Electromagnetic interference testing3 June 2020
Diagnostic electromagnetic compatibility testing of a safety- related digital control systems in a nuclear environment. By Youwei Yang, Quan Ma, Mingxing Liu, Peixun Chen, Xu Zhang.
NUCLEAR POWER PLANT INSTRUMENTATION & control (I&C) systems are divided into safety-related system and non-safety-related system. The safety-related I&C platform not only plays an important role in ensuring nuclear projects’ safety and stability, but also is critical for the safe shutdown and accident mitigation.
As a general trend, nuclear plants are replacing obsolete analogue I&C system with more efficient and economical digital systems. As analogue instruments have been gradually upgraded to digital instruments, digital control systems (DCS) are often used. But high-speed digital instruments can have electromagnetic problems. The design of the I&C system must consider and prevent electrical and magnetic interference.
Experimental investigations of the potential vulnerability of safety-related systems to electromagnetic interference (EMI) or radio-frequency interference (RFI), confirm the safety significance of electromagnetic compatibility (EMC) testing for analogue and digital technology.
The sources of interference are many and varied, ranging from welders to radio sets. If proper provisions have not been made during design, problems could appear after installation and may require troublesome and time-consuming investigation.
Since good EMI resistance is required in the design of instrument systems, the attention of the supplier should be drawn at an early stage to the specified levels of interference immunity and the ways in which the instruments are going to be tested in the factory and during plant commissioning. If the problems associated with EMI are not addressed at the design stage, they are difficult to locate and cure when the equipment is installed. Diagnostic testing is needed to see whether there are any ‘show stoppers’ before an item goes for full compliance testing. This test is to address EMC concerns for nuclear plant equipment.
A pre-compliance diagnostic electromagnetic compatibility test of a safety-related DCS was carried out in China. The main aim was to identify at-risk items and fix any problems with compliance tests, or with interference in the field.
The test used the military standards 151B of the Chinese department of defence as validation criteria of electromagnetic waves at the time of plant construction. The basic GJB151B test methods described here are similar to the basic MIL-STD-461E test method used by the US Department of Defense.
Nuclear plant digital control systems have dozens of cabinets. It is impossible to conduct an experiment for so many cabinets in a shielded enclosure. For this diagnostic EMC test, two typical function-integration cabinets were specially designed to be representative of the pressurised water reactor (PWR) protection systems. One is a power supply cabinet, and the other is a function cabinet, as shown in Figure 1.
There are two sets of AC power supplies for each cabinet. Each of the supplies has 40% spare capacity and appropriate voltage to meet the equipment load requirement.
The equipment under test (see Figure 2) was plugged into the line impedance stabilising networks (LISN) and the LISN was plugged into the power line.
The 220V AC voltage was converted to 24V DC voltages in the power supply cabinet.
The resistance signal provided by the resistor box represents the ambient temperature from the resistance temperature detector (RTD). The two voltage signals provided by a signal source represent temperatures from thermocouples at different locations in the reactor. In the immunity test, a common mode inductor is inserted in one path. Information is collected from local sensors and transmitted from the function cabinet to the maintenance test station via optical fibers.
The signal sources and the test station were outside the chamber. Each cabinet and station has redundant power supply switchover and protection devices. The computer single point ground (SPG) is connected in one location to the payload chassis ground to reduce shock hazard from the secondary power system. When the equipment to be tested is installed on a metallic ground plane, the DC resistance between metallic ground planes and the shielded enclosure is measured as less than 2.5 milliohms. It is oriented such that surfaces which produce maximum radiated emissions and respond most readily to radiated signals face the measurement antennas. Adequate room should be provided for cable arrangement.
To meet the test standard, 2m exposed length of power lead is separated from the bundle and routed to the LISNs.
Based on previous EMC test experience, the diagnostic test focused on the low-frequency conducted emission test, high-frequency radiated emission and high-frequency radiated susceptibility test in the 30-200MHz range.
Conducted Emission Test
The aim of the conducted emission test is to measure the amount of interference power transmitted from the equipment being tested. The conducted emission is divided into the low-frequency (CE101) and high-frequency (CE102) part. To make a harmonic measurement with the required accuracy, a source with very low distortion (a pure 50Hz sine-wave), high voltage stability and low impedance is needed. In general, the public mains supply will not meet these requirements and a special source will be required. LISNs, sometimes called artificial mains networks are the standard transducer for measuring conducted emissions in many test standards. The low-frequency spectrum contents from the power frequency (50Hz or 60Hz) to 2kHz is defined as harmonics which can strongly distort the input current and voltage waveforms of the systems — decreasing the power factor of the system and degrading power quality. Harmonic components of the AC supply input current to an item of equipment arise from nonlinearities of the load over a single cycle of the input voltage. The purpose of this test is to limit harmonics emission on power cables to ensure that new equipment does not adversely affect the quality of the power source to which it will be connected.
The CE101 test without harmonic filter was carried out and the results are shown in Figure 3A. The peak values were labelled with sequence number and listed in Table 1. According to the test standard, if the fundamental current I1 is greater than 1A, corresponding to 120dBμA, the limit shall be relaxed as 20lgI1. As we can see from Table 1, the fundamental current exceeds the limit 4.68dBμA so if the absolute value of the margin is less than this the result is within the CE threshold. As we can see from Table 1, the conducted emissions exceed the limit at the 5th and 7th harmonic. (The result for the live and neutral wires have almost the same interference, so only the live wire results are shown.)
A power system passive harmonic filter was developed for the CE101 test to filter out the harmonic component. The power lead wire used in the filter has 1mm thick insulation, so the turns can be tightly wound to the specified conductor spacing. The ground path from the ground layer to earth is as short as possible.
All the amplitudes are below the threshold limit of CE test specified by the regulating authority (see Figure 3B).
With the appropriate harmonic filter, the peak value of the odd harmonic decreased and the emission level met the standard, so complying with EMC.
Radiated Emission Test
Electrical equipment and related cabling should be designed and installed to ensure that they do not generate any harmful EMI when operating. This can be minimised if EMC principles are taken into account at the design stage.
This test checks that radiated electric field equipment emissions from new equipment emissions do not adversely affect existing plant equipment. The equipment used in nuclear plants should not radiate an electric field over the RE102 measurement reference. The frequency of radiated emission tests ranged from 0.01MHz to 200MHz. According to the previous test experience, the RE102 test only need to concern the frequency range between 30- 100MHz.
At frequencies above 30MHz, the test should be performed for both horizontally and vertically polarised fields. Setups greater than 3m in diameter should have several antenna positions. Two positions were chosen, one mainly facing the cables (Figure 5), and the other mainly facing the cabinets (Figure 6). The results show the radiated emission level is below the limit at least 15dBμV.
Radiated Susceptibility Test
Nuclear power plants are vulnerable to EMI because they contain many systems which depend on small signals in a hostile industrial environment. The most likely consequence of bad EMC is spurious shutdown and reduction of plant availability. Even when this does not happen, EMI can generate erroneous instrument readings, which may affect operator confidence. Malfunction or deterioration of equipment should not occur upon exposure to an electric field of the RS103 standard. As with RE102 test, the doors of the cabinets were closed during the RS103 test.
This test verifies the ability of equipment to withstand radiated electric fields. The electric field strength maintained 10 V/m during the test frequency range.
A diagnostic EMC test was carried out for trouble shooting or quality verification of the digital control system in nuclear power plants. Two typical function- integration cabinets were specially assembled to execute the key function of the reactor protection system in the nuclear reactor. A passive harmonic filter is designed to filter out the harmonic currents.
In view of plant owners’ concern on the open-door EMC property of the radiated test, (RE102 and RS103), the radiated emission and susceptibility tests were carried out with the door of the cabinet open, and the system showed good performance. Other techniques are seen to improve the quality of the system, and they may be employed in the future.
Author information: Youwei Yang, Quan Ma, Mingxing Liu, Peixun Chen & Xu Zhang, Science and Technology on Reactor System Design Technology Laboratory, Nuclear Power Institute of China, Chengdu 610213, China.