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  1. Configuration Principles of Protection To ensure both reliability and safety, and under the conditions of simplicity and economy, the protection of converter transformers can be configured as follows: Each converter transformer is equipped with two protection devices, each with independent power supply and input, and each device's output can reach both trip coils of the circuit breaker and both systems of DC control. Each device takes measures to prevent its own maloperation, while relying on the OR output of the two devices to prevent failure to operate in case of faults.

2. Configuration and Principles of Protection To avoid the impact of harmonics unique to converter stations on protection, protection devices should take measures in both hardware and software to ensure that protection only responds to power frequency components.

Main protections include steady-state ratio differential, differential fast-break, power frequency variation ratio differential, zero-sequence ratio differential, and overexcitation protection. Backup protections include overcurrent, zero-sequence overcurrent, overvoltage, zero-sequence overvoltage, and saturation protection.

2.1 Steady-State Ratio Differential Protection Due to differences in transformation ratio and connection group, when power transformers are in operation, the grounding continuity tester is a highly automated portable tester developed by our company, specifically used to measure the DC resistance of portable short-circuit grounding wires and personal protective grounding wires. The current magnitudes and phases on each side also differ. These influences must be eliminated before constructing the relay. The TA of converter transformers is generally installed on each side winding, so the currents in the primary and secondary windings have the same phase, and only the influence of the transformation ratio needs to be compensated. The following descriptions are based on the premise that the amplitude and phase differences on all sides of the transformer have been eliminated.

The steady-state ratio differential protection is used to distinguish whether the sensed differential current is caused by internal faults or unbalanced output (especially during external faults). The device adopts a variable-slope ratio braking characteristic with initial braking. The steady-state ratio differential element consists of a low-value ratio differential (sensitive) and a high-value ratio differential (insensitive). To ensure rapid removal of internal faults, only the low-value ratio differential element (sensitive) is equipped with TA saturation criterion, while the high-value ratio differential element (insensitive) is not equipped with TA saturation criterion.

For the differential current imbalance caused by tap changer adjustments of converter transformers, three methods can be used to solve it: first, avoid it by setting values; second, use floating threshold for adaptive adjustment; third, use tap position for adjustment. Methods one and two are simple and practical, while method three is complex to implement.

2.2 Power Frequency Variation Ratio Differential Protection The device discriminates phase by phase in sequence. When certain conditions are met, the power frequency variation ratio differential operates. The power frequency variation ratio differential protection is blocked by the inrush current discrimination element and the overexcitation blocking element before tripping.

Since the braking coefficient of the power frequency variation ratio differential can take a higher value, its inherent characteristics have strong ability to resist transient and steady-state saturation of TA during external faults. The power frequency variation ratio differential element improves the sensitivity of the device to slight inter-turn faults inside the transformer during normal operation. Moreover, the power frequency variation ratio differential protection is not affected by the differential current imbalance caused by tap changer adjustments of converter transformers.

2.3 Backup Protection Backup protection includes overcurrent, zero-sequence overcurrent, overvoltage, zero-sequence overvoltage, and saturation protection.