Article 379. Insulation Co-ordination

(1) Summary of insulation coordination study

Insulation coordination is a process determining the level of insulation for system equipment in accordance with the corresponding over-voltage protection (e.g. surge arresters). There are three basic elements for insulation coordination as follows.

-Determining the overvoltage from the power system

-Determining the insulation strength of specific equipment in the substation

-Selecting the technical parameters of surge arrester andlocations, or neutral grounding system, etc., to ensure the system-imposed overvoltage don't exceed the insulationstrength of the

equipment including an appropriate protective margin

The above mentioned overvoltage can be determined by simulation. In power systems, there are four general types of overvoltage thatshall be considered in studying the insulation coordination.These are:

-Maximum continuous operating overvoltage

-Temporary overvoltage (PFWV); caused by single-line to ground fault, load rejection, etc.

-Switching overvoltage (SIWV)

-Lightning overvoltage (LIWV)

(2)Basic idea of selecting the basic impulse insulation level (BIL)

For example, the basic idea of selection of the basic impulse insulation level) (BILforpower equipment in the power system with the nominal voltage 220kVof shown in the Table331-1 in Technical Regulation is as follows.

Firstly the BIL regulated in IEC aretwo types of 950kV and 1,050kV and the adopted BIL is normally selected from those values.

In practical selection of BIL, BIL shall be selected according to the maximum overvoltage applied to the target equipment based on the result of surge analysis (simulation in the above item (1)). Because the overvoltage applied to the actual target equipment vary accordingto various conditions (actual arrangement of equipment, equipment configuration, neutral grounding system, the specification and arrangement of surge arresters and etc.)Eventually,. the optimal specification and arrangement of surge arresters, and adopted BIL are determined by repeating the simulation while changing abovementioned conditions.

At this time, if applied overvoltage is low from the result of above analysis, adopted BIL can be chosen from low values. (e.g. from 950kV or 850kV) On the other hand, if applied overvoltage is high,

it is necessary to select from 1050kV or BIL adopted with the rated voltage of the higher rank, such as 1,175kV.

Chapter 4-2-6 Lightning Protection for Rotation Machine

Article 380. Connection of Overhead Line

As stipulated in Technical Regulation.

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Article 381. Protection of Rotation Machines Connected to Overhead Lines

For the protection of generator, synchronous compensator, electric motor with capacity more than 3MW (3MVA) connected to overhead line, appropriate lightning valve and capacitor with capacitance

not less than 0.5µF shall be installed in every phase. Besides, the overhead line sections connected to the power plant (or substation) shall be protected against lightning strikes at impulse current of not less than 50kA.

Lightning valve should be placed on the bus bar (or part of the bus bar) of generator to protect the generator ( or synchronous compensator) with capacity up to 15MW (15MVA), on the bus bar of distribution station to protect electric motor with capacity over 3MW, or on theoutlets of generator (synchronous compensator) with capacity over 15MW (15MVA).

As for the protection of generator (synchronous compensator) with isolated neutral and without winding insulation (insulation bar windings type) rated from 20MW (20MVA) and up, instead of using 0.5µF capacitor at every phase, lightning valve can be installed at the neutral conductor, with nominal voltage of the generator (synchronous compensator).

It is not required to install protection capacitor if the total capacitance of the cable system connected to the generator (synchronous compensator) with length up to 100m can derive a capacitance of 0.5µF or above for every phase.

Article 382. Protection of Overhead Lines with Rotation Electric Machine

If electric rotation machine and overhead line are jointly connected to the busbar of a power plant or a substation, such overhead line should be protected against lightning in accordance with the following requirements:

1. The first section of overhead line shall have lightning wire with length of at least 300m. The lightning arrester shall be installed on this section of the line (Figure382a). Conductors of overhead

line shall be placed on insulators with insulation class for35kV. Grounding resistance of lightning arrester shall not exceed 5Ω, grounding resistance of the pole where grounding wire is installed shall not exceed 10Ω.

At the beginning of the line, appropriate lightning arrester can be installed instead of line lightning arrester. In such case, grounding resistance of the lightning arrester shall not exceed 3Ω.

2.If the overhead line is connected to power plant or substation through cable section with length up to 0.5km, such section will be protected as an overhead line without connected cable (see point 1) and appropriate lightning arrester should be additionally installed at the junction between the cable and the overhead line. Lightning arrester shall be connected by the shortest path to metal sheath of cable or to the grounding system. Grounding resistance of the lightning arrester shall not exceed 5Ω.

3.If overhead line section with length of 300m or longer is already protected against direct lightning strike thanks to high buildings, trees or high structures, then it is not required to install lightning wire. In such case, at the terminal of overhead line section protected (at side of line), lightning arrester shall be installed. The grounding resistance of lightning arrester shall not exceed 3Ω.

4.If overhead line is connected through a line section of 100 - 150m to busbar of substation which has

protected by lightning wire, and lightning arrester shall be installed at the reactor. The grounding resistance of line lightning arrester shall not exceed 5Ω.

5. If the overhead line is connected to the busbar of distribution station where rotation machine is placed through reactor and cable with length of more than 50m, it is not be required to install lightning protection device. At the connection point of connecting cable and the overhead line, it needs to install lightning arrester with grounding resistance not exceeding 5Ω, and lightning arrester

shall be placed before reactor location. (Figure 382c).

6. If overhead line is connected through a line section with length not less than 0.5km to busbar of power plant (substation) which has electric rotation machine with capacity less than 3MW (3MVA) and pole grounding resistance not exceeding 5Ω, then lightning arrester shall be installed at distance

of 150m to power plant (substation). The grounding resistance of this lightning arrester shall be not more than 3Ω. In such case, it is not necessary to install lightning wire for that overhead line section.

Figure 382 Atmospheric overvoltage protection diagram for electric rotation machine

If generator (synchronous compensator) is connected with transformer through bare conducting system, the live parts of this system shall be located in the protection area of lightning poles or structures of power plant (substation) against direct lightning strikes. Distance from grounding points

of lightning poles to grounding points of power conducting systemll shabe not less than 20m (measured along the grounding wire).

In case that the power conducting system is not located in the area protected by the lightning arrester placed in the outdoor station, they shall be protected against direct lightning strikes byindependent lightning arrester poles or lightning wire hung on a separated poles with protection angle not over 200 degree. Independent lightning arrester poles and lightning wire poles should be connected to a separated grounding system. If such pole is connected to the common grounding system of the station,

the connection point shall be at least 20m far away from that of power conducting system.

In air distance between the independent lightning system or lightning wire poles and live parts or grounding parts of power conducting system shall be not less than 5m.

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Underground distance between the separated grounding system or underground parts of independent lightning poles to grounding system or underground parts of power conducting system shall be not less than 5m.

If the substation of an industrial enterprise is connected through overhead line to the distribution station of power plant where installed generators with capacity of up to 120MW for each unit, the protection against direct lightning strikes on that overhead line shall be implemented as stipulated in the above items.

If the bare conductor is connected to distribution equipment at generator’s voltage through reactor, appropriate lightning arrester shall be installed before the reactor.

In order to protect generator against lightningsurge transmitted along conducting bars as well as against induced over-voltage, proper lightning arrester and capacitor shall be installed for protection of all 3 phases. Capacitance of capacitor shall be not les 0.8µF for voltage level of 6kV, not less than 0.5µF for voltage level of 10kV and not less and 0.4µF for voltage level from 13.8 to 20kV.

It is not required to install protection capacitor if the total capacitance of the generator and cable network connected to the generator’s busbar is at required voltage value. For the calculation of capacitance value of the cable network in such case, only cables with length up to 750m are taken into account.

It is allowed not to install device for protection of overhead line section against direct lightning strike

if that overhead line is connected to electric motor with capacity up to 3MW fed by stable standby power source.

If the overhead line section with length of over 250m is connected into the substation and grounding resistance of poles of this section is not over 10Ω, lightning arrester is not be required.

If the overhead line is connected to the substation through cables with any length, the lightning arrester shall be installed before this cable section. Grounding of lightning arrester shall be connected

to the metal sheath of the cable.

For electric motors, appropriate lightning arrester and 0.5µF protection capacitor shall be installed in each phase.

Chapter 4-2-7 Internal Overvoltage Protection

Article 383. Internal Overvoltage Protection for 6-35kV

In the power grid at voltage level of 6 35kVto where installed arc suppression coil, capacitance between phase to ground should be balanced for each phase by rational placement of phases and highfrequency communications capacitors.

The difference of capacitance for each phase shall be not over 0.75%.

The location of arc suppression coil shall be determined based on power grid structure, the possibility

of dividing the power grid into separated areas, the probability of faulty modes, the effect on automatic circuits of railway and communications channel.

It is forbidden to place arc suppression coil into transformer in the following cases:

-Transformer is connected to the bus bar via fuse

-Transformer is connected to the power grid via only one transmission line.

Capacity of arc suppression coil shall be determined yb the total capacity to ground in consideration of the extension of the power network system in the future.

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Power grid at voltage level from 6 35kVto where installed arc suppression coil or generator (synchronous compensator) with directly water-cooled stator winding, it is not required to install the neutral displacement protector.

The above power grid, that is without arc suppression coil or water-cooled generator (synchronous compensator), or in the power grid where the equipment such as arc suppression coil and water-cooled generator (synchronous compensator) are disconnected when the power is automatically interrupted, it

removing that impeder shall be also equipped.

Moreover, in the diagram of generator-transformer and compensator-transformer, the second impeder

as mentioned above should be connected parallelly to the permanent impeder if the ferromagnetic resonance occurs.

If the power grid is not required to measure phase-to-ground voltage (insulation testing) or “zero” sequence voltage, then transformer with ungrounded primary winding should be utilized.

Article 384. Internal Overvoltage Protection for 110-220kV

As for 220kV transformers and autotransformers which have normal insulationlevel, they shall be protected from internal over voltage by arrangement of surge arresters in accordance with requirement in the Article 378.

In addition, the arrangement of surge arresters should be determined according to the following policy.

Protection of the equipment against over voltages should normally be provided by the surge arresters installed on the feeders(distribution and transmission lines). In some cases, the protection given by surge arresters may be inadequate. This situation arises mainly in the following configurations.

-Large distance between the surge arrester and the equipment

-The equipment connected to the surge arrester by means of power cables

-Long bus bars open at their ends

-Connection to overhead lines by means of insulated cables;

-Locations with high probability of lightning strikes.

For these configurations, the installation of additional surge arresters may be required. Theirlocation should be based on experience with similar situations or on calculations.

Article 385. Internal Overvoltage Protection for 500kV

In 500kV power grid, depending upon the total length and quantity of overhead lines, types of circuit breakers, capacities of transformers and other parameters, appropriate methods should be applied to prevent the substantial voltage rise in a long time and operating over voltage (temporary overvoltage,

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