6. Relay Currents

6.5 Reasons of Mismatch

Non-fault related currents or factors can indeed influence the operation of differential relays. These can cause a differential current to flow even in the absence of a fault. It is necessary to understand these factors and implement compensatory measures to ensure accurate and reliable relay operation. 

6.5.2 Unbalance caused by CT ratios 

It can be challenging to match Current Transformer (CT) ratios exactly on both sides of a transformer, even when the transformer has a fixed ratio. This mismatch results in differential currents flowing in the operating circuits of differential relays, potentially causing undesired relay operations.

• On-load tap changer (OLTC):

The OLTC dynamically adjusts the transformer's turn ratio to maintain a constant secondary voltage despite variations in the primary voltage or load. However, this introduces an additional layer of complexity as the CT ratios should be matched according to the OLTC position.

• De-energized tap changer (DETC):

The DETC is usually adjusted when the transformer is offline to adapt to changes in the overall system condition or load pattern. If the relay settings are not updated accordingly, it can cause a large discrepancy between the actual and expected CT ratios.

• Through-fault condition (a fault condition beyond the zone of protection):

The differential operating current due to mismatch can be significantly large. This could potentially lead to false tripping of the differential relay.

6.5.3 Magnetizing inrush 

Phenomenon:

Magnetizing inrush is a phenomenon that contradicts the fundamental principle of differential relaying. This occurs when the primary winding of a transformer is connected to a source and the secondary winding is connected to loads. In such a situation, magnetizing inrush currents flow from the source to the primary winding while little or no current flows from the transformer's secondary windings.

Situations:

• • Typically associated only with the energizing of a transformer 

  • Can actually be triggered by any sudden change in voltage at the transformer terminals. These voltage transients can be caused by a variety of events, including

    • The occurrence or clearance of a fault 

    • A change in the nature of a fault (for instance, a shift from a single-phase-to-ground fault to a two-phase-to-ground fault)

    • Out-of-phase synchronizing 

Severe magnetizing inrush phenomena:

Energizing of a transformer at a station where at least one other transformer is already energized. This inrush event affects both the already-energized transformers and the transformer being energized, and the transient inrush could potentially last for an extended duration.

Interestingly, the inrush into the transformer being energized happens during the opposite half-cycle to that of the already energized transformer. Consequently, the total inrush into all transformers might resemble into a sine wave of the base frequency. This means that the harmonic restraint element of a differential relay, which might be protecting both parallel-connected transformers, would not be activated.

While the inrush is not more severe in this situation than in a regular inrush, the issue is that there is inrush current flowing from the previously energized transformer to the incoming transformer. The combined inrush to both transformers has minimal second harmonic. As a result, it's preferable to equip each parallel-connected transformer with its own differential relay for optimal protection.

Harmonic content

The harmonic content of the inrush current is influenced by several factors, including 

• Residual flux in the core 

• The switching angle

• The load on the transformer

Analysis reveals that the second-harmonic content of the inrush current is particularly sensitive to these conditions. As the load at a lagging power factor increases, the second-harmonic content noticeably decreases.

Reduction in magnetizing inrush

A significant reduction in magnetizing inrush currents can be achieved by using three single-phase point-on-wave closing circuit breakers to switch a transformer. 

• The strategy in these situations involves estimating the residual flux in the transformer core for each phase. 

• The circuit breaker for each phase is then closed at a point where the maximum flux in the core doesn't substantially exceed the usual maximum flux. 

• This method can effectively manage and reduce the impact of magnetizing inrush currents.

6.5.4 Magnetizing current during overexcitation 

Power Plant Phenomena:

Several phenomena can cause overexcitation in a transformer, sometimes in conjunction with significant overvoltage at the nominal frequency. Some of these occurrences can happen which may involve an overvoltage significantly above nominal:

• During the startup or shutdown of unit-connected generators 

• During static start and pumped hydro starting, the field can be applied at very low frequencies

• Rotor warming operations 

• Excitation system runaway 

• Manual excitation control error

• Sudden loss of load 

Transmission Phenomena:

In the transmission system, overexcitation at nominal frequency with potentially substantial overvoltage can result from various control failures such as 

• Inappropriate activation of capacitor banks 

• Failure to compensate with shunt reactors when needed 

• Single-end breaker trip failures that lead to a voltage rise at the open end of a long line (Ferranti Effect)

• Malfunctioning load tap changers

If transformer saturation happens, substantial exciting current flows can overheat the transformer's core, tank, or structure, causing damage. The waveform in such instances is distorted, containing harmonic content and near zero-current periods. 

Overexcitation distorts the waveform, but the positive and negative half-cycle behavior remains the same, leading to only odd harmonics and no noticeable even harmonics that appear during inrush. Hence, the presence of third and fifth harmonics indicates overexcitation. The extent of these effects depends on the generator connections and the transformer design and connections.

Typical Transformers Overexcitation Scenarios:

• During Start-Up or Energization: When a transformer is first energized, the voltage and current waveform can be out of phase, leading to inrush current that can be several times the normal operating current. The transformer could temporarily enter an overexcited state, resulting in a high V/Hz ratio.

• Under Overvoltage Conditions: Overexcitation can also occur when the transformer's input voltage exceeds its rated value while the frequency remains within the normal range. This could be due to a fault condition, incorrect settings, or a malfunction in the voltage regulation equipment.

• Under Frequency Reduction: If the frequency of the power supply decreases while the voltage stays the same, the V/Hz ratio can increase and lead to overexcitation. This can happen in power systems where the load greatly exceeds the generation capacity, causing a drop in system frequency.

• Load Tap Changing (LTC) Operations: Overexcitation can occur during certain load tap changer operations, where the voltage is momentarily increased.

• Ferroresonance Conditions: These are complex phenomena that can happen when a system containing capacitance (like power cables or capacitor banks) is connected with a system that has iron-core inductance (like a transformer). The interaction can lead to overvoltages and overexcitation.

• Remedial Action Schemes (RAS): Some protective schemes may intentionally overexcite a transformer for a brief period during system stress conditions, in order to manage power flows and maintain stability.