What is the differential protection of a motor?

A phase-to-phase short circuit in the stator windings is detected by the longitudinal differential protection, while an inter-turn short circuit in the stator windings is detected by the transverse differential protection. The principle behind these protections is to detect faults by comparing the difference in current phasors at both ends of the transmission line. A deterioration in the motor’s insulation can also trigger the differential protection. Last edited by liuqj on 2008-10-30 08:03]

Differential protection is primarily used for protecting electrical equipment installed between two sets of current transformers from various faults related to current parameters.

According to relay protection specifications, longitudinal differential protection must be installed for motors with a power rating of >2000 kW; it should also be installed when the current quick-break protection does not meet the sensitivity requirements for motors with a power rating of less than 2000 kW.

To protect against inter-phase and turn-to-turn short circuits in motors, similar to internal protection for transformers, two sets of current transformers are required. For motors with a capacity of over 2000 KW, self-balancing protection should be implemented, as it provides better protection effects.

Differential protection uses the difference in the CT current vectors at the two inputs; it activates the operating element when this difference reaches a set threshold value. The equipment (motor) whose protection range is between the CTs at both ends of the input experiences active reverse power, which is reverse power rather than reverse phase sequence; this is generally used in generator protection. Current differential protection is a type of protection used in relay protection; in the positive phase sequence, A leads B by 120 degrees, and B leads C by 120 degrees as well. Reverse phase sequence (that is, inverted phase sequence) means that A leads C by 120 degrees, and C leads B by 120 degrees. A reversal of the active power direction simply means that the angle between voltage and current is increased by 180 degrees; this results in out-of-phase power, rather than an inverted phase sequence.

Differential protection involves installing current transformers on both sides of a transformer or high-voltage motor; the secondary windings of these transformers are connected in series to form a loop, with a differential relay connected across this loop. The current flowing into the relay is equal to the difference between the currents in the secondary windings of the current transformers on either side of the transformer or high-voltage motor. When the transformer or high-voltage motor is operating normally, or when a short circuit occurs outside the protection zone of the differential protection, the unbalanced current flowing into the differential relay is less than the relay’s operating current, so the protection does not activate. When a short circuit occurs within the protected area, the current flowing into the differential relay is much greater than the relay’s operating current, causing the relay to operate instantaneously. Trip the circuit breaker to isolate the fault.

Principle of differential protection: A relay protection device that compares the magnitude and/or phase of the currents at various ports of the equipment being protected. When the protected equipment is operating normally, or in the event of an external short circuit or system oscillation, since the sum of the currents at all ports of the protected equipment is zero, the differential protection will not malfunction ； When an internal short circuit occurs in the protected equipment itself, the sum of the currents at all ports will equal the total short-circuit current, causing the differential protection to activate sensitively. 　　To implement differential protection, current transformers must be installed at each port of the equipment to be protected (see transformers), and secondary cables of a length corresponding to that equipment must be laid, which greatly limits the application of differential protection in ultra-high voltage long-distance transmission lines. In China, the application of differential protection for 110–220 kV transmission lines is limited to 5–7 km, and it is referred to as leader line protection ； For the differential protection of longer ultra-high voltage transmission lines, high-frequency carrier channels are used to transmit electrical quantities between the two ends of the line; this is known as carrier protection. To simplify the protection devices and save secondary cables, in the guide wire protection and carrier protection of ultra-high voltage transmission lines, the three-phase currents and/or voltages are usually first converted into single-phase symmetric component currents and voltages using symmetric component filters. To ensure the safety of the guide wire itself and the reliability of the guide wire protection device, overvoltage protection and breakage monitoring devices for the guide wire should also be installed. 　　Other main electrical equipment in power systems, other than transmission lines (such as generators, transformers, reactors, motors, busbars, etc.), have a relatively short extension length, generally not exceeding a few hundred meters, which makes differential protection an excellent choice for their primary protection. Therefore, differential protection has become a widely used relay protection device for main electrical equipment. Busbar protection: The main issue with busbar differential protection is that during external faults, the current in the faulty branch is extremely high, causing the corresponding current transformers to become severely saturated. In contrast, the current transformers of the other, non-faulty branches become only slightly saturated; as a result, a large imbalance in currents can occur, leading to incorrect operation of the protection system. For this reason, bus differential protection requires that the current transformers of all branches have the same transformation ratio and sufficient saturation margin. An effective solution is to switch to voltage-type differential protection, that is, to use high-impedance voltage relays as the sensing elements for differential protection. By appropriately selecting the resistance value and saturation voltage of the current transformer’s secondary circuit, it is possible to avoid both false trips and failed operations. Although this voltage-type differential protection allows current transformers to saturate, it still requires that all transformers have the same turns ratio, which imposes practical limitations. A bus protection scheme that allows for different turns ratios of current transformers is phase differential protection, which utilizes the fact that during an internal short circuit, the currents in various branches are nearly in phase, while during an external short circuit, the current in the faulty branch is almost 180° out of phase with the currents in the non-faulty branches; this enables a differential protection system that compares only phases rather than amplitudes. This scheme cannot be used for a one-and-a-half switch wiring or polygonal busbar. 　　Transformer protection: Due to the magnetic coupling between the windings on different sides of the transformer, its differential protection differs significantly from that of other main electrical equipment. That is, in the absence of any internal faults in the transformer, the Icd value equals the excitation current and is not zero. Especially when the transformer is switched on suddenly, a large excitation surge occurs, which can easily cause the transformer’s differential protection to malfunction. Currently, the main methods used to prevent protective device malfunctioning due to inrush current are as follows: 3. 　　①Taking advantage of the fact that in inrush currents there are often large amounts of aperiodic components, a fast-saturation converter is used to prevent the excitation inrush currents from flowing into the actuator. The disadvantage of this method is that when a three-phase transformer is switched on suddenly, there is often a non-periodic component in the inrush current of one phase. In such cases, it is necessary to increase the setting current in order to prevent false trips, which reduces sensitivity. Additionally, since there are also non-periodic components in the current caused by internal faults, the response speed of this protection mechanism is not fast either. 　　②Based on the fact that the ratio of the second harmonic to the fundamental wave in the excitation inrush current of single-phase transformers is not less than 17%, a second harmonic braking scheme is adopted. However, studies show that in the excitation inrush current of three-phase transformers, the second harmonic component of the inrush current in one or two phases is often less than 15%. To this end, the approach is adopted of immediately disabling the three-phase differential protection whenever the ratio of the second harmonic to the fundamental frequency of any one of the three phases in the inrush current exceeds 15%. 　　③A transformer differential protection based on the principle that the discontinuity angle of the excitation inrush waveform must be greater than 60° is constructed. 　　Given the influence of the distributed capacitance in modern ultra-high voltage long-distance transmission systems and static compensation devices, short circuits within the transformer differential protection zone generate currents with frequencies close to the second harmonic in significant amounts, which makes it even more difficult to prevent false trips caused by inrush currents. Utilizing the excitation characteristics of the transformer core to distinguish between short-circuit current and inrush current will provide new possibilities for the design of microcomputer-based transformer protection systems.

The main fan you mentioned is commonly used in chemical plants for large-scale catalytic equipment. This differential protection (longitudinal differential protection) is primarily used to detect any issues with the windings or the stator of the motor. There are two ways to protect the motor, as illustrated below. 1. Balanced type: In some cases, a balance-like connection method is used, whereby the beginning and end of the coil pass through a core-passing transformer at the same time, but in opposite directions; after passing through the transformer, the end is short-circuited to form a star shape. 2. Differential type: This method is widely used. It involves installing three current transformers at the end of the motor and shorting them together; discrimination is made based on the differences in current and angle between these transformers and those located in the switchgear. In summary, whether it is the former or the latter, the main reason is that the direction must be toward the motor, which is on the load side. Be careful, otherwise it will lead to wrong judgments

Please explain the magnetic balance differential protection of synchronous motors.

Question: Should the grounding wire of the zero-sequence current transformer’s through-cable be passed through the zero-sequence current transformer together with the cable?