Search

Premium Membership

Get the essence of electrical engineering in the form of specialized HV/MV/LV technical articles, electrical guides, and papers.

Home / Download Center / Electrical Engineering Books and Technical Guides / Relay control and protection guides / Advanced protection scheme for power transformers based on IEC 61850 standard

Introduction to transformer protection

Power transformers of medium and large sizes are very critical and vital components for power systems. Due to its significance and cost, its protection needs to be appropriately addressed. Transformer protection should be fast and reliable. To provide early cautioning of electrical failures and prevent disastrous losses, appropriate monitoring of power transformer should be selected.

The IEC 61850 standard-based protection scheme for power transformers
The IEC 61850 standard-based protection scheme for power transformers (photo credit: SEL)

This results in the damage limit and the reliability improvement of the power supply. The requirements of the protective relays include dependability (no missing operations), security (no false tripping), speed of operation (short fault clearing time) and stability. Therefore, a transformer differential relay is used to meet the protection requirements of the medium and large power transformers.

The differential scheme approach compares the currents at the primary and secondary on the protected zone of the transformer by calculating and monitoring a differential current.

In case the computed value of the differential current is greater than the set value, this indicates an internal fault.

A switching-in or an external fault recovery can cause a sudden change in the input terminal voltage of a transformer and the large current drawn by the transformer from the supply is known as inrush current. Energising a transformer which is in parallel with a transformer that is already in service can cause a similar magnetizing inrush condition, known as “sympathetic inrush”.

The inrush condition results in the saturation of the transformer core. Magnetizing inrush current that arises in a transformer is identified by comparing the polarity and magnitude of residual flux which does not correspond to polarity and magnitude of an ideal instantaneous value of steady-state flux.

Magnetizing inrush current can be as high as ten times of full load current. The inrush condition phenomenon would typically cause the trip element of the transformer differential to mis-operate, if not adequately blocked or restrained.

Magnetizing inrush current phenomenon (no residual flux initially in transformer)
Figure 1 – Magnetizing inrush current phenomenon (no residual flux initially in transformer)

Transformer inrush currents usually are rich in harmonics in general and in second harmonic particularly. The second-harmonic ratio is traditionally used for transformer differential protection in order to block or restrain the differential trip elements during transformer magnetizing inrush current conditions.

Presently there are three types of schemes that are being used for the magnetizing inrush current determination:

  1. First scheme makes use of data obtained from the transformer incoming currents only. The method is based on the principle of second harmonics restraint.
  2. Second scheme makes use of information that is obtained from the transformer terminal voltage variation. This method is based on the voltage restraint principle.
  3. Third scheme makes use of information that is obtained from both the transformer’s currents and voltages. This method is based on the flux characteristic principle using the low-voltage acceleration criterion.

This thesis used the first scheme which is the second harmonic restraint method for magnetizing inrush current determination. Extra reliability to the power system is provided by the backup overcurrent protection schemes.

Backup overcurrent relaying scheme is attached to the main protection with its own relaying system. The primary role of the backup overcurrent relay is to operate in case of any failure or tripping of the circuit breakers due to the main protection.

The main protection which is the transformer differential scheme may fail due to:

  1. The mechanical defect of moving regions of the transformer differential relay,
  2. Transformer differential relay DC supply failure
  3. Tripping pulse of the transformer differential relay failure to the breaker
  4. Current or voltage supply failure to the transformer differential relay from CT or VT circuits

However, this thesis is not considering the above failure conditions of the transformer differential scheme. In this specific situation, another type of protection called backup overcurrent relaying scheme is applied. Hence, backup overcurrent relaying scheme has every configuration setting separate from the main transformer differential protection.

Reverse harmonic blocking scheme for transformer protection
Figure 2 – Reverse harmonic blocking scheme for transformer protection

The reason is the backup overcurrent relay must not fail to operate in case of the failure of the main protection scheme. As a backup overcurrent protection scheme, it must be slower in action than the main differential protection one, so that it should only work in case the main differential protection scheme of the transformer fails.

With reference to Figure 2 above, the transformer differential relay has the magnetizing inrush current function, which blocks the differential relay from tripping for inrush conditions. Nevertheless, the overcurrent relay employed as backup protection to the transformer differential protection scheme does not have the inrush current function, and it will trip during inrush conditions.

This thesis used the differential relay SEL-487E as the main protection and backup overcurrent protection SEL-751A.

Left: Differential relay SEL-487E as the main protection; Right: Backup overcurrent protection relay SEL-751A
Figure 3 – Left: Differential relay SEL-487E as the main protection; Right: Backup overcurrent protection relay SEL-751A

In order to restrain SEL 751A overcurrent relay from tripping during inrush conditions, a blocking scheme based on second harmonic restraint current is employed. The second harmonic restraint scheme uses the harmonic blocking element (87HB) of the SEL-487E to send a blocking signal to the SEL 751A to inhibit it from tripping during inrush current conditions.

The IEEE 14-bus system is simulated in the DIgSILENT and RSCAD software environments to generate a fault and inrush current conditions. The lab-scale test bench setup is implemented to test the phase percentage differential protection schemes using SEL-487E IED and Omicron CMC 356 test injection device.

Omicron CMC 356 test injection device
Figure 4 – Omicron CMC 356 test injection device

The demonstration of the IEC 61850 standard-based reverse harmonic blocking scheme is implemented in the CSAEMS laboratory using Omicron test injection device CMC 356, SEL-487E and 751A protection IEDs. Finally, implementation and testing of the IEC 61850 standard-based hardware-in-the-loop simulation are performed using RTDS and protection IEDs.

The hardware-in-the-loop simulation is conducted for external and internal faults and inrush current conditions

Transformer differential and overcurrent protection scheme test bench
Figure 5 – Transformer differential and overcurrent protection scheme test bench

The research aim is to investigate hardwired and IEC 61850 GOOSE applications to the transformer protection schemes. The test bench setup provides the lab scale illustration of how the physical power transformer is protected by using differential protection (SEL-487E), and backup overcurrent (SEL-751A) relays as shown in Figure 5.

The current signals (CT2 and CT3) on both (LV and HV) sides of the power transformer are injected into the S and T windings current channels of the SEL-487E using the OMICRON test injection device. The CT1 current signals are injected into the SEL-751A current channels. Each end of the power transformer is connected to the switching device circuit breakers (CB1 and CB2) respectively.

For internal events, the SEL-487E and SEL-751A IEDs current coils are energised and send trip signals to the binary contact of the test injection device as shown in Figure 1.2.

The pickup and trip signal of the circuit breakers are represented using the binary signal connected to the output port (OUT101) of the SEL-487E and (OUT102 and OUT103) of the SEL-751A, which are mapped to the binary inputs 1 and 2 of the test injection device as shown in Figure 5.

Title:Advanced protection scheme for power transformers based on IEC 61850 standard – Bwandakassy Elenga Baningobera; Thesis submitted in fulfilment of the requirements for the degree Master of Engineering: Electrical Engineering in the Faculty of Engineering At the Cape Peninsula University of Technology
Format:PDF
Size:6.7 MB
Pages:357
Download:Right here | Get Download Updates | Get More With Premium Membership
Advanced protection scheme for power transformers based on IEC 61850 standard
The IEC 61850 standard-based protection scheme for power transformers

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

2 Comments


  1. Ajetomobi Balogun Olagbenga
    Sep 16, 2020

    Good article, very educative.

    • Edvard
      Edvard
      Sep 16, 2020

      Thank you Ajetomobi, feel free to download the Study (PDF).

Leave a Comment

Tell us what you're thinking... we care about your opinion!

Subscribe to Weekly Digest

Subscribe to free HV/MV/LV technical articles, electrical guides and papers.