RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY.docx

RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY摘要：Abstract: Research and application progress of NARI-RELAYS in protection based on DPFC were reviewed, key technology employed in line, busbar, transformer and generator protection were presented. It was pointed out that integration of main and backup protection in one device and duplicate allocation was reasonable and beneficial. Key words: Protection; DPFC; Main protection; Backup protection; Duplicate protection 关键词：Research 0. IntroductionElectric power network in China has step into a new stage with large-scale networks, super power plants and high-rating generators. By the end of year 2002, total installed generation capacity is 353000MW, total length of 35kV and above transmission lines is 806500kM, total installed transformer capacity is 1194000MVA, and generated energy of the year is 1641010kWh。As an important part of electric power system, protection is the technology measures to ensure the security and stability of power networks. Development of electric power system requires new functions and higher performance of protection. New progress in microelectronics, computer and communication technology makes it possible to improve protection further more.NARI-RELAYS Electrics Co., Ltd. is the main base of research on protection principle and technology, and among the leading suppliers of protection and automation products in China. She has made great progress in research and application of new protection technology, especially series protection based on Deviation of Power Frequency Component (DPFC). She also possesses many invention patents. The author summarized these research and application progress, presented his own viewpoint on protection allocation, and look forward to discuss this issue with experts in protection area.1. Application of DPFC protectionDPFC protection is a kind of fault components protection. Fault components have two characteristics: (1) Fault components only appear during fault and disappear under normal condition; (2) All fault components are produced by the same electromotive force at the fault point, so that fault components at fault point are larger than elsewhere. Normal load current does not influence protection based on DPFC principle at all. Fault transient resistance slightly affects DPFC protection. In many cases, DPFC protection has excellent performances.The first DPFC protection, DPFC directional comparison protection was invented in 1982. In the following years, the whole DPFC protection system was set up successively, DPFC distance protection and DPFC current differential protection were developed, at the same time this technology migrates from line protection to busbar, transformer and generator protection. Large amount of real on-site operation records proved that DPFC line, busbar, transformer and generator protection are of high security, high reliability and fast operation speed, and play an important role in improving electric power system security, stability and transmission capacity.1.1. DPFC directional comparison protectionNo matter what type of fault, which fault phase, whether or not a transferring fault, whether or not under phase discrepancy condition, the phase angle between voltage DPFC and current DPFC only depends on system impedance backward from where the protection installed, and has nothing to do with system electromotive force and fault transient resistance. Line fast protection based on DPFC directional comparison protection can distinguish between positive direction and negative direction unambiguously. This protection was proved reliable by practice and has been applied to electric power system widely and successfully 357.1.2. DPFC distance protectionFault on and near busbar is the most serious fault for electric power system transient state stability. Clearing such kind of fault quickly is key to keep system stability. As to line protection, main protection, i.e. pilot protection, cannot trip related breaker quickly enough with the limitation of signal transmission time delay. Operation time of distance zone should not be less than one cycle for the sake of security of itself. DPFC distance protection is a kind of distance protection using fault component, and has ultra fast operation speed along with high security. In fact, DPFC distance protection has set up a record of 3 ms operation time on site.1.3. DPFC current differential protectionCurrent differential protection based on DPFC principle bas been applied to line, busbar, transformer and generator successfully 911.RCS-931 ultra high voltage series protection employed DPFC current differential protection with adaptive floating threshold, which can withstand system unbalance and disturbance. The measuring element can operate with ultra high speed and high security. The pickup element based the same principle has high sensitivity, and yet it will not pick up frequently under normal condition even with unbalance and disturbance thanks to floating threshold. Inter-turn fault is the main kind of fault in electric power transformer. It is meaningful that employing sensitive and secure protection to identify inter-turn fault inside power transformer. In RCS-978 series power transformer protection, DPFC percentage restraint differential protection makes use of DPFC in phase current on each side of transformer and DPFC in differential current, with a high setting of percentage restraint factor. The unwanted operation caused by TA saturation during external fault can be prevented. The sensitivity during internal fault is not affected by pre-fault load current and is high enough to identify internal slight fault, such as phase-to-ground fault near neutral point and low percentage inter-turn fault on the same phase under heavy load.When slight fault occurs in generator or power transformer, traditional current differential protection cannot identify such fault for the sake of sensitivity limited by normal load current. RCS-985 Series generator-transformer unit protection is equipped with transformer DPFC percentage restraint differential protection and generator DPFC percentage restraint differential protection. Comparing with traditional percentage restraint differential protection, the DPFC percentage restraint differential protection can identify the slight fault with higher sensitivity, because they are not influenced by normal load current at all and are affected by fault transient resistance slightly. Higher restraint percentage is selected to deal with TA saturation; floating threshold prevents unwanted operation during system swing and frequency shifting. 2. Key technology in line protection2.1. Series-capacitor compensated lineSeries-capacitor compensation equipment installed on long-distance transmission line can improve stability allowance, optimize load distribution between parallel lines and increase the transmission power capacity. But the line impedance and its distribution is change by series-capacitor, which will affect the operation characteristics of protection 81014.RCS-902XS series distance carrier protection and RCS-931XS optical fiber current differential protection make up the whole protection scheme for Series-capacitor compensated line.Split-phase current differential protection is not affected by series-capacitor at all.For zero-sequence directional comparison protection, so-called compensated impedance setting is introduced to ensure comparing direction correctly during asymmetrical phase-to-ground fault ahead.DPFC distance protection keeps unambiguous when installed on series-capacitor compensated line 14, but may overreach when fault occurs ahead after capacitor. Enhance the threshold by additional MOV protection-level voltage to prevent protection from overreaching, at the same time the fast operation speed needed when near and serious fault occurs is not affected at all. For traditional distance protection, polarization voltage vector is introduced to prevent missing operation during fault at the very beginning of line after capacitor. Logic combination of two distance relays with different attenuation time constant each is designed to prevent unwanted operation during backwards near fault after capacitor. So-called forwards protection-level voltage is introduced to protection. Operation curve of reactance relay is adjusted according to measured current in order to accommodate various power system operation modes. In this way, under-reaching distance protection will not overreach when fault occurs at the end of line or the beginning of the next line after series capacitor. The whole scheme takes into account the influence caused by operation of protective devices of series capacitor unit, for example MOV break-over or protective gap break-over, and ensures correct operation of protection under any operating condition of MOV and gap. Based on the information only from one line, the mentioned measures ensure correct operation of protection without compromise of operation speed, are appropriate for the line with series capacitor installed on it or on its neighboring line, no matter where TV installed. RCS-902XS and RCS-931XS series protection devices have been applied to several series capacitor compensated lines and their neighboring lines. 2.2. Double-circuit lines on the same towerMore and more double-circuit lines on the same tower appear in Chinese electric power network for their high transmission capacity and saving of line corridor. Protection for double-circuit lines on the same tower should identify any kind of faults especially those faults cross over double-circuit lines, select fault phase correctly and trip breaker as quickly as possible. Auto-reclosing logic for those important double-circuit lines on the same tower should be designed carefully to increase their transmission reliability, to improve auto-reclosing success percentage and reduce risks of reclosing onto serious permanent fault. 2.2.1. ProtectionThe whole protection scheme for double-circuit lines on the same tower consists of split-phase current differential protection or split-phase composite distance carrier protection and zero-sequence directional comparison protection as main protection, and DPFC distance protection, step zones distance protection and zero-sequence directional over-current protection as backup protection. Split-phase current differential protection has excellent capability of fault phase selection no matter whether the fault occurs on the same line or cross over lines. For composite distance carrier protection and zero-sequence directional comparison protection, phase selection results are exchanged between two ends of the same line, thus each end can recognize the fault phase whether the fault occurs on the same line or cross over lines. Communication channel-independent DPFC distance protection, step zones distance protection and zero-sequence directional over-current act as backup protection. Their phase selection and trip logic can distinguish single, two or three phases fault correctly, so that only the fault phase is cut off.In order to meet the demand of adaptive auto-reclosure, only the fault phase of double-circuit lines on the same tower is cut off under any condition. 2.2.2. Auto-reclosingTo prevent severe strike on electric power system cause by reclosing onto permanent fault or fault at the very beginning of line, novel adaptive auto-reclosure scheme is brought forward here. Auto-reclosure of double-circuit lines on the same tower is designed as one device. After fault, only one phase of the double-circuit lines is closed first, then the left opened phases are closed in turn according to the determinate rules, each at one time. Measures are researched to distinguish between transient fault and permanent fault in order to prevent blind re-closing. For transient fault, auto-reclosure will operate only after arc at the fault point has been extinguished. For permanent fault, auto-reclosure will not operate at all.For fault at the very beginning of line, auto-reclosure at the remote end will operate first. If it succeeds, the local auto-reclosure will operate subsequently. If it fails, i.e. it re-closes onto permanent fault and all three phases are tripped by protection at the remote end of line, local auto-reclosure will not operate at all.Such an auto-reclosing logic keeps links of electric power system at the greatest extent during fault, improves auto-reclosing success percentage and prevent sever strikes on electric power system caused by reclosing onto serious permanent fault. Split-phase current differential protection RCS-931, split-phase composite distance carrier protection and zero-sequence directional comparison protection RCS-902C have been widely applied to 220kV and above lines in Chinese electric power system. RCS-902E, RCS-931E with the same protection scheme as RCS-931 and RCS-902C respectively, and with additional auto-reclosing logic for double-circuit lines on the same tower, have been applied to LongWan-HongGou transmission line in SiChuan, which is the first 500kV double-circuit lines on the same tower along the whole length in China. 2.3. Power-swing-blocking relayFor distance protection, plenty attention must be paid on influence caused by power swing. To prevent unwanted operation of distance protection during power swing, power-swing-blocking relay must: a) block distance protection during power swing without internal fault; b) not block distance protection during internal fault; c) ensure correct operation of protection for fault during power swing; d) keep selectivity of distance protection during power swing period after fault. The main protection of RCS-900 series protection devices, i.e. current differential protection, directional comparison protection and composite distance carrier protection dont need power-swing-blocking logic in principle. On the other hand, step zones distance protection needs power swing blocking relay. Power-swing-block relay consists of four parts 4: enabling element in short-term duty; enabling element for asymmetrical fault; enabling element for symmetrical fault; enabling element during phase discrepancy period. The power-swing-block relay can enable distance protection quickly for internal faults under any condition even during power-swing, and block distance protection during power-swing with or without external fault. 3. Key technology in busbar protectionThe main difficulty busbar protection facing is to prevent unwanted operation caused by TA saturation during external fault. Traditional mid-impedance and high-impedance differential protection inserts appropriate impedance in differential circuit, while traditional low-impedance differential protection makes use of rapid saturation principle to prevent unwanted operation. Digital busbar protection sample currents of each branch circuit separately so that only low-impedance percentage restraint current differential principle is available. Some digital busbar protection take countermeasures such as increasing percentage restraint factor or delaying operation after a short term enabling, to prevent unwanted operation caused by TA saturation during external fault, thus the sensitivity and operation speed of protection are harmed. RCS-915 series busbar protection employs adaptive weighting busbar differential protection principle based on DPFC for the first time in the world 9, and takes steady state percentage restraint current differential protection with waveform identification element as assistant criterion. With ultra fast operation speed, high sensitivity and high anti-saturation ability, this protection keeps best balance between sensitivity and security. For external fault, unwanted operation will not appear even if TA is saturated as quickly as in no more than 2ms. For internal fault the protection can operate quickly. 4. Key technology in power transformer protectionFor power transformer protection, a great challenge is to prevent unwanted operation caused by inrush current and to operate quickly when switching onto internal fault especially those slight ones. RCS-978 series power transformer protection devices employ a novel differential current phase angle adjusting method. After phase angle adjusted, fault character of fault phase current is more obvious, and the current of left phases contains more component of inrush current. When switching onto internal fault, operation speed of fault phase protection will not be slowed by inrush current of healthy phases thanks to split-phase inrush current restraint. Inrush current that appears when switching on transformer with no fault, when external fault is cleared and when the parallel transformer is switched on can be distinguished more easily. 5. Key technology in generator-transformer unit protectionThe scheme of large generator-transformer unit protection must ensure the security of whole unit and reduce the damage in case of fault. The scheme must be perfect, reasonable, and not complex. RCS-985 series generator-transformer unit protection devices employ DPFC principle, variable slope percentage restraint current differential protection, asynchronous anti-TA saturation identifying criterion, zero-sequence transverses differential protection with current restraint and floating threshold, meet the requirements of protection for large generator-transformer unit 17. DPFC principle has been described in the former part of this paper.Percentage restraint current differential protection has a variable slope percentage restraint curve, which has a none-zero percentage at the beginning of curve. The curve well adapts to unbalanced differential current and the initial setting of current differential protection can be lower, thus the