G. Ziegler. Digital Differential Protection

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G. Ziegler, 10/2005 page 221

Differential Protection Symposium

Belo HorizonteNovember 2005

BB 2

BB 1

∆I1

∆I2

Isolator replica, Principle, (stabilising circuit not shown)

G. Ziegler, 10/2005 page 222

Differential Protection Symposium

Belo HorizonteNovember 2005

Optic fibre-communication

-HDLC protocol

-1,2 MBaud

Wired

connections

Decentralised BB-protection 7SS52, Structure

G. Ziegler, 10/2005 page 223

Differential Protection Symposium

Belo HorizonteNovember 2005

12345

Transfer bus

7SS50/52: Maximum configuration

7SS50 7SS52

centraliseddecentralised

Bays3248

BB-zones8 12

couplings4 16

G. Ziegler, 10/2005 page 224

Differential Protection Symposium

Belo HorizonteNovember 2005

7SS50/52: Acquisition and supervision of isolator positions

Connection to

central unit

G. Ziegler, 10/2005 page 225

Differential Protection Symposium

Belo HorizonteNovember 2005

Res

Internal faults (k=1)

Relay characteristic

(k =0.1 -0.8)

Optional extension

for networks with limited earth

current (impedance earthing)

(release by U

∆I >

Res

∆I

Operating characteristic of BB protection 7SS52

G. Ziegler, 10/2005 page 226

Differential Protection Symposium

Belo HorizonteNovember 2005

110 /10 kV

40 MVA

35 MW

= 6 Ohm (I

= 1 kA)

kA 1kV/6Ω310/

kA 2

310kV

35MVA

≈==

⋅

Restraint current:

kA 1

IΣI

I ===

kA 21)(2IΣ

Res

I ++==

2.0

===

Operating current:

Res

Relay operating characteristic

(k =0.1 -0.8)

Additional, for earth faults

(release by U

∆I>

Res

<= 7kA

∆I

>= 500 A

Res-L

= 4kA

hres-E

= 1kA

op-E

= I

= 1kA

BB-Protection 7SS52

Performance in networks with earth current limitation

G. Ziegler, 10/2005 page 227

Differential Protection Symposium

Belo HorizonteNovember 2005

∆I

SS1

∆I

SS2

∆I

Check

Check zone:

Ø in the past used with HI protection on EHV level (needs separate CT cores)

Ø in general not used with traditional low impedance busbarprotection (too expensive)

Ø However, now integrated as software function in full scheme versions 7SS51 and 7SS52

Check zone of busbarprotection

G. Ziegler, 10/2005 page 228

Differential Protection Symposium

Belo HorizonteNovember 2005

I1I2

I3 + I4

2·(|I

| +|I

| )

Res

+ I

Special restraint algorithm:

Positive and negative currents are added separately.

4I3I2I1I

IΣ +++=

4I3I

IΣ +=

Normal restraint:

Sum of all current magnitudes

Check zone

7SS52: special restraint algorithm avoids over-stabilisation

4I3I4I3I2I1IIΣ

Res

I +++++==

4I3I

IΣ

Res

I +==

Smaller value is then taken:

G. Ziegler, 10/2005 page 229

Differential Protection Symposium

Belo HorizonteNovember 2005

Bus coupler CB aux. contacts not connected:

•87-A trips bus coupler

•Current inverted in 87-B after T-BF.

Subsequently 87-B trips BB-B

Coupler CB auxiliary contacts connected:

•87-B trips immediately after opening of bus

coupler CB because coupler current is

removed from bus protection.

→ time reduction (T-BF saved)

BB-A

BB-B

7SS52: Special treatment of dead zone faults (1)

G. Ziegler, 10/2005 page 230

Differential Protection Symposium

Belo HorizonteNovember 2005

A) Bus coupler CB aux. contacts not connected:

•87-A overfunctionsand trips unnecessarily.

•87-B „sees“an external fault and only trips finally

through BF function (delay T-BF!)

B) Bus coupler CB aux. contacts connected:

•87-A remains stable („sees“no fault current)

•87-B trips immediately

BB-A

BB-B

7SS52: Special treatment of dead zone faults (2)

Coupler CB auxiliary contacts connected to 7SS52:

Coupler current is removed from 87-A and 87-B current

comparison with open coupler CB.