Senior Design 2014-2015

SCE PSCAD Library Component Third Quarter Report

Prepared by: Student Team

5/29/15

Supported by:

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SCE PSCAD Library Component Senior Design 2014-2015

Design Review

5/31/15

Student Team Members:

Cho Wai Leung, Eric Hy, Jeff Ge, Adolfo San Juan Santiago

Advisor & Liaison:

Dr. Nagy Abed

ABSTRACT:

Power system computer aided design or PSCAD is a professional simulation tool used for

analyzing power system transients. It allows engineers to construct circuits, run simulations,

and analyze the results in a highly efficient way. From the previous winter quarter, the student

team was successful in designing wind and solar generation models integrated with a battery

storage element to smooth the fluctuating power output generated by renewable energy. The

objective of this quarter is to create a custom library that contains the models of wind, solar,

battery storage, and previous designs by utilizing PSCAD’s custom module techniques. The

report concludes that the custom library component was created successfully and any user

interested in designing a renewable system, can use the library created by the team to aid them

in their design. Future work that will be passed onto the next student team will be to

investigate and improve the control systems of the battery element.

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Table of Contents 1 Background and Requirements………………………………………………………………………………………..3-4

2 Technical Progress…………………………………………………………………………………………………………..4-7

3 System Components…………………………………………………………………………………………………………..7

- 3.1: Unbalanced Distribution Lines………………………………………………………………………………………8-9

- 3.2: Loads Model…………………………………………………………………………………………………………….9-11

- 3.3: Voltage Regulators and Controls…………………………………………………………………………….11-13

- 3.4: Photovoltaic Transient Modeling……………………………………………………………………………13-26

- 3.5: Wind Generation System……………………………………………………………………………………….26-45

- 3.6: Battery Energy Storage System………………………………………………………………………………45-57

4 Project Planning……………………………………………………………………………………………………………….58

5 Conclusion…………………………………………………………………………………………………………………….….59

6 Reference…………………………………………………………………………………………………………………….60-61

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1. BACKGROUND AND REQUIREMENT

Before it was known as PSCAD, it was known as Electromagnetic transients including

direct current, or EMTDC. It was initially used to study the Nelson River High Voltage Direct

Current Power system. Due to the success of that study, further advancement of EMTDC was

passed on and led to PSCAD. PSCAD stands for Power Systems Computer Aided Design which is

a powerful and flexible graphical user interface to the EMTDC engine. PSCAD enables the user

to schematically construct a circuit, run a simulation, analyze the results, and manage the data

in a completely integrated, graphical environment.

From PSCAD, a collection of components can be dragged from the library tab and be

paste onto to workspace to model a circuit. These components include the following and the

master library from PSCAD is shown below in Figure 1.1.

Network Components:

RLC components Transmission lines models

induction machines Transformers with saturation

Synchronous machines Motors with exciter governor and turbine

models

Faults Circuit Breakers

Surge arresters Current voltage sources

Multiple harmonic injectors Underground cables models

Power Electronics Components:

•Thyristors •Diodes •GTOs •HVDC valve group •SVS •FACTS devices

Control Blocks:

•Derivative •Delay •Differential lag •Integrator •Limit •Complex pole

•Real pole •Lead lag •Filter •Amplifier •Switch •Boolean functions

Meters:

•Voltmeters •Ammeters •Real reactive power meters •Peak detectors

•Phase angle •Frequency meters

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Figure 1.1: Master Library in PSCAD

Due to the increase penetration of renewable energy into utilities networks, the need for

a way to store this energy during off peak periods is in demand. This project intends to create a

component library to simulate and integrate renewable energy with battery energy storage

system. One of the important factors with renewable energy such as wind and solar is the power

output variation to the utility grid due to unpredictable input from nature resources. The power

output quality and variability can be simulated and evaluated by utilizing PSCAD applications to

investigate the case.

2. TECHNICAL PROGRESS

As the project title implies, the student team is creating a custom component library that any user of PSCAD can utilize to aid in their design process. The reason of creating custom components is that it is useful in simplifying and organizing the overall project. However, data signals and electrical nodes that once connected the module circuit to the greater circuit will sever, and these must be reconnected.

The process of transporting data and connecting nodes between a module circuit and the

greater circuit, is referred to as porting. A method the student team has selected to create these custom modules is by using the component wizard application of PSCAD, which is shown below in Figure 2.1.

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Figure 2.1: Component Wizard used to create custom modules

As you can see from Figure 2.1, we can model components, transmission lines, and

cables by using this technique. The component wizard generates a new definition, and then

creates a single instance of it for placement on the Schematic canvas. After entering the

desired parameters, a custom module can be created and an example of how the student team

created a custom module will be represented below in Figure 2.2. Furthermore, clicking into the

custom module box, an external node component is shown inside where it can be used to make

an external electrical connection to an electrical system from within a module.

Figure 2.2: Custom Module

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Shown below in Figure 2.3, an example of how we can connect the greater circuit to a node shown by ‘a’ to represent the module that we create.

Figure 2.3: Example of utilizing component wizard

Through this technique, the student team came up with the main three custom component library which contains the solar, wind, and battery storage elements which are shown below in Figure 2.4. These custom components’ models and control systems will be described in further detail within the report.

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Figure 2.4: Custom Component Libraries of Wind, Solar, and Battery element

3. SYSTEM COMPONENTS

To validate the necessity of integrating a battery storage to a renewable system, the IEEE 34-Bus Feeder was used as the testing benchmark. However, due to limitations of the student team’s educational version of PSCAD, modifications had to be done in order to simulate our design. Displayed in Figure 3.1, is the modified IEEE Bus which contains 23 Buses and the varying components modeled in PSCAD.

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Figure 3.1: IEEE Modified Bus System with system parameters

As with any system, the IEEE Bus system contains important system components that allow

the system to operate efficiently and correctly. These components includes unbalanced

distribution lines, load types, voltage regulators, wind/solar generation, and battery storage

elements.

Next, our report will introduce a more in depth discussion of each system component and

custom component library models in sections that were studied and designed by each of our

team members.

UNBALANCED DISTRIBUTION LINE

Starting off with the unbalanced distribution line, it is design in PSCAD by Figure 3.2 shown

below,

Figure 3.2: Unbalanced Line modeled in PSCAD.

This component models two wires that are mutually coupled where Z & B matrices are

directly entered as shown below in Figure 3.3,

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Figure 3.3: Configuration of unbalanced lines

Each type of standard construction was modeled and six types of construction models that

are included are 300, 301, 302, 303, 304, and 305. The difference between each model was the

configuration that was inputted in each model.

LOADS MODEL

The generic term for a load is anything in the circuit that consumes power. These loads can

either be reactive, capacitive, or inductive, but more commonly a combination of all. The ones

that were focused on for the IEEE Bus includes phase loads and phase to phase loads, where the

difference between them is simply the rated voltage. Starting off with phase loads, these loads

can be constant power loads, constant impedance loads, and constant current loads.

A constant power load is a load that varies its impedance to change of input to keep its power

constant. A constant impedance load is simply a load that presents unchanging impedance like a

resistor. Lastly, a constant current load is a load that varies its internal resistance to achieve a

constant current, regardless of the input voltage that is fed to the load. In the real world and

shown in the IEEE system, the combinations of these loads are present.

These loads are modeled in PSCAD by Figure 3.4, which represents the load characteristics

as a function of voltage and frequency.

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Figure 3.4: Load connected to the system in Different configurations in PSCAD

Also, some important relations to these loads are the following equations that are a

function of voltage and frequency:

𝑃 = 𝑃𝑜 (𝑉

𝑉𝑜)

𝑁𝑃

∗ (1 + 𝐾𝑃𝐹 ∗ 𝑑𝐹) (1)

𝑄 = 𝑄𝑜 (𝑉

𝑉𝑜)

𝑁𝑄

∗ (1 + 𝐾𝑄𝐹 ∗ 𝑑𝐹) (2)

Constant Power: Constant Z: Constant Current:

-Np=Nq=0 -Np=Nq=2 -Np=Nq=1

-Kpf=kqf=0 -Kpf=Kqf=0 -Kpf=Kqf=0

Where the following variables are represented by the following:

P = Equivalent load real power

Po = Rated real power per phase

V = Load Voltage

Vo = Rated load voltage (RMS L-G)

NP = dP/dV Voltage index for real power

KPF = dP/dF Frequency index for real power

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Q = Equivalent load reactive power

Qo = Rated reactive power (+inductive) per phase

NQ = dQ/dV Voltage index for reactive power

KQF = dQ/dF Frequency Index for reactive power

As we can see from Eq. (1) and (2), by entering different values for the variables, the

presence of different load types is seen. Real life application of a constant power load can be a

motor load with variable frequency drive, a constant impedance load can be an incandescent

lightbulb, and a constant current load can be an electronic load.

Frequency and voltage dependent loads are loads that their active and reactive power

requirements are functions of the system voltage and frequency. In PSCAD it is modeled by a

variable resistor and inductor and is shown in Figure 3.5.

Figure 3.5: Phase to Phase load representation in PSCAD

The variable resistor is a resistor whose electrical resistance can be varied to a required

value. It’s commonly used to allow for finer control over current by changing the amount of

resistance [1], so as you increase resistance, current would decrease giving an overall voltage

gain. Likewise, a variable inductor varies its inductance to a desired value to oppose changes in

current.

VOLTAGE REGULATORS AND ITS CONTROLS

An important system component included in the IEEE Bus is the regulator and its control,

where its main function is maintaining a relatively constant output voltage within a range of

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varying input voltage [2]. In PSCAD a custom design is created and is represented with a tap

changer with its regulation control, shown below in Figure 3.6.

Figure 3.6: Regulator and its controls modeled in PSCAD

The importance of the regulator is to measure how efficient the system is performing and

gives an indication of whether any adjustments are needed to improve the system. A tap changer

is a device within a power transformer to regulate the output voltage to a required level. This is

normally achieved by changing the turn ratios of the transformer by changing the turn ratios of

one side of the windings.

Regulator Controls includes:

Table 3.1: Additional functions of the Regulator control

Set Voltage Control time delay

Initial tap setting Tap position calculator

Bandwidth Timer reset

Out-of-band detector Line compensation

Now that the types of loads and regulators have been selected, now we will introduce

and discussed how to integrate battery storage component and how to use it in smoothing the

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power output for wind and solar generation. Before the integrating of the solar and wind with

battery storage, their full transient models designed will be presented and discussed. This

include the hardware (e.g. Inverter, Induction generators and solar plans) and the control

system.

PHOTOVOLTAIC TRANSIENT MODELING

We now start describing the custom component library that we have developed from

PSCAD starting with the solar generation. Represented below in Figure 3.7, is the custom

component library and a discussion of its components within the library will be explained.

Figure 3.7: Solar Generation Library with its components

Photovoltaic system is the most popular and efficiency solar system conversion method

recently. This system can transfer the solar energy to the electricity directly. Compared to

thermal power system and nuclear power system, photovoltaic panel system cannot cause

environment pollution. Figure 3.8, shows the world largest solar farm, Desert Sun light located

in the center of the desert in Riverside County. This solar farm can provide 550 MW power to

support 170,000 homes. Furthermore, it can reduce 614,000 tons of carbon-dioxide emission.

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Figure 3.8: Desert Sun light Farm World largest solar farm [3]

The solar farm is just a large scale photovoltaic system designed for supply of merchant

power into electricity grid. The detail photovoltaic system model is shown in Figure 3.9.

Figure 3.9: PV transient model in PSCAD

This transient model contains controllable photovoltaic panel, DC-DC converter, DC-AC

inverter, transformer and equivalent load. The photovoltaic panel, DC-DC converter and DC-AC

inverter are the three main parts in this model.

Photovoltaic Panel

All the photovoltaic panels are made by semiconductor material and they are combined

with photovoltaic cells. The equivalent circuit of the PV cell can be shown in Figure 3.10.

Figure 3.10: Photovoltaic cell equivalent circuit [4]

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The process of transferring solar energy to electricity is called photovoltaic effect. When

the sunlight hits the surface of the panels, the electrons present in the valence band absorb

energy, and being excited, jumping to the conduction band and become free. These highly

excited, non-thermal electrons diffuse, and some reach a junction where they are accelerated

into a different material by a built-in potential. This generates an electromotive force, and thus

some of the light energy is converted into electric energy. From Figure 3.10, the Eq. 3 of output

current (I) can be expressed as:

I=ISC-Idiode*(e(q*(V+I*Rseries)/n*k*t)-1)-(V+I*Rseries)/Rshunt (3)

Where, Idiode is diode saturation current, q is electron charge, k is Boltzmann constant (1.38*10-

23 J/K), n is ideal factor (1~2) and T is cells temperature. ISC is the photo current and it is a function

of the solar radiation on the plane of the solar cell G and the cell temperature TC. The photo

current Eq. 4 is as follows:

ISC=ISCR*G/GR[1+αT*(TC-TCR)] (4)

Where, ISCR is the short circuit current at the reference solar radiation GR and the reference cell

temperature. The parameter αT is the temperature coefficient of photo current.

Photovoltaic panel has a long life time because all parts in the panel are fixed. Also, the

most efficiency photovoltaic panel can improve the conversion rate to 24%. Figure 3.11 below

shows the photovoltaic panel model in PSCAD.

Figure 3.11: Photovoltaic panel represented in PSCAD

Figure 3.12 below shows the parameters in photovoltaic panel model in PSCAD.

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Figure 3.12: Photovoltaic panel parameter from PSCAD

We can set the number of cells to a large amount to make the photovoltaic panel model

act as a solar farm.

DC-DC Converter

The core component in the DC-DC converter is called maximum power point track or

(MPPT). The output power from photovoltaic panels changes based on the solar radiation, cells

temperature and also the value of the loads. Figure 3.13 below represents the relationship

between output voltage and output current from photovoltaic panels under 600 W/m2, 800

W/m2 and 1000 W/m2 and figure 3.14 represents the relationship between output voltage and

output current from photovoltaic panels under 25 Celsius, 50 Celsius and 75 Celsius. The

increasing solar radiation increases the short circuit current and the increasing cells

temperature decreases the open circuit voltage.

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Figure 3.13: Relationship between voltage and current under different solar radiation [4]

Figure 3.14: Relationship between voltage and current under different cells temperature [4]

Figure 3.15 below shows that under certain cells temperature and solar radiation,

different values of load affect the output power from the photovoltaic panel.

Figure 3.15: Relationship between photovoltac panel output power and values of load [4]

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The main benefit of maximum power point track (MPPT) is that it can make the

photovoltaic panels ignore the effect of loads and find the maximum power on the power curve

under certain solar radiation and cells temperature. Then it helps the system generate the

corresponding power. From Figure 3.16 below, shows the procedure of tracking maximum

power point.

Figure 3.16: Relationship between photovoltac panel output power and values of load [5]

Figure 3.17 below shows the MPPT custom library component represented in PSCAD.

Figure 3.17: Maximum power point component in PSCAD

There are two algorithm options which are the Perturb and Observe (P&O) and Incremental

Conductance. The concept behind P&O algorithm is based on observation of PV array output

power and its perturbation by changing the current or the voltage of PV array operation. The

algorithm increments or decrements continuously the reference voltage or current based on

the previous value of power until it reaches the maximum power point. When the dP/dV>0, it

means the perturbation moves the operating point of PV array to the maximum power point

and P&O method will continue to perturb the PV voltage in the same direction. When the

dP/dV<0 which means the perturbation moves the operating point of PV array away from the

maximum power point and P&O method will reverse the direction of the perturbation. Figure

3.18 below shows the state flow chart implementation of P&O method.

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Figure 3.18: P&O algorithm stateflow chat [6]

The other algorithm, Incremental Conductance is based on the derivative of PV output

power and the equations are shown below in (5) and (6),

(5)

(6)

I/V is the instantaneous conductance of PV array and dI/dV is the instantaneous change in

Incremental Conductance. Comparing these two quantities, it shows the position of the

currently operating point in relation with maximum power point. The flowchart is shown in

Figure 3.19 below,

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Figure 3.19: Incremental Conductance flowchat [7]

The results of these simulation for tthe two algorithms are shown below in Figure 3.20.

Figure 3.20: Simulation result of P&O and incremental conductance for step irradiance [6]

The advantage of Incremental Conductance over P&O, is that it can stop and determine

when the maximum power point is reached without having to oscillate around this value. Also,

it can perform MPPT under rapidly varying radiation conditions with higher accuracy than the

P&O method. Due to the advantages and results of the simulation, the Incremental

Conductance algorithm is selected in our solar generation. From PSCAD simulation, when

radiation, temperature, and algorithm is incremental conductance, the results between the

system with MPPT and without MPPT have an obvious difference. Figure 3.21 below shows the

output power from photovoltaic panel of the system with and without MPPT.

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Figure 3.21: Output power difference between the system with MPPT and without MPPT

The data above is exported from the original graphs made using excel. When the time

increases, the PV panel which contains MPPT generates more and more power than the one

without MPPT. The results from the simulation identifies how important MPPT techniques are

for a PV system.

DC-AC Inverter In order to use the power from solar generation, the DC-AC converter is necessary and is

placed between the grid and DC-DC converter. There are six high frequency switches which are

called insulated-gate bipolar transistors or IGBT which converts DC to AC. These IGBTs are

controlled by a controller which is the core component in the inverter. Before we go through

the control system in the inverter, the inputs of the control systems should be introduced.

Figure 3.22 shows the simple real power (P) and reactive power (Q) regulation in our solar

generation.

Figure 3.22: Simple P and Q regulation in PSCAD

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For real power regulation, the measuring DC voltage is compared to the reference DC

voltage. The error between the two values is processed as an angle voltage (Ang) through a

proportional-integral (PI) controller. Similarly for reactive power regulation, the error is

compared between the desired and actual value of reactive power, then processed into

magnitude voltage (Mag) through a proportional-integral (PI) controller [8]. After that, the

angle voltage and magnitude voltage signals are sent to the controller in the inverter. Now, the

controller in the inverter will be introduced.

There are two methods of controller to control the IGBTs in the inverter in our system. The

first method is called firing pulse generation which is shown in Figure 3.23 below and it shows

the firing pulse generation model used in our solar generation.

Figure 3.23: Firing pulse generation in PSCAD

The controller starts with creating three sinusoidal modulating waves with a frequency of

60 Hz and a phase shift equal to the output of the previous PI controller (Ang) with additional

shifting of -120 and 120 degrees. The magnitude of the modulating waves is equal to (Mag)

from the previous PI controller. Then, the three sinusoidal modulating waves are compared

with a triangular carrier wave with magnitude ranging between -1 and 1. Switching signals gt1,

gt3 and gt5 were generated by setting the output of the comparator to 1 whenever the

modulating wave is greater than the carrier wave and 0 otherwise. Since the operation of the

two switches in each of the three legs of the inverter bridge should be complementary to

produce the final sinusoidal wave, the switching signals gt4, gt6 and gt2 were generated by

inverting the switching signals gt1, gt3 and gt5, respectively [9].

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The other method is called phase locked loop controller. Figure 3.24 below shows the

phase locked loop controller used in our solar generation.

Figure 3.24: Phase looked loop controller in PSCAD

The greatest advantage of the phase locked loop controller, is that it can constantly

synchronize the output frequency and phase of grid voltage with grid current. This means the

output phase and input phase are always matched up. This will improve the quality of output

power to the grid and Figure 3.25 below shows the results of two methods.

Figure 3.25: Simulation result by different control methods in PSCAD

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The red line represents the output power by using the phase locked loop controller. The

blue line represents the output power by using firing pulse generation. The DC-AC converter

which is controlled by phase locked loop generates more power than the firing pulse

generation. Due to the advantages and results of the simulation, the phase looked loop

controller is selected in our solar generation.

Solar Radiation representation

Lastly, the solar radiation variation over the full day cycle and its effects on the power

generated from the photovoltaic system will be modeled in PSCAD and discussed. Although

solar PV system is an important source for renewable energy with great advantages, it also has

its disadvantages. A disadvantage is that the PV panels can only generate power when the solar

radiation is present. Figure 3.26 below is the simulation from PSCAD, which shows the output

power from the solar panel in a 24 hour time frame.

Figure 3.26: Output power from solar panel

Referring again to Figure 3.26, before 6 A.M and after 6 P.M, the solar panel doesn’t

generate any power and the highest power that is generated is around 12 p.m. This is due to

the fact that before 6 A.M and after 6 P.M, there is no sun light at all and solar intensity reaches

the highest peak value during noon time. Our indication of output power from the solar panel is

that the more solar radiation the solar panel can detect, the more power it can generate and

vice versa. To prove this indication, a real time data graph of the power produced by solar

radiation is used, which is shown in Figure 3.27.

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Figure 3.27: Daily power that the renewable sources generate [10]

This graph is from California Independent System Operator (CAISO) website which shows

the daily output power that the renewable sources generated on May 29, 2015. Notice that

there is no power generated from solar panel before 5 A.M and after 9 P.M, which identifies

that the results from simulation matches up with real world data.

Now that the modeling of solar generation using PSCAD has been discussed, another

renewable energy model that will be introduced is wind generation and its controls.

WIND GENERATION SYSTEM (WGS)

People started using wind power centuries ago with windmills, which pumped water,

ground grain, and did other work. Today’s wind turbine is a highly evolved version of a

windmill. A modern wind turbine takes the wind’s kinetic energy and converts it into

mechanical energy and then in electricity. Wind power is an affordable, efficient and abundant

source of domestic electricity. For Southern California Edison (SCE), it has been 30 years since

the first wind turbines were installed in the Tehachapi-Mojave Wind Resource Area shown in

Figure 3.28 [11].This location is considered one of the premier places in the nation for wind

power generation, and it is one of the windiest places in the world.

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Figure 3.28: SCE wind Farm Located in Tehachapi, CA [11]

The Tehachapi Renewable Transmission Project (TRTP) is a critical piece of infrastructure

necessary to bring clean, renewable energy to customers throughout Southern California.

However, this is still only one part of SCE’s effort to achieve this important goal. SCE officially

began the construction of its Tehachapi Renewable Transmission Project (TRTP) in 2008. The

first phase of the project is high-voltage transmission system capable of delivering 4,500 MW of

clean energy into the Los Angeles area.

The following design, represented below by Figure 3.29, represents the wind generation

library component model designed using PSCAD. The parameters needed in PSCAD were

specified based on the functions and ratings for each of these components. Among these parts,

the wind turbine component and double-fed induction generator (DFIG) component are

available in the PSCAD library directly. The model in the PSCAD library was used with proper

parameters based on the ratings.

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Figure 3.29: Wind Generation Library Component Module Designed in PSCAD with DFIG

In recent years, wind energy has become one of the most important and promising

sources of renewable energy, which demands additional transmission capacity and system

stability. Therefore, for our project, we used the 34 bus system to test the integration of a wind

generation system with the implementation of a Double-Fed Induction Generator (DFIG). Our

focus was to comprehend the electromagnetic transient analysis behavior of the given system

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for utility applications through different simulations. Figure 3.30 below, represents the PSCAD

design for a wind turbine model. Three models related to wind energy are available in the

PSCAD library as shown in Figure 3.30, including the wind source model, wind turbine model

and wind governor model. In our project, the wind source model and the wind turbine model

were used. In PSCAD, the wind source model generates the wind speed variable. The wind

speed variable can be composed by four components as shown in equation (7). The values of

those four components can be independently set when necessary.

The wind turbine governor in the PSCAD library is designed for enabling the pitch

control function when necessary. Its output pitch angle value denoted as beta (β) is shown in

Figure 3.30.

Figure 3.30: Wind Turbine Model from PSCAD

Wind power equation:

Vwind= Vbase + Vgust + Vramp + Vnoise (7)

Where;

Vbase= base wind speed (m/s)

Vgust= gust wind component (m/s)

Vramp= ramp wind component (m/s)

Vnoise= noise wind component (m/s)

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𝑃𝑚 = (1

2) 𝜌𝐴𝐶𝑝𝑉𝑤

2 (8)

Where; Pm: Output mechanical power [W], 𝜌: Air density (1.225 kg/m3), A: Rotor’s Blades area [m2], Cp: Power coefficient, Vw: Wind speed [m/s] represented in (Figure: 3.31)

From equation 8, one can find the mechanical power output.

The following equations, Cp depends on two parameters:

1. Tip Speed ratio (λ) 2. Blade pitch Angle (β)

Cp= 0.5(λ-0.02 2 β2-5.6) 𝑒−.17𝜆 (9)

λ = (1

𝑉𝑤) 𝑊𝑡𝑅𝑡 (10)

Where:

𝑊𝑡 is the Turbine Speed [rad/sec]

Rt is the Radius of the blade [m].

Figure 3.31: Variations of power coefficient vs. tip speed ratio for β = 0 [18]

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In order to obtain the maximum Cp we have to control λ to be λopt so 𝑊𝑡 must be controlled in each wind speed shown below in Figure 3.32.

Wind Turbine V-I Characteristic

The wind turbine control implements the V-I characteristic illustrated in Figure 3.33

below. As long as the reactive current stays within the maximum current values (-Imax, Imax)

imposed by the converter rating, the voltage is regulated at the reference voltage verve. A

voltage drop equation is used for the V-I characteristic (3% at maximum reactive power

output).

V= Vref. + I* Xs (11)

Where;

V Positive sequence voltage (p.u.)

I Reactive current (p.u./PNOM) (I > 0 indicates an inductive current)

XS Slope or drop reactance (p.u./PNOM)

PNOM Three-phase nominal power of the converter specified in the block dialog

box

Figure 3.32: Output power of the wind turbine vs. rotor speed. [16]

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Figure 3.33: Wind Turbine V-I Characteristic [13]

We used an example simulation in PSCAD where we used the standard 60Hz as our

frequency. The circuit simulates a low voltage system with an induction generator to analyze

each component and find the variation of the active and reactive power based on the variability

of the wind speed and pitch angle of the wind turbine Figure 3.34. We also analyze the

components of the wind source, their effect on the active and reactive power, and the response

of the control system. Each input and output is indicated on Table 3.2.

Figure 3.34: PSCAD Simulation design for an induction generator

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Shown below in Figure 3.35, is the typical layout of a wind turbine.

The main purpose of the transient analysis simulation is to reduce the wind turbine

power output fluctuation to less than ten percent for a low voltage ride through. In our

simulation we used a constant wind speed at 15 m/s. Our results indicate that as the wind

speed increases, the pitch angle must be adjusted also for maximum power output. Since the

generator starts in the per unit control mode, the active and reactive power fluctuate until the

control switches to the torque control mode and the machine rotates at a constant speed.

When the speed and pitch angle are constant, the active and reactive power also become

constant as shown in Figure 3.36 and Figure 3.37.

Wind Generator Input/output Turbine Inputs Turbine Outputs Wind Parameters

W: Speed control mode of the machine [p.u. speed]

Vw: Wind speed (must be a positive value) [m/s]

Tm: Output torque of the turbine [pu]

Mean Wind Speed

T: Torque control mode. Speed calculated based on machine’s inertia

W: Machine mechanical speed [rad/s]

P: Output power of the turbine [pu]

Gust

S: Switch to select the speed mode. Beta: Pitch angle [deg]

Ramp/Ramp

Figure 3.35: Major components of a typical horizontal axis, three-bladed, upwind wind turbine

Table 3.2: Wind generator components input

[7]and output. [6]

33

The power is controlled in order to follow a pre-defined power-speed characteristic,

named tracking characteristic. This characteristic is illustrated by the ABCD curve shown in

Figure 3.38 below.

Figure 3.36: PSCAD Simulation design for an induction generator

Figure 3.37: PSCAD Simulation design for an induction generator

34

Figure 3.38: Turbine Characteristics and Tracking Characteristic [13]

The actual speed of the turbine ωr is measured and the corresponding mechanical

power of the tracking characteristic is used as the reference power for the power control loop.

The tracking characteristic is defined by four points: A, B, C and D. From zero speed to speed of

point A the reference power is zero. Between point A and point B the tracking characteristic is a

straight line. Between point B and point C the tracking characteristic is the focus of the

maximum power of the turbine (maxima of the turbine power vs turbine speed curves). The

tracking characteristic is a straight line from point C and point D. The power at point D is one

per unit and beyond point D; the reference power is a constant equal to one per unit.

Wind Turbine Generator

The generator is the component of the wind turbine responsible for converting the

mechanical motion of the rotor into electrical energy. There are many different types and sizes

of electric generators for a wide range of applications. Depending on the size of the rotor and

the amount of mechanical energy removed from the wind, a generator may be chosen to

produce either AC or DC voltage over a variety of power outputs. There are four major types of

electrical generators for converting mechanical energy; the squirrel cage induction generator,

the wounded-rotor induction generator, the doubly-fed induction generator, and the

synchronous generator. For our project, we decided to use the doubly-fed induction generator

simply because it can perform as well as the synchronous generator in controlling the active

and reactive power for variable wind speed, and it is less expensive.

35

Double-Fed Induction Generator

To control the wind power efficiently, the most reliable system in the present era is the

grid connected DFIG. There are some important characteristics of the DFIG that we must take

into consideration. The DFIG brings the advantage of utilizing the turns ratio of the machine, so

the converter does not need to be rated for the machine’s full rated power. The rotor-side

converter (RSC) usually provides active and reactive power control of the machine while the

grid-side converter (GSC) keeps the voltage of the DC-link constant. The DFIG technology allows

extracting maximum energy from the wind for low wind speeds by optimizing the turbine

speed, while minimizing mechanical stresses on the turbine during gusts of wind. The optimum

turbine speed producing maximum mechanical energy for a given wind speed is proportional to

the wind speed. Another advantage of the DFIG technology is the ability for power electronic

converters to generate or absorb reactive power, thus eliminating the need for installing

capacitor banks [12].

The main electronic component controllers of the DFIG is shown in Figure 3.39 are:

The Rotor Side Converter (RSC)

The Grid Side Converter (GSC)

The DC-Link

The Crowbar

Figure 3.39: Double-Fed Induction generator Diagram

The following design represented below by Figure 3.40, represents the wind generation

model designed using PSCAD.

36

Figure 3.40: Wind Generation Module in PSCAD with DFIG

Wind Generator Control System

In this section, aspects of implementing the control systems of the DFIG are named.

Controlling of DFIG depends upon the requirement, type of study and method to be used. The

classical technique based on proportional integral control has been used for this system. A

cascade control structure has been used to command the rotor-side converter (RSC) and grid

side converter (GSC). The rotor-side converter provides the excitation for the induction

machine rotor and a varying excitation frequency depending on the wind speed conditions. The

control objectives must be achieved by the manipulation of currents, thus the inner control

loops are current loops as shown in Figure 3.41. With a pulse with modulation (PWM) converter

it is possible to control the torque, the speed of the DFIG, and also the power factor at the

stator terminals shown in Figure 3.42. They receive the reference from the outer control loop,

which relates to the process variable defined in the control objective.

37

Figure 3.41: Generation of Current References Control System Model from PSCAD

Figure 3.42: Generation of Current References PMW Control System Model from PSCAD

The choice of a synchronously rotating reference frame has the advantage that all the

currents, voltages and flux-linkages associated with the stator and rotor dq-windings are DC in a

balanced sinusoidal steady state as presented in Figure 3.43. Since the objective of the grid-side

converter (GSC) is to keep the dc-link voltage constant regardless of the magnitude and

direction of the rotor power, a vector control method is used as well. This method enables

independent control of the active and reactive power flowing between the grid and the

38

converter. The PWM converter is current regulated, with the d-axis current used to regulate the

dc-link voltage and the q-axis current component to regulate the reactive power. While the GSC

control scheme always uses grid voltage space vector alignment, two possibilities exist for the

alignment of the RSC reference frame.

Figure 3.43: Stator to Rotor Control System from PSCAD

It can be aligned with the stator flux space vector as shown in Figure 3.44. Its purpose

rely in the identification of main stator flux by integrating the stator voltage after the removal

of resistive drop.

Figure 3.44: Stator Flux Control System from PSCAD

The rotor position and slip angle control has a purpose of determining the relative

difference between stator flux and rotor position for resolving the rotor currents. The angle

resolver cleans the angle signal so that jumps of 2π do not show up as referenced in Figure

3.45.

39

Figure 3.45: Rotor Position and Slip Angle Control from PSCAD

The vector control for the generator can be embedded in an optimal power tracking

controller for maximum energy capture in a wind power application. By controlling the active

power of the converter, it is possible to vary the rotational speed of the generator, and thus the

speed of the rotor of the wind turbine. This can then be used to track the optimum tip-speed

ratio as the incident wind speed changes thereby extracting the maximum power from the

incident wind. The grid-side converter control gives potential for optimizing the grid integration

with respect to steady-state operation conditions, power quality and voltage stability. The

optimal power tracking control referenced in Figure 3.46, determines the reference speed to

maintain the optimal tip speed ratio (Max. power tracking) Assuming constant tip speed ratio:

wind speed/Wpu = 12/1.1 = 10.909 = constant. The Initial wind speed is 12 m/s and the initial

speed of machine is 1.1 pu.

Figure 3.46: Optimal Power Tracking Control System Model from PSCAD

The Rotor-Side Converter (RSC)

The rotor-side converter representation in PSCAD is shown below in Figure 3.47, is used

to control the wind turbine output power and the voltage measured at the grid terminals. The

purpose of the rotor-side converter is to control the rotor currents such that the rotor flux

position is optimally oriented with respect to the stator flux in order that the desired torque is

developed at the shaft of the machine.

40

Figure 3.47: Rotor Side Converter Model from PSCAD

The Grid-Side Converter (GSC)

The GSC model in PSCAD is shown in figure 3.48, is used to regulate the voltage of the

DC-Link between the two converters and allows generating or absorbing reactive power for

voltage support requirements. During the steady state, and low voltage periods, the GSC takes

place in the generation of reactive power. It supplies the reactive power quickly while the rotor

side converter results in delay as it passes the current trough the machine.

Figure 3.48: Grid Side Converter Model from PSCAD

41

The DC-Link

The dc-link model describes the dc-link capacitor voltage variations as a function of the

input power to the dc-link. The energy stored in the dc capacitor is as followed in the equation,

Wdc = ∫ 𝑃𝑑𝑐𝑑𝑡 = 1

2𝐶𝑉𝑑𝑐

2 (12)

Where the following variables represents,

C: Capacitance Vdc: Voltage Wdc: Stored energy Pdc: Input power to the dc link

The voltage and energy derivatives are as follows, 𝑑𝑉𝑑𝑐

𝑑𝑡 =

𝑃𝑑𝑐

𝐶𝑉𝑑𝑐

𝑑𝑊𝑑𝑐

𝑑𝑡 (13)

Where; Pdc: Pdc = Pin –Pc. Pin: Input power from rotor-side converter Pc: Grid-side converter output power

The dc-link voltage varies as Pdc and is a constant when Pdc = 0 [15].

A braking resistor is provided in the dc-link bus shown in Figure 3.49 below, as a form of

protection to dissipate excess energy during a grid fault. The resistor is connected to the dc-link

bus in series with an insulated gate bipolar transistor (IGBT), and is referred as either the dc-link

chopper or the dc crowbar. The IGBT can control the amount of energy dissipated in the

resistor, with the resistor or set of parallel resistors, sized to handle a specific amount of energy

during a grid fault. The larger the fault that is desirable to ride through, the larger the physical

size of the resistor or resistors must be. This design decision can be a function of utility

requirements for fault ride through, as the energy dissipation during a fault is a function of the

voltage dip magnitude and fault duration [12].

Figure 3.49: DC-Link model From PSCAD

42

The AC Crowbar

An AC crowbar modeled in PSCAD is shown in Figure 3.50, is implemented on the rotor

side of the RSC, which acts to bypass the RSC by applying short-circuit to the rotor terminals. This

acts to protect the RSC from over currents as well as to protect the DC-link from over voltages.

The crowbar is an important aspect in not only the protection of the converter components, but

in understanding the behavior of the DFIG machine during grid faults. The crowbar avoids the

voltage bus exceed its maximum value once the RSC loses current control providing a path for

the rotor currents. The crowbar short-circuits the rotor and the machine operates as a squirrel

cage machine.

Figure 3.50: AC Crowbar model From PSCAD

To avoid the bus voltage from reaching the converter limits, it is necessary to break this

energy flow, and the simplest method is to short-circuit the rotor when the bus voltage reaches

a limiting value. With a passive control, the crowbar act as a protection system; the time

necessary to open the stator breaker is approximately 100 milliseconds, causing at the end the

disconnection of the wind turbine. When the control objective is to keep the wind turbine

connected to the grid during fault, it is necessary to control the bus voltage. The simplest

technique consists of comparing the bus voltage with its maximum and normal operation

reference values and, depending on that comparison, keeping the crowbar circuit open or

closed.

43

Low Voltage ride through (LVRT)

The performance of the DFIG during grid faults is attracting much interest due to the

proliferation of wind turbines that employ this technology. Grid codes specify that the

generator must exhibit a fault-ride-through (FRT) capability by remaining connected and

contributing to network stability during a fault. The classical solution to fulfill LVRT

requirements is the use of the rotor crowbar method referred to Figure 3.51. It is the

conventional system adopted by manufacturers to ride through grid faults. Although the

crowbar is a cost-effective method able to protect the generator and the converter during the

faults, it has some disadvantages that cannot be overlooked. Its major disadvantage is that, the

DFIG loses its controllability once the crowbar is triggered, due to the rotor-side converter

deactivating. In such a situation, the DFIG absorbs a large amount of reactive power from the

grid, leading to further grid voltage degradation [13].

Figure 3.51: Low Voltage Ride Through graph [16]

In order to reduce the operation time of the crowbar, an improved hysteresis control

strategy is adopted. The maximum absolute value of rotor current |Ir| max is compared with a

threshold value Ith and a safety value Isa. If |Ir| max is greater than Isa, the crowbar is activated

for protecting the power converters. Immediately after |Ir| max decreases to be less than Isa,

the crowbar cuts off [14]. During the grid fault the power converters remain connected in order

44

to continuously control the reactive power. In this way the DFIG is able to supply reactive

power to the grid, as required by recent grid codes, to help the grid voltage recovery as seen in

Figure 3.52.

Figure 3.52: System Response under Fault Conditions

Now that the models of solar and wind generations have been presented, the

importance of the integration of the battery with these renewable energy source will be

presented.

BATTERY ENERGY STORAGE SYSTEM (BESS)

Advantages of BESS have already been briefly established in the beginning of the report.

Further research and simulations were conducted regarding battery’s characteristics and

integration in power grids. Simulations fulfilled the purpose of integration of battery storage in

power grids, which is to smooth the power output due to fluctuating nature of renewable

resources. In spring quarter, two battery models were finalized and put into a custom

component library.

California legislature put an energy storage revolution into motion with the passage of

AB 2514 to meet the goal of 1.3 gigawatts by 2022 [19]. In order to achieve the goal, Southern

California Edison (SCE) has been implementing energy storage years ago and especially the 32

megawatt-hours (MWh) battery energy storage system at SCE’s Monolith substation in

Tehachapi, California Figure 3.53. The area is where the Tehachapi wind resource area that is

45

projected to generate up to 4,500 megawatts (MW) of wind energy by 2016 [20]. The battery

storage for wind integration can provide 8 MW for 4 hours.

Figure 3.53: SCE’s Monolith substation in Tehachapi

The electrochemical battery model in PSCAD was given by Dr. Abed Nagy in order to

study the implementation of BESS with Solar and Wind generations. This section of the report

introduces a battery component in PSCAD which is based on electrochemical battery model.

Here are the sections:

Electrical characteristics of batteries: Description of battery main characteristics

of battery.

Battery model in PSCAD: Description of battery model in PSCAD and its

parameters.

Improvement to wind generation output: Battery integration smooth out power

output profile of wind generation.

Improvement to solar generation output: Battery integration smooth out

voltage output profile during low sun radiance.

Electrical characteristics of batteries:

There are many different common types of batteries and other factors to be considered

regarding performance. The four main rechargeable battery cell types include Lithium-Ion, Lead

Acid, Nickel-Zinc (Ni-Zn), Nickel-Cadmium (Ni-Cd), and Nickel metal hydride (Ni-MH). They all

have very similar percentage of capacity discharge characteristic Figure 3.54. Lithium-ion (Li-

ion) battery type has better advantages over other types. It offers excellent performance, which

is related to their high specific energy, energy density, specific power, efficiency, and long life

[21].

46

Figure 3.54: Percentage Capacity Discharged [22]

The battery model is created for PSCAD based on a research paper from IEEE [22]. The

model has a series of assumptions which includes constant resistance, same charge and

discharge characteristics, no effect on the capacity at different current, no temperature effect,

no self-discharge effect and no hysteresis. The reason for neglecting hysteresis effect is that the

charge and discharge characteristic for any generic battery is similar Figure 3.55. Fully charged

voltage at 100% as indicated in Figure 3.55 is higher than nominal or rated voltage. On the

hand, voltage after end of nominal zone is lower than rated voltage. In real life, it is common to

run battery between 40% and 80% of state of charge to extend battery lifespan and to obtain

more stable voltage output.

47

Figure 3.55: Charge and discharge characteristic [22]

Before modeling the PSCAD battery model, one must understand the fundamental and the

important terminology of batteries:

1. Rated Capacity (ah): The ampere-hour when a battery is fully charged.

2. Nominal Voltage (V): Voltage that is under normal operating conditions.

3. State of Charge (SOC): SOC is in percentage and represents the instantaneous battery

capacity over the total rated capacity.

4. Charge Rate (C rate): The amount of current that a battery can deliver for 1 hour A

1000mah or 1ah battery that is rated at 1C means that it can be fully discharged in one

hour ideally.

5. Internal Resistance: The Thevenin resistance in the battery Figure 3.55. The internal

resistance varies based on manufacture’s datasheet.

48

The equivalent circuit of a general battery is shown below in Figure 3.56,

Figure 3.56: Equivalent circuit of the battery [22]

The battery voltage equation can be written as:

𝐸 = 𝐸𝑜 − 𝐾 (𝑄

𝑄−𝑖𝑡) + 𝐴𝑒−𝐵𝑖𝑡 (14)

In the equation above:

E No-load voltage (v)

Eo Battery constant voltage (V)

K Polarization voltage (V)

Q Battery capacity (Ah)

∫ 𝑖. 𝑑𝑡 Actual battery charge (Ah)

A Exponential zone amplitude (V)

B Exponential zone time constant inverse 1/(Ah)

Vbat Battery Voltage (V)

Rbat Internal Resistance (Ω)

Ibat Battery current (A)

49

The battery voltage equation can be modified as following in terms of State of Charge

(SOC):𝐸 = 𝐸𝑜 − 𝐾 (1

𝑺𝑶𝑪) + 𝐴𝑒−𝐵𝑄(1−𝑺𝑶𝑪) (15)

Therefore, SOC will affect the battery voltage exponentially and non-linearly.

Battery model in PSCAD:

In order to form the voltage equation of battery, the model parameters can be obtained using

Table 3.3.

Table 3.3: Battery parameters

The parameters appeared in the equation above can be changed in PSCAD accordingly as

shown in Figure 3.57 and Figure 3.58.

Figure 3.57: PSCAD battery model menu 1 Figure 3.58: PSCAD battery model menu 2

50

The battery component in PSCAD is shown in Figure 3.59:

Figure 3.59: Battery component in PSCAD

Improvement to wind generation output:

The battery model is integrated to wind generation’s module in order to smooth out the

wind generation power output as wind speed changes. The battery in red box in Figure 3.60 is

integrated in between Grid Side Converter (GSC) and Rotor Side Converter (RSC). GSC converts

Alternating Current (AC) voltage from the grid to Direct Current (DC) voltage to the rotor side.

Because electrochemical battery produces DC signal, it is a common practice to integrate

battery energy storage system in between GSC and RSC.

Figure 3.60: Wind generation with battery integration

In the Double-Fed Induction motor (DFIG) wind generator model, wind is speed is

varying over time and is affecting the output power shown in Figure 3.61. The simulation shows

that output active power fluctuate between 0.30 MW up to 0.7 MW. The fluctuation is 0.5 MW

GSC RSC

51

Figure 3.61: Wind output power without battery support

However, simulations show a significant improvement and the output power is much

smoother after battery integration in Figure 3.62.The output power in Figure 3.62 varies

between 0.5MW and 0.65MW. The fluctuation is improved and reduced from 0.5MW to

0.15MW.

Figure 3.62 Wind output power with battery support

52

Improvement to solar generation output:

Figure 3

.63

: Solar gen

eratio

n w

ith b

attery Integratio

n

53

In Figure 3.63, battery is integrated to solar generation’s module to smooth out the

voltage output profile whenever sun radiance becomes low. The model is placed and indicated

by a red box in the circuit in Figure 3.63. This circuit has designated output Root Mean Squire

(RMS) voltage to be 11kV.

To demonstrate the effect of sun radiance on the output RMS voltage, sun radiance is

manually reduced during simulation from 622.22 (Watts/meter^2) to zero without battery

integration. Indicated in red box in Figure 3.64, the RMS voltage drops below rated 11kV when

sun radiance becomes 0 (W/M^2) at around 1.4 seconds.

Figure 3.64: Solar Generation without battery integration

54

In order to mitigate and smooth out the output RMS voltage profile of solar generation,

simulation is repeated counting the effect of battery integration in Figure 3.65. At around 2

seconds where sun radiance becomes zero, the output RMS voltage remains close to rated

11kV as indicated. It is proven that battery integration helps smoothing the output profile of

solar generation.

Figure 3.65: Solar generation with battery integration

55

Custom energy storage library

In this quarter, two battery models were put finalized and modulated into a custom

component library called battery energy storage library shown in Figure 3.66.

Figure 3.66: Battery Energy Storage

Upon expanding this module, the user will find two versions of battery model shown in

Figure 3.66. Battery version_1 is one chosen to be integrated with solar and wind generations

since it is a newer version and has better control regarding state of charge. On the other hand,

battery version_2 has charge protection and charge/discharge control as shown in Figure 3.67.

User can choose whichever version based on their applications.

56

Figure 3.67: Charge protection and charge control of battery version_2

57

4. PROJECT PLANNING

Overall, the student team was able to work effectively and efficiently to complete their

custom component library which included the solar, wind, and battery storage elements.

Represented below in Figure 4.1, is the organization chart that indicates who was responsible for

what.

Figure 4.1: Team Organization Chart

From Figure 4.1, each member was in charge of a specific task, but overall each member

helped one another to complete a task. This last quarter was more focused in synchronous work

due to the fact that every component had to be placed within a custom component library that

contained the wind, solar, and battery elements.

58

YE

AR

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UN

D S

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Figu

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. Year ro

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59

5. CONCLUSION

PSCAD is a powerful tool that can enable its user to analyze their system’s transient

behavior. With PSCAD, the student team was able to understand the transient behavior from

renewable energy and was able to develop custom component libraries which contained these

elements. These elements included solar generation, wind generation, and battery storage

elements. With the custom component libraries that has been developed, new users who decides

to model a renewable energy system, can just recall the libraries created by the student team to

help them develop their design. The student team has already validated their results by using the

IEEE Bus system to validate their results and the need for integration of a battery storage element

to smooth out the power fluctuations generated by renewable sources. If future work is involved

next quarter, the analysis and implementation of a more efficient control system should be

analyzed.

60

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[19]. Tweed, Katherine. "SCE, PG&E Issue First Energy Storage Requests to Comply With AB 2514." Katherine, n.d. Web. 08 Mar. 2015. <http://www.greentechmedia.com/articles/read/sce-pge-issue-first-energy-storage-requests-to-meet-ab-2514>.

[20]. "News Releases." SCE Unveils Largest Battery Energy Storage Project in North America. David Song, 24 Sept. 2014. Web. 08 Mar. 2015. <http://newsroom.edison.com/releases/sce-unveils-largest-battery-energy-storage-project-in-

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north-america>.

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