Modern Analysis of Power Flow and Network Stability of Transmission Lines for Nuclear Power Dispatch

This article presents an general approach to improve the power system planning, load flow pattern and stability of transmission lines using modern analysis concept. The objective is to categorize and characterize the existing system reliability concerns inherited from the adopted deterministic criteria, so that power utilities can accordingly adjust their reliability criteria to manage with real-life system uncertainties and hence to improve the overall system reliability. In the past many wide spread blackouts had occurred in interconnected power systems. Therefore it is necessary to ensure that grid should be operated economically and reliably. Contingency analysis is a well-known function in modern power system management. The aim of this analysis is to give the operator information about the static security, power flow pattern and stability. In general an outage largest capacity of one transmission line or transformer may lead to disturb the vital parameters in other lines. Modern analysis is used to calculate the violation on the network and improvement. Nuclear power plant is a base unit and best way to dispatch the power to the grid with minimum disturbance. This paper also shows the network stability, power flow management for 500MW Nuclear power plant transmission lines and connected to southern region network of India.


Introduction
BharatiyaNabhikiyaVidyut Nigam Limited (BHAVINI) is currently constructing a 500MWe Prototype Fast Breeder Reactor at Kalpakkam (KPFBR), 70 km away from Chennai. The KPFBR is the forerunner of the future Fast Breeder Reactors and is expected to provide energy security to the country. The KPFBR is being built with the design and technology developed at the Indira Gandhi Center for Atomic Research (IGCAR) located at Kalpakkam.
The plant is connected to the Tamil Nadu / Southern Regional Grid to transmit the power generated. A 230 kV substation with six numbers of transmission lines namely One Double Circuit (D/C) to Sirucheri (30 km), One Double Circuit (D/C) to Kancheepuram (70 km) and One Double Circuit (D/C) to Arani (85 km). These substations are in turn connected to the 400 / 230 kV substations at Melakottaiyur and Sriperumbudur which are connected to North Chennai Thermal Power Station and the 400 kV grids. The interconnections planned provide reliable off-site power supply to the nuclear station.
The power system study to be carried out to ensure at least one of the off-site connections of switchyard is available even under various transients in the power system. The following prescribed transient one at a time shall be analyzed.
(a) Loss of the generating unit of KPFBR (500MWe Nuclear Power Plant) (b) Loss of the largest generating unit of the concerned grid (c) Loss of the largest transmission line or Inter -tie (d) Loss of a double circuit from KPFBR Switchyard. In addition to the above, maximum fault level at the generating plant of switchyard and fault levels at various grid nodes, and Dynamic over voltage are needs to be studied. In each of the above cases, it shall be checked that system should remain stable and at least one of the transmission lines remains connected to our 220kV KPFBR switchyard [1]. This study and analysis has been done based on the existing generation capacity and future generation envisaged in the Stability of Transmission Lines for Nuclear Power Dispatch area.

Evolution of Transmission System Procedure
The transmission system planning exercise is done based on power system studies on all the credible alternatives. These studies are performed utilizing power system analysis software in which entire All India electrical network up to 220 kV level is simulated [3]. The transmission scheme for evacuation of power from KPFBR was planned in accordance with "Manual on Transmission Planning Criteria" of Central Electricity Authority (CEA) [17]. The salient criteria with regard to security and transient stability are:

Criteria for Single Contingency ('N-1') -Steady State
All the equipment in the transmission system shall remain within their normal thermal and voltage ratings after a disturbance involving loss of any one of the following elements (called single contingency or 'N-1'condition), but without load shedding / rescheduling of generation: (a) Outage of a 132kV or 110kV single circuit, or (b) Outage of a 220kV or 230kV single circuit, or (c) Outage of a 400kV single circuit, or (d) Outage of a 400kV single circuit with fixed series capacitor(FSC), or (e) Outage of an Inter-Connecting Transformer(ICT), or (f) Outage of a 765kV single circuit (g) Outage of one pole of HVDC bipole. The angular separation between adjacent buses under ('N-1') conditions shall not exceed 30 degree.

Criteria for Single Contingency ('N-1') -Transient Stability
The transmission system shall be stable after it is subjected to one of the following disturbances: 1. The system shall be able to survive a permanent three phase to ground fault on a 765kV line close to the bus to be cleared in 100 ms. 2. The system shall be able to survive a permanent single phase to ground fault on a 765kV line close to the bus. Accordingly, single pole opening (100 ms) of the faulted phase and unsuccessful re-closure (dead time 1 second) followed by 3-pole opening (100 ms) of the faulted line shall be considered. 3. The system shall be able to survive a permanent three phase to ground fault on a 400kV line close to the bus to be cleared in 100 ms. 4. The system shall be able to survive a permanent single phase to ground fault on a 400kV line close to the bus. Accordingly, single pole opening (100 ms) of the faulted phase and unsuccessful re-closure (dead time 1 second) followed by 3-pole opening (100 ms) of the faulted line shall be considered. 5. In case of 220kV / 132 kV networks, the system shall be able to survive a permanent three phase fault on one circuit, close to a bus, with a fault clearing time of 160 ms (8 cycles) assuming 3-pole opening. 6. The system shall be able to survive a fault in HVDC convertor station, resulting in permanent outage of one of the poles of HVDC Bipole. 7. The system shall remain stable under the contingency of outage of single largest generating unit or a critical generating unit (choice of candidate critical generating unit is left to the transmission planner).

Criteria for Second Contingency ('N-1-1')
Under the scenario where a contingency as defined at above has already happened, the system may be subjected to one of the following subsequent contingencies (called 'N-1-1' condition [8]): (a) The system shall be able to survive a temporary single phase to ground fault on a 765kV line close to the bus. Accordingly, single pole opening (100 ms) of the faulted phase and successful re-closure (dead time 1 second) shall be considered. (b) The system shall be able to survive a permanent single phase to ground fault on a 400kV line close to the bus. Accordingly, single pole opening (100 ms) of the faulted phase and unsuccessful re-closure (dead time 1 second) followed by 3-pole opening (100 ms) of the faulted line shall be considered. (c) In case of 220kV / 132kV networks, the system shall be able to survive a permanent three phase fault on one circuit, close to a bus, with a fault clearing time of 160 ms (8 cycles) assuming 3-pole opening. In the 'N-1-1' contingency condition as stated above, if there is a temporary fault, the system shall not lose the second element after clearing of fault but shall successfully survive the disturbance.
In case of permanent fault, the system shall loose the second element as a result of fault clearing and thereafter, shall asymptotically reach to a new steady state without losing synchronism. In this new state the system parameters (i.e. voltages and line loadings) shall not exceed emergency limits, however, there may be requirement of load shedding /rescheduling of generation so as to bring system parameters within the normal limits.

Additional Criteria for Nuclear Power Stations
(a) In case of transmission system associated with a nuclear power station there shall be two independent sources of power supply for the purpose of providing start-up power. Further, the angle between start-up power source and the generation switchyard should be, as far as possible, maintained within 10 degrees. (b) The evacuation system for sensitive power stations viz., nuclear power stations, shall generally be planned so as to terminate it at large load centers to facilitate islanding of the power station in case of contingency. (c) Permissible normal and emergency limits. (d) As per planning criteria of CEA [17], India, the permissible normal and emergency voltage limits are mentioned in table 1 below: The studies are done by carrying out load flow studies for normal as well as contingent operating conditions on all the identified alternatives. Results of these studies are compared based on power flow, estimated cost and overall system losses to arrive at the most techno-economic option [16]. The chosen alternative is then studied for transient disturbances through stability studies to determine the system behavior under transient faults and to ensure that the system is stable under all the contingencies prescribed in the Transmission Planning Criteria [17]. The results for the dynamic studies are presented in the form of machine angle variations following the disturbance to ascertain that the system remains stable. Further, the short circuit analysis is performed for subtransient (t=0) and transient (t=0.2 Secs) time frames on the chosen alternative to determine the short circuit currents under single and/or three phase short circuit conditions.

Major Considerations
(a) The KPFBR has been envisaged with capacity of 500 MW. Considering the large unit capacity of 500 MW, alternative for 400kV transmission system has also been considered. (b) The transmission system for evacuation of power from nuclear project should be constructed on separate corridors so that the power could be evacuated even under extreme contingency of tower outage.

Data Considered for the Studies
As mentioned above, for evolution of transmission system for evacuation of power from KPFBR, the entire all India system has been simulated for peak demand scenario. Southern region power system has been simulated in detail up to 220 kV level. The load flow studies has been performed on the above-simulated data and checked for violations with respect to the bus voltages and transmission line loadings. The voltage limits considered for the bus voltages and thermal limits for the transmission lines are given at Table 2 andTable 3 [2]. Generation units in Southern Region are also considered. Load Generation Balance Report (LGBR) is also considered for the studies. Transfer Function Graph pertaining to Exciter, Stabilizer and Speed Governor is also considered along with plant (KPFBR) parameters.  Total station loads considered at KPFBR is 50 MW and generation voltage level is 21 kV. For load flow study, the maximum active power dispatch is considered at KPFBR i.e. 500 MW while the reactive power limits are considered as -433 MVAR to +250 MVAR in line with the machine data of the manufacturer. Auto re-closer scheme for all the transmission lines at KPFBR end has dead time of 600ms.

Transmission System Alternatives
To evolve evacuation system of KPFBR, power system studies have been carried out for 2017-18 time frames where 500 MW from KPFBR is available. While carrying out the studies, adequacy of the transmission system has been checked under normal and contingent conditions for different alternatives. These alternatives are compared on the basis of overall system losses and cost. For evacuation of power from KPFBR two alternatives have been considered.

Alternative-I (400kV)
To evacuate power from 500 MW unit of Kalpakkam KPFBR, following 400kV and 230kV lines to nearby load centers have been considered: (  [1]. The system is able to cater to normal condition but under 400kV line outage, 230kV line is being critically loaded.

Alternative-II (230kV)
In this alternative, 230kV lines to other load centers have been considered. Accordingly, following system has been considered under this alternative: (  From the studies, it can be seen that the system is able to cater to normal condition and contingency conditions and no constraints are envisaged in evacuation of power from KPFBR.

Comparison of Alternatives
It has been observed from the enclosed study that under alternative-II, power from KPFBR is being evacuated reliably under normal as well as contingency condition. In case of alternative-I only about 170MW power is dispatched through 400kV corridor while 270MW flows through 230kV line [1]. Further, under outage of one ckt of 230kV D/c line, other circuit get loaded to about 200 MW and under outage of both ckt of KPFBR -Malekottaiyur 400kV D/c line, the entire power is being evacuated by 230kV line. Hence, in case of alternative-I, system is able to cater to normal condition, however under contingency condition, 230kV line is being critically loaded. While alternative-II connects KPFBR generation complex with Arni, Kanchipuram and Sirucheri which is connected further to Malekottaiyur and Thiruvalam. This system is more reliable, as under contingency, alternate paths are available for evacuation of power. This connectivity would help in evacuation of KPFBR power to load centers. In view of the above, Alternative-II has been finalized for evacuation of KPFBR power.

Load Flow Studies -Under Light Load Conditions
The load flow studies have been conducted for light load conditions for the Alternative-II that has been chosen as the most techno-economic option. The case for light load condition has been simulated by considering regional load demand approximately 75% of the peak load demand. The reduction in load has been matched by equivalent reduction in generation, mostly hydro and Gas based. All the bus reactors andline reactors are in operation. The results of the load flow studies have been enclosed at Exhibit-ALT-II-LIGHT. From the studies, it has been observed that the voltages at different kV buses are well within their steady operating limits(1.05 p.u). The light load condition studies provide vital information about the reactive power absorption that the machine may be subjected under condition when the grid is expected to surplus in reactive power. The data at the generator terminals, for the KPFBR machines for light load conditions are as mentioned below in Table 5and Table 6 [11].

Stability Studies
The stability studies were performed on the Alternative -II that has been chosen as the most techno-economic option to study its behavior under transient/fault conditions [13]. As per the transmission planning criteria the system should be stable when subjected to following disturbances as prescribed in the Transmission Planning Criteria of CEA [17]: (a) The system shall be able to survive a permanent three phase to ground fault on a 765kV line close to the bus to be cleared in 100 ms. (b) The system shall be able to survive a permanent single phase to ground fault on a 765kV line close to the bus. Accordingly, single pole opening (100 ms) of the faulted phase and unsuccessful re-closure (dead time 1 second) followed by 3-pole opening (100 ms) of the faulted line shall be considered. (c) The system shall be able to survive a permanent three phase to ground fault on a 400kV line close to the bus to be cleared in 100 ms. (d) The system shall be able to survive a permanent single phase to ground fault on a 400kV line close to the bus. Accordingly, single pole opening (100 ms) of the faulted phase and unsuccessful re-closure (dead time 1 second) followed by 3-pole opening (100 ms) of the faulted line shall be considered. (e) In case of 220kV / 132 kV networks, the system shall be able to survive a permanent three phase fault on one circuit, close to a bus, with a fault clearing time of 160 ms (8 cycles) assuming 3-pole opening. (f) The system shall be able to survive a fault in HVDC convertor station, resulting in permanent outage of one of the poles of HVDC Bipole. (g) Contingency of loss of generation: The system shall remain stable under the contingency of outage of single largest generating unit or a critical generating unit. Following studies has been conducted to ascertain the dynamic performance of the chosen alternatives. The result of the studies has been presented in the form of machine angle and power flows with respect to time as shown in Figure 4 to 12 and the Table 7 [9].  The mathematical model of speed governing system is shown in Figure 1. The corresponding parameter are tabulated as per the IEEE standard model in Table 8.  The mathematical model of stabilizing model is shown in Figure 2. The corresponding parameter are tabulated as per the IEEE standard model in Table 9.  The mathematical model of Excitation system model is shown in Figure 3. The corresponding parameter is tabulated as per the IEEE standard model in table 10.     The initial variation in machine angle depends on severity of fault [10]. However in general if the oscillations damp out to10-15% after 20-30 secs of fault, it is considered stable. From the above results Table 11, it has been observed that alternative-II is stable system under transient/ fault conditions for above faults [6].

Short Circuit Studies
Short circuit studies have been carried out foralternative-II. Short circuit currents for the single and three phase faults at major 230 kV stations are calculated. Short circuit currents summary for the single and three phase faults at major stations close KPFBR are calculated t=0 and t=0.2 time frames and same have been enclosed at Exhibit -SC1 and SC2 respectively. From the studies, it is observed that short circuit levels at all the substations near KPFBR are well within the designed limits [5]. Further, the short circuit level along with their contributions has been determined for 220 and 400kV buses in the vicinity of KPFBR and Graphical plot for three phase and single phase fault for t=0 and t=0.2 is also arrived at Exhibit-SC1-3ph, Exhibit-SC1-1ph, Exhibit-SC2-3ph and Exhibit-SC2-1ph [12].

Dynamic over Voltage Studies
Dynamic over Voltages are the power frequency over voltages that are experienced at the receiving end of transmission lines following a sudden "load rejection". The magnitude of these voltages depends on the length of transmission line, strength of the system at the sending end and active and reactive power flow prior to the load rejection. These voltages are generally controlled through line reactors at the receiving end. As per the Manual on Transmission Planning Criteria [17], DOV for 230 kV transmission lines should be limited to 1.8 p.u (360kV) (peak) phase to neutral [4,7].
The DOV studies in the present studies have been performed using PSCAD software. The results of the DOV studies for the different transmission lines considered for the evacuation of KPFBR is as show in Figure 7(a), (b) and (c) and Table 12.  It has been observed from the results that DOV on the transmission lines are well below the limit stipulated in the Transmission Planning Criteria [17].

Conclusion
The stability, power flow, short-circuitand DOV studies have been performed in line with the Transmission Planning criteriain the KPFBR transmission lines and the test results are well within the stipulated limits. The results of the load flow studies have been observed that the voltages at different buses (in kV) are well within their steady operating limits (1.05 p.u) [14]. The initial variation in machine angle depends on severity of fault and in general if the oscillations damp out to 10-15% after 20-30 secs of fault, it is considered stable.
From the short circuit studies, it is confirmed that short circuit levels at all the substations near KPFBR are well within the designed limits. The Dynamic Over voltage studies it is confirmed from the results are well below the limit stipulated in the Transmission Planning criteria. Finally it has been confirmed that the prosed system is a stable system under any transient or fault conditions [15] by using the modern analysis technics. The simulation result also showsthat the modeling has enough accuracy to meet the demand of power system calculation.