Setting Guidelines; Restrained And Unrestrained Differential Protection - ABB RELION 650 Series Applications Manual

Section 6
Differential protection
6.1.2

Setting guidelines

The parameters for the Transformer differential protection function are set via the local HMI or
Protection and Control IED Manager (PCM600).
6.1.2.1

Restrained and unrestrained differential protection

To make a differential IED as sensitive and stable as possible, restrained differential
protections have been developed and are now adopted as the general practice in the
protection of power transformers. The protection should be provided with a proportional bias,
which makes the protection operate for a certain percentage differential current related to the
current through the transformer. This stabilizes the protection under through fault conditions
while still permitting the system to have good basic sensitivity. The bias current can be
defined in many different ways. One classical way of defining the bias current has been Ibias =
(I1 + I2) / 2, where I1 is the magnitude of the power transformer primary current, and I2 the
magnitude of the power transformer secondary current. However, it has been found that if the
bias current is defined as the highest power transformer current this will reflect the difficulties
met by the current transformers much better. The differential protection function uses the
highest current of all restrain inputs as bias current. For applications where the power
transformer rated current and the CT primary rated current can differ considerably,
(applications with T-connections), measured currents in the T connections are converted to pu
value using the rated primary current of the CT, but one additional "measuring" point is
introduced as sum of this two T currents. This summed current is converted to pu value using
the power transformer winding rated currents. After that the highest pu value is taken as bias
current in pu. In this way the best possible combination between sensitivity and security for
differential protection function with T connection is obtained. The main philosophy behind the
principle with the operate bias characteristic is to increase the pickup level when the current
transformers have difficult operating conditions. This bias quantity gives the best stability
against an unwanted operation during external faults.
The usual practice for transformer protection is to set the bias characteristic to a value of at
least twice the value of the expected spill current under through faults conditions. These
criteria can vary considerably from application to application and are often a matter of
judgment. The second slope is increased to ensure stability under heavy through fault
conditions which could lead to increased differential current due to saturation of current
transformers. Default settings for the operating characteristic with
transformer rated current can be recommended as a default setting in normal applications. If
the conditions are known more in detail, higher or lower sensitivity can be chosen. The
selection of suitable characteristic should in such cases be based on the knowledge of the
class of the current transformers, availability of information on the load tap changer position,
short circuit power of the systems, and so on.
The second section of the restrain characteristic has an increased slope in order to deal with
increased differential current due to additional power transformer losses during heavy loading
of the transformer and external fault currents. The third section of the restrain characteristic
decreases the sensitivity of the restrained differential function further in order to cope with CT
saturation and transformer losses during heavy through faults. A default setting for the
operating characteristic with
conditions are known in more detail, higher or lower sensitivity can be chosen. The selection of
suitable characteristic should in such cases be based on the knowledge of the class of the
current transformers, availability of information on the tap changer position, short circuit
power of the systems, and so on.
64
The three winding transformer differential protection function can be also used
in variety of application for two winding transformer protection. In such
application, it is suggested to set the tertiary winding (W3) rated current and
rated voltage to the exact same rating as secondary winding (W2).
IdMin = 0.3 * IBase is recommended in normal applications. If the
1MRK 504 158-UEN A
IP14936-1 v2
M15266-4 v5
M15266-289 v10
IdMin = 0.3pu of the power
Application manual