Transformer differential protection

Transformer differential protection – SIPROTEC 7UT87


The SIPROTEC 7UT87 transformer differential protection has been designed specifically for the protection of multi-winding transformers (up to 5 sides). Furthermore, it is to be used where numerous measuring points (up to 9 3-phase current measuring points) are required. Another application is simultaneous protection of two parallel transformers (additional fast backup protection). The SIPROTEC 7UT87 is the main protection for the trans- former and contains many other protection and monitoring functions. The additional protection functions can also be used as backup protection for subsequent protected objects (such as cables or lines). With its modular structure, flexibility and the powerful DIGSI 5 engineering tool, SIPROTEC 7UT87 offers future-oriented system solutions with high investment security and low operating costs.

Features & functions

Transformer differential protection SIPROTEC 7UT87


Main function:

  • Up to 3 differential protection functions with additional stabilization (in different transformer function groups); up to 5 ground fault differential protection functions.
  • For auto transformer applications, two differential protection functions can be processed in an Auto transformer function group.
  • Interoperability of SIPROTEC 5 and SIPROTEC 4 line protection devices

Usable measuring points:

  • 9 x 3-phase current measuring points,
  • 5 x 1- phase current measuring points,
  • 5 x 3-phase voltage measuring points

Inputs and outputs:

2 predefined standard variants with

  • 20 current transformers, 
  • 4 voltage transformers, 
  • 15 to 27 binary inputs, 
  • 22 to 38 binary outputs

Hardware flexibility:

  • Flexibly adjustable and expandable I/O quantity structure within the scope of the SIPROTEC 5 modular system.

Width of housing:

  • 2/3 × 19 inches to 2/1 × 19 inches

As standard:


DIGSI 5 permits all functions to be configured and combined as required. In SIPROTEC 7UT87, two transformer function groups can be used.

  • Transformer differential protection for multi-winding transformers with versatile, additional protection functions (multi-winding transformers are typical in power converter applications (such as HVDC))
  • Transformer differential protection for phase angle regulating transformers of the single core and two core types, and special transformers
  • Transformer protection applications with up to seven three-phase current measuring points
  • Simultaneous differential protection for 2 transformers (such as two 2-winding transformers)
  • Universal usability of the permissible measuring points
  • Applicable from average up to extra high  voltage
  • Protection of standard power transformers, auto transformers and motors
  • Typical properties of a transformer differential protection  such as flexible adaptation to the transformer vector group, control of inrush and overexcitation  processes, safe behavior in case of current-transformer saturation with different degrees of saturation
  • Fault locator plus for accurate fault location with inhomogenous line sections and targeted automatic overhead-line section reclosing (AREC)
  • Arc protection
  • Voltage controller  function ANSI 90V for two-winding transformers, three-winding transformers and grid coupling transformers with parallel control (master/follower, circulating reactive current minimization)
  • Adaptive adaptation of the operate curve to the transformer tap position
  • Increased sensitivity with near-neutral-point ground faults through a separate restricted ground-fault  protection
  • Point-on-wave switching
  • Additional current and voltage inputs can be supplements for standard protection functions, such as overcurrent, voltage frequency, etc.
  • Graphical logic editor to create powerful automation functions in the device
  • Up to 4 pluggable communication modules, usable for different and redundant protocols (IEC 61850-8-1, IEC 61850-9-2 Client, IEC 61850-9-2 Merging Unit, IEC 60870-5-103, IEC 60870-5-104, Modbus TCP, DNP3 serial and TCP, PROFINET IO)
  • Reliable data transmission via PRP and HSR redundancy protocols
  • Extensive cyber security functionality, such as role-based access control (RBAC), protocolling security-related events or signed firmware
  • Simple, quick and secure access to device data via a standard Web browser – without additional software
  • Secure serial protection data communication, also over great distances and all available physical media (optical fiber, two-wire connections and communication networks)
  • Capturing operational measured variables and protection function measured values for the evaluation of the system, to support commissioning, and to analyze faults
  • Frequency tracked protection functions over a wide frequency range (10 Hz to 80 Hz) and the option to assign the protection functions in a single device to different frequency tracking groups 
  • Phasor Measurement Unit (PMU) for synchrophasor measured values and IEEE C37.118 protocol
  • Powerful fault recording (buffer for a max. record time of 80 sec. at 8 kHz or 320 sec. at 2 kHz)
  • Auxiliary functions for simple tests and commissioning
  • Flexibly adjustable I/O quantity structure within the scope of the SIPROTEC 5 modular system


The SIPROTEC 7UT87 is intended for special applications (up to 5 sides and 7 measuring points), therefore, you must create your own application template depending on the application and store it in the device. To make work easier, you can use an available template and modify it.




Auto transformer with stabilizing winding in breaker-and-a-half layout

  • Differential protection for the complete transformer (auto transformer winding + stabilizing winding)
  • Nodal point protection for the auto winding for recording ground faults (3-phase recording of the neutral-point current)
  • Overload protection, backup protection for the downstream power system
  • Circuit-breaker failure protection
  • Frequency and voltage protection on the upper-voltage side.

Line protection


  • Safe and reliable automation and control of your systems
  • Purposeful and easy handling of devices and software thanks to a user-friendly design
  • Cyber security in accordance with NERC CIP and BDEW Whitepaper requirements
  • Highest availability even under extreme environmental conditions  by “conformal coating“ of electronic boards
  • Powerful communication components warrant safe and effective solutions
  • High investment security and low operating costs due to future-oriented system solutions