Protection principle of comprehensive protection transformers

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This post was last edited by chenjinfeng on 2020-3-27 at 13:58. Principles of transformer protection: The common protection methods used for transformers include addressing abnormal operating conditions such as overloading, overcurrents caused by external short circuits, neutral point overvoltage resulting from external ground faults, a decrease in oil level due to oil leaks in the tank, and temperature increases caused by failures in the cooling system. Furthermore, large-capacity transformers, due to their higher rated operating magnetic flux density – with the operating magnetic flux density being proportional to the voltage frequency – may experience over-excitation faults when operating under overvoltage or low-frequency conditions. In light of the above situations, large transformers generally employ the following protection methods: 1. Transformer longitudinal differential protection. This type of protection is used to detect inter-phase short circuits, single-phase ground faults on the high-voltage side, as well as turn-to-turn short circuits. Its protection scope includes the transformer bushings and their connecting wires.  The over-excitation current that occurs when a transformer is switched on under no-load conditions can be several times to more than 10 times In; such a large excitation current is commonly referred to as inrush current. II. Backup protection against inter-phase short circuits in transformers mainly includes overcurrent protection and low impedance protection. III. Overload protection of transformers
Overloads in transformers are, in most cases, three-phase symmetric. Therefore, overload protection can be implemented by connecting to just one phase, using a current relay. This type of protection usually operates with a delay and sends out a signal. For two-winding step-up transformers, it is installed on the side connected to the generator voltage; for three-winding step-up transformers, it is installed on the side connected to the generator voltage when one side has no power supply, and on all three sides when power is available on all sides. IV. Single-phase ground fault protection for transformers: 1. Ground fault backup protection for ordinary transformers with directly grounded neutral points; 2. Ground fault backup protection for transformers that can operate with either a grounded or ungrounded neutral point. Transformers with fully insulated neutral points, and transformers with graded insulation and discharge gaps at the neutral point. V. Overcurrent protection: Used to protect against external phase-to-phase short circuits, and serves as a backup protection for gas protection and differential protection (or instantaneous current protection). VI. Zero-sequence current protection: Protects against external single-phase ground faults in systems with high ground current. VII. Overload protection: Protects against symmetric overloads, and operates only on signals. The second common protection method used for transformers is longitudinal differential protection or instantaneous current protection, which are designed to detect short-circuit faults in the transformer’s leads, bushings, and internal components. The protective instantaneous action operates to disconnect the circuit breakers on all sides of the transformer. 1. For station service transformers of 6.3 MVA and below as well as transformers operating in parallel, as well as station service standby transformers of 10 MVA and below and transformers operating independently, current quick-break protection shall be installed when the backup protection time is greater than 0.5 s. 2. For station service transformers of 6.3 MVA and above that are operated in parallel, as well as for station service standby transformers of 10 MVA and above that operate alone, and for transformers of 2 MVA and above whose current quick-break protection does not meet the required sensitivity, longitudinal differential protection shall be installed. 3. For transformers with a high-voltage side voltage of 330 kV and above, dual longitudinal differential protection can be installed. 4. For generator-transformer units, when there is a circuit breaker between the generator and the transformer, the generator is equipped with separate longitudinal differential protection. When there is no circuit breaker between the generator and the transformer, generators of 100 MVA and below share a longitudinal differential protection system with the transformer; generators larger than 100 MVA use their own such system. In addition to sharing the longitudinal differential protection with the generator transformer, the generator should also be equipped with its own separate longitudinal differential protection. For generator-transformer units of 200–300 MVA, a separate longitudinal differential protection device can also be installed on the transformer. This involves using dual protection mechanisms: one to detect external phase-to-phase short circuits in the transformer, along with overcurrent protection that serves as a backup for gas protection and longitudinal differential protection (or instantaneous current protection); overcurrent protection that operates upon low voltage; overcurrent protection that operates based on composite voltage levels; negative sequence current protection; and impedance protection. In the event of a fault, these protections should trigger tripping after a specified time delay. 1. Overcurrent protection is suitable for step-down transformers. 2. Overcurrent protection with composite voltage starting is suitable for step-up transformers, system interconnection transformers, and step-down transformers for which the overcurrent protection does not meet the sensitivity requirements. 3. Negative sequence current and single-phase low voltage starting overcurrent protection, suitable for step-up transformers of 63 MVA and above. 4. When the protection methods described in 2 and 3 above cannot meet the requirements for sensitivity and selectivity, impedance protection can be used. Fourth common protection method for transformers: Zero-sequence current protection, which is used to detect the zero-sequence current resulting from an external ground short circuit in transformers within a system with high ground current. In large grounding current systems of 110 kV and above, where the transformer neutral point may operate in a grounded condition, zero-sequence current protection should be installed on step-up or step-down transformers with two or three power supplies. This protection serves as a backup to the main protection of the transformer, as well as a backup for adjacent components. What is zero-sequence current protection? It is a device that utilizes the zero-sequence current generated during a ground fault to activate the protection mechanism; this is what is known as zero-sequence current protection. Special zero-sequence current transformers are used in cable circuits to provide grounding protection. The zero-sequence current transformer is installed on the three-core cable grounded to earth, and the current relay is connected to the secondary coil of the transformer. During normal operation or in the absence of a ground fault, since the vector sum of the currents in the three phases of the cable is zero, the current in the secondary coil of the zero-sequence transformer is also zero (with only a very small amount of unbalanced current), so the current relay does not activate. When a ground fault occurs, a large current will flow in the secondary coil of the zero-sequence transformer, causing the current relay to activate in order to send a signal or isolate the fault. Common protection method 5 for transformers: Overload protection, which responds to symmetrical overload conditions in the transformer. For transformers of 400 kVA and above, when operated in parallel or individually as a backup power source for other loads, overload protection should be installed to address potential overload conditions. For autotransformers and multi-winding transformers, the protection devices should be able to detect overloading in the common winding as well as on each side. In most cases, the overload current in transformers is three-phase symmetric; therefore, overload protection can be implemented using a current relay that monitors just one phase of current, with a certain delay applied to the signal. When selecting on which side the protection should be installed, it is necessary to consider that it should be able to reflect the overload conditions of the windings on all sides of the transformer. In substations without regular personnel on duty, the overload protection can operate to trip or disconnect part of the load when necessary. Transformer protection mode 6: Over-excitation protection, which responds to over-excitation of the transformer. In current large-scale transformer designs, in order to save materials, reduce costs, and lower transportation weight, the rated operating flux density of the core is set relatively high, at around 1.7–1.8 T, which is close to the saturation flux density (1.9–2 T). As a result, overexcitation can easily occur under overvoltage conditions. Furthermore, due to the relatively \"stiff\" magnetization curve, during over-excitation, core saturation causes the excitation impedance to decrease, resulting in a rapid increase in the excitation current. When the operating magnetic flux density reaches 1.3 to 1.4 times the normal value, the excitation current can reach the rated current level. Secondly, since the excitation current is a non-sinusoidal wave containing many high-order harmonic components, the eddy current losses in the core and other metal components are proportional to the square of the frequency, which can cause severe overheating of these components as well as the insulating materials. If the over-excitation level is high and persists for a long time, it may damage the transformer. Therefore, transformers with a high-voltage side of 500 kV should be equipped with over-excitation protection. The purpose of installing over-excitation protection on transformers is to detect over-excitation conditions in the transformer, to send signals in a timely manner or to trigger a trip, thereby ensuring that the over-excitation level does not exceed acceptable limits and preventing damage to the transformer due to over-excitation.