Verifying The Zero-Sequence Current Reduction From The Winding One Side - Hitachi RET670 Commissioning Manual

Table of Contents

1MRK 504 165-UEN Rev. K

The PSTPDIF function is self-adaptive. Thus, special measures and procedures shall be used during

testing. Off-the-shelf available test sets with associated software and their testing routines for

standard transformer differential protection will not work, in general, for this function. They may work

for special case, when the phase angle shift is zero degree (i.e. as for Yy0 standard power

transformer). However, for specific phase angle shift (e.g. tap position) of the protected PST, which is

different from zero degree, this standard routines cannot be used.

To test this function, it is strongly recommended to use a test set with six current sources and then

follow the sequence-wise testing procedure as explained for the traditional transformer differential

protection in the paper Easy and Intuitive Method for Testing Transformer Differential Relays

published by Hitachi Power grids. This paper is available on the Hitachi Power grids website.

To prepare the function to be tested for specific phase angle shift (e.g. specific tap position) of the

protected PST, proceed as follows:

Make sure that the binary input STOPCOMP into PSTPDIF function has the logical value zero.

Wait for five seconds and inject the rated load using the positive sequence current systems

only on both sides of the PST, which correspond to the set default transformation ratio and

phase angle shift (typically, zero degree). Instructions on magnitude calculation and phase

angles for the six currents to be injected are available in the above mentioned paper. When

currents are injected correctly, check that all three differential currents are approximately 0%

and the bias current is 100%. These values are available as outputs and service values from

the PSTPDIF function.

Slowly (for example, one degree per second) rotate the positive sequence currents (i.e. all

three phases together) for winding two side until the required phase angle shift is achieved.

Note that the rotation in anti-clock-wise direction of winding two positive sequence current will

result in corresponding but negative value of the estimated phase angle shift displayed on the

IED. When PST is of an asymmetrical design, the magnitudes of the currents on the winding

two side shall also be adjusted accordingly. Once the desired tap is reached, make sure that all

three differential currents are still approximately 0% and the bias current is still 100%. Also,

check that DIFRATIO and DIFANGLE outputs have value that corresponds to the PST tap

position at which the function needs to be tested.

Apply the logical value one to the binary input STOPCOMP into PSTPDIF function. The

function will now remember the transformation ratio and phase angle shift, which corresponds

to this particular PST tap. These two values will be stored and used as long as the input

STOPCOMP has a logical value one. To perform this, the IED shall be configured accordingly

beforehand. Typically, one front LHMI key or one binary input can be configured in the

configuration tool to perform this task. If additional security is needed, this action can be only

allowed when the IED is in Testmode.

Interrupt all six currents from the test set and verify that DIFRATIO and DIFANGLE outputs still

have value that corresponds to the PST tap position, at which the function needs to be tested.

The function is now prepared to be tested at this particular tap position in a similar way as the

standard transformer differential protection.

The following sections explain how the PSTPDIF shall be tested for this particular tap position. Note

that, the above procedure and the following tests shall be repeated for every PST tap position, at

which the function needs to be tested.

Note that the following tests can be used for any dedicated tap-position available in the protected

PST. In actual installation, perform these tests for two extrema tap positions, mid-tap position and

one or two more freely selected tap-position(s).

11.3.5.2