Case Study 1. Power System Planning and Design: Power Plant, Transmission Lines, and Substations

Transcription

1 Case Study 1 Power System Planning and Design: Power Plant, Transmission Lines, and Substations Lindsay Thompson, Presented to Riadh Habash ELG /10/2013

2 1.0 ABSTRACT A power plant delivers a transmission line with 500kV and 1800A at 60Hz, which is feeding a city 500km away. A Substation lies halfway between the city and the generator. The city requires a minimum of 120 kv with a peak current of 1600 A. This report discusses the design of the power plant, substations, transformers and transmission lines specifications required to meet these criteria. The layout is shown below. 2.0 POWER PLANT SPECIFICATIONS Hydroelectric power plant converts gravitational potential energy of elevated water into electrical energy. The water is used to turn a turbine, who'se kinetic energy does work on an electric generator connected to the turbine by a metal drive shaft, who then turns the work done into electrical energy. This electrical energy can then be transmitted over power lines and distributed to the customers. The hydro power plant layout used in this case study is shown in Figure Turbine and Shaft Specifications The chosen turbine is the Francis Impulse Turbine model, manufactured by Gikes. This model was chosen due to its robust design, speed ranges and power output [1]. The corresponding Francis Turbine specifications are displayed below. Manufacturer Speed Range Power Output Head Range Gikes 83 to 1000 rpm 20MW Up to 400 m The turbine is also very efficient, reaching efficiencies close to 100% when operating at 80% of full flow as shown in the graph below [2]: 2.2 Synchronous Generator Specifications Below are typical characteristics of water-cooled synchronous generators offered by GE Energy: [3]

3 In order to decide which option to chose, we can calculate required number of poles of the generator. The chosen Francis Turbine has a nominal output speed of 1000 rpm, which means we require a generator that has the following number of poles to be compatible with this turbine system: P = 120*f/n = (120*60Hz)/1000rmp P=7.2 = 8 poles Thus, we can choose an 8-pole generator from GE, with characteristics similar to the models shown above, which are listed in the following table: Manufacturer Type Frequency #Poles Voltage Range Output Range PF GE Energy Synchronous 60Hz 8 poles 20kV 450 MVA In order to meet the system criteria, the power plant will require 3 synchronous generators, each rater at 450MVA, with 3 corresponding Francis turbines and metal generator shaft. Together, the 3 turbine, generator shaft and generator sets will provide the following total output: Total Power Plant Capacity = 3*(Generator output rating) Total Power Plant Capacity = 3*450MVA Total Power Plant Capacity = 1350MVA Since the transmission line requirements are 900MVA, the above total capacity allows an added safety factor of 1.5. Additionally, we will only allow the generators to work at 80% of their total capacity will provide (450MVA*0.8) *3 = 1080MVA, which is still sufficient. As an additional safety and risk factor, we can have an additional turbine and generator on standby to be used in case of emergency or maintenance. 3.0 GENERATION SUBSTATION From the power plant, we can use generator step-up transformers to step up the voltage from 20kV to 500kV for transmission. This requires a step-up ratio of 1:25. Such transformers can be purchased from Siemens. We require 900MVA, thus the substation will have: 900MVA = X*(200MVA*0.8) X = 6 transformers + 1 on standby for safety and maintenance X = 6 Transformers working at 80% of full capacity + 1 extra The transformer specifications are listed below: Transformer Size MEDIUM (30-200MVA and V 72.5kV) Manufacturer Siemens Rated Power 200MVA Turns Ratio 1:25 (V p = 20kV, V s = 500kV) Frequency 60Hz Required Quantity 6 (+1 in Standby) = 7 total Cooling Method Oil Forced Water Forced Cooling [10] (For transformers of few hundreds of MVA) Design 3-Phase design Connection Y - Δ Transformer Efficiency ~ 98%

4 In addition to power transformers, generation, transmission and distribution substations also require additional components for different reasons, which are listed below [4] Substation Equipment Reason/Functionality Busbars To connect various incoming and outgoing circuits Surge Arrestors Protect the equipment insulation from switching surges Isolators Provide isolation from live parts during maintenance Earth Switch Discharge the voltage on the circuit to the earth for safety Current/Voltage Transformer Step down current/voltage for measurement, protection and control Circuit Breaker Used for switching during operating conditions Shunt Reactors/Capacitance For long HV transmission lines to control voltage/compensate reactive power Series Capacitor/Reactors Used to improve power transferability/limit short circuit current Lightning Protection Protect equipment from direct lightning strokes Monitors, Sensors and Control To monitor, sense and control parameters, such as V, I, P, and f. 4.0 TOWER STRUCTURE SPECIFICATIONS Our system requires stable transmission towers that can support high voltage overhead conductors rated at 500kV. The waist-type towers are the most common, and are generally used for voltages ranging from 110 to 735 kv. [5] The Dimensions and specifications are shown below. [6] 4.1 CABLE SPECIFICATIONS The selected cable for the design is a Cardinal-type Aluminum cable manufactured by Aluminum Company of America (ACSR). Aluminum cables were chosen because of its conductivity, strength, light weight and corrosion resistance. It was decided that a 2 conductor, single circuit system would be used, which would split the current into 2 conductors instead of one, thus minimizing the cable size requirements. The Cardinal Cable specifications are displayed is the table to the right [7]. The current capacity of an individual conductor is 1010A, which means we require two conductors for a total of 2020A, which meets design requirements with added safety factor. Since it is a three phase single circuit 2 conductor system, we will require a total of 6 cables that each measure 250km in length Below are the corresponding transmission line reactance and admittance using the following:

5 Zline = (Resistance distance) + j(inductive Reactance distance) Zline = ( miles) + j( miles) = = j60.58 Ω 1 Yline = = 1 = j11.24x10 6 s = j7.2x10 8 s Shunt Chapacitive reactance j miles 4.2 INSULATOR SPECIFICATIONS The insulator material chosen was toughened glass, for which the reasons and specifications are listed below. [8] Type of Insulator Material Quantity Reasons for choosing toughened glass Toughened Glass For a 500kV line, standard is 24 insulators/string - Typical for 500kV Range - Good past experience, resistance to ageing - "Healthy" insulator; facilitates the inspection of electrical lines 4.3 TRANSMISSION LINE SPECIFICATIONS A transmission line can be represented by an ABDC Network, which relates the input and output voltages and currents as shown below. To meet our design criteria, we set Vs = 500kV, Is = 1800=2000 for safety. Since the design is a 2 conductor system, the ABCD Network will be determined for only one circuit, where Vs=500kV, Is=1010A, Z line = j60.58ω, and Y line = -j7.2x10-8 s as determine in the cable specifications. Thus, using the transmission line impedance Z line and Y line, we can determine the ADBC parameters: Z Y A = = = [( j60.58) ( j7.2x10 8 )] = 1 + j5.52x10 7 Ω B = Z = j60.58 Ω Z Y C = Y (1 + 4 ) C = ( j7.2x10 8 ) { [( j60.58) ( j7.2x10 8 )]} = 2x10 14 j7.23x10 8 Ω Z Y D = = = [( j60.58) ( j7.2x10 8 )] = 1 + j5.52x10 7 Ω Using the ABCD Network parameters as previously found, we can now determine the voltage V R and current I R at the receiving end of the 250km transmission line using the two following equations with two unknowns: Vs = A V R + B I R Is = C V R + D I R We find: V R = j = kv I R = j A = A Power Factor = cos( ) = leading We can also find the efficiency of the first part of the transmission line:

6 Output Power ƞ = Input Power x100 = V RI R cos Ɵ R x100 V s I s cos Ɵ s ( ) (1010) cos( ) ƞ = x100 (500000) (1010)cos(0) ƞ= 96.92% Now, we can determine, working backwards, the voltage and current at the sending end of the transmission substation by setting the following parameters at the receiving end of the distribution substation: V R = 500kV and I R = 1010 A Using the same ABCD parameters since the same conductor is used, we obtain the following values at the sending end of the transmission substation: V s = j = kv I s = j A = A Power Factor = cos( (-0.002)) = lagging Which has the following corresponding efficiency: Output Power ƞ = Input Power x100 = V RI R cos Ɵ R x100 V s I s cos Ɵ s (500000) (1010) cos(0) ƞ = (519025) (1010)cos(6.77 ( 0.002)) x100 ƞ= 97.01% 5.0 TRANSMISSION SUBSTATION From previous calculations, we have found that the primary and secondary voltage requirements are kV and kV respectively. Here, the substation and transformers are mainly used to improve voltage regulation and efficiency. Siemens manufacturers power transformers that have tapped windings, which means that the ratio of the power transformer can be changed gradually, either in noload or full-load conditions by means of on-load-tap-changers [9]. The variable turns ratio enables voltage regulation of the output voltage. The tap-transformer specifications are below. The voltage regulation is determined using the following: With a longer cable, V out will decrease due to a voltage drop. If VR = 0, then V in = V out which is ideal. Thus, we want the smallest voltage regulation possible (smaller then 20%). The following corresponds to the voltage regulation of the system: VR(First 250km) = x 100% = 2.37 % VR(Last 250km) = x 100% = 5.8 % 500 Thus, we can conclude that the system has good voltage regulation.

7 6.0 DISTRIBUTION SUBSTATION At the distribution substation, we need to step down the voltage from 500kV to 120kV for distribution within the city. Here we have a Δ - Y connection, since the 4th neutral-wire is essential for distribution in order to ground the line within a distribution network. The specifications listed below are similar to that of the generation and transmission transformers, but with a different turns ratio of 5:1 to enable the step-down [9]. 7.0 Power System Design Layout The overall power system design layout is displayed below:

8 8.0 HVDC DESIGN HVDC Design is becoming more popular because of following advantages [11]: HVDC ADVANTAGES Smaller Transmission Towers; Narrower rights of way Lower wire losses Resulting cost savings = offset additional converter station costs In order to convert from AC line to DC line, we require Rectifier, and to convert from DC to AC, we require Inverters. Below is a block diagram representing a typical AC to DC Line-Commutated Current Source Converter (CSC) design for an HVDC System:

9 9.0 REFERENCES [1]WIKIPEDIA, 'Francis Turbine'. Accessed on October 2, 2013 [2] GIKES, 'Francis Turbines' Accessed on October 2, 2013 [3] GE Energy, 'Generator Products' Accessed on October 3, 2013 [4] HubPages, 'Substation Components' Accessed on October 5, 2013 [5]Hydro Quebec, 'Power Transmission Towers' Accessed on October 7, 2013 [6] EHV-UHV Transmission Systems, 'EHV Line Characteristics' Accessed on October 6, 2013 [7] GLOVER., Duncan, 'Power System Analysis & Design' SI Edition, Accessed on October 7, 2013 [8] SCD, 'Glass Insulators' Accessed on October 5, 2013 [9] SIEMENS, ' Power Transformers' Accessed on October 8, 2013 [10]HubPages, 'Transformer Cooling Methods' Accessed on October 9, 2013 [11] GLOVER., Duncan, 'Power System Analysis & Design' SI Edition, Accessed on October 10, 2013