Experimental Study on Electromagnetic Interference on control cable from two unbalanced 10 kV Power Cables

The occurrence of unpredicted and sudden failures in power cables seriously threatened the nearby control cables. Two cases of unbalance in the three-phase current load originate from a high horsepower pump and a sudden power off in an MW pump, resulting in an error signal detected in the adjacent control cable. However, these two cases just occurred once during the plant operation; no more history records can be a reference. In this paper, a typical power cable and control cable structure was constructed, typical accidents in power cables were considered, and electromagnetic interference was considered. The results show that the unbalance of the three-phase load results in a fold increase in interference current. Compared with the two-phase unbalance, the one-phase would bring a higher current. In other words, power off in a sudden situation brought a higher current than three-phase unbalance, especially a faster drop in current load. Finally, more attention should be given to the power cable operation, including unbalance in the power cable and an urgent power off.


Introduction
With the improvement in nuclear power plants, increasing electrical power consumption and more power cables have been installed in trays.Generally, power and other signal cables are installed vertically based on standard regulations [1].In an actual application, the variation in current load from power cable operation gives more effects on such signal cables, especially in some unpredictable cases from power cables, which is out of the range of regular operation [2].The limitation in installment spaces contributes to the instruction of cable operation [3].Previously, multiple interferences on control cables in nuclear power are mainly focused on the amplitude of current and voltage, or distance, grounding, and frequency et al., and the effects from the failures of power cables or instruments are rarely reported [4,5].In this case, power cable and sign cable models were made, typical laying models are considered based on two accidental events, several accident situations were carried out, and the effects of interferences from the power cables on the signal cables are discussed.

Cable selection
There are various specifications in control cables in a nuclear power plant, and the power cable size is also the same as the power grid.In this paper, the parameters of the selected cables were selected from a real application case, and the cross-sectional structure is illustrated in Figure 1.Important specifications are summarized in Table 1.

Laying model
A typical model originates from the standard instruction of GB 13286 and GB-Z30556.2Y2017,as shown in Figure 2. Generally, the distance between the HV tray and the I&C tray is long enough, but the most severe case was that no other trays were blocked between them, so the cable installment graph was simplified, as illustrated in the right part.In this case, the minimum distance between the two trays is 15 cm.Beside the width of the cable tray in a wide range and multiple cable lines in the trays, just one power cable and one control cable were considered, respectively.Importantly, the cable tray is strictly grounded every 1 m, and the power cable and the control cable are also grounded, respectively.They share the same ground system, and the measured ground resistance is distributed in a small range but totally below 0.5 Ω.

Interference path
A good grounding for the power and the control cable is to ensure lower interference on control cable.From the conductor of the power cable to the conductor of the control cable, the interference path is shown in Figure 3.

Case Study
In this paper, two typical cases were considered.The first one is the current unbalance in the power cable, and the other is sudden power-off occurrence with different drop rates in the power cable.The model was constructed by CST suite software, and the interference current on the control cable was calculated.

Effect of three-phase load unbalance on the interference voltage and current
Figure 4 compares the interference current at different three-phase loads, including single-phase and two-phase unbalance.In other words, two phases unbalance, which is also called three phases unbalance.A similar result showed that the current peak increased as the unbalance strength increased.Under the same magnitude in the Y axis, it is seen that the two different cases brought higher interference current as the increasing unbalance rate.The detailed comparison is calculated based on the current increment to the reference current; the changes are illustrated in Figure 5.In the early stage of the unbalance rate, it is widely known that the two-phase unbalance brought higher interference current than the one-phase unbalance [6].When the unbalance rate reaches 15% and 20%, the difference becomes weak, because such a combination of three phases gets a similar intensity.

Emergent power off on interference current
This part originates from an accident that a sudden power off in the power cable by the failure of a pump, and an interference current was generated in the control cable and resulted in malfunction in the connected instrument.Figure 6 shows the interference current from a sudden power off at the same rate but in a different current load.It is seen that the interference current increases rapidly and drops slowly.
When the current load is powered off regularly, the interference current would be a tiny value with a magnitude of 10 -6 , which could be ignored compared with the response current [7].In an ideal critical situation, the interference current would appear at the initial and ending moments of such a drop phase [8].From the current at 300 A, which is lower than the rated current load corresponding to this cable given in Table 1, the sudden power drop gives a significant value, which doubles when the current load is 500 A.
Figure 7 compares the interference current with different drop times when the current load is at 500 A. It is more obvious that the drop rate gives more remarkable results [9].When the current drop is at a half cycle, the current is tiny with a magnitude of 10 -5 A, and the peak is up to 5.8 10 -4 A when the drop time is low at 0.002 s [10].However, such a drop rate is strongly related to the inductive effects of the electrical system [11].

Conclusions
A typical situation including a 10 kV power cable and signal cable installed in trays was considered, the effect of unbalance in the power cable and sudden power off on the signal cable was simulated, and the following conclusions were obtained: 1) Three two-unbalance, which is the three-phase unbalance in the power cable, brought the most serious effects on the control cable, and the increment in interference current becomes weaker when the unbalanced rate reaches 20%.
2) Sudden current drop brought higher interference current than load unbalance, and the magnitude reached 10 -4 A compared with the latter of 10 -5 A.

Figure 1 .
Figure 1.Cross-sectional graph of the two cables (Left is the control cable, and right is the power cable)

Figure 3 .
Figure 3. Diagram of interference path

Figure 4 .
Figure 4. Interference current at different three-phase loads.(Left is one-phase unbalance, and right is two-phase unbalance; The base current in each phase is 300 A)

Figure 5 .
Figure 5.Comparison of the peak values of the interference current at the two different unbalanced situations.

Figure 7 .
Figure 7. Interference current from power drops at different levels

Table 1 .
Important specifications of the two cables