Abstract: This paper analyzes the fuse protection of distribution transformers and the overcurrent protection of Y/Y0 connected transformers and Δ/Y0 connected transformers, and proposes improvement measures.
Keywords: Transformer, Overcurrent Protection, Sensitivity
Foreword
Transformers are key equipment in power distribution networks, widely used and numerous, and their safe operation directly affects the safety and reliability of power supply. Currently, there are still some problems in the overcurrent protection of distribution transformers, such as protection mismatch or dead zones, which can cause super tripping or failure to operate, exacerbating or worsening accidents and reducing power supply reliability.
Currently, in my country, 6-10kV distribution transformers are protected by either fuses or relays, depending on their capacity. Regardless of the protection method, the sensitivity of the protection should be considered.
1. Fuse protection
For transformers with a capacity below 400kVA, fuses are generally used for protection. When using fuses, outdoor drop-out fuses are typically used on the high-voltage side for overcurrent protection, while fuses on the low-voltage side provide overload protection. When selecting the rated capacity of a drop-out fuse, it is essential to consider both the matching of its upper breaking capacity with the maximum short-circuit current at the installation location and the relationship between its lower breaking capacity and the minimum short-circuit current at the installation location. The lower breaking capacity of a drop-out fuse is generally 10% to 20% of its rated capacity. If the rated capacity of the fuse is too large, its lower breaking value will also be correspondingly large, which is not conducive to interrupting small fault currents.
Considering that the high-voltage fuse serves as the primary protection against internal transformer faults, and includes a lead from the low-voltage bushing to the fuse (or air switch), and is also a backup protection for the low-voltage fuse, its lower breaking capacity should be selected based on the minimum short-circuit current of a two-phase short circuit at the low-voltage outlet. To ensure the selectivity or sensitivity of the fuse operation, a certain relationship should be maintained between the high- and low-voltage fuse elements. To prevent overload damage to the transformer, the rated current of the selected low-voltage fuse element can be equal to (or slightly greater than) the rated current of the transformer; for transformers with a high-voltage fuse element capacity below 150kVA, it can be selected at 2 to 3 times its rated current, and for those above 150kVA, at 1.5 to 2 times its rated current.
2. Relay protection
2.1 Overcurrent protection for Y/Y0-12 connected transformers
In the past, the distribution transformers we selected were all Y/Y0-12 connected, with three-phase four-wire power supply on the low-voltage side. The substation was generally AC operated, using circuit breaker control with spring energy storage operating mechanisms, and two-phase or three-phase overcurrent protection, with inverse-time relays. The advantages of this scheme are simple protection, economy, and small footprint. However, for single-phase ground fault protection on the low-voltage side, GBJ "Design Code for Relay Protection and Automatic Devices of Industrial and Civil Power Installations" stipulates that one of the following protection methods should be used: a. Utilizing overcurrent protection on the high-voltage side. In this case, a three-phase protection device is recommended to improve sensitivity. b. Zero-sequence current protection connected to the neutral line on the low-voltage side. c. Three-phase current protection connected to the low-voltage side.
In practical applications, three-phase overcurrent protection is used on the high-voltage side, but sometimes it cannot meet the sensitivity requirements. Here is an example: When a single-phase ground fault occurs on the low-voltage side of a Y/Y0-12 transformer, the through current flowing through the high-voltage side can be calculated from the relationship in Figure 1, where Ld is the short-circuit current and K is the transformer turns ratio.
From the current distribution on the high and low voltage sides, it can be seen that when the low voltage side is single-phase grounded or connected to neutral, the three-phase overcurrent protection device on the high voltage side can be tested according to 2/3 of the short-circuit current, while the two-phase overcurrent protection is tested according to 1/3 of the short circuit. Therefore, the three-phase type is used to improve sensitivity. The single-phase short-circuit current can be obtained according to the following general formula. In the formula, U is the rated voltage of the network, V; R<sub>XeN</sub>, Z<sub>eN</sub> are the total resistance, total reactance, and total impedance of the phase-zero circuit, mΩ. The zero-sequence impedance of a three-phase four-wire low-voltage power grid is the sum of the zero-sequence impedance of the phase line and three times the zero-sequence impedance of the neutral line, that is, the relationship between the phase-zero circuit impedance and each sequence impedance, generally: The zero-sequence impedance of the Y/Y0-12 transformer is greatly related to the materials and structural form used, and should be obtained through actual measurement. Its value is much larger than the positive sequence impedance.
Based on the above formula, the calculated short-circuit currents of the low-voltage busbars for the SJI1-560/10 (Ud=4%, △Pd=8.99kW) and SJLl-1000/10 (Ud=4.5%, △Pd=13.93kW) transformers installed at a short-circuit capacity of 100MVA are 4.226kA and 7.95kA, respectively.
For the overcurrent protection device installed on the high-voltage side of the aforementioned transformer, a 100/5 current transformer and a GL11 type relay are selected. The operating current values are calculated using conventional formulas, with Idz being 7A and 9A, respectively, meaning the primary operating currents are 140A and 180A.
When using the high-voltage side three-phase overcurrent protection for low-voltage single-phase grounding protection, its sensitivity is limited for a 560kVA transformer. This demonstrates that using the high-voltage side three-phase overcurrent protection device for low-voltage single-phase grounding protection does not meet the sensitivity requirements for either 560kVA or 1000kVA transformers. In other words, when an internal single-phase grounding fault occurs in the low-voltage winding of this transformer, the protection device may not trip, potentially burning out the transformer and increasing the risk of electric shock, endangering personal safety. For safety reasons, when the high-voltage side overcurrent protection cannot meet the requirements, additional protection should be installed on the low-voltage side as specified in the regulations, or a zero-sequence current protection should be installed on the low-voltage neutral line to trip the high-voltage side circuit breaker. Its operating current can be set so that the neutral line unbalanced current does not exceed 25% of the transformer's rated current.
For example, for a 560kVA transformer, when the zero-sequence current transformer ratio is 200/5, the starting current is: Idz=Kk×0.25/n=1.2×0.25×850/40=6.4A. Taking the primary operating current of a 7A protection device as 240A, its sensitivity coefficient is KIm=4226/280=15.09>2. A GL-11/10 type relay can be selected, and the operating time of the protection device is increased by 0.5s compared to the next level.
In the actual operation of our power supply system, from the design department to the operation department, there is insufficient understanding of the need to install zero-sequence current protection on the neutral line of the low-voltage side of Y/Y0 connected transformers. There are still cases where this protection has not been installed or has been installed but not put into operation, thus leaving potential safety hazards.
2.2 Overcurrent protection for Δ/Y0-11 connected transformers
With the continuous development of urban construction, more and more substations are being designed in high-rise buildings. For fire and explosion prevention requirements, Δ/Y0-11 connected transformers are generally selected. The advantages of this connection are: the delta winding filters out a large amount of third harmonics generated by the ballast of lighting equipment, preventing harmonic interference to the power grid; since the zero-sequence reactance of the Δ/Y0 connected transformer is equal to the positive-sequence reactance, for a single-phase ground fault on the low-voltage side, the sensitivity of the three-phase overcurrent protection on the high-voltage side meets the requirements. For example: with a Δ/Y0-11 transformer, when a single-phase ground fault occurs on the low-voltage side, the current flowing through the high-voltage side can be calculated. When a single-phase ground fault occurs on the low-voltage side or is connected to neutral, the three-phase overcurrent protection on the high-voltage side is tested according to 1/Id/K.
For example, if an SCR-1600/10 (Ud=5.55%, △Pd=13.93kW) DYN-11 distribution transformer with a short-circuit capacity of 134MVA is installed, the short-circuit current Id of the single-phase grounding of the low-voltage side bus is 36.4kA. When a 200/5 current transformer is selected on the high-voltage side of the transformer, its overcurrent protection action value is calculated according to the conventional formula, and Idz is 7A, that is, the primary action current is 280A.
3. Conclusion
In practical work, it is essential to carefully verify the protection sensitivity. For distribution transformers with Y/Y0-12 connections, when a single-phase ground fault occurs on the low-voltage side and the sensitivity of the three-phase overcurrent protection on the high-voltage side does not meet the requirements, zero-sequence current protection should be installed on the low-voltage neutral line. However, for distribution transformers with Y/Y0-11 connections, the overcurrent protection sensitivity on the high-voltage side is sufficient, and zero-sequence current protection is not necessary on the low-voltage neutral line.
