RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY(3)

4. Key technology in power transformer protection

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.

5.  Key technology in generator-transformer unit protection

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 [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 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.

6.    Integration of main and backup protec-

tion, duplicate allocation

DL/T769-2001, Technical guide for microprocessor-base protection equipment of power system [18] 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.

6.1.  Line protection

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.

6.2.  Busbar protection [12]

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.

6.3.  Transformer protection [15]

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.

6.4.  Generator-transformer unit protection [16]

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.

7.  Prospect

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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