ABB SPAD 346 C3 Differential Protection

ABB SPAD 346 C3 Differential Protection

Scope and Introduction

Document purpose: To introduce the differential protection setting calculation of SPAD 346 C3 protection relay module SPCD 3D53, which is applicable to two winding power transformers and involves differential protection of three winding power transformers, motors, generators, etc., including setting suggestions and discussion of intermediate current transformer (CT) requirements.

Protection principle: By comparing the phase currents on both sides of the protected object, when the differential current of a certain phase current exceeds the starting value of the stable action characteristic or the instantaneous protection stage value, the relay sends an action signal, which has the characteristics of fast action speed, high stability of faults outside the area, and high sensitivity to faults inside the area. CT selection and relay setting should be cautious.

Two winding power transformer protection

（1） Vector Group Matching (SGF1)

By using SGF1/1... 8 switches, the vector groups of power transformers are numerically matched on the high voltage (HV) and low voltage (LV) sides. Based on the phase shift and delta connection inside the relay, there is no need for intermediate CT, and the zero sequence component in the phase current can be automatically eliminated. Different vector groups correspond to different switch positions and check sums.

（2） CT ratio correction (I ₁/In, I ₂/In)

When the CT secondary current is different from the rated current under the rated load of the power transformer, the CT ratio on both sides of the transformer needs to be corrected. Calculate the rated load of the power transformer firstI nT=S n/(3 × U n), then calculate the transformation ratio correction settings I 1/In=I nT/I p (HV side) and I 2/In=I T/I p (LV side), where I p Rated primary current for CT, and the rated input current (1A or 5A) on the HV and LV sides of the relay can be different, using 1A secondary current can improve CT performance.

（3） Startup ratio (S)

Due to the inaccuracy of CT and changes in the position of the tap changer, an increase in load current will cause differential current to increase by the same percentage. The setting of the start-up ratio (S) affects the slope of the relay action characteristics between the first (fixed 0.5 × In) and second turning point (set I 2tp), which is calculated as the sum of CT accuracy on both sides, tap changer adjustment range, relay action accuracy (4%), and required margin (usually 5%).

（4） Basic Startup Settings (P/In)

The basic setting (P) defines the minimum sensitivity of protection, taking into account the no-load current of power transformers, which is generally calculated asP=0.5 × S+P, where P 'represents the no-load loss of the transformer at maximum voltage, usually used when the actual value is unknown

P ′=10%。

（5） Second turning point (I ₂ tp/In)

The second turning point defines the point at which the influence of the activation ratio S in the action characteristics ends and the slope begins at 100%. Its setting needs to balance stability and sensitivity. In power transformer protection applications, the range of 1.5-2 is usually selected, with 1.5 being more stable for out of zone faults and 2.0 being more sensitive for in zone faults.

（6） Second harmonic blocking (Id2f/Id1f>)

The excitation inrush current of power transformers during excitation contains a large amount of second harmonic. Differential protection is locked by detecting the content of second harmonic (low setting stage). The recommended setting for second harmonic locking in power transformer protection is 15%, which can be enabled by setting the switch SGF2/1=1, and the setting can be reduced to 10% during the first excitation.

（7） Instantaneous differential current stage (Id/In>>)

It is recommended to use it together with the low setting stage to provide faster protection in case of severe faults, and is not subject to harmonic blocking. Its setting needs to be high enough to prevent the differential relay module from tripping when the transformer is excited, usually 6-10.

（8） Fifth harmonic blocking and unlocking (Id5f/Id1f>, Id5f/Id1f>>)

Used to lock the relay action when there is a sudden voltage rise (or frequency drop), based on the fifth harmonic component of the transformer excitation current to monitor overexcitation. Due to the need to know the magnetization characteristics of the transformer, it is usually not enabled, that is, SGF2/3 and SGF2/4 are set to 0.

（9） Interference recorder

The internal interference recording function of the relay module is a powerful tool for analyzing the causes of transformer inrush current and tripping. The factory default settings are used during normal operation, and the serial communication parameters V241 and V245 need to be changed during inrush current research.

Three winding power transformer protection

The SPAD 346 C relay can be used for three winding transformers or two winding transformers with two output feeders. On the dual feeder side of the transformer, the two CT currents of each phase must be summed through parallel connections, usually requiring an intermediate CT to handle vector group and/or ratio mismatches, and at least 75% of the short-circuit power should be fed from the transformer side with only one connection to the relay, otherwise it may cause unstable protection.

Motor and generator protection

The calculation process for setting up differential protection applications for motors or generators is very similar to that of power transformers, but the vector group should be set to Yy0. The starting ratio (S) is calculated as the sum of the CT accuracy on both sides, the relay action accuracy (4%), and the required margin (usually 2.5-5%). The second turning point (I ₂ tp/In) may cause CT saturation due to current during motor start-up, with a typical setting value of 1.0. The generator is usually between 1.0-1.5, and the motor and generator usually do not require second or fifth harmonic blocking.

Combination protection of motor and autotransformer

The differential relay measures the phase current on both sides of the protected object, and the protection area includes the autotransformer and motor. The current between all autotransformer/motor combinations and the network must be measured, and its setting is the same as that of ordinary motor protection applications. The autotransformer will have a typical starting sequence during the motor starting process, from power supply to acting as a parallel reactor and then to bypass.

Variable frequency motor and its power transformer protection

SPAD 346 C can only be used to protect power transformers that supply power to frequency converters, and is not suitable for protecting power transformers or motors powered by frequency converters, as the fundamental frequency component in the relay is numerically filtered through a Fourier filter and is not suitable for measuring the output of frequency converters.

Short overhead or cable protection

SPAD 346 C can be used for differential protection of overhead or cable lines. When the distance between measurement points is long, an intermediate CT may be required to reduce CT load. Using 1A secondary current can reduce CT load, and the actual accuracy limit factor (Fa) of CT can be calculated to ensure compliance with requirements.

Summary

This guide describes how to select and calculate differential protection settings for SPAD 346 C protection relays, introduces the working principle and setting effect of relays, and presents the calculation process through examples. It also describes protection application examples for three winding power transformers, motors, and generators, discusses the applicability of SPAD 346 C relays in frequency converter applications, and finally describes differential protection for short overhead or cable lines through examples of calculating the actual accuracy limit factor of intermediate CT.