Stray Flux and its Influence on Protection Relays

Authors: Z. Gajic, S. Holst, ABB AB, SA Products, Sweden, D. Bonmann, ABB AG, Transformers, Germany, and D. Baars, ELEQ bv. Netherlands

The bar-primary (i.e. ring-core; window type) current transformers are typically designed assuming that the flux in the CT magnetic core is homogeneous and only caused by the current flowing in the CT primary conductor. Thus, this means that:

  • The primary CT conductor is ideally centered in the middle of the CT toroidal magnetic core
  • The primary CT conductor is straight and infinitely long; and
  • There are not external magnetic fields which can cause additional flux in any part of the CT core

However, in practice the primary conductor is never straight and infinitely long and the CTs are commonly installed in a three-phase system. Thus, at least the magnetic fields from the other two phases are present in the vicinity of the CT. These “external magnetic fields” may under certain circumstances produce significant stray flux in the CT magnetic core, which can cause problems for protection systems connected to that CT.

As shown in Figure 1, the stray flux will split into two paths through the CT core. Thus, at one side of the CT core the resultant flux will be equal to the sum of the “usual flux” caused by the CT primary current and the stray flux, while at the other side of the CT core the resultant flux will be equal to the difference between the “usual flux” and the stray flux. Obviously the resultant flux will have different values in different parts of the CT core and a partial CT saturation may occur.

There are quite a number of papers published regarding CT accuracy under such operating conditions. Surprisingly very few papers discuss the influence of the stray flux on the relay protection systems. Even in some of the above mentioned papers it is stated that stray flux should not produce big impact on the relay protection. This might be true for the relays with time delayed operation such as phase or ground overcurrent relays. However, stray flux can easily cause unwanted operation of the instantaneous and sensitive relays like differential protection. Note that both high impedance and low impedance differential protection relays can be affected by this phenomenon.

Testing in the Laboratory
The laboratory testing was performed on the two CT cores designated CT #1 and CT #2 as shown in Figure 2.  Both CT cores have the ratio 800/1A with a relative small core cross section. The only important difference between the two CT cores is rating data and consequently the magnetic core cross section area. These two CT cores had the following ratings:

  • CT #1: 800/1A; 5P30; 10VA with  core cross section area of 17.1cm2
  • CT #2: 800/1; 5P5; 10VA with core cross section area of 1.9cm2

As shown in Figure 3, the stray flux influence is tested by positioning the CT core close to an adjacent primary conductor. Figure 3a was taken by digital camera during laboratory testing, while Figure 3b represents the simplified geometrical view of the test setup. The distance X is 6cm during these tests.  The test was done by applying the 50Hz, AC current with the RMS value of 6.5kA. The primary current could be injected with or without a DC offset. The applied current through the primary conductor and the CT secondary current were recorded by an oscilloscope as Channel 1 and Channel 2 respectively. These two waveforms are given in Figure 4. In the Figure 4a, the two waveforms are given when the DC offset is present in the primary current. The CT secondary current with maximum peak of 1.5A was recorded during this test. In Figure 4b, the two waveforms are given for the symmetrical primary current with AC RMS magnitude of 6.5kA. The recorded peak of the CT secondary current during this test reaches 0.2A. Note that secondary current spikes are only observed during testing of CT #2 and not during testing of CT #1. Consequently it can be concluded that CT core with bigger cross section area of the core has potentially less problems in the actual installation.

Relion advanced protection & control.
BeijingSifang June 2016