Cause analysis of zero-sequence operation in 10kV switching operations
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The 110kV power supply system features a main transformer with a capacity of 31,500 kVA and an yd11 connection configuration. The 110kV neutral point is grounded either with just one main transformer according to regional dispatch instructions, or with neither main transformer grounded at times. Whenever the main substation closes the bus coupler at the 10kV switchyard, zero-sequence current signals are generated simultaneously in the two 10kV feed lines running from the main substation to that switchyard. These zero-line signals trigger an alarm, and the line trip protection is disabled. Once the switching operation is completed and the busbar switch in the substation is opened, the zero-sequence alarm signals for both lines disappear simultaneously. 1. This phenomenon occurs in 2 provinces and autonomous regions, during switching operations at the boundaries of three sets of 110KV substations. 2. The main transformers and line parameters are the same; both main transformers originate from the same upstream 110 kV substation. 3. Relay protection includes zero-sequence electronic relays and microcomputer-based integrated protectors. 4. There is no grounding or resonance in the 10KV system. 5. (1) Two power supply systems, with the 10KV system being a system with an ungrounded neutral point; (2) One power supply system, where the 10KV neutral point is grounded through arc-suppression coils, and arc-suppression coil grounded transformers are installed on each section of the main transformer. 6. (1) For 2 power supply systems, three single-core cables are used for the two 10 kV line cables; (2) For 1 power supply system, one three-core cable is used for the two 10 kV line cables. 7. The relay protection setting values are calculated by professional engineers; the primary alarm setting value for zero sequence current is 5A (7A), while the secondary action value is determined based on the transformation ratio of the zero sequence current transformer and relevant tests. 8. This issue has been discussed with many senior electrical engineers and chief engineers, but opinions vary widely, making it difficult to provide a reasonable explanation. Please share your opinions.
It’s so hard. Have you never experienced it? Waiting for an answer. How long is the 10kV feeder? How is zero-sequence current sampled?
The 10kV line is approximately 0.5 kilometers long; the zero-sequence current is sampled by a zero-sequence current transformer and sent to the zero-sequence protection unit of the microcomputer-based integrated protector or to the zero-sequence current relay.
Is it a zero-sequence current transformer through which three single-core cables pass simultaneously?
Yes, the three power cables must pass through the zero-sequence current transformer simultaneously, while the grounding wire depends on the location of the cable terminals to determine whether it passes through the zero-sequence current transformer or not.
My personal understanding: First, by using the 10KV busbar switch, you are essentially creating a closed loop in your system. Are the voltages of your two main transformers equal? Are the loads on your two main transformers equal? Your closing point is at the 10KV bus coupler switch. Are the loads on the two 10KV buses similar? If not, then it’s normal for a zero-sequence alarm to occur.
110kV main transformers are generally equipped with on-load tap changers, and the voltage settings of the two main transformers are the same when the 10kV circuits are connected. When the loads of the two main transformers and those of the two busbars differ, line circulating currents are generated, but no zero-sequence current is produced.
When the power system operates in an asymmetric manner, zero-sequence current also appears. Examples include asymmetric operation caused by different parameters among the three phases of a transformer, two-phase operation during single-phase reclosing, asynchronous operation of the three-phase circuit breakers during three-phase reclosing or manual closing, zero-sequence circulating currents resulting from the parallel connection of circuit breakers and isolators during busbar switching operations or when circuit breakers are operating in a normal closed-loop configuration due to inconsistent contact resistances among the isolators or circuit breakers. Additionally, unbalanced excitation inrush currents can occur when a transformer is connected to the system suddenly; especially when there are transformers with grounded neutral points operating on the busbar where the new transformer is connected, unbalanced excitation inrush currents and DC components may persist for an extended period of time. All these conditions can trigger the operation of zero-sequence current protection.
Asymmetry generates negative-sequence current, while grounding generates zero-sequence current, right?
Negative sequence and zero sequence can be extracted as long as there is three-phase asymmetry.
None of these phenomena you mentioned actually exist; zero-sequence current only appears when two circuits operate in parallel.