Why does differential protection malfunction?
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This post was last edited by zyz8438 on 2010-8-24 at 08:55. The differential protection of one of our 4500KW (10KV) motors tripped a few days ago. The reason was the activation of the incoming cable cabinet of a 20000KVA (35/10.5) main transformer in the upstream power grid (unit #1); this caused an alarm in the control room related to the differential protection of this motor, which led to its tripping. What caused this? This motor is connected to unit #2 main transformer, and the secondary busbars of units #1 and #2 main transformers had not yet been activated. We only activated the primary side switch cabinet of unit #1 main transformer (35KV); the secondary side was not activated, nor were the secondary busbars of units #1 and #2. So how could the differential protection of a motor connected to unit #2 main transformer be triggered, while all other equipment remained fine, including the other 6 transformers (10/0.4) connected to unit #2 main transformer? After restarting the motor, everything worked normally and no alarm was triggered; there’s really nothing to do, it’s frustrating. I hope the experts can give me some advice. Thank you!
According to the original poster, there is no electrical connection in the primary circuit between the motor and the high-voltage side switch of transformer #1, which was being operated at that time. Is it possible that there is a problem with the secondary circuit? What are the operating principles for the differential protection of large motors and that of transformers? Is it a centralized panel layout or individual installation in switchgear? Is it possible for the wiring of two differential circuits (including the differential current circuit and the protection output trip circuit) to intersect? (Although this kind of error is very basic and the likelihood is low)
The poster needs to provide detailed information on various aspects such as the specifics of motor differential protection, so that everyone can help with the analysis!
Check the wiring of the zero-sequence CT or the motor differential protection CT, including the grounding leads.
Provide a detailed description of the situation, including what devices are used and how many turns the transformer has, etc.!
Although the information you provided isn’t very detailed, through the analysis of relevant cases, I have a general understanding of the issue you’re referring to. Firstly, a key criterion for determining differential protection in electric motors is the method used and the turning points involved. The method refers to whether magnetic balance or longitudinal differential protection is employed; if magnetic balance is used, there are no turning points, only overcurrent differences. In the case of proper longitudinal differential protection, six current transformers are used, along with input and neutral point transformers, to make the determination, resulting in what is known as proper motor differential protection (this type of differential protection operates in the same way as that used in transformers). Now, you need to provide the parameters related to the protection system: the starting current for differential protection, the differential current, the braking current, the current at the first turning point, the current at the second turning point, as well as the slope of the braking curve and the internal fixed coefficients. These are all important factors for making such determinations, if my guess is correct. There are several possible causes for such problems: first, whether there were any fluctuations in the power grid at that time; second, whether there was any synchronization with the higher-level power grid; third, check the parameters of the motor; fourth, conduct protection tests using differential protection
Could the poster communicate with your electrical staff to confirm whether differential protection actually activated at that time? The differential protection operation information you have is displayed in the DCS in the control room, or within the microcomputer-based comprehensive protection system of the motor? Once, we also experienced an unit shutdown incident; the DCS received information indicating that differential protection had activated, but there was no record of such an event in the motor’s microcomputer-based protection system. It was finally determined that someone accidentally turned off the power supply for the DCS signals, which caused the DCS to shut down as a result of interlock mechanisms.
1. The poster should take a look at the current and voltage curves at that time, and compare them with the settings of the comprehensive protection parameters, to see if there were significant fluctuations in voltage and current during the shutdown, resulting in an abnormal shutdown. It may be related to the fluctuations in grid current and voltage caused by the installation of large-capacity transformers in the power grid mentioned in the previous paragraph. It is recommended to set the comprehensive protection parameters reasonably. 2. Check the zero-sequence current leakage protection device to see if it is reporting any faults.
Why is it related to zero-sequence current? Could someone upstairs explain it?
When the cable is installed with a grounding wire in a non-compliant manner, if a charging fault or grounding fault occurs in one of the devices, it can easily cause the device with incorrectly installed cable grounding wire to trip.
We have also encountered situations where the wiring of the cable ground wire was not proper – that is, the ground wire did not pass back through the zero-sequence CT – which led to the motor tripping. But it seems irrelevant to this case: 1. The original poster in this case is referring to the differential protection mechanism of the motor, rather than the zero-sequence protection mechanism ； 2. I don’t know what type of neutral-point grounding system the 10kV system to which the 4500KW (10KV) motor in question is connected is In some low-current grounding systems, the zero-sequence protection only serves to issue alarms and signals.