Package Information:
- Trainer: Dr. Saeed Roostaee (Profile)
- Recorded video with offline access (lifetime access)
- Certificate of completion
- Total time: 9 hours & 52 minutes
- Course creation: Feb 2025
- Last update: Feb 2025
Training Content:
Case Study SIPROTEC 5 7UT Configuration
Single-line diagram and case study diagram study
SIPROTEC 5 7UT8 Configuration (Measuring points, functions, FG connection, Masking IO, settings, LCD config)
Application Templates
7UT8 Diff settings and signals
7UT8 REF settings and signals
7UT8 87N or REF
SIPROTEC 5 7UT Testing and commissioning
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SIPROTEC 5 7UT Notes
Basic Principle of Differential Protection
Depending on the basic type of device selected, it can protect different system configurations. In the simplest implementation (7UT82), this is a two-winding transformer. The 7U T 85 protects a two-winding transformer or an autotransformer. The 7UT86 processes 3 sides or 3 measuring points. In a function group, 2 differential protection functions can run in parallel for auto-transformer applications. In the maximum implementation (7UT87), up to 5 sides or 7 measuring points can be processed. Furthermore, the 7UT87 can process 2 function groups of Transformer differential protection.
Additional Stabilization with External Short Circuits
In an external short circuit, high short-circuit currents flowing through can lead to current transformer saturation. This saturation can be more or less strongly pronounced at the measuring points, and in this way simulate a differential current. The differential current can lie in the tripping range of the characteristic curve for a certain time and lead to unwanted tripping without any particular measures.
on One Side
- High-current, external short circuit
First, the current on both sides is transferred and leads to a large restraint current. After this, a transformer becomes saturated, resulting in a differential current Idiff that can exceed the operating curve. Simultaneously, the restraint current Irest. drops out. Once the transformer is no longer saturated, the
differential current decreases and falls below the characteristic curve. - Low-current, external short circuit
Current transformers can also become saturated through the large direct-current time constant (switching of transformers, motors), but small currents flow. This manifests itself as a phase angle rotation of the current. Exceeding the characteristic curve in a non-stabilized area is possible.
Intelligent saturation detection methods capture this state and lead to a temporally limited blocking of the Differential protection function.
Inrush Current and Overexcitation Stabilization
When a transformer is switched on, an inrush current with a DC component flows, since the flow cannot change itself erratically. The inrush current develops in all phases and appears immediately as a differential current. The direct-current time constant can amount to several seconds with large transformers. The following figure shows a typical situation.
If a transformer is operated with too high of a voltage, it can be modulated up into the non-linear part of the magnetization characteristic curve. An increased magnetizing current results, which manifests itself as an increased differential current and can lead to an overfunction.
For this reason, additional functions are required that block the differential protection with closure and overexcitation processes.
Reference winding
The largest power is used as reference winding with a multi-winding transformer. If several windings have the same absolute value of power, then, the highest current winding is selected as the reference winding.
The absolute value of the neutral-point transformer is also adapted according to the previous formula. In this case, the absolute value should be adapted to the side with the greatest power.
Inrush-Current Detection
The inrush-current detection evaluates the calculated instantaneous values of the differential current. The inrush current develops with Sympathetic Inrush – connection of a parallel transformer or Recovery Inrush – inrush current after returning voltage after a fault in the system in the differential current considerably higher than in the side currents. An assignment to the corresponding blocking phases is possible only in the differential current due to the vector-group correction. The inrush-current detection is activated when the pickup characteristic is exceeded. It is deactivated when it is blocked by an external fault.
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