Load Angle Measurement Using SCADA

Load Angle Measurement using SCADA

A unique tool for Grid Operation

V.K.Agrawal, AGM
SRLDC, Bangalore / P.R.Raghuram, DGM
SRLDC, Bangalore / S.P.Kumar, Manager
SRLDC, Bangalore

Abstract:- Monitoring stability of the grid is the mostonerous function for the grid operator.With complexity of the network increasing and new mechanisms like Open Access etc. in place,the operator has to make a distinct balance between maximizing the network loadability vis-à-vis keeping the security intact. To address this, monitoring of load angle between the adjacent buses and coherent groups of generators is one of the most viable alternative as this single information can be used as good indicator about the system stability as well as the availablemargins in the transmission network. In discharging its responsibility, Southern Regional Load Despatch Centre (SRLDC) has been using this practice most optimally by virtue of the most modern and powerful state of the art SCADA and EMS systeminstalled under the Unified Load Despatch & Communication (ULDC) scheme in POWERGRIDanda user-friendlytool SRLDC has developed using the on-line data. This has enhanced the network usage and at the same time enabled the operator in fast visualization of the grid and saving it from disturbances on many occasions. The paper describes about this tool, its implementation and experience gained as well as the specific benefits and limitations.

1. INTRODUCTION

Mission of system operator is to ensure stable and secure power supply to end user at least possible cost. A most important aspect concerning security of the grid is its ability to withstand the effects of various contingencies viz. generation/load loss or network failures resulting into system disintegration, voltage collapse/rise and cascaded trippings. These if not checked in time, have the potential to cause a major disturbance leading to even total black out in large areas. The bestways to avert and/or to mitigate effect of such occurrence is early visualization of the problem and takeproactive action.

With the fast development of economy and due to environmental and commercial reasons the bigger plants are generally set up at pit head locations. This coupled with deregulation of power market and concentration of load at central locationsis resulting into wheeling of large power for long distances, leading to operation of thenetwork closer to its stability limit. Under such situation, when an abnormal condition/failure is not addressed and resolved immediately, the result could be catastrophic. To reduce the effect of such occurrences, the system operator should be equipped to know about these abnormalities at the earliest in order to take corrective action well in

time.[1]. Wide area monitoring using SCADA/EMS offers real time accurate information about the power systems’ condition and such monitoring not only helps informing an opinion about system stability but also provides a good instinct of the available transfer margin.

The specific advantages available on this account include power flow optimization, reduction in losses, averting the chances of black out, reducing the impact of disturbance and improved planning of network.

II. SOUTHERN REGION GRID–AN OVERVIEW

Southern Regional (SR) grid is a large system coveringapproximately6,51,000sq.km area and comprising the systems of four States viz. Andhra Pradesh, Karnataka, Kerala and Tamil Nadu and Union Territory of Pondicherry, interconnected with each other mainly through 400 KV grid network and few 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.

Fig.1 – Southern Regional Grid Map

The installed capacity of Southern Regional Grid is 32,659 MW (as on 31st March 2005) with thermal (including Nuclear and Gas) and hydro mix in the ratio of 67:33. The regional grid experiences a peak demand of around 22000 MW and a large component of load is predominantly agricultural in nature.

Since the load in the northern part of the Southern Grid where Ramagundam and other plants are equipped 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 1250 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.

1. OVERVIEW OF POWER SYSTEM STABILITY

While monitoring and controlling grid stability, it can be said that frequency instability, angular instability and voltage instability are the three main reasons which can generally be encountered and endanger the system.[2]-[3], [6]

A. Frequency Instability

Frequency instability occurs due to the mismatch between load and generation caused by tripping of generators and / or rejection of loads giving rise to a sudden change in frequency. In Southern Region, by operating maximum possible number of units with their governors in free mode, by automatic under frequency load shedding and by way of using special protection schemes, etc. the frequency stability is being ensured.

B. Voltage Instability:

Lack of adequate reactive power support is the main cause for the voltage instability. It could be due to (a) sudden change in the network topology redirecting the power flows, and/or due to (b) gradual increase of power demand in such a way that VAR requirement of some of the buses may not be met locally. Voltage collapse getting initiated from a node/set of nodes could result into wide area voltage instability and can be classified as Transient (varying from 1 to 3 sec.) or Steady-state (varying from tens of seconds to several minutes). Transient voltage instability remedial measures require fast and automatic actions viz. power system stabilizers and static VAR compensators, etc., while the steady-state voltage instability which occurs mainly due to gradual VAR deficit can be controlled to a large extent by the timely and prompt intervention of the system operator and utilities.

C. Angular Instability :

The third and the last instability reason, viz. the Angular instability is mainly responsible for causing power oscillations in the system. Lightly meshed networks, large power flows on long distance power lines are some of the main reasons for the angular instability[5].Transient angular instability caused due to sudden tripping of load/generator etc can be prevented only by automatic actions like fast auto reclosing, dynamic breaking, switching of series capacitors, power system stabilizer and static VAR compensators etc. However, the other form of angular instability, causing the inter-area modes oscillations and resulting into swinging of many machines in one part of the system against machines in other part can be addressed to a large extent by controlling the network loadability. These oscillations are mainly experienced when two or more large areas are interconnected using some weak links. Southern Region used to face such oscillations very frequently in the past resulting into system separations and disturbances. This was generally used to be caused due to increase of power transfer beyond certain limits from the northern part consisting of two States control area viz. Andhra Pradesh and Karnataka to southern part consisting of another two States viz. Tamil Nadu and Kerala. In the various incidences, initially, with the gradual increase in power flow, the frequency of oscillations used to be less, however if not controlled,these oscillations used to further increase leading to even tripping of the tie lines from northern part to southern part and a typical case is shown in Fig. 2.

Fig.2 – Oscillation on Tie-Lines in SR

The tool developed and used by SRLDC using the State of the art SCADA/EMS system was mainly to address this issue and has been further elaborated in the following paragraphs.

IV. SCADA SYSTEM AT SRLDC

SRLDC has been provided with a state-of-the-art SCADA/EMS system. This system has three tier hierarchy viz area level, state level and regional level. The data transmission from substations to control center(s) is through a combination of PLCC, Microwave and Optical fiber links and use of dedicated wideband communication has immensely improved the reliability of data. The database at SRLDC comprises of more than 30000data values from more than 300nodes across the region.

On-line monitoring of all this data enables the SRLDC operator to have wide area monitoring of the SR Grid as well as inter-regional links. Similar datais available in all State Control centersalso, facilitating the operatorsto take the requisite measures for controlling the safety and security of their system and take other actions in a most interactive and transparent manner. A simple sketch enumerating the complexities involved in the scheme is shown below in figure -3.

Fig.3Complexity of Monitoring

1. TOOL FOR ANGLE MEASUREMENT

Analyses of thedisturbances occurred in southern region in the past, say during last ten (10) years indicate that most of the disturbances were due to voltage instability/inter-area oscillation. In order to visualize the system and to detect voltage instability/inter-area oscillation the system operator can have the following options:

A. SCADA

Parameters such as voltage of the bus, power flow on the lines, line length, etc. are available to estimate the stability limit proximity of a line. Under real time conditions, however, even with all the data available from SCADA, it would require special expertise to visualize the wide area power system and to detect the starting of voltage instability/inter-area oscillation.

B. Real time network analysis

Carrying out online system studies is another widely used method to get the full status of the Power system.Carrying out on-line studies require a special skill set and as it would require certain time to run each case, under emergency situations ,at times, it may exceed the time target

C. Phasor measurement

Using phase measurement units for locating the voltage instability is a new tool [4].This technology is however just picking up and is more suitable for detecting transient instability for which the system is equipped for taking automatic mitigation action are to be taken.

D. Load angle measurement

In order to empower the System Operator to address the issue ofvoltage instability/inter-area oscillation, a simple tool has been devised by SRLDC using the data collected through SCADA system. This tool uses the simple and established equation of power flow between the two buses.

P / = / V1V2
X / Sinδ
Where / P / = / Power flow
V1
V2 / |
| / voltage at sending and receiving bus
δ / Angular difference between two buses.
X / Impedance of the line

The voltage angular difference between the two buses can therefore be worked as

δ / = / Sin-1 / XP
V1*V2

As can be seen above, the single value δ depends on a number of factors like powertransfer, voltages, length of the line, etc. Monitoring of this single value enables the system operator to visualize the power system condition and enabling him to take fast proactive action in respect of ensuring the grid stability as well as optimally utilizing any available margins. Following figures 4 and 5, indicate the variation of δ with respect to power level, voltages and for different line lengths.

Fig.4 Load Angle vs Power for different line lengths

Fig.5 Load Angle vs Voltage for different line lengths

VI. IMPLEMENTATION AND LOGISTICS

As indicated earlier, Southern Regional Grid can generally be bifurcated into two large areas, the northern area comprising of generation cluster around Ramagundam and Simhadri and the southern area comprising of generation cluster around Neyveli and during disturbancesthe power swings were generally being experienced between these two areas. In order to address this, the angle difference between the adjacent buses are calculatedusing the power flow equation. Adding of all such differences between two important nodes, say between Ramagundam and Neyveli buses through the set of intervening buses (Ramagundam-Nagarjunasagar – Cuddapah – Sriperumbudur-Neyveli) and between Simhadri and Neyveli through the set of other intervening buses(Simhadri – Gazuwaka – Vijayawada – Nellore- Sriperumbudur - Neyveli), thenet angular difference between these coherent groups of generators is calculated and displayed on the screen. Based on constant observation and past experience, it has been seen that power swings between the coherentgroupsof generators of Ramagundam/Simhadri with Neyveli generally starts whenever the angular difference has exceeded 70°. A typical Curve of Load angle during a system separation is shown below in Fig 6

Fig.6Graph of Load Angle during a system separation

On this analogy, the load despatchercontinuously monitors this parameter and once the angular difference crosses 50° to 55°, he immediately cautions the constituentsfor taking specific actions stated below

(1) Load shedding in the Tamil Nadu-Kerala area particularly if they are overdrawing from grid.

(2) Reduction of generation in the Andhra Pradesh-Karnataka particularly if they are under drawing from the grid.

(3) Exporting power to Western and Eastern Regions on opportunity basis.

(4) Re-routing of power from Gazuwaka HVDC linkthroughKolar HVDC.

(5) Requesting constituents to reduce reactive power drawal in specific areas.

(6) Opening of bus reactors at Cuddapah, etc.

By keeping a watch on the angular difference, theOperator is continuously apprised of the changing situation and totake further steps with conformity.

1. TYPICAL EXPERIENCES

The angles observed through this tool is comparable with those obtained through loadflow studies The figures 7 and 8 shows the variation of δ for various power flow between AP-KAR and TN_KER system

Fig.7Chart showing load angle with reference to Ramagundam bus at various stations

Fig.8Chart showing load angle with reference to Simhadri bus at various stations

This tool has helped the system operators in Southern Region in managing the grid better. Atypical plot of load angle between Ramagundam and Neyveli bus is shown in Fig. 9 below which indicates the trend over the day. At around 18.30 hrs. when the angle started crossing above 55°, the action as mentioned in section VI was taken to mitigate the problem and thereby reducing the risk of the disturbance.

Fig.9Load Angle Between Ramagundam and Neyveli typical day plot

VIII. LIMITATIONS OF THE TOOL

(1)Since the SCADA data updates after every10-15 secs and operator’s intervention is required for taking corrective action, this tool cannot be used optimally for addressing transient instability problem.

(2)Non-availability of data from any substation in the path will result in non-updation of the angle value and in such an event there would be requirement to obtain these values through alternate paths, if available. This type of occurrences, however, are very less owing to reliable communication channels and SCADA system.

(3)This tool is used assuming that load on transmission system has not reached thermal limits. In Southern Region as of now generally no 400 KV line loading exceeds thermal limit.

References

1. Reducing the risk of major blackouts through improved power system visualizationThomas J. Overbye Douglas A. Wiegmann University of Illinois Submitted to 15th Power System Computation Conference, August 2005, Liège, Belgium
2. Special Protection Schemes in Electric PowerSystem Literature survey Marek Zima 6th June 2002 eeh power systems laboratory
3. Power System Stability and control Kundur.P McGrawhill ,Inc,Ney York 1994
4. System for wide area protection control and optimization based on phasor measurementsC.Rehtanz.M.Larsson,M.Kaba.J.Bertsch Power system and communication system infrastructure for the future. Bejing, September 2002
5. Protection strategies to mitigate major power system breakdowns by Mattias Jonsson Thesis for the degree of Doctor of Philosophy ,Chalmer Univercity of Technology ,Sweden 2003
6. Consequence and Impact of electric Utility Industry Restructuring on Transient Stability and small signal StabilityAnalysisVijayVittal PowerSystemEngineeringResearchCenter Web site

Bibliography:-

V.K. Agrawal is presently working as Additional General Manager at SRLDC and is responsible for operation of the power system grid in Southern Region and for maintaining its safety and security. Mr Agrawal has done his graduation in Electrical Engineering in 1977 from Delhi College of Engineering followed by M. Tech in Power Apparatus and Systems from I.I.T., Delhi. He has a long experience in respect of Regional Grid Operation and Commercial aspects in Grid Management is his area of special interest.

Mr. P.R.Raghuram is presently working as Dy.General Manager at SRLDC and is responsible for operation of the power system grid in Southern Region and for maintaining its safety and security.He has done his graduation in Electronics and Communications Engineering in 1978 from Malnad College of Engineering.He is a senior member of IEEE. His interests are Power system Economics and Protection

Mr. S.P.Kumar is presently working as a Manager at SRLDC and is responsible for SCADA-EMS functions. He has done his graduation in Electrical Engineering in 1987 from BMS College of Engineering. His areas of special interest are Power Markets and Energy Management System.