Relay protection configuration for large units

Large capacity units are usually connected directly to high voltage or ultra-high voltage power grid by generator - transformer unit connection, which plays a very important role in power system. Due to its complex, expensive, once damaged due to fault, a lot of damage on the economy will inevitably, so considering the large capacity generator - transformer relay protection configuration, the whole group should be emphasized to ensure its security and maximize the narrowing the scope of power failure, as far as possible, avoid unnecessary stop suddenly, for some automatic processing device for the abnormal condition, especially to avoid protection misoperation and refused to move. So not only requires high reliability, sensitivity and selectivity, rapidity, good protective relay, it also requires on the overall configuration of relay protection try to be perfect and reasonable, and strive to avoid cumbersome and complex.

1. The traditional relay protection method and its disadvantages

Traditional internal fault of generators: the main protection scheme often USES traditional longitudinal differential protection, transverse differential protection, longitudinal zero sequence fundamental voltage protection, rotor second harmonic current protection, scalar product brake type longitudinal differential protection, etc. The traditional way of protection to a certain extent, played the selectively to fault component removal from power system, reduce the damage for electric power system, to restore the normal operation of trouble-free part, electrical components abnormal operation condition, but because of many reasons, always exist in the power system, generator, transformer and other equipment to protect components correct operation rate is generally low, many are less than 70% of the problem, so it is necessary for the protection mode analysis, slightly in order to find improvement measures.

1.1 traditional longitudinal differential protection
Traditional differential protection applies only to interphase short circuit, the phase of generator stator winding turns or short circuit between the layers and welding (break) failure are no protection, thus protecting the function is not comprehensive, coupled with the existing components of product quality and low maintenance and management level of many problems, must be improved.

1.2 traditional horizontal difference protection
Previous unit transverse differential protection, the current action is about (0.2 ~ 0.3), In the current transformer neutral point of attachment of change than choose 0.25 In / 5 A, so the old unit transverse differential protection for generator rated current of the secondary current action about values (5 A), as A result of the generator internal fault with old transverse differential relay of three harmonic filter is less than 15, so the normal setting limits the protection action sensitivity enhancement.

1.3 longitudinal zero-sequence overvoltage protection of base wave
For a generator with only U, V and W three phases leading to three terminals on the neutral side, it is customary to install longitudinal differential protection to reflect inter-phase short circuit. In order to protect the generator stator winding interturn short circuit or open welding failure, can add longitudinal zero sequence over-voltage protection, but the longitudinal zero sequence over-voltage protection device is complex, low sensitivity, and may cause some mistrip. Only when the three phases on the neutral side of the generator lead out only three terminals and the longitudinal differential protection is installed.

1.4 protection of secondary harmonic current of rotor
In the rotor harmonic secondary current protection, the starting element and the selected element have strict requirements for cooperation, which is easy to misoperate and difficult to adjust. Special rotor circuit reactance transformer is required. Only when the generator has determined that the neutral side only leads to three phases and three terminals, and decided to install longitudinal differential protection rather than high sensitive transverse differential protection.

1.5 scalar braking longitudinal differential protection
The scalar braking longitudinal differential protection can not reflect the turn-to-turn short circuit and the stator winding soldering fault, and this protection also requires the generator neutral side to introduce three phase terminals.

At present domestic 300 mw and above the steam turbine generator adopts two parallel branches, each phase neutral only leads to the three terminal of three-phase, generally used generator differential protection and generator - transformer differential protection for generator - transformer group internal short circuit main protection. When there are only three phase and three phase lead terminals on the neutral side of the generator, the transverse differential protection of the unit cannot be installed, which will affect the reliability and sensitivity of the whole relay protection device.

Improvement scheme of main failure protection for large units

2.1 improvement scheme
For large unit relay protection configuration principle is: strengthen main protection, simplify backup protection. Therefore, in view of the problems of traditional relay protection, a dual main protection configuration scheme consisting of incomplete longitudinal differential protection and high sensitive element transverse differential protection is proposed.

Will change the way you lead generator neutral side, three phase six branches winding is divided into two groups, one group only its neutral N1 derivation, another set of three-phase terminal lead respectively, and in the second neutral N2 generator external into, N1 and N2 connection to furnish unit transverse differential protection, generator transformer TA1 and TA2 incomplete differential protection. Many theoretical research and practical experience has proven: high sensitive unit a transverse differential protection with the stator winding of generator turn-to-turn short circuit and interphase short circuit, open welding protection function, very simple, high sensitivity, can be used as the first main protection of all kinds of generators. Incomplete differential protection overcomes the traditional differential protection does not reflect diagnosing.hardware circuit and welding problems of defects, is the second main generator internal various fault protection, easily double of the main protection in the large sets of requirements. The necessary precondition of this scheme is that there should be four outlet terminals on the neutral side of the generator.

2.2 improvement measures
A) for steam turbine generators of 300MW or above, as long as the neutral point extraction method is slightly changed during the manufacture of the generator, the high-sensitivity element transverse differential protection can be used, whose function exceeds the longitudinal differential protection.
B) the replacement of transformer, reduce current transformer variable ratio, improve the harmonic filtration than three times, through a conventional generator short circuit test, the measured transverse differential protection is not balance the fundamental wave and harmonic current three times to correct setting current action, to transform the traditional transverse differential protection into high sensitive transverse differential protection.
C) incomplete differential protection are the two parallel branches for each phase of generator, USES the ratio braking type relay, the machine end transformer made variable than In / 5 a, choose change ratio 0.5 In branch transformer / 5 a, this way can all interphase short circuit inside the generators, winding interturn short circuit and branches open welding failure, as shown In figure 2. This kind of protection can protect the short circuit of the generator lead line. But should be paid attention to in every branch number many (more than 2), if one does not furnish transformer branch fails, to furnish the branch of the fault current of transformer may be small, incomplete differential protection may refuse to action, and therefore to be cautious with branch number is large.
D) generally speaking, when the generator transformer itself is configured with dual main protection, not need to set the generator, transformer short-circuit backup protection, then all can be installed on the machine end impedance or offset impedance protection, balancing machine end and high voltage busbar interphase short circuit fault, the step-up transformer high voltage side also typically have a zero sequence current and zero sequence voltage as backup protection of high side grounding short circuit.

2.3 the role of double main protection
The main protection scheme can make the all interphase short circuit inside the generators, turn-to-turn short circuit and dual stator winding open welding failure will receive quick protection, at the same time also can make the timing generator run independently and end of the short circuit has rapid protection. When the number of branches of each phase is greater than 2, the number of branches of the ct installed at the neutral side of each phase should be greater than or equal to n/2(n is the number of branches per phase).

When the main protection configuration scheme is adopted, the vertical zero sequence voltage protection and the secondary harmonic rotor current protection can be completely abandoned.

3. Prospect of relay protection

The development history of relay protection way and direction comparison, phase comparison, current differential phase, such as the use of relay from electromagnetic to simulate the stationary type, develop to digital motionless, with the development of digital technology and the emergence of a microcomputer and microprocessors, digital opened up broad prospects for relay protection, appeared on the basis of microcomputer and optical transmission technology of full digital control protection system.

Microcomputer protection has the following characteristics:

A) the protection function is implemented by software;
B) adopt digital signal processing technology;
C) it has digital storage functions, such as process memory, recording of waves, etc.;
D) easy remote communication and simple interface;
E) automatic testing and monitoring;
F) software and hardware standardization;
G) public data can be reused to realize different functions.

China has successfully developed a number of large unit microcomputer protection devices, and has been put into trial operation or formal operation. Many 35kV and 110 kV substations adopt many kinds of microcomputer monitoring and protection devices, but there are not many examples of microcomputer protection devices used in 300MW or above large generating sets. Due to its flexibility, high performance, microcomputer relay protection operation maintenance convenience, good reliability, hardware, the advantages of small size, light burden of hardware, the relay protection can predict the future development direction will be the main protection by micro processor or scattered small computer installed in a protected components, the system control center backup protection substation computer to realize the centralized control center computer. It is believed that the microcomputer protection of large units will have a broad development prospect and will lead the field of relay protection with its superior performance.