Protection Application Handbook (англ. яз.)

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SELECTIVITY PLAN

Setting of Protection

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Figure 19. A Differential relay can be indicated in the selectivity plan with

the operating time and a current range starting from maximum sensitivity.

An impedance protection is more difﬁcult to indicate as the oper-

ation is dependent of the quotient Z = U/I which means depen-

dence of the power S = U

+ Z

) where Z

is source

impedance and Z

is distance protection setting for each zone.

is rated line to line voltage. Representation of a distance pro-

tection in a log-log diagram will be as below.

SELECTIVITY PLAN

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Figure 20. A Distance protection with three stages can be indicated with the

time stages and impedance stages recalculated to MVA or current at nomi-

nal voltage and possible some other voltage level as a range for each stage.

Special problems at setting of protection relays

Figure 21. A generating station with many infeeds to a fault on an outgoing

feeder.

It should be noted that for a system with many infeeds such as

H.V. grid systems, the currents through relays of different loca-

tions can be quite different, which can give a situation like this.

In this case the same setting can be used on all relays with main-

tained selectivity since current through faulty object always is big-

gest. The same situation also appears for earth-current relays in

SELECTIVITY PLAN

Setting of Protection

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solidly grounded systems with many grounding points. Also for

one of many generator feeders a similar situation occurs.

Figure 22. Similar settings can give selectivity due to differences in fault cur-

rents at different locations.

SETTING EXAMPLES

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5. SETTING EXAMPLES

Setting of short circuit protection relays in a distribution network.

For a power system the settings of short circuit protection relays

shall be calculated. The network is a distribution network as

shown in below ﬁgure.

Figure 23. The selectivity plan must cover setting of all the protection relays

in the network.

Start to-

1) assemble data

a) One-line diagram see ﬁg.

b) Rated data see ﬁg.

SETTING EXAMPLES

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c) protection equipment. All relays are of type RXIDK 2H with

normal inverse characteristic. Recovery (Overtravel) time is < 80

ms. Directional relay is type RXPE 42 with deﬁnite time delay

0.2-3 s.

d) Calculate fault power at all locations. Max and min values are

required. See ﬁg.

e) All breakers have tripping time 60 ms

Fuse is of type SHB. 100 A = 1.9 MVA

2) Draw 100 A fuse characteristic on the log-log paper.

3) Calculate setting of short circuit protection A. Check required

setting with achieved selectivity. Note that A protection is only re-

quired to cover 11 kV faults as a fuse is a “secure break point”.

Fault clearance time of fuse is 10 ms. A setting 15 MVA k=0.05

gives selectivity. Time margin 140 ms at max fault current on 11

kV (t1).

4) Calculate required setting for relay B. Relay B must be selec-

tive to relay A allowing for load on other objects on 11 kV busbar.

B set

≥ I

Aset

+ I

LOAD

Set = 20 MVA, k=0.15.

5) Calculate required setting of relay C.

Same setting of inverse type element can be used as for relay B

as transformer any way always is disconnected and on an

achieved selectivity will delay backup tripping 300-400 ms for

fault within transformer and for faults higher up in system. As

transformer is D/Y a phase to phase fault on low voltage side will

have only 0.866 p.u. of current when H.V side has 1.0 p.u. in one

phase and 0.5 p.u. in the other two phases. This must be com-

pensated for by selecting a little higher setting. Set 25 MVA,

k=0.15.

6) Select setting of instantaneous element of relay C.

Maximum fault power fed through the transformer for a low volt-

age fault will be 87 MVA. Select a setting of 120% of max current

plus margin for transient overreach. RXIDK 2H transient over-

reach for instantaneous step is < 18%.

Set 1.2-1.18-87=125 MVA.

7) Select setting of relay D.

D must be given a current setting I

LOAD

A setting I

= 30 MVA gives a sufﬁcient margin not to operate

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during normal load, and also fulﬁls above requirement since fault

and load currents are not in phase, and as load transmitting pos-

sibility is limited because the voltage has collapsed. A check with

different k values and setting 30 MVA shows that k = 0.2 is a suit-

able value.

8) Select setting of relay E.

In this case relay E is required to be selective to relay D although

disconnecting the same line. Only the breaker closest to the fault

may trip as stations are unmanned with quite a distance between.

E relay is set to I

= 34 MVA which gives margin for differences

in current transformers and relays. Time curve k=0.3 gives selec-

tivity to relay D. Time margin 0.3 sec. with max fault current at sta-

tion B.

9) Setting of relay F delayed stage.

Relay F shall be set on I

Fset

≥ I

Eset

+ I

LOAD

(other object) and >

1.4-1.5 • In for the transformer. Note that at maximum service two

transformers are connected in parallel. Fault current will also be

split up between two transformers at maximum service but at

minimum service only one transformer is feeding.

Set I

Fset

= 60 MVA k=0.4 which gives a suitable curve.

10) Select setting of relay F instantaneous stage.

Instantaneous stage is set as under item 6.

Set I

FInst

= 1.2 -1.18*345 MVA = 490 MVA.

11) Set directional short circuit protection relays G. Protection G

are intended to operate for fault current fed from parallel trans-

former for a fault in one transformer low voltage bay. Relays must

operate selective to relay F for all occurring fault current fed from

parallel transformer (Maximum 690/2 MVA)

Protection has an operating angle of 60° and can be set on rated

power of transformer or even below if required.

Set I

Gset

= 40 MVA t = 0.4 sec. which gives time selectivity to pro-

tection on high voltage side (F) for all fault currents.

12) Setting is now completed and all settings can be calculated

in current for the appropriate voltage level and then to secondary

current for setting of the relay. Minor adjustments might be nec-

essary if settings are not steplessly. The required time margin be-

tween selective relays is checked according to item 2.2.4.

Required time difference will be (simpliﬁed).

This is fulﬁlled for all steps.

SETTING EXAMPLES

Setting of Protection

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Figure 24. The selectivity plan will have all relays in the network indicated

and the time margin must be checked for all selective relays. The sensitivity

for back-up protection to fulﬁl tripping criteria must also be checked.

SETTING EXAMPLES

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Setting of short circuit protection relays in a generation net-

work

To illustrate use of inverse type characteristics we shall set the

protection relays the net as shown on ﬁg.

1) Assemble data.

a) One-line diagram see ﬁg.

b) Rated data see ﬁg.

c) protection equipment. All relays are type RXIDK 2H with “nor-

mal inverse characteristic”. Recovery (overtravel) time < 80 ms

d) Calculate fault power for busbar fault.

Figure 25. The selectivity plan shows that selectivity is achieved with similar

settings due to the differences in fault currents.

SETTING EXAMPLES

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Max and min value required. Min generation is 4 generators. e)

Breakers tripping time is 60 ms.

2) Draw selected settings of outgoing object on log-log paper.

Only highest setting need to be shown i.e. draw Al setting.

3) Max operating time for fault on outgoing feeder A1 is about

0.75 sec. i.e. min operating time on each generator must be high-

er than 750 ms + C.B. (60 ms) + RXIDK recovery (80 ms) + mar-

gin i.e. > 1.0 s.

Current setting on generator is set with margin to max load.

Select 52 MVA (1.3-In) k=0.2 which gives a suitable curve.

It is noted that although generator setting is lower than for outgo-

ing feeders selectivity is still achieved because of the big differ-

ence in fault current for each object compared to the total current.

Setting of Back-up earth fault protection relays in a transmission

network

RXIDG 2H inverse characteristic.

Back-up earth fault protection relays shall be set in system as

shown in ﬁgure.

1) Assemble data

a) One line diagram see ﬁg.

b) protection equipment. All relays are ABB type RXIDG 2H with

specially designed inverse characteristic for back-up earth-fault

protection relays in a H.V. meshed system.

c) Calculate fault current contributions from the different objects

under different occurring service conditions.

d) Set RXIDG 2H operation level on required level for system lev-

el. Select 120 A primary. Time curve is ﬁxed and will always give

selectivity if biggest infeed is less than 80% of fault current on

faulty feeder. Inverse time characteristic has a deﬁnite min level

which is selected to allow for single-pole tripping and autoreclos-

ing. Set this level to 1.2 sec.

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Figure 26. The logaritmic inverse characteristic will give selectivity if the big-

gest object fault current contribution is lower than 80% of the faulty object

fault current.

Final conclusion

The setting of time and current selective relays is a quite compli-

cated task. The preparation is very important in order to make a

good selectivity plan. It must be remembered that a incorrect set-

ting can cause big disturbances in the power network with high

costs (missing earnings) as a result. The work should thus be

performed carefully and the future should be predicted as far as

possible to ensure that selected settings are suitable also when

network is extended and changed.

Modern tools are today available to perform a selectivity plan, but

they must always be used with human control, where all results

are checked and conﬁrmed by a experienced relay engineer, as

tools are never better than the programmer of the tool. The same

philosophy is adapted when using a tool as described above. The

tool is just taking away the need for semi-transparent paper and

the work of drawing the characteristic.