ABB RELION 620 Series Technical Manual page 669

1MAC504801-IB E
620 series ANSI
Technical Manual
Tap position decoder
When there is a wired connection to the TAP_POS input connector, the corresponding tap
changer position is decoded from the mA or RTD input. When there is no wired connection
to the TAP_POS connector, the binary inputs are expected to be used for the tap changer
position information. The tap changer position value and quality are internally shared to
other functions. The value is available through the Monitored data view.
The function has three alternative user selectable operation modes: "NAT2INT",
"BCD2INT" and "GRAY2INT". The operation mode is selected with the Operation mode
setting. Each operation mode can be used to convert a maximum of 6–bit coded input to
an 8–bit signed short integer output. For less than 6–bit input, for example 19 positions
with 5 bits when the BCD coding is used, the rest of the bits can be set to FALSE (0).
The operation mode "NAT2INT" is selected when the natural binary coding is used for
showing the position of the transformer tap changer. The basic principle of the natural
binary coding is to calculate the sum of the bits set to TRUE (1). The LSB has the factor
1. Each following bit has the previous factor multiplied by 2. This is also called dual
coding.
The operation mode "BCD2INT" is selected when the binary-coded decimal coding is
used for showing the position of the transformer tap changer. The basic principle with the
binary-coded decimal coding is to calculate the sum of the bits set to TRUE (1). The four
bits nibble (BI3...BI0) have a typical factor to the natural binary coding. The sum of the
values should not be more than 9. If the nibble sum is greater than 9, the tap position output
validity is regarded as bad.
The operation mode "GRAY2INT" is selected when the binary-reflected Gray coding is
used for showing the position of the transformer tap changer. The basic principle of the
Gray coding is that only one actual bit changes value with consecutive positions. This
function is based on the common binary-reflected Gray code which is used with some tap
changers. Changing the bit closest to the right side bit gives a new pattern.
An additional separate input, SIGN_BIT, can be used for negative values. If the values
are positive, the input is set to FALSE (0). If the SIGN_BIT is set to TRUE (1) making
the number negative, the remaining bits are identical to those of the coded positive
number.
The tap position validity is set to good in all valid cases. The quality is set to bad in invalid
combinations in the binary inputs. For example, when the "BCD2INT" mode is selected
and the input binary combination is "0001101", the quality is set to bad. For negative
values, when the SIGN_BIT is set to TRUE (1) and the input binary combination is
"1011011", the quality is set to bad.
Section 7
Measurement functions
663