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

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

First Stage Logic Implementation Station Name YeraguntalaKolarHoodySalemSriperambadurHosur Connected Load 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 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 SchemeNo of times SPS operated when not to operate Expected to Operate Operated correctly Failed to Operate 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 Operations Reasons FiveWhen 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. OneDuring the initial stages of the logic development, checking the monopolar/Bipolar status was missed in the logic. This problem was later rectified OneDue 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 FourThe 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 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

STATESUBSTATIONSRELIEFREMARKS A.P Chinakampalli 150MW Signal through wide band. Kar Kolar ICT 250MW Through local pilot wiring HoodyThrough PLCC TN Hosur 300MW Through PLCC SriperumbudurSignal through wide band. Salem STATESUBSTATIONSRELIEFREMAR K A.PGTY Switching Stn, Ananthapur Somayajulapalli Kurnool 200Signal will be sent through wide band KarSomanahalli200 KeralaTrichur North, Kozhikode200 TNMadurai,Karaikudi,Thiruvarur,Trichy230 Ingur200 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 Scheme

No. of times System Protection SchemeNo of times SPS operated when not to operate Expected to Operate Operated correctly Failed to Operate 9812 Details of Improper operations Improper operation No.of operation Reasons Non Operation1Due to Problems in HVDC controls and matter referred to the supplier Mal operations 2Problems in HVDC controls and the logic Same has been rectified Due to fluctuations in power flow during tripping of converter transformer