Anti Interference Technology. Of Relay Protection System

Transcription

1 Anti Interference Technology Of Relay Protection System Author s name: Liulyu Address: No.5, Shanghai Road, Xiangshan District, Guilin City, Guangxi,China @qq.com ABSTRACT Relay protection is an extremely important line of defense to prevent failure caused by the power system failure, and ensuring the normal operation of power systems. With the development of power system, the scope of the fault occurring in the system are increasing, thus the demand of the power system to relay protection in stability, rapidity, sensitivity are increasing. And all kinds of interference in relay protection become the major factor that it can not operate stable, rapidly, sensitively. This paper describes the interference sources into different category, listing and analyzing substation relay protection anti-interference measures, discussing how to solve the problem of interference, strengthen immunity in relay protection, to achieve safe operation of power system. KEYWORDS relay protection, anti-interference, interference sources 0. Introduction With the development of computer automation, communication and substation equipment manufacturing technology, automatic protection and monitoring devices constantly upgrading in relay protection, power system automation level has been gradually improved, substation control are also moving toward to digital, centralized control even unattended direction []. The full advent of the digital age, to put forward new requirements for protection, also for electromagnetic compatibility(emc) and protection class. However, when the microcomputer is controlling substation in real-time, interference from all kinds of sources will inevitable through the swich and analog input channel of the computer control system or other means into the internal computer, once these interference have impacts on system, which can cause data transmission errors or even misoperation, causing electrical power system accident, threat the safe operation of the grid. In addition, when there is a big electrical equipment leakage or poor grounding, the input channel of the microcomputer control system will string into high common mode or differential mode voltage, if poorly handled, it will couse distortion of the input signal even drown [2]. Therefore, to ensure safe and reliable power supply system, it is necessary to improve the anti-interference capability of the microcomputer control system.. Sources of interference In addition to normal signals, those are affecting the normal operation of the relay protection system, and will cause a certain degree of abnormal to the system is called interference [3-4]. Sources of interference in relay protection system are the following:. overvoltage interference Overvoltage interference mainly affects the secondary circuit of the relay protection system, there are many different coils, resistance element in the secondary circuit. There is a certain inductance between the coils. At the

2 same time there is distributed capacitance between the coils. If using a equivalent lumped capacitance to replace the distributed capacitance connecting to the coils in parallel, then it constitutes a ringing RLC circuit (Fig.), the relay contact acts as a switch in the circuit. Fig. shows that, when the circuit switch is off, it tends to produce larger back EMF. Voltage across the capacitor can be regularly changed, the voltage waveform is a sinusoidal ringing voltage. Since this voltage is out of control of the power supply, called free component voltage, this voltage is /4 period slightly longer time to reach the maximum. When R<2 L / C and is small, the voltage across the capacitor is large, often several times or ten times higher than the supply voltage, which is the overvoltage generated in the secondary circuit. In Fig. U C =Ae t sin( 2 2 ω δ t θ o ), in which A and θ are constant to be determinedδ=r/2l, ω 0 =/ LC. Figure Second order oscillation damping circuit Due to the presence of overvoltage, the contacts have been breakdown when the distance have not reached large enough. High voltage enter into the DC operating power system, the semiconductor device will be subject to different degrees of damage due to the lower voltage level, which will cause elements do not work, so the secondary overvoltage has lager impact on microcomputer system protection..2 Inductive(magnetic) coupling interference Transformer, large current through the cable, capacitors and reactors in substation has a strong alternating magnetic field. Secondary device in the alternating magnetic field, including lead, network communication loop will has induction, this induction forms interference voltage. As shown in Fig.2, two adjacent wires exists distribution of the mutual inductance M,M= Φ 2 /I (in which, I is the current through the wire, Φ 2 is the flux generated by I and wire 2), the mutual coupling voltage is U 2 =2 ω M I, the voltage on the wire are connected in series. The formula shows that interference frequency is proportional to the amount of mutual inductance. This interference can cause secondary devices operation error in CPU, memory date change, the display image warping and flickering of local monitoring, data change in network communication or communication interrupted, resulting in abnormal equipment operation, which causes the most destruction to the control system.

3 Figure 2 Magnetic coupling between wires.3 Capactive (electric field) coupling interference Since the primary equipment carrier fluid and secondary circuit exists capacitance, the primary voltage and secondary cables will produce capacitivety (electric field) coupling, causing interference voltage on secondary devices. In addition, there is mutual coupling between lines, power lines and system in station. These electic field is one of the reasons interfering secondary equipment work. Fig.3 is the diagram of the capacitive coupling between the parallel wires. In Fig.3A, the wire and wire 2 are two parallel lines, C and are the distributed capacitance of each wire, is the coupling capacitance between two wires, U is the voltage between wire and ground, R is resistance between wire 2 and ground. Fig.3 is equivalent circuit. Wire couple the wire 2 producing U 2,as show in Fig.3B. U 2=j ω R () Can be simplified as follow while R>>/jω( U 2 = U /[jω( C2 U /( ) U2 partial voltage by capacity at this time, this kind of situation is serious. While R<</[jω( )], () can be simplified as U 2=jω 2 )R] () ) (2) C RU (3) From ()-(3) it is obvious that capacitive coupling interference increases with coupling capacitance. Figure 3 The block of capactive coupling between wires.4 ground voltage difference interference In the power system, due to poor insulation, it will produce an unstable leakage current, ground current flowing in the earth will produce voltage difference, which will cause the current interference in station cable ends and shield. If the secondary equipment choose improper grounding point, the leakage current will produce voltage difference between every point, cause secondary equipment malfunction..5 Interference produced by power supply system Many secondary equipment use DC power supply, generally filtered by power grid, sometimes it is poorly

4 purified and will cause interference to the system. If the voltage of the filter capacitor could not meet the design by energy storage, it will judge as power failure or malfunction, cause the lock of the device, malfunction or refused to move..6 Natural interference Natural interference refers to interference caused by natural phenomena and electromagnetic interference cosmic radiation, such as lightning, atmospheric low-level electric field, ionizing electromagnetic radiation change and sunspots, these are the interference that can not be eliminated. The lighting interference is the most serious, direct lightning strike member or outdoor line, there will be a large current inflow to ground, different access point of the shield in secondary cable, will produce transient current because the presence of the resistance between the grid and ground, it will cause interference voltage in secondary cable, which will not only cause damage to the secondary cable, but also burned communication port and input modules. The microcomputer protection and monitor device in relay protection system have digital components and analog components, interference can cause different result to them. In the analog components interference tends to cause the switch flip by mistake, if not have the lockout will cause action; In the digital components interference tends to cause data and address transport mistake, which will cause operation malfunction and malfunction. Thus, Interference ranging from causing data transmission errors to protection tripping or malfunction, these will affect the reliability of the power system. The secondary device in station disruption is not only those talked above, but those are the most common, we must take operation to weaken the interference. 2. Microcomputer protection EMC standards and anti-interference technology measures 2. EMC protection professional international standards International Electrotechnical Commission (IEC) developed IEC80 and IEC of EMC (electro-magnetic compatibility) standards on relay protection, referred to as electromagnetic compatibility standards, as shown in Table. Table Electro-magnetic interference test standard of static relay and protective device IEC standard number name country(china) standard (80-) MHZ pulse interference test GB (80-2) electrostatic discharge test ZBK (80-3) radiated electromagnetic interference test GB (80-4) fast transient disturbance test GB/T EMC standardization has become an important aspect of the current international standardization. To do EMC test in according to international standards in power system with serious EMC is important to improve the immunity of the relay protection. 2.2 Specific measures for anti-interference technology 2.2. Secondary prevention for overvoltage Contactor ends parallel circuit RCD contactor ends parallel circuit (Fig.4) connect both ends of resistance R and diode D in contactor K and then connect capacitor C in parallel. When the contact is open, magnetic energy stored in the coil generate EMF charging the diode and the capacitor though power supply, its fast and current decays rapidly to zero, it will not cause connect sparking; when the contact is close, the capacitor C discharge though the resistance R, to limit discharge current and avoid closed contact K welding.

5 Figure 4 RCD contacts parallel branch The coil connect resistance and capacitance branch in parallel This approach can apply not only to the DC circuit but also suitable for AC circuit, the wiring is connect the resistance and capacitance branch to the coil in parallel. In increased parallel circuit, the resistance is adjusted to the critical value, so that when the switch is off the carrier coil current does not oscillate. On the other hand, since the same time constant of the two branches, no matter how total current change over time, two-way free current component is always equal and opposite. Free current component in power supply is zero; that is, when the switch is off, the voltage between the coil is zero. So this wiring can not only eliminate overvoltage, but also cutting the inductive load generated between the switch contacts Inductive coupling, capacitive coupling interference prevention Prevention of such interference mainly use cable having a metal shield. When the secondary loop cables are brought near, in order to reduce coupling, preferably a metal shielded cables, and both ends of the shield grounded, respectively, so that the lines of magnetic force of the voltage mostly concentrate in the shield, it will not enter the internal cables, thus avoiding the transfer of the voltage. If the cable dose not have a metal shield, connecting the inside the spare cable core to the ground, it will have the same effect Voltage difference interference prevention The right choice of the secondary equipment grounding locations, is the main measure to prevent voltage difference interference. There are different sets of general substation grounding system: Electrical grounding system, used for UPS and isolation transformer shield grounded, to prevent clutter fleeing into the grid system. 2 Indoor substation shielding and anti-static grounding system, used for the inner shield grounding, anti-static grounding systems and equipment chassis grounding. 3 Substation lightning protection and grounding system, used to prevent lightning damage. 4 Control system dedicated system, is secondary device-specific measures, do not allow any other connected devices, in order to avoid interference. Four said grounding system is absolutely not allowed to mix with each other, it needs to ensure a certain safe distance in the ground position power supply system interference prevention For the power supply system interference we can take the following precautions: To ensure a stable supply voltage waveform, UPS can be used to stabilize the power supply, and use DC power supply if possible, use DC power supply which has smaller ripple factor in substation. 2 Isolation transformers should be used to isolate the common mode interference, to prevent noise interference fleeing into the grid control system, or strong lightning voltage causing equipment damage. 3 The output circuit should as short as possible to reduce inductive noise, the use of the cable core can not be too small, reducing pressure drop, or increasing its power supply Correct construction process is an important means of interference Installation of substation equipment should note the following:

6 Cable shield is installed correctly, using the control cable with a shield. 2 Weak signal conductors must not share cable with a strong electric wire, placing them separately if possible; AC and DC circuit never share the same cable, to prevent causing interference with each other. 3 Authority control cable laying, substation design should consider good cable channel to avoid too close to the power cable. 4 Designing dedicated ground copper bar configuration equipotential surface for secondary and secondary cabling equipment. 5 The secondary circuit of the current transformers, voltage transformer should ensure grounding. 6 Voltage transformer circuit should be independent of each other. 7 installing passive filters in signal input terminal to weak the interference signal fleeing. 8 All switching substation of input and output contacts (tripping exports and monitor signal) and digital output (such as serial) should adopt optical isolation Software anti-interference measures Software interference means taking specific measures in software design, to prevent signal interference fleeing computer protection and control devices inside. Although the software is dependent on hardware, the software has a positive sense on the system interference immunity [5], the software anti-interference measures are: For those do not have hardware watchdog system software watchdog should be properly designed, the system program can automatically reset after "running out". 2 Rational designing software trap service routine, the fact that the vast majority of system program "run off" is running into unknown interrupted, how to design software traps inside the program is crucial. 3 Rational design of algorithms for various disturbances has a certain extent, such as digital filtering algorithm, it has great adaptability. There are arithmetic mean filtering method and weighted smoothing method. Arithmetic mean filtering method is continuous sampling N times in one point, calculate the average value as a result of the sampling point.the accuracy of X depends on the number of detections N, the higher N the larger value of precision. However, increasing the value of N will add overhead storage space and computing time, in general, the value of N is Practical application is the general weighted smoothing method, the algorithm formula is: = = 2 2 X 4 8 N 2 = (4) From the formula, the current sample value and historical value is associated,which do favor of weakening transient disturbances, and has a fast speed, occupy a small storage features. This approach not only eliminates harmonic interference AC voltage signal, and will also reduce random noise signal in some degree. 4 Take into account all hardware error conditions and alarm handling procedures. 5 Due to a hardware error of uncertainty and unpredictability, so the redundancy design of important data such as reference values, etc. can greatly reduce the chances of device malfunction. 3. Conclusion Interference is always inevitable, in order to achieve the purpose of suppressing interference, we must take specific measure to eleminate interference against interference from different sources. Due to different device structure, function, the design of anti-interference is different. Relay Protection System anti-interference technology is a matter of technology, high-speed signal EMC complicated subject, needing theoretical support in digital signals, analog signal, the proposed method has a certain universality, but still needs large number of pratical tests, and seek an optimal anti-interference method from the comparison of theoretic and experimental

7 data. Reference []. Peng Honghai, Zhou Youqing, Wang Hongtao, et al. Anti-interference technology for microprocessor -based protection[j]. High Voltage Engineering, 2007,33(0): [2]. Bu Haiyan. Designation of anti-interference of the microprocessor based relay protection[j]. Power System Technology, 2007,3(7): [3]. Liu Aiqin. Power adaptor and antijamming tech in computer monitoring system[j].relay, 2003,3(): [4]. Jiang Lin, Shen Youchang, Yang Qixun. Research on anti-interference ability of microprocessor-based protection[j]. Automation of electric power systems, 998,22(2): [5]. Ke Hezhen. The hardware and software combined measures of enhancing the anti-interference capability of microcomputer safe-devices[j]. Electrotechnical Application, 2004, 24(5):7-20.