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.
DPFC 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.
No 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 .
Fault 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 Ⅰ 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.
Current differential protection based on DPFC principle bas been applied to line, busbar, transformer and generator successfully .
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.
Series-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 .
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 , 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.
More 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.
The 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.
To 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.
For 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 don't 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 : ①enabling element in short-term duty; ②enabling element for asymmetrical fault; ③enabling element for symmetrical fault; ④enabling 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.
The 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 , 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.
For 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.
The 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 . 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 differential protection sensitivity is high when internal slight fault occurs even during unit start-stop process.
It was a misunderstanding that the unbalanced differential current in secondary coil of TA is low because the TAs installed on head and tail of generator are of the same type and both are P class, and the fault current passing through during external fault is low, thus the initial setting can be set lower properly to improve the sensitivity for internal slight fault. In fact, TA wiring cable lengths from head and tail of generator may be not the same though TAs are of the same type, TAs are of different type in some cases, the aperiodic components decays slowly though the whole amplitude of current is not high under external fault which leads to TA saturation. All these factors lead to no acceptable unbalanced current, which causes unwanted operation repeatedly.
Lifting differential setting or introducing differential protection with product- quantity restraint will lose sensitivity or even lead to missing operation. With identifying harmonic component and waveform character of differential current asynchronous anti-TA saturation criterion can distinguish between internal fault and TA saturation under external fault.
So far inter-turn fault protection, such as negative-sequence direction blocking longitudinal zero-sequence voltage protection, negative-sequence direction blocking second harmonic component protection, third harmonic component blocking longitudinal zero-sequence voltage protection, transverse differential protection still have some defects in sensitivity and security. Take transverse differential protection as an example, the setting must be higher than the maximum unbalanced current under three-phase fault at generator terminal in order to prevent unwanted operation during external fault or power-swing, thus the sensitivity is not enough for inter-turn fault. RCS-985 introduces longitudinal zero-sequence voltage inter-turn fault protection and high sensitive transverses differential protection for the first time; both have current restraint and floating threshold. Setting of zero-sequence voltage only need to be higher than maximum unbalanced fundamental harmonic voltage under normal operation mode of generator, and setting of transverse differential protection be higher than maximum unbalanced differential current under normal condition, thus the sensitivity is high for internal low percentage inter-turn fault without risks of unwanted operation under external fault.
DL/T769-2001, Technical guide for microprocessor-base protection equipment of power system  laid down the guideline for digital protection devices allocation. Digital protection may integrate main and backup protection of protected main equipment or line into one device, main and backup protection share the same DC power supply, TAs and TVs. For those lines and main equipments that need duplicate protection system, each system may be a digital protection device with main and backup protection integrated together.
Main protection is the protection to selectively clear fault on protected main equipment or line in definite time short enough to guarantee system stability and equipment security. Backup protection clears the fault when they cannot be cleared by main protection or breaker fails to trip. Backup protection falls into two categories, remote backup protection or near backup protection.
Based on information sharing and powerful calculation capacity of embedded system, one digital protection device can integrates main and backup protection together, so that different protection elements can coordinate with each other on sensitivity. For those lines or main equipments that need duplicate protection, two such devices can be installed, thus loss of protection will be prevented even if one device fails. In each device, two independent measuring and deciding circuits are installed, thus failure of any one circuit will not lead to unwanted operation. Integration of main and backup protection and duplicate allocation make it convenient to test, maintain, operate, and design the protection and it's secondary winding.
Integration of main and backup protection and duplicate allocation have been achieved first years ago in ultra high-voltage line protection, which provides convenience for operation, management and maintenance of line protection and improve rate of correct operation largely. In recent years, this concept has been applied to main equipment protection, which brings convenience for design and operation.
In each device of RCS-900 series line protection, pilot protection such as differential protection, directional comparison protection and composite distance protection act as main protection, while step zones distance protection, over-current and zero-sequence over-current as backup protection. Double devices are installed on line of 220kV and above, and in some cases, three devices altogether.
Traditionally, busbar protection and breaker failure protection are independent, each has its' own TAs. RCS-915 series busbar protection integrates busbar protection and breaker failure protection together on the basis of correctly recognizing operation mode; two protections share the same TAs. Double devices are installed on busbar of 220kV and above, thus both busbar and breaker failure protections are doubled with no need of additional TAs. This scheme is of high reliability, simple secondary winding, flexile operation mode, i.e. maintenance on each protection device can be carried out without switching off the busbar.
Each device of RCS-978 series transformer protection integrates all electric quantities protection for power transformer, including steady state percentage restraint differential protection, differential instantaneous protection, DPFC percentage restraint differential protection, zero-sequence or winding percentage restraint differential protection, combined voltage (low-voltage plus negative-sequence over-voltage) blocking directional over-current, zero-sequence directional over-current, over-excitation protection, phase-to-phase impedance, zero-sequence over-voltage, and gap zero-sequence over-current. Main and backup protection share the same TAs; the concept of integration of both in one device is achieved. Installing two such devices on one power transformer, thus the main and backup protections are doubled.
Integration of main and backup protection with duplicate allocation reduces the total number of power transformer protection devices, and the secondary winding is clear, simple and independent. Main protection has been enhanced, so the backup protection can be simplified. Duplicate allocation eliminates the case that failure or time scheduled maintenance of protection take power transformer out of operation. All electric quantities of power transformer can be sampled and recorded during fault by protection, it is meaningful to make use of the record to analyze the fault in details and find the fault causation quickly.
Traditionally, for 300MW generator unit, only differential protection of main transformer, generator and generator-transformer unit are doubled. The rest is of single arrangement. But some fault type can only be protected by one dedicated protection element, when the dedicated protection is out of operation because of failure of wiring or device itself, the generator has to be stopped or run with defects, thus the loss and risks are high.
RCS-985 series protection integrates all electric quantities protection for one generator-transformer unit together, i.e. main protection, backup protection and abnormal operation protection. The protected range includes main transformer, generator, high-voltage house-service transformer and exciting transformer or field exciter. For large generator-transformer unit, double protection devices are needed, each has its' own TAs and independent trip outputs.
This arrangement results in convenience of operation, setting, testing and maintenance, high security and reliability, simple design and clear wiring. Main protection and backup protection share the same TAs, so the total number of TAs needed is the same as or less a little than before.
The first group of generators in Three-Gorge hydraulic power plant has passed acceptance test and put into service. The structure of linked networks spanning large area is coming into being in China. More higher voltage level 750kV network will soon appear in northwest China. Fast developing electric power industry in China offers wide scope of activities for research and application on protection. NARI-RELAYS will continue to explore and innovate in electric power protection and automation area, and contribute her efforts to the development of protection technology.
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