Operational Experience of system Protection Scheme of Talcher – Kolar HVDC Link

MISSION OF POWER SYSTEM ENGINEERS

To provide

Reliable,Secured,and stable power supply

AT

Least possible cost

AND

Maximizing profit to all Stake holders

Security Monitoring

Preventive Control

Emergency Control

Normal

Restorative

In extremis

Restoration

Alert

Emergency

Why System protection Schemes

• Changing rules of operating electric power– Due to Deregulation of Electricity market– Seeking better Profits by

• Optimising maintenance• Over utilising the asset

• System Operators have to cope up with following “mores” resulting in accommodating transactions reducing the security margin– Capacity and complexity of the power grid– Inter-regional power exchange– Information to be handled– Uncertainty in operating conditions– Frequency of fault to be a trigger

• System Operator has to manage the contingencies in addition to handling above Mores and maintain integrated operation. System protection Schemes is a tool to achive this

System Protection Scheme Definition

• a protection scheme that is designed to detect a particular system condition that is known to cause unusual stress to the power system and to take some type of predetermined action to counteract the observed condition in a controlled manner. In some cases, SPSs are designed to detect a system condition that is known to cause instability, overload, or voltage collapse. The action prescribed may require the opening of one or more lines, tripping of generators, ramping of HVDC power transfers, intentional shedding of load, or other measures that will alleviate the problem of concern. Common types of line or apparatus protection are not in the scope of interest here”

Characteristic of SPS• These protections are also known by other names depending on the

originator like– Special stability controls– Dynamic security controls– Contingency arming schemes– Remedial action schemes– Adaptive protection schemes– Corrective action schemes– Security enhancement schemes

• Depending on the type of the problem, action such as tripping of generators, intentional islanding of the system at predetermined locations, tripping of loads will be taken.

Characteristic of SPS

• SPS are dynamic security control systems and are designed to control power system stability in case where the uncontrolled response is likely to be more damaging than the controlled response.

• SPS are devised by off –line analysis as opposed to on – line real time control. The reason behind being power system response is too fast to allow sequential control system logic. Involvement of a grid operator (human intervention for corrective action) should be kept to minimum contrary to SCADA/EMS (i.e. dispatchers’ actions and maintenance effort).

Types of Power system instability and the corrective actions

• Frequency Instability – The power system frequency going beyond operating limits

• Tripping of generators• Fast generation reduction through fast-valving or water

diversion• HVDC power transfer control• Load shedding• Controlled opening of interconnection to neighboring

systems to prevent spreading of disturbance• Controlled islanding of local system into separate areas

with matching generation and load

• Voltage Instability - Inability of the power system to maintain steady acceptable voltages at all buses in the system

• Change of the generator voltage set point• Automatic shunt switching• Control of series compensation• Blocking of Tap Changer of transformers• Under voltage load shedding• Transient Angular Instability - Inability of the system to

maintain synchronism – Braking resistor, FACTS devices etc. – Reducing the mechanical power driving the generator:– Fast-valving, disconnection of the generator etc.

• SR grid Over view

SR grid Over view

SR grid Over view

First Stage Logic Implementation

Station Name

Yeraguntala Kolar Hoody Salem Sriperambadur Hosur

ConnectedLoad

150 250 100 100 100

MODE OF OPERATION POWER LEVEL FOR GENERATING INTERTRIP SIGNAL

MONO POLAR >400 MW AND THE POLE TRIPS

BI-POLAR

IF BOTH POLE ARE CARRYING > 800 MW EACH AND ANY ONE POLE TRIPS

IF POWER FLOW ON EACH > 200 MW AND BOTH POLE TRIP

IF EACH POLE WAS CARRYING POWER IN THE RANGE OF 400-800 MW AND ONE POLE TRIPS, TRIP IS EXTENDED IF AFTER A DELAY OF 2 SECONDS THE POWER OF SURVIVING POLE IS LESS THAN 600 MW.

First stage Logic

List of stations to which the trip signals are sent and the quantum of connectedLoad

Performance Analysis of the Stage I Scheme

No. of times System Protection Scheme No of times SPS

operated when not to

operate

Expected to Operate

Operated correctly

Failed to Operate

29 18 11 7

• Mal operations

– On seven occasions the scheme operated when not required to operate due to failure of the HVDC measuring equipment like Optodyne/control errors(During initial period of Operation) due to which the power flow by the healthy pole could not be compensated with in two seconds after one pole trip

Performance

Analysis of Improper operation

Reasons for non operation Number of Non

OperationsReasons

Five When one pole trips on line fault the surviving pole goes to 150MW as per the present control scheme and this type of power ramp down is very slow(>>2sec). Since the logic checks the power loss only after 2 seconds of the incident, the logic failed to operate. This has been taken care in the Stage –II SPS logic development by introducing a line fault signals.

One During the initial stages of the logic development, checking the monopolar/Bipolar status was missed in the logic. This problem was later rectified

One Due to communication failure between Talcher and Kolar the HVDC system went in to shutdown mode and the power was ramped dowm very slowly .this was not envisaged in the logic design

Four The power level signals were derived from HVDC control which was changing only in steps (bit form). Therefore decisions were influenced by the power levels in specified steps which at times, could not detect the actual loss of power of more than 400MW.

This has been taken care in the Stage –II SPS logic development

Reasons for going in for revisions in the scheme

• Increased Power flow through Talcher –Kolar HVDC link

– more quantum of load shedding

– Non availability of sufficient quantum of sheddable loads around Kolar

• To prevent excess load shedding

– graded inter trip signals corresponding to the power loss were required

• To improve upon the deficiency noticed in stage I

• Implementation like

– To take Analog signals for the pole power status instead of bit status in the earlier logic

– Providing Two trip signals for different power loss

– To consider line faults in the logics

Logic of Presently improved SPS

POWER FLOW > 1500MW

AND

OR

OR AND

AND

REDUCTION IN POWER > 500&<1000MW

POWER FLOW >1000&<1500MW

DC LINE FAULT

SIGNAL A

REDUCTION IN POWER > 1000MW

SIGNAL B

TRIP SIGNAL 2

TRIP SIGNAL 1

STATE SUBSTATIONS RELIEF REMARKS

A.P Chinakampalli 150MW Signal through wide band.

KarKolar ICT

250MWThrough local pilot wiring

Hoody Through PLCC

TN

Hosur

300MW

Through PLCC

Sriperumbudur Signal through wide band.

Salem

STATE SUBSTATIONS RELIEF REMARK

A.P GTY Switching Stn, Ananthapur Somayajulapalli Kurnool

200 Signal will be sent through wide band

Kar Somanahalli 200

Kerala Trichur North , Kozhikode 200

TN Madurai,Karaikudi,Thiruvarur,Trichy230 Ingur 200

Substations to which Trip Signal II is sent and the expected relief

Substations to which Trip Signal I is sent and the expected relief

Performance of the SchemeFREQUENCY DIP DURING KOLAR HVDC TRIPING AND DURING SIMHADRI GENERATION LOSS

48.5

48.7

48.9

49.1

49.3

49.5

49.7

49.9

50.1

T-30 Minutes T-25 Minutes T-20 Minutes T-15 Minutes T-10 Minutes T-5 Minutes T=0 Minutes T+5 Minutes

Time

FR

EQ

IN

HZ

TAL-KOL TRIP ON15-09-06 AT 16:52 HRS LOSS IS 1887 MW

SIMHADRI GEN LOSS OF APPROX 950 MW ON 16-01-07

AT 1812 HRS

Performance of the Scheme

No. of times System Protection Scheme No of times SPS operated when not to operateExpected

to OperateOperated

correctlyFailed to

Operate

9 8 1 2

Details of Improper operations

Improper operation

No.of operation

Reasons

Non Operation 1 Due to Problems in HVDC controls and matter referred to the supplier

Mal operations 2 Problems in HVDC controls and the logic Same has been rectified

Due to fluctuations in power flow during tripping of converter transformer