Beckwith M-3311A Transformer Protection Relay Debugging Guide

Engineering Tip: The standard mode has covered the vast majority of common transformer connections (such as Y/d1, Y/d11, etc.). After changing the wiring settings, it is necessary to ensure that the CT polarity is correct, otherwise the differential current may not be zero under normal load.

2.4 Current Summing

Partial protection functions (such as 49, 50, 51, 50N, 51N) can choose the current source as a single winding or two "summation groups" (Sum1, Sum2). Each summation group can contain the vector sum of any winding current, facilitating the overall overcurrent protection of multi winding transformers.

2.5 Fixed value group (profiles)

Supports four independent fixed values, which can be switched through external input (IN5/IN6) or HMI/communication. For example, two sets of fixed values, "normal" and "maintenance", can be defined. When switching, the relay briefly exits the protection (about 1 second), ensuring the safety of the unit status.

Key points for setting core protection functions

3.1 Phase Differential (87T) - Transformer Main Protection

87T is a percentage differential relay that adopts dual slope braking characteristics (Slope1 and Slope2), and can choose 2nd and 4th harmonic braking (to suppress excitation inrush current) and 5th harmonic braking (to suppress overexcitation).

Setting steps:

CT Tap Calculation: Convert the current of each winding to the same reference (per unit value). Calculation formula:

CTTapWn=MVA×10003×kVLL×CTRWn

CTR stands for CT transformation ratio. For example, for a three winding transformer (example in Figure 4-51) with W1 of 8000:5, voltage of 17.1kV, and capacity of 392.8MVA, TapW1=392.8 × 1000/(√ 3 × 17.1 × 1600) ≈ 8.29. Manually input the Tap values for each winding.

Minimum operating current (Pickup): usually set at 0.2-0.4 pu, higher than the excitation current.

Slope 1: Considering CT error and the influence of voltage regulating switch, it is usually taken as 30% to 40%.

Slope 2: Used in high braking current areas to cope with CT saturation, set at 60% to 100%.

Braking breakpoint: usually 2.0 pu, which means it switches to Slope2 when it exceeds this value.

Harmonic braking:

Second harmonic braking: typically 10% to 15%, suppresses excitation inrush current. You can choose 'cross averaging' to improve reliability.

Fourth harmonic: merged with the second harmonic to form "even harmonic" braking, set the same as the second harmonic.

Fifth harmonic braking: typical 30%, suppresses overexcitation. Can be enabled or disabled separately.

Pick up at 5th restraint: When there is a fifth harmonic, the action value can be raised above the rated value (such as 0.75 pu) to prevent overexcitation misoperation.

High setting differential (87H): no braking, no harmonic blocking, usually set at 8-12 pu, delayed by 1-2 cycles, used for quickly clearing serious internal faults.

3.2 Grounding Differential (87GD)

Suitable for low impedance grounding systems, internal grounding faults can be identified by comparing the polarity of the neutral point grounding CT of the winding with the zero sequence current of the phase CT. Need to set:

Action current: Typical 0.2 A (secondary value)

Directional component: Can be enabled/disabled, automatically switches to non directional if zero sequence current<0.2A

CT transformation ratio correction: If the phase CT and neutral point CT transformation ratios are different, the correction coefficient (phase CT transformation ratio/neutral point CT transformation ratio) needs to be input

Attention: The delay should not be less than 2 cycles, and it is recommended to be ≥ 6 cycles to avoid external fault CT saturation misoperation.

3.3 Overexcitation protection (24) - optional voltage pack

Contains two timed elements and one inverse timed element (four sets of curves are optional). When setting, it is necessary to integrate the V/Hz withstand curves of the transformer and generator (if any). The typical inverse time start value is 105%, and the timed limit # 1 can be set to 110% delay seconds, while # 2 can be set as an alarm. The reset rate is set according to the cooling time.

3.4 Frequency protection (81O/U) - optional voltage pack

Provide 4 frequency components, which can be set as overclocking or underflocking respectively. Underfrequency is commonly used for load shedding, and the fixed value is set according to the system frequency stability requirements. It is recommended to avoid transients with a delay of ≥ 6 cycles.

3.5 Winding thermal protection (49)

Based on the current thermal accumulation model, it is necessary to input the thermal time constant (τ) and the maximum allowable continuous overload current (Imax). The protection adopts the IEC-255-8 standard curve, and the preload current is taken from the average current of the most recent demand interval (15/30/60 minutes). For transformers with forced air cooling, parameters can be switched between different cooling levels using a fixed value group.

CT wiring and polarity verification (key on-site)

The normal operation of differential protection depends on the matching of CT polarity and transformer wiring. Sections 4.6 and 5.4 of the manual provide detailed guidance.