OPERATIONAL EXPERIENCE OF SYSTEM PROTECTION

SCHEME OF TALCHER KOLAR HVDC LINK

V.K.Agrawal P.R.Raghuram C.S.Tomar Oomen Chandy P.Ranga Rao

SRLDC Bangalore SRTSII HVDC Kolar

Power Grid Corporation of India

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ABSTRACT:

The growth of power demandin India is veryhighand the Electric Power System in the country is undergoing major structural changes. In order to meet the growing demand, generating units of higher capacities (1000 MW) and HVDC interconnections of large transfer capability are being added to the grid. Further the Implementation of a scientific settlement system (Availability Based Tariff)and introduction of Open Access in transmission has been of much help in increasing the flows across the State/Regional boundaries and has resulted in merit-order based generation dispatch and bringing economy across the entire country.

Though the higher capacity elements are extremely helpful in bringing around the economy and efficiency, howeverin the event of outage of theseelements the normal operation of the power system gets endangered and can result into a blackout if automatic fast suitable corrective action is not taken. In order to address the above issues, particularly with the increased complexity of the power system and reduced margin,System Protection Schemes (SPS) have becomeessential. Classical protection schemes protect the power system elements from damage but may not help preventing the total system from a collapse and in order to combat such an eventuality SPS are extremely important.

SPS in India are in their infancy and are yet to be recognized formally as an important faculty. The first such major schemecoveringa large area, implemented in the country is the SPS designed to take care of the contingency of tripping of Single pole/Bi pole of 2000 MW Talcher Kolar HVDC Link and has proved to be extremely useful. The present paper talks of the implementation of this scheme in different stages and its operating experiences.

Key Words:Power System Operation-System Protection Schemes-Southern Regional Grid.

1.0Introduction

The power system in India has grown from a small stand-alone system (individual self sufficient units along with matching load of area like palace, towns etc) to a large integrated and complex combination of generating units and transmission network covering larger geographical areas even beyond national boundaries. Restructuring of the electric supply industry has brought in competition and better service to the end-user. However this has also resulted in additional strain on power system operation and has pushed asset utilization to the maximum extent, reducing security margins and demanding more surveillance and quick remedial action in shortest possible time. Utilities in India are using Protection Scheme / System Islanding Schemes in one form or other and a wealth of experience is available in this field. However, with the continued expansion and integration of different systems / grids and the prevailing practices in the grid operation, following are some of the reasons that why a dedicated scheme to handle a typical situation/contingency inpredetermined manner to be precisely named as System Protection Scheme is required in Indian Power system:

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  • The standard protection schemes available are rightly designed to protect men and the individual elements from damage. However many a times such operations may result in widespread disturbance.
  • The modern trend in power system is to achieve economy through economy of scale i.e. by installing high capacity Generating Units, constructing pit-head power plants and having HVDC interconnection of large transfer capability. Outage of such high capacity units endangers the stability of the grid
  • Introduction of new commercial settlement mechanism in India (Availability Based Tariff Mechanism) has brought in overall economy in the whole country. This however also brings in at times the skewed generation and load pattern to which the grid has not been designed.
  • With unbundling of state utilities and advent of open access, there is a desired thrust towards competition in order to bring the cost of power down. This is also resulting in pressure to utilize all available transfer margins in a transmission system.
  • At times, generators coming up earlier than its planned evacuation or any backlog in the planned transmission network result in a security threat till the planned transmission systems are fully commissioned
  • During Planningstage, at times, due to investment constraint the network augmentation decisions are postponed to a latter date resulting in over utilization of existing assets
  • The cyclic nature of generation and load causes seasonal overloading and transmission lines get over loaded during some seasons.
  • The staggering and rostering of loads which is a common phenomenon in India also causes over loading of corridors during particular hours of the day.
  • At many places environmental problems do not allow more transmission corridors to be built for power evacuation and thereby the existing corridors continue to face excessive loading.

Due to the above mentioned reasons there always remains a possibility to encroach on the available transmission margin while an alert system operator would alwayslikes to work with more margins to take care of the contingencies.This at times results in clash of interests between security and economic operation. System Protection Scheme is a tool which largely addresses this issue and keeping the system security intact it give confidence to the system operator to enhance the power flow through the system and hence resulting in maximization of the asset use and reducing per unit cost.

1.1System Protection System (SPS) - Definition

According to P.M.Anderson [2] SPS is defined as “ 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”

2.0The Southern Regional Grid – An overview

Southern Regional (SR) grid is a large system having a installed capacity of 37,054 MW, meeting a peak demand of over 23,000MW and covering approximately 6,51,000 sq.km area. It comprises of the systems of four States viz. Andhra Pradesh, Karnataka, Kerala and TamilNadu and Union Territory of Pondicherry, interconnected with each other mainly through 400 KV grid network and some 220 KV inter-State lines. There are two major Central Generating Stations connected to the 400 KV grid, viz. Ramagundam Super Thermal Power Station (3x200 + 4x500 = 2600 MW) in the northern part and NeyveliI and II (6x50 + 3x100 + 7x210 + 2x210 = 2490 MW) in the southern part.

Since the load in the northern part of the Southern Grid where Ramagundam and other plants are situated is comparatively low, in the past excessively high quantum of power used to flow from north to south corridor of the Region, resulting into critically low voltages in the Central part viz. around Bangalore and thereby causing system instability. This problem has largely been addressed now with the commissioning of 1368 km. long, +/- 500 KV, 2000 MW HVDC bi-pole link between Talcher (Orissa) to Kolar (Karnataka) and the addition of some more 400 KV lines in the parallel AC network. Further, the augmentation of both SR-WR and SR-ER inter-regional links to 1000MW,each by commissioning of 2x500 MW HVDC back-to-back links at Bhadrawati and Gazuwaka respectively have also resulted into considerable strengthening of the system.

3.0 Necessity of SPS

Talcher – Kolar HVDC link is a 2000 MW asynchronous link between Eastern and Southern regions. Power system in India works on a floating frequency range of 49 to 50.5 Hz. and as the grids are operating without any spinning reserve or the primary control of generation (most of the generators do not run on free governor mode of operation)any generation loss / tie line tripping results in sharp drop in frequency. Therefore when the SR grid is operating at lower end of the operating band, say between 49.0 Hz. – 49.5Hz, tripping of HVDC Talcher-Kolar link may result in sharp drop in the operating frequency and possibly in cascade tripping of the generating units. To avoid such incidence of grid instability a system protection scheme has been implemented for fast load relief during contingency of tripping of Singlepole/Bipole of the HVDC link.

4.0 Implementation of the scheme

The scheme was implemented in two stages. In the first stage the logic was implemented on 20/04/2003and was in service till 24/03/2006. In the Second stage an improved version of the scheme was implemented on 24/03/2006 to take care of increased power flow on the Talcher Kolar HVDC link and to over come some deficiencies of the existing scheme.

4.1 First stage of Implementation

In the first stagematching with the generation of Talcher stage II at that time, the scheme was designed to give a load relief of about 500 to 700MW by shedding the loads in the near-by substations of Andhra Pradesh, Karnataka, and TamilNadu detailed below:

Station Name / Yeraguntala / Kolar / Hoody / Salem / Sriperambadur / Hosur
ConnectedLoad / 150 / 250 / 100 / 100 / 100

Loss/reduction of power due to one/bipole trip was sensed at Kolar and the trip signal was generated by the SPS based on the logics explained below. This Trip signal was sent to the above mentioned stations through PLCC. Initially after the HVDC station was commissioned,the instantaneous power levels of HVDC poles were available in the form of 4bits(Digital Form)and the status of the bit changed for every 200MW change in the power. This information was used to develop the following logic:

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, THEN TRIP IS EXTENDED IF AFTER A DELAY OF 2 SECONDS, THE POWER OF SURVIVING POLE IS LESS THAN 600 MW.

Following table indicate the performance of the Scheme in initial three years:

No. of times System Protection Scheme / No of times SPS operated when not required to operate
Expected to Operate / Operated correctly / Failed to Operate
29 / 18 / 11 / 7

Improper operation was there mainly due to the following reasons:

  • Non Operation: On eleven occasions defense mechanism did not operate when requiredto operate due to following reasons:
  • On five occasions (one occasion relay failure) when one of the pole tripped on line- ground fault, inter trip signal was not generated.This is due to the reason that when one pole trips on line fault the surviving pole goes to 150MW with a slow power rampdown (>2sec). Since the logic checks the powerloss after 2 seconds of the incident, the logic could not identify it as a failure. This has been taken care in the Stage –II SPS logic development by introducing a line fault signals.
  • In one case, during the initial stages of the logic development, checking the monopolar/Bipolar status was missed in the logic. This problem was later corrected.
  • On one occasion due to failure of communication between Talcher and Kolar a controlled shutdown of both the poles were carried out and the ramp down of the Power was very slow .This type of condition not was envisaged in the logic.
  • In the remaining fourcases non operation was due to the signal input. 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.
  • Maloperation 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.

4.2 Second stage of Implementation

Improved version of the scheme was required to be implemented due to the following reasons:

  • Increase in the flow on Talcher –Kolar HVDC link due to commissioning of all the generators at Talcher stage IIand thereby resulting in increase of the quantum of load shedding.
  • Due to non availability of sufficient shedable loads in the substation surrounding Kolar it was required to send tripping signals to far locations using wideband communication links.
  • To avoid excess load shedding,graded signals in two stageswere required to be generated to match the outage of single pole, Bi pole etc.
  • The deficiencies noticed in the 1st stage implementation were to be rectified immediately since the improper operation during high power flow would have badly affect the power system

4.3 Logic for initiating signals

To overcome the above shortcomings the logics were suitably modified involving the following changes :

  • Analog signals were taken for the pole power status instead of bit status in the earlier logic
  • Line fault signal was introduced in the logic
  • Two trip signals were generated for different power loss as explained below

Trip Signal 1

Trip Signal 1 would be initiated to obtain a relief of around 700 MW from the stations listed under Para5.1 under any one of the following conditions:

(i)If loss of power flow on the HVDC link at any instant compared with the power flow 2 seconds prior to current instant is more than 500 MW but less than or equal to 1000 MW.

(ii)If one of the HVDC pole block on a line fault and the power flow on the HVDC link just prior to that instant was more than 1000 MW but less than or equal to 1500 MW.

(iii)When Trip signal 2 is generated

Trip Signal 2

Trip Signal 2 would be initiated to obtain an additional relief of around 800 MW (in addition to 700 MW as stated above) from the stations listed under Para 5.2 under any one of the following two conditions:

(i)If loss of power flow on the HVDC link at any instant compared with the power flow 2 seconds prior to current instant is more than 1000 MW.

(ii)If one of the HVDC pole block on a line fault and the power flow on the HVDC link just prior to that instant was more than 1500 MW.

Note: In the event of occurrence of Trip signal 2, the total relief would be of the order of around 1500 MW, consisting of around 700 MW from the stations listed under Para 5.1 and additional 800 MW from the stations listed under Para 5.2.

Block Diagram

A simple block diagram depicting the logic for generation of Trip Signal 1 and Trip Signal 2 and transmission of such signal to the respective locations is given below.

Fig.2 – Kolar Inter Trip Logic Diagram

5.0Logistics for obtaining relief at different locations

On the basis of above stated logic Trip Signal 1 and Trip Signal 2 were extended to the following substations through wideband /PLCC:

5.1 Substations activated with Trip Signal I

STATE / SUBSTATIONS / RELIEF / REMARKS
A.P / Chinakampalli / 150MW / Signal through wide band.
KARNATAKA / Kolar ICT / 250MW / Through local pilot wiring
Hoody / Through PLCC
TAMIL NADU / Hosur / 300MW / Through PLCC
Sriperumbudur / Signal through wide band.
Salem

5.2 Substations activated with Trip Signal II

STATE / SUBSTATIONS / RELIEF / REMARK
A.P / Gooty Switching Stn,AnanthapurSomayajulapalliKurnool / 200 / Signal will be sent through wide band
Karnataka / Somanahalli / 200
Kerala / Trichur North , Kannur(Kanhirode) / 200
Tamil Nadu / Madurai,Karaikudi,Thiruvarur,Trichy230,Ingur / 200

6.0 Performance of the Scheme

The trend curve given below indicates the effectiveness of this SPS scheme. The drop in frequency for a generation loss of about 950 MWto the SR grid (Tripping of two generators of 500MW capacity at Simhadri on 16.1.2007) was more than 1 Hz. whereas in the event of tripping of HVDC Talcher-Kolar bipole on 15.9.2006 carrying a power, even of the order of 1887 MW, resulted in drop of about 0.7 Hz onlydue to the load relief obtained by the operation of System Protection Scheme

After modifications under Stage 2, the performance of the scheme,(since implementation till date) indicate that the operation of the scheme has improved compared to the 1st stage implementation.

Performance is detailed in the following table

No. of times System Protection Scheme / No of times SPS operated when not required to operate
Expected to Operate / Operated correctly / Failed to Operate
9 / 8 / 1 / 2

On one occasion maloperation was due to error in the HVDC system and the Logic. Same has been corrected now. The other instance of mal operation was due to fluctuation in the power flow during tripping of converter transformer. One instance of non operation was due to problem in HVDC control system which is being looked in to by the HVDC supplier

Acknowledgement Authors acknowledge the support given by HVDC Kolar Station, POWERGRID to publish this paper.

The opinions expressed here is that of authors only and not necessarily of that of POWERGRID.

References

  1. System Protection Schemes in Electric Power System Literature survey Marek Zima 6th June 2002 eeh power systems laboratory
  2. Industry Experience with System Protection Schemes IEEE/CIGRE committee Report P.M.Anderson ,B.K. Lereverend IEEE transaction on Power system ,Vol11,no3,August 1996
  3. Power System Protection P.M.Anderson IEEE Press Chapter 21 page 902-909
  4. Power System Stability and control Kundur.P McGrawhill ,Inc,New York 1994 Chapter 17
  5. WSCC Relay workgroup Guide for Remedial Action Schemes WSCC website

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