Power System Analysis of Steam Power Plant Using ETAP at ULPL Indralaya, South Sumatra

ABSTRACT


INTRODUCTION
PT. PLN ULPL Indralaya is a combined cycle plant with a high level of efficiency that utilizes the residual heat or exhaust gas of GT 1.1 and GT 1.2 to heat feed water so as to produce steam, which is then flowed to rotate the ST 1.0 steam turbine [1].The source of https://doi.org/10.24036/tip.v16i2.785 2 P.ISSN: 2086 -4981 E.ISSN: 2620 -6390 tip.ppj.unp.ac.id production water for the Indralaya PLTGU is supplied from the river resource [2], water from the Klekat River in Klekat Village and Tanjung Seteko Village, North Indralaya District, Ogan Ilir Regency.PLN UPDK Keramasan Indralaya ULPL has 3 generator units : 2x40 MW (gas turbine generator) [3] and 1x40 MW (steam turbine generator) [4].The steam turbine generator unit (ST 0.1) often experiences shutdowns due to several factors.One of them is out of service for the protection operating system on the 4 MVA, 11.5/6.9kV transformer unit during a disturbance, so that an interruption occurs to the upstream system, which results in a blackout system [5].In electric power systems, disturbances often occur, including symmetrical and asymmetrical [6].
Electrical energy in the house usually comes from PLN.Where in general, the generating company uses a steam power plant system to distribute electrical energy to each region.The steam turbine is an important component that cannot be ignored in an electricity system.In the steam turbine, the steam pressure energy will be converted into kinetic energy as the steam flows through the nozzle.The conversion of kinetic energy is done mechanically by working on the blades and rotors connected to the steam turbine generator, which functions as a mediator.The turbine generator collects mechanical energy from the rotor and converts it into electrical energy [7].
In a power system, to keep it safe to operate even though one or several elements are experiencing a disturbance, a power system analysis is needed.Power system analysis aims [8] as follows: 1. To model or perform phase analysis of power system components 2. To regulate voltage at different buses, real and reactive power flow between buses 3. To plan the future development of the current system 4. To analyze the system under different fault conditions and based on different scenarios 5. To design protection devices, as well as to investigate the ability of the system to handle small and large disturbances or any faults.Many software packages [9], [10] are available for power system analysis studies, including ETAP [11], [12], Easy Power, DigSilent, Matlab, and others.Many things must be considered in an electrical system, including the distribution network (from the power source to the load).In this system, the power generated from several sources (PLN or generator) is not used at 100% load.This is due to the resistance of the line, which causes power losses to be converted into heat, and the type of load connected to the line (resistive, inductive, or capacitive).ETAP functions allow us to work with or analyze many things on a single line graph, including power flow analysis, short circuit, and overcurrent relay coordination.The software is designed with three main concepts, viz: virtual reality functionality, full data integration, and ease of data entry.ETAP 19.0.1 has several benchmarks, such as: standards used in the installation (ANSI or IEC), frequencies, and cell types (AC cells, instrument cells, or DC cells), different types of alarms and security types.ETAP 19.0.1 is the best software to analyze the entire power system [12]- [14].https://doi.org/10.24036/tip.v16i2.785 3 P.ISSN: 2086 -4981 E.ISSN: 2620 -6390 tip.ppj.unp.ac.id

Load Flow
Load flow is the power generated by generators and transmitted to the load through transmission lines.Load flow studies are the first step in designing a new system and also the first step in determining the appropriate equipment for the input power.It also has other functions to determine the bus voltage and phase angle as well as knowing the active and reactive power of the line.
There are 3 types of buses in the system, where buses can be defined as follows [8]. 1. Slack Bus is connected to the generator and has a fixed magnitude and phase angle.
This bus serves to include losses and is used to facilitate calculations.2. PV bus (generator side), for this type of bus, the voltage and active power, reactive power and voltage phase angle are determined from the calculation results.The bus is connected to the generator.3. BUS PQ for this type of bus is known active power and reactive power, the other two are obtained from the calculation results.The bus is connected to the load.
As following is the general power flow equation [6]: The Newton Raphson method has advantages in mathematical calculations over other methods.Large power systems are suitable for using the Newton-Raphson method because it is more efficient and practical.The Newton Raphson method is a method used to solve power flow equations with one variable and multiple variables.
In this case, the separation of active power and reactive power at bus-i.This separation will produce a set of nonlinear simultaneous equations in polar coordinates [15].
Because of  (  −  +  ) = cos (  −   +   so, the separation of power at bus-i into real and imaginary components is [15]. The active power (Pi), reactive power (Qi) values are known, but the voltage (Vi) and degree (i) values are unknown except at the slack bus.The two nonlinear equations can be decomposed into a set of linear simultaneous equations by expressing the relationship between changes in active power ∆Pi reactive power ∆Qi to changes in voltage magnitude ∆Vi voltage phase degree ∆i.

Short Circuit
Short circuit faults are one of the causes of electrical system faults.This fault is caused by phase wires connected to the ground or other phase wires.As a result of this short circuit, a large current is generated at the fault point.This current comes from additional currents from the power grid, generators and AC electric motors, so the fault has a very high current value and can damage electrical equipment around the fault.As a result, the voltage around the fault can drop significantly.Short circuits can be caused by lightning, insulation failure or interference from tree branches and animals.Short faults can occur two phase, three phase, one phase to ground, two phase to ground or three phase to ground.The short circuit pattern itself can be classified into two groups, namely symmetrical short circuit patterns and asymmetrical short circuit patterns [8], [16].
Figure 1 shows the symmetry short-circuit fault or balanced short-circuit fault is a fault that occurs on all three phases, so that the current and voltage of each phase remain balanced after the fault occurs.This fault can be analyzed using the positive sequence component only [17], as in equation ( 8): Where : Isc3∅ = 3-phase short circuit current VLN = Nominal voltage Z1 = Positive sequence impedance viewed from the fault point P.ISSN: 2086 -4981 E.ISSN: 2620 -6390 tip.ppj.unp.ac.id

Coordinating Relay
Coordinating studies of electrical protection equipment or systems are studies or analyses to determine the settings of overcurrent protection relays and circuit breakers that have the main objective of obtaining an optimal compromise between protection and selectivity.The study includes the determination of fault clearing time and coordination of electrical protection equipment on the upstream side.Proper coordination and fault clearing time can help reduce damage to electrical equipment and protect workers from harm.The study and analysis of protection equipment coordination is one part of the power system study.Protection relay coordination analysis is performed after the power flow study and short circuit fault analysis are performed [8].
The function of the relay is to detect and identify faults and then instruct the circuit breaker to isolate the faulty device quickly.Overcurrent relays are very widely used for overcurrent protection in primary distribution networks at the downstream end of medium voltage (TM) power supplies, and are also used as ground fault overcurrent protection.This relay requires a protected flow in the form of current obtained through a current transformer (CT).The reference element (comparator) of the relay compares it with the limit value/current mark at which the relay operates (as a trigger), if the input current exceeds the set point and continues for different periods of time after detecting the overcurrent, then this is when the relay should be activated [18].

Electric Transient Analyzer Program (ETAP)
In Study and analyzing a power system, an application software is needed to represent the real conditions before a system is realized.ETAP is a software used for power systems.It can work offline for power simulation, and online for real time data management.
ETAP is used in this study as a supporting application to analyse disturbances and coordination of power system protection at PT PLN ULPL Indralaya, especially steam turbine generator unit-01.The type of ETAP used is ETAP 19.0.1.For ease of analysis calculation results can be displayed on a one-line diagram.ETAP Power Station makes it possible to work directly with a single line diagram image display [19].

RESEARCH METHOD
This study uses a quantitative approach.This method was chosen because quantitative research is research that uses numbers, starting with data collection, analysis, and testing the results.This method was chosen because the quantitative research method is a research method that uses data in the form of numbers, emphasizing objective measurement [20] and analysis of the results shown in   Scenario 2 has one unit of generator STG 1.0 inactive and energising resources from the PLN 20 kV network without delivering power to the 150 kV grid and only supply to the STG 1.0 self-use system with a power of 1.44 MW on the LV system (0.415 kV).
Scenario 3 has one unit of generator STG 1.0 inactive and energises the resource from the Diesel Generator without delivering power to the 150 kV grid and only supplies to the STG 1.0 self-use system with a power of 1.45 MW on the LV network (0.415 kV).
Figure 4 and Table 1 showed from the simulation results there is an alert view before performing only in scenario 3 which occurs over operating the generator set by 145% of the 1 MW generator rating.

Short Circuit
There are three scenarios (Table 2 to 4) in the simulation with several areas with fault types of line to ground (LG), double line to ground (LLG), and three phases (LLL) which include: 1. Area no.1: A short circuit occurred at the 11.5kV bus bar. 2. Area no.2: short circuit occurred at the 6.9kV switchgear MV bus bar. 3. Area no.3: The short circuit occurred at the LV switchgear 0.415kV SUS 1 bus bar. 4. Area no.4: The short circuit occurred at the LV switchgear 0.415kV SUS 2 bus bar. 5. Area no.5: The short circuit occurred at the LV switchgear 0.415kV bus bar of the ATS unit.

Coordinating Relay
This simulation is carried out to ensure the protection system on the existing equipment is correct by considering several relays M-3425 as generator protection, M-3311 as protection on incoming transformer 4 MVA and the characteristics of all relays shown in Figure 5. From the relay coordination curve before tunning in the 4 MVA transformer area using beck with electric relay.
If the LLL, LG and LLG fault currents occur in the 4 MVA transformer based on the actual case as the cause of the blackout system, the TRIP relay sequence operates as shown in the Figure 6

Short Circuit
Short circuit in each area is simulated with several types of interference, including 3 phases (LLL), line to ground (LG) and line to line to ground (LLG) carried out in each scenario showed on Table 6 to 8.   In Figure 8 from the coordination relay to actual tunning, we obtained the protection relay setting in the area of the 4 MVA transformer and 11.5 kV generator, it is replaced using ABB relay type REG630 which has a choice of over current performing.

CONCLUSION
Power system analysis for STG 1.0 using ETAP, there are three results: 1. Load flow analysis in the third scenario by removing some loads by 63% so that the power source backup (Diesel generator) from 1.45 MW to 0.91 MW from SUS 1 & 2. 2. Short-circuit analysis in all scenarios in each fault area are still lower than shortcircuit withstand current rating < 65 kA in LV switchgear, < 25 kA in MV switchgear and < 54 kA in GCB. 3. Relay coordinating analysis in the first scenario, in order to reach the protection against interference, it is necessary to replace the solid state relay (Beckwith Electric) with a multifunction relay (ABB) which has a curve selection function from IDMT (inverse definite minimum time), namely Long Time Inverse.

Figure 7 .
Figure 7. Transformer Coordinated Relay System 4MVA for actual Tunning

Figure 8 .
Figure 8. Characteristics performing of 4MVA Phase-Phase Transformer for actual Tunning

Table 1 .
Load Flow Condition initial Tunning

Table 2 .
Fault Currents at Different Buses Considering LG Fault Condition Initial Tunning

Table 3 .
Fault Currents at Different Buses Considering LLG Fault Condition initial Tunning

Table 4 .
Fault Currents at Different Buses Considering LLL Fault Condition initial Tunning

Table 5 .
Load Flow Condition Actual Tunning

Table 7 .
Fault Currents at Different Buses Considering LLG Fault Condition Actual Tunning

Table 8 .
Fault Currents at Different Buses Considering LLL Fault Condition Actual Tunning

.2.3. Coordinating Relay Coordinating
Relay used to protect area during fault simulation is only carried out on the 4 MVA transformer as in the actual case which causes a system blackout that are illustrated on Figure 7. https://doi.org/10.24036/tip.v16i2.78517 P.ISSN: 2086 -4981 E.ISSN: 2620 -6390 tip.ppj.unp.ac.id