Influence of CT Saturation on the Protection System

Authors: Dr. Peter Meinhardt, Dr. Michael Freiburg, OMICRON electronics GmbH, Austria

Due to internal or external events, the sinusoidal primary current is sometimes overlain by a DC component (e.g. fault occurrence, switching event, and lightning impact) that temporarily drives the iron core into saturation or causes a residual flux and endangers a proper signal transformation to the secondary side.
In addition to the mentioned internal events, testing and maintenance using DC measurements can also lead to a residual flux in the iron core resulting in the same behaviour: the sinusoidal primary currents with a transient DC component during fault occurrence as well as residual magnetism due to testing, switching or lightning impact may result in secondary distortion. ( Figure 1.)

Depending on the CT design, its power and the connected burden (including the wiring between CT and relay(s) and the burden of the relay input circuit), CT saturation may corrupt the transformed currents up to a point where proper relay performance is impaired, especially during the first few cycles where fast and reliable operation is expected.

There are well-known calculation methods to find out if transient CT saturation will occur for given burden and fault conditions. These theoretical methods have several advantages but they are not considering the real life condition resp. behaviour of the CT and the connected secondary equipment. In practice, often CT designs are (or have to be) chosen that will saturate under adverse conditions.

The questions to be answered in this article are:

  • What are typical CT designs and their design criteria?
  • How can installed CTs be easily tested (test of the burden, CT parameters, the behaviour at fault condition and residual magnetism included)?
  • How do the dedicated CTs behave in real life operation with the connected burden (commissioning test)?
  • How will the connected relays cope with these non-ideal signals? Can a certain amount of saturation be acceptable, i.e. will the relay still trip with acceptable trip time and reach tolerance under all realistic fault conditions?

CT Design Criteria
Depending on the purpose of the CT - metering or protection - different requirements regarding their operation have to be fulfilled. In general, the magnetic characteristics as well as the secondary elements (secondary winding resistance and the secondary leakage flux) are the most important CT design parameters. In terms of accuracy, the primary signal should be ideally transformed to the secondary side according to the turns ratio of the CT. The excitation current bypasses parts of the ideally transformed secondary current from the secondary circuit of the CT and therefore defines the CT error. The higher the magnetic induction (voltage-time-area across the main inductance) the higher the excitation current. This dependence becomes nonlinear when the iron core is saturated. The voltage-time-area is defined by the secondary impedance of the CT which consists of the winding resistance and the leakage inductance in addition to the impedance of the connected burden. The larger the impedance for a given standardized secondary current, the larger the voltage-time-area across the magnetizing inductance and the larger the excitation current (the higher the operating point).

Depending on the applicable standard (IEC 61869-2, IEEE C57.13), additional excitation characteristics requirements, time constants or terminal voltage need to be considered. Considering the aspects above, it is crucial to operate the right CT for intended purpose and to operate CTs with a correct burden. The CT behavior for the operating burden should be determined as part of commissioning tests.

 

Approach
A proper assessment of the reliability of the protection performance in case of CT saturation is only possible by considering all relevant conditions of the site. The most realistic test method would be related to real life operation of the system. Additionally, impedance measurement devices must be available to determine the connected burden including the connections and wires. This test reconstructs real life operation most properly but is connected to several drawbacks. The laboratory approach to thread n windings (e.g. 50) through the primary window of a CT in order to multiply the output current of a testing device in this way usually is not applicable on site either due to the CT construction type or due to the insufficient space remaining with the primary conductor in place. Therefore, very bulky equipment would have to be used. Beside this, it would simply take too much time during commissioning.

The testing solution presented in this article is based on four steps:
  Measurement of the actual burden on site as seen by the CT with connected relay(s) and cable connection
   Measurement of the actual CT data (e.g. on site, with disconnected burden)
  Selection of symptomatic fault data as given by the infeed conditions at the primary CT connection
   Transient simulation of the fault currents (and voltages if needed), including the transient and steady-state saturation derived from the CT and burden data (CT model required in the simulation tool, the model parameters are determined on site)

These signals are injected into the relay in the same way as conventional stepped steady-state test currents and voltages. The relay operation may now be assessed to verify if operation is acceptable under these real-world related conditions.

The described measurements can nowadays be carried out to a large extent in an automated fashion and very efficiently. The transfer of the measured data to the test system is quite straightforward, and the actual simulation is done practically in real-time on location. The readily available equipment required for measurement and test injection is compact and lightweight for easy commissioning use.

The benefits of this 'virtual primary injection test' are the greatly increased knowledge and resulting trust in the actual worst-case relay behavior as affected by the conditions on site. The commissioning staff will know if the possibly unavoidable CT saturation will be acceptable with regards to the relay performance, or if additional measures have to be taken. This approach is valid for distance protection as well as for overcurrent protection. It is also applicable for differential protection in order to verify the proper settings of stabilizing functions against false tripping due to CT saturation.

Measuring Actual CT Burden
A compact CT analyzer supports the measurement of both burden and CT data. For all measurements described in the following, the CT is disconnected from mains and therefore the system is “offline”. For the burden measurement, the burden has to be disconnected from the CT. The burden circuit is connected to the analyzer (Figure2).  In this way the full external burden (including leads and relay input circuit) is included in the measurement, the CT testing device injecting the test current (depending on the rated secondary current) with a 4-wire or Kelvin measurement.

The result of the measurement is a complex impedance. Its angle, though usually fairly small, substantially influences the transient saturation devolution and should be taken into account for the subsequent simulation. It must be noted that a precondition for this simulation approach is the linearity of the system under investigation, i.e. the burden should not change its impedance with varying injected current. The leads fulfill this requirement, same as all current input circuits of relays with auxiliary power supply ('static' / electronic and numeric). This is not necessarily true for electromechanical and other self-powered relays.

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