SPAD 346 C Stabilized Differential Relay ABB

Features Integrated three-phase differential relay, overcurrent relay and earth-fault relay Stabilized three-phase differential relay providing winding short-circuit and interturn fault protection for two-winding transformers and generator-transformer units and winding shortcircuit protection for generators. Earth-fault protection for transformer HV and LV side windings according to the desired principle: the stabilized differential current principle, the high-impedance principle, the calculated or measured residual current principle or the neutral current principle Three-stage overcurrent protection for transformers and generators and two-stage back-up protection for earth-fault protection The operation characteristic of the differential relay easily adapted for different applications Short operate times, even with partially saturated current transformers Stabilization prevents unwanted operations at faults outside the protected area and transformer inrush currents Blocking based on the ratio of the second harmonic and the fundamental component of the differential current prevents unwanted operations at transformer inrush currents Blocking based on the ratio of the fifth harmonic and the basic frequency component of the differential current prevents operation in harmless situations of transformer overexcitation - can be eliminated if the ratio of the fifth harmonic and the basic frequency component increases at high overvoltages No interposing transformers are needed for the protection of two-winding transformers - numerical vector group matching on HV and LV side Wide CT ratio correction range - accurate correction allowed by digital setting Sensitive phase current and phase angle displays facilitate the checking of measurement circuit connection and vector group matching Four trip and four signal relay outputs available to the protection design engineer Five programmable external control inputs intended for the indication and retransmission of alarm and trip signals of gas relays, oil temperature sensors and other sensors of transformer auxiliary devices Adjustable CBFP operate time to improve reliability of operation Integrated disturbance recorder capable of recording currents and digital signals - signals to be used for triggering selectable High immunity to electrical and electromagnetic interference allows the relay to be used in severe environments High availability and system reliability due to continuous supervision of hardware and software 4 Application The stabilized differential relay SPAD 346 C is designed to be used to protect two-winding transformers and generator-transformer units against winding short-circuit, interturn fault, earth fault and short circuit and to protect generators against winding short-circuit and short circuit. The relay can also be used for the protection of a three-winding transformer provided 75% of the short circuit power is fed from the same direction. Description of operation The integrated differential relay SPAD 346 C includes three independent relay modules: a three-phase stabilized differential relay module SPCD 3D53, an earth-fault relay module SPCD 2D55 and a combined overcurrent and earthfault relay module SPCJ 4D28. The rated currents of the relay are 1 A and 5 A. The HV and LV side may use the same or different rated currents. Below a short description of the features of the protection relay modules. The manuals for the separate relay modules describe the modules more in detail. In power transformer protection differential current is caused by CT errors, varying tap changer positions, transformer no-load current, transformer inrush currents, transformer overexcitation in overvoltage and underfrequency situations, and CT saturation at high currents passing through the transformer. Differential current caused by CT errors and tapchanger position grows at the same per cent ratio as the load current increases. In the protection of generators the differential current is caused by CT errors and saturation of the CTs in situations where high currents pass through the transformer. High currents passing through the object to be protected may be caused by short circuits outside the protected area, large currents fed by the transformer or the generator in motor start-up The differential relay module SPCD 3D53 provides protection for winding short-circuit and interturn faults. The differential relay compares the phase currents on both sides of the object to be protected. Should the differential current of the phase currents in one of the phases exceed the setting of the stabilized operation characteristic or the instantaneous protection stage of the module, the module provides an operate signal. Different amplitudes or phase difference of the currents may be the reason for the differential current. Interposing current transformers have normally been used in the differential protection of transformers to obtain vector group matching and to match the secondary currents of the main transformers. Interposing CTs have also been used to eliminate the zero-sequence components of the phase currents at earth faults occurring outside the protected area. The differential current relay SPAD 346 C eliminates the use of interposing transformers for the protection of two-winding transformers as the differential relay module allows the transformer vector group matching, the CT ratio correction and the elimination of the zero-sequence component of the phase currents to be digitally implemented on the HV and/ or the LV side. Three-phase stabilized differential relay module SPCD 3D53 Stabilized differential current stage or transformer inrush situations. Due to these circumstances the operation of the differential relay has been stabilized in respect of the load current. In a stabilized differential relay the differential current required for relay operation is higher, the higher the load current is. The stabilized operation characteristic of the differential relay module and the setting range of the characteristic is presented in the description of the differential relay module SPCD 3D53. The operation of the differential relay module SPCD 3D53 is based on the fundamental frequency components. Operation based on fundamental frequency components is accurate and stable: the DC component and harmonics of the current do not cause unwanted operation of the protection stage. 5 Blocking based on the second harmonic of the differential current The blocking also prevents unwanted operation at recovery and sympathetic magnetizing inrush. At recovery inrush the magnetizing current of the transformer to be protected increases momentarily when the voltage returns to normal after clearance of a fault outside the protected area. Sympathetic inrush is caused by a transformer, which runs in parallel with the protected transformer already connected to the network, being energized. The connection of the power transformer against a fault inside the protected area does not delay the operation of the relay module, because in such a situation the blocking based on the second harmonic of the differential current is prevented by a separate algorithm based on the waveform and the rate of change of the differential current. Transformer magnetizing inrush currents occur when energizing the transformer after a period of deenergization. The inrush current may be many times the rated current and the halving time may be up to several seconds. To the differential relay inrush current represents differential current, which would cause the relay to operate almost always when the transformer is connected to the network. Typically, the inrush current contains a large amount of second harmonics. Blocking of the operation of the stabilized stage of the relay at magnetizing inrush current is based on the ratio of the amplitudes of the second harmonic digitally filtered from the differential current and the fundamental frequency Id2f/ Id1f. Inhibition of relay operation in situations of overexcitation is based on the ratio of the fifth harmonic and the fundamental component of the differential current Id5f/ Id1f. At dangerous levels of overvoltage which may cause damage to the transformer, the blocking can be automatically eliminated by a separate blocking inhibiting setting Id5f/ Id1f> > . When required, the blocking based on the second and fifth harmonic of the differential current can be disabled. In addition to the stabilized stage the differential relay module SPCD 3D53 has a separate adjustable instantaneous stage the operation of which is not stabilized. The instantaneous differential current stage operates when the fundamental component calculated from the differential current exceeds the set operate limit Id/ In> > or when the instantaneous value of the differential current exceeds the level 2.5 x Id/ In> > . The setting range of the instantaneous stage Id/ In> > is 5...30. Should the stabilizing current be less than 30% of the differential current, there is most certainly a fault in the protected area. In such a situation the set operate value Id/ In> > will be halved and the blockings of the stabilized stage are automatically prevented. The differential relay module SPCD 3D53 is provided with an integrated disturbance recorder that is capable of recording six phase currents, the internal trip and blocking signals of the module and the control input signals. Recording can be triggered by the rising or falling edge of these signal. The recording length is 38 cycles. The recording memory has the capacity of storing one recording at a time. Sampling frequency is 40 samples/ cycle. The recording is downloaded by using a PC program. The recording memory has to be reset before a new recording is possible.