Rotor DC Current Measurement by Using Current Transformers

Authors:Z. Gajić, ABB SwedenD. Trišić, Drinsko-Limske HE, SerbiaS.Roxenborg, ABB Sweden

Presented by Roger Hedding

Objectives

Information about typical excitation systems

Static excitation What is the role of an excitation

system New measurement principle for

rotor DC current Information about the trial

installation Show some captured recordings DC current use for the protection Conclusions

Power Station

CONTROL ROOM

CONTROL SYSTEMS

STEP UP TRANSFORMER

HV SYSTEM

STARPOINTCUBICLE

HV- SUBSTATION

TURBINEGOVERNOR

STATIC STARTER

SYNCHRONIZATION

1

1

Power Island

GEXCITATIONTRANS-FORMERTURBINE

MV SWITCHGEAR

PT’sCT’sSA’s

AUXILIARYTRANS-FORMER

MVDRIVES

LV SWITCHGEAR

EXCITATION SYSTEM

UNITPROTECTION

Two main types of excitation system

AVR

Regulation & Control Electronics

Excitation transformer

Converter

De-excitation

DC-breaker*

Field flashing

AC-breaker*

*AC- or DC breaker

Main Components Static Excitation Systems

Static Excitation System “Inside View”

ULVPh-Ph

Thyristor Controlled Rectifier

Note that phases are marked with letters R, S, T instead of A, B, C

Static excitation System

AVR

Excitation Transformer

Converter

ACCB

PT

CTController

Field Discharge

Provide variable DC current with short time overload capability

Control terminal voltage with suitable accuracy

Ensure stable operation with network and / or other machines

Contribute to transient stability subsequent to a fault

Communicate with the power plant control system

Keep machine within allowed operating range on P-Q diagram

HPP Bajina Bašta (Low Lake)

View of RHE from the dam

HPP Bajina Bašta (Upper Lake)

Overall Single Line Diagram of the Plant

315MVA11kV

100MVA15,75kV

0,2km 220kV OHL

Hydro Power Plant

220kV S/S Zaugline

8km, 220kV OHL (4 off)

Generator/Pump Unit Data

Turbine Francis 428.6rpm

Synchronous Machine 315MW pf=0.95 11kV; 50Hz

Unit Transformer 2*160MVA 242kV/11kV Yd5

Special Things about this Installation

Operating Modes Generator Motor (Pump) Electrical braking Synchronous start

(i.e. Back-to-Back)

Single Line Diagram of Machine Used in Trial Installation

Excitation System Three-Line-Diagram

Traditional Possibilities to measure DC current

Resistive Shunt Hall-effect sensor

Typically not possible to be used by the numerical protection relays

Primary connections

Connections to transducer

Required CT Connections for DC Current Measurement

100/5

1500/5

1400kVA11kV/0,55kV

Yd1

How the DC Current is Calculated ?

When HV CTs are used: IA, IB, IC are measured By using properties of

excitation transformer re-calculate Ia, Ib, Ic first

Then use formulas to the right

When LV CTs are used: Ia, Ib, Ic are measured Calculate instantaneous

DC current by using the following formula:

Calculate equivalent RMS value of the DC current by using the following formula:

Recordings from the Trial Installation

100/5

1500/5

1400kVA11kV/0,55kV

Yd1

IR_HV, IS_HV, IT_HV

IR_LV, IS_LV, IT_LV

HV_DC_I

LV_DC_I

R, S, T <=> A, B, C

Filed Record No 1

Filed Record No 2

Filed Record No 3

Filed Record No 4

Filed Record No 5

DC Current Value on the built-in HMI, via Comms, etc.

Conclusion

Patented DC current measurement in the numerical IED will improve the synchronous generator protection and monitoring.

Current based protection for the rotor winding can be done in an easy way (e.g. thermal overload).

Possible to record and monitor the rotor winding DC current together with Stator U and I in the same COMTRADE file!

Practical Advise

When more than one power source is used for the excitation equipment supply, use the CT located just in front of the six-pulse rectifier bridge in order to measure DC current under all operating conditions of the machine

5

2

3

6

4

1

7

89

220kV Transmission Network

Questions

Recording from Another Machine

8 10 12 1440-

20-

0

20

40 IAIBIC

W1 Current Waveforms

Time [cycles]

Cur

rent

[Am

ps P

rimar

y]

8 10 12 14

200

400

600

800HV_side

DC Current Integrated Value

Time [cycles]

DC

Cur

rent

[Am

pere

s]