manual or coordination software, generate the TCC curve based on your settings. The curve will show how the breaker will respond to different fault current magnitudes.

Coordination Study

Perform a coordination study to ensure that your breaker's TCC curve is coordinated with other protective devices in the system. This means that the breaker closest to the fault should trip first, isolating the fault while keeping the rest of the system online.

Testing and Commissioning

Once the settings are determined, thoroughly test the breaker and MPU to verify that they are operating correctly. Use a relay test set to inject simulated fault currents and confirm the trip times.

Important Considerations



Coordination

Coordination is critical in MV systems. Improper coordination can lead to unnecessary outages and equipment damage.

Ground Fault Protection

Pay close attention to ground fault protection (50G/51G). MV systems are often grounded using various methods (e.g., solid grounding, resistance grounding), and the ground fault protection settings should be appropriate for the grounding method.

Motor Starting

If the breaker is protecting a motor, you must consider the motor starting current (inrush current). The overcurrent protection settings must be high enough to allow the motor to start without tripping the breaker.

Arc Flash

Arc flash is a significant hazard in MV systems. Protection schemes should be designed to minimize arc flash duration and incident energy. Many modern MPUs have arc flash mitigation features.

Regular Testing

MV breakers and their protection systems should be tested and maintained regularly to ensure reliable operation.

In Summary: MV breaker manufacturers provide comprehensive manuals for the MPUs used in their breakers. These manuals are the key to understanding the available protection functions, setting ranges, and TCC curves. Coordination software is essential for performing protection coordination studies and generating TCC curves.* Proper settings and coordination are critical to ensure reliable system protection and minimize the impact of faults. Flag for review