SELECTION OF MEASURING CURRENT TRANSFORMERS USED BY DIFFERENTIAL RELAY PROTECTION
The operating principles of measuring current transformers used by differential relay protection and their classification are reviewed. Recommendations for the selection of current transformers are provided.
Keywords: current transformer, differential relay protection, short circuit.
The selection of measuring current transformers plays an important role in the implementation of differential protection. For accurate comparison of currents by differential relay protection, primary currents must be transformed with sufficient accuracy and with correct polarity.
Due to incorrect polarity connection or the presence of transformation errors, false differential currents arise. This leads to a threat to the stability of protection functioning due to the flow of through-fault currents through the protected object. Significant errors can also occur as a result of current transformer saturation. When implementing differential protection, it is always necessary to strive to select current transformers with identical characteristics and construction. If the protection does not provide additional restraint during current transformer saturation, the above-mentioned points must be taken into account. This remark is valid for protection implemented based on electromechanical and most static analog relays. However, despite this, digital protection devices allow a high degree of current transformer saturation. This is possible due to a built-in saturation detector, which prevents incorrect protection actions.
The operating principle of a power transformer and a current transformer are identical. In normal operation, the magnetic induction is insignificant compared to the saturation magnetic induction. With an increase in the primary current and a voltage drop across the connected secondary load, the magnetic induction also increases proportionally. Therefore, when selecting a current transformer, it is necessary to consider the requirements for transforming the periodic component of the fault current without saturating the current transformer.
When considering protection systems, it is possible to neglect the leakage magnetic induction of the current transformer. The simplified equivalent circuit is shown in the figure.
Equivalent circuit of a current transformer
IEC 60044-8-2010 standard establishes requirements for current transformers in transient mode when transforming fault current, taking into account the aperiodic component. Depending on the design of their core, this standard distinguishes four classes of current transformers:
1. Class TPS – current transformer with a closed core with very low leakage reactance. For this class, the magnetization characteristic and the secondary winding resistance determine the transformation capability.
2. Class TPX – current transformer with a closed core without limitation of residual magnetic induction. Its selection requires additional determination of transient performance requirements.
3. Class TPZ – current transformer with a linearized core (residual induction is negligibly small). The specified accuracy is applicable only when transforming the periodic component of the current. However, the aperiodic component is characterized by strong damping.
4. Class TPY – current transformer with an air gap to reduce the level of residual induction (residual induction <10%). Like Class TPX, its selection requires determining transient performance requirements.
The following limiting errors correspond to the highlighted classes (Table 1).
Current Transformer Classes According to IEC 60044-8-2010 Standard
Table 1
| Class | Погрешность при ном. токе: | Maximum instantaneous error at the limiting primary current value | |
|---|---|---|---|
| токовая | угловая | ||
| ТРХ | ±0,5% | ±30 мин | ϵ<10% |
| ТРY | ±1,0% | ±60 мин | ϵ<10% |
| ТРZ | ±1,0% | ±180 ±18 мин | ϵ<10% (Periodic component) |
Current transformers of the first two classes transform both aperiodic and periodic components within a certain range of values with high accuracy. These types of current transformers are characterized by a very high level of residual magnetic induction, which can exceed 80%. When transforming current with an aperiodic component, almost all generated magnetic induction remains in the core. To eliminate it, the current transformer must be demagnetized.
This can lead to a situation where, during a subsequent auto-reclosure (ARC), if the power breaker contacts close at an unfavorable moment, the magnetic induction will double in value. Consequently, for ARC application, a current transformer with a twice larger core cross-section must be selected.
The presence of air gaps in the current transformer core significantly reduces the level of residual induction. They reduce the demagnetization time to 1 second or less. Nevertheless, the secondary circuit time constant cannot be significantly reduced. This is because, in such a case, the aperiodic component of the short-circuit current will not be transformed correctly. Thus, the lower limit is approximately 200 - 300 ms. As a result, a Class TPY current transformer is only partially demagnetized during the currentless pause.
The presence of large air gaps, characteristic of Class TPZ current transformers, leads to a fairly strong damping of the aperiodic current component during transformation. This significantly reduces the increase in magnetic induction. Due to the fact that the current transformer's time constant is approximately 60 ms, the core demagnetizes in less than 200 ms. Consequently, by the time ARC is performed, the magnetic induction will decrease to 0. This allows for significant savings on core cross-section.
The frequency and amplitude of the aperiodic component of the short-circuit current, as well as the level of residual magnetic induction of the current transformer, are considered only in a small number of statistical analyses.
Fault current contains only an aperiodic component in very rare cases. A low probability of such a situation exists during lightning strikes. In most cases, the percentage of aperiodic component content does not exceed 70%.
Canadian researchers analyzed the residual induction level of current transformers. According to their results, only for 27% of 141 current transformers were residual induction values found to be equal to 60...79% The worst case is the superposition of the maximum aperiodic component and maximum residual induction. This situation is quite rare in practice. In such a case, if the use of a current transformer with a closed magnetic core is planned, such a current transformer will be very cumbersome and expensive. Therefore, such conditions are usually not considered in practice. To account for the possible auto-reclosure onto a sustained short circuit, a current transformer with characteristics that allow operation without saturation is selected. For this purpose, current transformers with air gaps of Class TPY or TPZ are used. The required limiting transformation ratio of the current transformer is determined based on the nature of the change in magnetic induction.
IEC 60044-8-2010 provides detailed recommendations for calculations, as well as examples thereof.
For the selection of current transformer characteristics, it is first necessary to consider the recommendations of the manufacturers of the protection devices used. They allow determining the minimum required limiting ratio or the corresponding knee point voltage for the maximum through-fault current and according to the minimum required operating time without saturation after the occurrence of a short circuit. Line differential protection additionally requires that the limiting ratios for current transformers at both ends of the line do not differ significantly.
When selecting measuring current transformers for differential relay protection, the stability of protection operation during through-fault current flow (external short circuit mode) must be considered. That is, it is necessary to ensure a minimum limiting operating ratio, depending on the magnitude of the maximum through-fault current.
A current transformer is primarily required to be able to transform the maximum periodic component of the through-fault current without saturation, which allows for a significant degree of saturation in the presence of an aperiodic component.
In the case of busbar differential relay protection, the situation is more complex. This is due to the fact that fault currents from several power sources flow into the protected object via separate connections and flow out of it as the sum of short-circuit currents from all power sources through the damaged connection. For this reason, the busbar protection device must allow a significant degree of saturation of the measuring current transformer. For busbar differential protection, correct current transformation is allowed only for (3) ms to ensure stable non-operation. This means that the current transformer must be capable of transforming half of the maximum total fault current.
To achieve tripping stability during internal short circuits, it is necessary to ensure correct current transformation without saturation for the set minimum time to allow the formation of a trip command without a time delay. This condition can become decisive in cases where the short-circuit current is quite significant. This situation is relevant for differential relay protection of a power transformer. For an internal short circuit close to the bushings, the fault current will be very large. In contrast, during an external short circuit, the fault current will be significantly smaller due to the consideration of the power transformer's resistance.
More detailed recommendations for the selection of current transformers are provided in the documentation for protection devices or in the calculation guidelines provided by manufacturers.