A Case Study on the Validation of Renewable Energy Grid Code Compliance for a Large-Scale Wind Power Plant Grid-Connected Mode of Operation in Real-Time Simulation

Abstract

In this study, the grid-connected mode of operation was evaluated, considering the renewable energy grid codes. First, the renewable energy grid code specifications have been revisited, focusing mainly on large-scale renewable power plants. An interconnected system has been developed by combining the traditional electrical power grid with a large-scale wind power plant. The modeling method used in this study and how the evaluation has been conducted can be used for other power system evaluation studies. The results obtained from the interconnected system show a significant improvement in substation (or busbar) voltages.

1. Introduction

Wind power plants (WPPs) have continuously evolved globally and have become a significant participant in enhancing grid operation in most countries, showcasing the immense potential of renewable energy sources [1,2,3,4,5,6,7,8]. WPPs can be operated in one of two modes: standalone or grid-connected. In grid-connected mode, they provide extra power when the demand for loads in the power grid increases. In standalone mode, they act as an emergency supply for individual loads during power system outages [9,10].

WPPs are classified as small-scale or large-scale, depending on the size of the wind turbine generator units (WTGUs) they comprise. The size of these WTGUs is based on the rotor diameter (in meters), swept area (in square meters), and the standard power (in kilowatts) produced by the WTGU.

Table 1. Horizontal-axis wind turbine generator unit classification based on rotor diameter, swept area, and power rating.

		Rotor Diameter (m)	Swept Area (Square Meters)	Standard Power Rating (Kilowatts)
Small-scale	Micro	0.5–1.25	0.2–1.2	0.004–0.25
	Mini	1.25–3	1.2–7.1	0.25–1.4
	Household	3–10	7–79	1.4–16
Small commercial		10–20	79–314	25–100
Medium commercial		20–50	314–1963	100–1000
Large commercial		50–100	1963–7854	1000–3000

shows the lists of different sizes of WTGUs [11].

Large-scale wind power plants (LSWPPs) comprise large commercial wind turbine generator units (LCWTGUs).

On the other hand, WTGUs are classified into two by the axis at which wind turbine blades (WTs) rotate. They are horizontal-axis wind turbine generator units (HAWTGUs) and vertical-axis wind turbine generator units (VAWTGUs). HAWTGUs are the most popular class and are either upwind or downwind. Figure 1 and Figure 2 show the upwind and downwind HAWTGUs [12]. Upwind HAWTGUs are the most popular class and are applied mostly in large-scale wind power plants (LSWPPs).

The operation of LSWPPs in a grid-connected mode has been leading over the past ten years [13,14,15]. Many studies have been conducted regarding LSWPPs in grid-connected modes of operation. However, few studies have examined the renewable energy grid code (REGC) specifications while integrating these LSWPPs into the power grid. Among the few, none have evaluated various operating conditions of the power grid into which the LSWPP is (or is to be) integrated.

For instance, a study in [16] conducted an assessment of the grid-connected mode of operation for an offshore LSWPP. The study compared the cost margin when the power transmission from an offshore LSWPP is alternating current (AC) or direct current (DC). The study aimed to prove little significance in choosing DC over AC when transmitting power from an offshore LSWPP to the power grid. However, it did not consider ensuring the LSWPP meets the grid code specifications before integration. This oversight underscores the crucial role of REGC in the operation of LSWPPs.

A study of integrating a LSWPP into a weak distribution network was conducted in [17]. A 6 MW single WTGU that was sufficiently sized to approximate the acceptable megawatt capacity of a WPP was used in the study. Additionally modeled in this study was a 9 MW single WTGU for grid integration using the static synchronous compensator (STATCOM) [18]. The impact integration of wind power plants (WPPs) and wind turbine generator units (WTGUs) has on power system stability has been studied in the references [19,20,21]. None of these models were initially performed independently for grid compliance tests as required by the REGCs.

Considering the models used in [17,18,19,20,21], it is understood that the computation of each component consumes time. However, these models may not provide adequate validation to align the REGCs. This violates the design and operational requirements of WPPs mentioned in [22,23,24].

An interconnected system was developed by integrating a LSWPP with the IEEE 9 Bus Power System in [25]. The study mainly focused on developing the point of common coupling (PCC) protection system that adapts to the conditions of the interconnected system and responds to the faulty conditions for both normal and increased loading conditions.

The work published in references [1,3,4,14,26] emphasized the significance of REGCs for LSWPPs in the grid-connected mode of operation.

The significant gap in all the studies reviewed above is that none have studied the LSWPP alone to ensure its operation meets the REGC specifications before integrating it into the considered power grid. This work stands out in the field due to its unique approach of combining REGC discussions with case studies. This approach provides a better understanding of the implications of integrating large-scale WPPs into the power grid. The significance of this work lies in its focus on the REGCs requirements, a crucial yet underexplored subject in the realm of renewable energy. Our revisit to these requirements, with the evaluation performed using a case study, promises to shed new light on their role in integrating LSWPPs, underscoring their importance in the field.

Following the introduction of this article, Section 2 revisits REGCs with an emphasis on WPPs. The overview of the proposed test system is in Section 3. This includes the modeling and simulation consisting of a few case studies of the power grid into which the LSWPP is integrated and the LSWPP itself. In contrast, the integration of the LSWPP into the power grid is dealt with in Section 4. Section 5 is dedicated to the crucial evaluation of the operation of the interconnected system, which also formulates an essential aspect of our research. Section 6 discusses the results. Finally, conclusions and future work are drawn and stated in Section 7.

2. Overview of Renewable Energy Grid Code Specifications

The REGCs are a set of technical standards and requirements to be followed when connecting renewable energy sources to the power grid [10,23,24].

Since the voltage level at which the RPPs are integrated into the power grid is significant, we first recap the standard voltage levels. We will then categorize the RPPs at a later stage. It should be noted that standard voltage levels differ depending on the country. For example, one of the Asian articles that categorized the voltage levels at which various WPP categories are integrated also provided some information on the Saudi Arabian standard voltages. This information is presented in

Table 2. Saudi Arabian standard grid voltages at which various types of wind power plants are integrated.

Category		Wind Power Plant Group	Voltage (kV)
Transmission	Normal transmission	Offshore: Group 1 or Group 2	400
			220
			132
	Sub-transmission	Onshore	66
Distribution		Small wind turbines	33
			11

[1].

However, this article defines voltage levels based on the South African power grid. The National Energy Regulator of South Africa (NERSA) [24] and ESKOM [27] categorizes the grid standard voltage as shown in

Table 3. South African standard grid voltages.

Category		Voltage (kV)	Level
Transmission	Normal transmission	765	Extra-high
		400
		275
	Sub-transmission	132	High
Distribution		66
		50
		33	Medium
		25
		22
		11

.

RPPs are categorized into three, based on their minimum and maximum apparent power production and the voltage level at which they can be integrated into the power grid. The first category is A (with subcategories A1, A2, and A3), followed by B and C.

Table 4. South African renewable power plant categories.

Category		Minimum Capacity (kVA)	Maximum Capacity (kVA)	Integration Level Voltage
A	A1	0	13.8000	LV
	A2	13.8000	100	LV
	A3	100	1000	LV
B		0	20,000	MV
C		>20,000	-	MV/HV

lists the RPP categories and specifies the voltage levels at which the RPP category can be integrated [23].

WPPs made of squirrel-cage induction generators (SCIGs) may have reactive power support in the power grid since their stator terminals are coupled with capacitor bank exciting circuits. However, this reactive power is minimal [1].

The work presented in this article mainly focuses on WPPs, whilst the discussions regarding renewable energy grid codes (REGCs) are based entirely on RPPS. Therefore, in this article, for discussions on REGCs, the term RPP instead of WPP is used.

2.1. Tolerance of Frequency and Voltage Deviations

2.1.1. Normal Operating Conditions

RPPs must be designed to operate continuously within the point of common coupling (PCC) voltage range specified in

Table 5. Minimum and maximum operating voltages at the point of common coupling.

Nominal Voltage (${\mathbf{V}}_{\mathbf{N}}$) in Kilovolt	Minimum ${\mathbf{V}}_{\mathbf{N}}$ (PU)	Maximum ${\mathbf{V}}_{\mathbf{N}}$ (PU)
132.0000	0.9000	1.0985
88.0000	0.9000	1.0985
66.0000	0.9000	1.0985
44.0000	0.9000	1.0800
33.0000	0.9000	1.0800
22.0000	0.9000	1.0800
11.0000	0.9000	1.0800

[24,28], measured at the PCC.

RPPs must be designed to operate within the minimum range illustrated in Figure 3 during a system frequency disturbance. It must be disconnected from the power grid should it experience a frequency higher than 51.5000 Hz for longer than 4 s or a frequency lower than 47.0000 Hz for longer than 200 ms. Also, the RPPs must remain connected to the power grid when they experience a rate of frequency change with values of 1.5000 Hz per second as long its minimum frequency is still within the minimum and maximum frequencies of 49.0000 HZ and 51.0000 Hz [24,28].

RPPs must only be connected to the power grid as early as 3 s after the following conditions have been met [24,28]:

• For transmission level-connected RPPs, the voltage at the PCC is within +/−5% of the nominal voltage.
• For distribution level-connected RPPs, the PCC voltage is within the minimum and maximum ${\mathrm{V}}_{\mathrm{N}}$ specified in Table 5.
• Frequency in the RPP is within 49.0000 Hz and 50.2000 Hz or otherwise stated as agreed with the system operator (SO).

2.1.2. Abnormal Operating Conditions

RPPs must be built to tolerate abrupt phase changes of 20 degrees at the PCC without disconnecting or lowering its output. They must return to normal production within 5 s of the operating parameters of the PCC returning to normal following the settling time. They must also be designed to withstand and fulfill the PCC voltage conditions described in Table 5 and Figure 4. Area D in the figure is mainly for Category C RPPs. Furthermore, RPPs must withstand PCC voltage drops for a minimum duration of 0.150 s without disconnecting, as shown in Figure 4 [24,28].

RPPs must be designed to withstand voltage droops and peaks, as illustrated in Figure 5 [24,28]. As the figure illustrates, it must supply or absorb reactive current without disconnecting.

2.2. Frequency Response

RPPs must be built to offer a power–frequency response to stabilize the power grid frequency in the event of frequency deviations. RPPs must be able to meet the required active power reduction needs to stabilize the frequency during high-frequency operating situations (see Figure 6) [24,28].

The accuracy of grid frequency measuring must be at least +/−10 mHz. When the power grid frequency increases above 50.5000 Hz, RPPs must reduce the active power proportionate to the frequency change, as illustrated in Figure 6. RPPs must be disconnected if the frequency exceeds 51.5000 Hz for more than four seconds to protect the electrical grid [24,28].

2.3. Reactive Power Capabilities for Category C

RPPs must be designed to operate in one of three control modes: power factor, voltage, or reactive power (Q or Mvar). It is necessary to agree on the control operating mode (Q, power factor, or V control) and the network service provider (NSP). When operating between 5% and 100% of rated power Pn (MW), the RPPs must be able to adjust reactive power (Mvar) support at the PCC within the reactive power capacity ranges depicted in Figure 7, where Qmin and Qmax are voltage-dependent, as seen in Figure 8 [24,28].

Figure 9 shows the necessary RPP reactive power capability (determined at the PCC) at nominal voltage. No reactive power capability is needed when operating at less than 5% of the rated power Pn (MW). As Figure 9 illustrates, the RPP can only function within the reactive power tolerance range not over +/−5% of rated power in Areas A, B, C, and D [24,28].

2.4. Voltage and Reactive Power Control Functions

Reactive power control functions that can regulate the reactive power that the RPP supplies at the PCC and a voltage control function that can regulate the voltage at the PCC through commands utilizing setpoints and gradients are required for RPPs. Reactive power and voltage control are mutually incompatible, so the three functions, voltage control, power factor control, and reactive power control, can only be engaged simultaneously [24,28]. Moreover, the RPP generator will be employed to carry out the control function and apply parameter settings for reactive power and voltage control functions, which the NSP will decide in coordination with the SO. The operational agreement must include documentation of the agreed-upon control functions [24,28].

• Reactive power control

Regardless of the active power and voltage, Q control regulates the reactive power supply and absorption at the PCC. Figure 10 shows this control function as a vertical line. The RPP generator must respond to any changes made to the Q control setpoint by the NSP, SO, or their agent within two seconds by updating its echo analog setpoint value. When an instruction to modify the setpoint is received, the RPP must react to the new setpoint within 30 s. The setpoint and the accuracy of the control must not differ by more than ±2% of the setpoint value or by ±0.5% of the maximum reactive power, whichever produces the highest tolerance. The RPP must receive a Q setpoint with a minimum precision of 1 kVAr [24,28].

• Power factor control

The power factor control function regulates reactive power at the PCC proportionately to active power. An example is a line with a constant gradient in Figure 10. If the NSP, SO, or their agent modifies the power factor setpoint, the RPP has to update its echo analog setpoint value within 2 s. When a command to change the setpoint is received, the RPP has 30 s to respond with the updated setpoint. The maximum difference between the control accuracy and the setpoint is ±0.02 [24,28].

• Voltage control

One feature that regulates the voltage at the PCC is voltage control. If a modification is to be made to the voltage setpoint, it must start within 2 s and be finished within 30 s after receiving the command. Droop characteristics, as seen in Figure 11, require that the accuracy of the voltage setpoint be within ±0.5% of the nominal voltage and that the precision of the control be within ±2% of the required injection or absorption of reactive power [24,28].

With the droop setup as in Figure 11, each RPP can regulate within its dynamic range and voltage limit. Droop is defined here as the change in voltage (per unit) brought on by a change in reactive power (per unit). The control function will wait for potential overall control from the tap changer or other voltage control functions once the voltage control has reached the dynamic design limits of the RPP. Together with the SO, the NSP will be in charge of total voltage coordination [24,28].

2.5. Comparison with International Grid Codes

This study focused primarily on the renewable energy grid code (REGC) specifications of South Africa (SA), as regulated by the National Energy Regulator of South Africa (NERSA) and Electrical Supply Commission (ESKOM). Recognizing that REGCs for renewable power plant (RPP) integration vary across different electrical power systems worldwide is significant. Nonetheless, most international REGCs share fundamental operational requirements concerning continuous operating voltage and frequency ranges, active power control capabilities, reactive power support and control functions, fault ride-through (FRT) capabilities, frequency response, and stability requirements. For instance, the German BDEW Medium Voltage Directive, the UK National Grid Code, and the IEEE 1547 Standard for Interconnection of Distributed Energy Resources in the USA set comparable technical requirements for voltage and frequency tolerance, reactive power capability, and grid fault behavior [29,30,31].

However, differences typically arise in specific voltage ride-through curves, the permissible ranges of power factor adjustment, the exact frequency thresholds and response times, and the operational control modes, such as voltage versus power factor versus reactive power control priority.

The LSWPP model developed in this study was validated against REGCs of SA, which serve as a representative framework for Category C LSWPP. Given the similarity in core requirements across most international standards, the model can be adapted for other regions by adjusting the reactive power control ranges and setpoints, modifying the acceptable frequency and voltage operation limits, tuning the FRT capability curves and thresholds according to the REGCs of the target region.

This adaptability is possible because the model was built using flexible, parameterized control components within the Real-time Simulation Computer-Aided Design (RSCAD) simulation environment. Therefore, while the present study validated compliance based on SA standards, the methodology and simulation framework remain applicable for evaluating LSWPPs against other regional grid codes with minor parameter adjustments.

3. Overview and Development of the Proposed Test System Considered for the Study

This study evaluated the grid-connected mode of operation of the large-scale wind power plant (LSWPP). Operating the LSWPP in a grid-connected mode forms the modern power grid. On the other hand, a modern power grid combines any renewable power plant (RPP) and the traditional electrical power grid.

This section describes the traditional electrical power grid and the LSWPP considered in this study.

3.1. The Power Grid System Considered for the Study

The IEEE 9 Bus Power System is used in this study due to its simplicity, well-documented data availability, and established use in renewable integration studies. There are other publications in which the system has been used. For instance, in reference [19], it was used for another study dealing with renewable power plant (RPP) integration. The IEEE 9 Bus Power System allows the performance of power grid case studies that significantly complement the need for a large-scale wind power plant (LSWPP) integration.

3.1.1. Preparation of the IEEE 9 Bus Power System

The model of the IEEE 9 Bus Power System is available in the Real-time Simulator Computer-Aided Design (RSCAD) software, version FX 1.3 instruction manual from RTDS Technologies Inc., referenced in [32]. Below is a significant recap of how the components were modified from the original IEEE 9 Bus Power System in the RSCAD software instruction manual.

The components were rearranged and renamed, as shown in

Table 6. Renaming of components for the modified IEEE 9 Bus Power System.

Original Name		Modified Names
Busbar	Component Connected to It	Busbar	Component Connected to It
1	G1	G1Bus	Gen1
2	G2	G2Bus	Gen2
3	G3	G3Bus	Gen3
4	None	Bus1	None
5	Unnamed load 1	Bus2	DLoad1
6	Unnamed load 2	Bus3	DLoad2
7	None	Bus4	None
8	Unnamed load 3	Bus5	DLoad3
9	None	Bus6	None

. The modified system is shown in Figure 12.

Area 1 and Area 2 in Figure 12 represent subsystems on RSCAD. These real-time simulations for an electrical system of two subsystems are performed in two RTDS hardware racks, Rack 1 and Rack 2. In the rest of this article, the two terms subsystems and racks are used interchangeably because they have the same meaning.

The transmission line parameters of the IEEE 9 Bus Power System have been listed in

Table 7. IEEE 9 Bus Power System transmission line parameters.

Lines	Busbar to Which Line Is Connected	Length (km)	Resistance (PU)	Reactance (PU)	Susceptance (PU)
Line12	Bus1–Bus2	89.9300	0.0100	0.0850	0.1760
Line13	Bus1–Bus3	97.3360	0.0170	0.0920	0.1580
Line24	Bus2–Bus4	170.3380	0.0320	0.1610	0.3060
Line36	Bus3–Bus6	179.8600	0.0390	0.1700	0.3580
Line45	Bus4–Bus5	76.1760	0.0085	0.0720	0.1490
Line56	Bus5–Bus6	106.6460	0.0119	0.1008	0.2090

. All per unit values have been derived from a base apparent power of 100 MVA [32].

RSCAD consists of two modules, namely the draft and runtime modules. The draft module deals with the modeling of the system itself, while the runtime module deals with the real-time simulations of the modeled system. The modeling on the draft module deals with gathering components that make up a power system model. This includes control components, such as control switches, sliders, pushbuttons, and other operating systems.

After the completion of the model, the draft load flow simulation was executed to verify data accuracy and the initial conditions of the modeled system. Results were obtained and recorded in

Table 8. Initial conditions of the IEEE 9 Bus Power System.

Bus	Bus Type	Voltage (PU)	${\mathbf{P}}_{\mathbf{G}}$ (MW)	${\mathbf{Q}}_{\mathbf{G}}$ (MVAr)	${\mathbf{P}}_{\mathbf{L}}$ (MW)	${\mathbf{Q}}_{\mathbf{L}}$ (MVAr)
G1Bus	Slack	1.0400∠0.0000°	71.7798	36.2675	-	-
G2Bus	PV	1.0250∠8.3639°	163.0000	11.2300	-	-
G3Bus	PV	1.0250∠4.0221°	85.0000	−3.7204	-	-
Bus1	PQ	1.0207∠27.7678°	-	-	-	-
Bus2	PQ	0.9928∠26.3247°	-	-	125.0000	50.0000
Bus3	PQ	1.0064∠26.5655°	-	-	90.0000	30.0000
Bus4	PQ	1.0230∠32.7884°	-	-	-	-
Bus5	PQ	1.0128∠30.2929°	-	-	100.0000	35.0000
Bus6	PQ	1.0283∠31.3133°	-	-	-	-

. These results are the same as those found in [32].

It has been stated in the references [33,34,35], that the reactive power transfer through substations (busbars) causes voltage stability challenges in the power grid. In this study, the assumption has been made that the increase in load demand means an increase in active and reactive power. We, therefore, configured the load increase simulation logic, as shown in Figure 13.

PDLoad1Set and QDLoad1Set in the figure are signal names to control the three loads, DLoad1, DLoad2, and DLoad3. DLoadSchedStart is a digital signal from a binary switch that controls the status (ON/OFF) of the scheduler component. The “scheduler” component multiplies the starting power levels by the specified multiplier to increase the load demand. The active power input signal of the dynamic loads is multiplied by the “tan (deg)” component. Reactive power to the load is shown by the output signal QDload1Set, which is based on the simultaneous rise in variables P and Q to maintain a constant load power factor. The power demand in the runtime module simulation platform will remain in its initial state until the switch DLoadSchedStartSW is turned on. The scheduler for each bulk load DLoad1, DLoad2, and DLoad3 is set to increase the load demand by 5.0000 MW every 0.1 s. An example is DLoad1, which has an initial active and reactive power of 125.0000 MW and 50.0000 MVAr. After 0.1 s, the load starts to draw 130.0000 MW and 51.9986 MVAr. And after 0.2 s, it starts to draw 135.0000 MW and 53.9986 MVAr.

To summarize the operation of the logic in Figure 13, the Expressions (1) to (3) are used.

Incremental increase in active power and reactive power:

$$P_{New}=P_{Old}+\Delta P$$

(1)

$\Delta P=5 \mathrm{M}\mathrm{W}$ every 0.1 s.

Reactive power is proportional to active power:

$$Q=P \times \tan (\mathrm{\varnothing })$$

(2)

The power factor $\cos (\mathrm{\varnothing })$ is calculated from the initial conditions. For example, for DLoad1 $P_0=125 \mathrm{M}\mathrm{W}$, $Q_0=50 \mathrm{M}\mathrm{V}\mathrm{A}\mathrm{r}$, then

$$\tan (\mathrm{\varnothing })={\displaystyle \frac{Q_0}{P_0}}=0.4$$

So,

$$Q_{New}=Q_{Old}+0.4$$

(3)

In real-world scenarios, this is when the loads connected to the electrical power grid increase. These simulations of load increase every 0.1 s. This simulation looks into what might happen regarding electrical power grid loading in the future.

3.1.2. IEEE 9 Bus Power System Simulations

• Initial load demand simulations for the power grid

An initial load demand was simulated for the power grid.

Table 9. Active power on load busbars and their loads under initial load demand for the power grid.

	Load Active Power (MW)	Busbar Active Power (MW)
DLoad1|Bus2	125.0000	125.0000
DLoad2|Bus3	90.0000	90.0000
DLoad3|Bua5	100.0000	100.0000

and

Table 10. Reactive power on load busbars and their loads under initial load demand for the power grid.

	Load Reactive Power (MVAr)	Busbar Reactive Power (MVAr)
DLoad1|Bus2	50.0000	50.0000
DLoad2|Bus3	30.0000	30.0000
DLoad3|Bua5	35.0000	35.0000

show the active and reactive power of the loads and busbars under initial load conditions, while

Table 11. Busbar voltages under initial load demand for the power grid.

Busbars	Voltage (PU)
Bus1	1.0210
Bus2	0.9928
Bus3	1.0060
Bus4	1.0230
Bus5	1.0130
Bus6	1.0280

shows the voltages on the busbars Bus1, Bus2, Bus3, Bus4, Bus5, and Bus6.

We used tables to present these results because the time parameter is insignificant since no disturbances were simulated.

• Increased load demand simulations for the power grid

An increase in load demand was simulated to identify the voltage stability collapse point, which might be any voltage outside the range of 0.9500 and 1.0500 per unit based on power system grid codes. The voltages were observed in this simulation for busbars Bus1 to Bus6 in response to the load demand increase in different loads DLoad1, DLoad2, and DLoad3. It was identified that the voltage at Bus2, where DLoad1 is connected, could not recover and, therefore, is critical. This is because it settled and fell to 0.9486 per unit, out of the range of 0.9500 and 1.0500 per unit. This behavior is when the load demand of DLoad1 was raised by 5.0000 MW and 1.9999 MVAr every 0.1 s, which in total ended up in the value of 160.0000 MW and 65.0535 MVAr.

Figure 14, Figure 15, Figure 16, Figure 17, Figure 18 and Figure 19 plot the active and reactive power demands of the loads DLoad1, DLoad2, and DLoad3 and the corresponding power at specific buses Bus2, Bus3, and Bus5. The voltage plots due to the active and reactive power demands of these loads are shown in Figure 20, Figure 21, Figure 22, Figure 23, Figure 24 and Figure 25.

In the simulation of the incremental load demand simulation performed above, we assessed the voltage stability thresholds and the compensation needed. This is how we studied the need to integrate the large-scale wind power plant (LSWPP) described below.

3.2. The Large-Scale Wind Power Plant Considered for the Study

The LSWPP used in this study is adapted from our previous study by the reference [36]. It consists of eighteen Type I upwind horizontal-axis wind turbine generator units (HAWTGUs) modeled from a combination of V117-4.2 MW and 4.0000 MW squirrel-cage induction generators (SCIGs) from Vestas’ brochure and Appendix B Generator Parameters, respectively [37,38]. In total, the LSWPP has a capacity of 72.0000 MW. This capacity complements Category C WPP, which this document already refers to as the LSWPP.

Significantly, only simulation-related control components and other operating systems are described in this section so that the simulation of the system itself can be easily understood.

3.2.1. Preparation of the Large-Scale Wind Power Plant

The simulation of the LSWPP requires fundamental tuning of wind speed (m/s) and pitch angle adjustment (degrees). See Figure 26 for logic adapted from the same reference [36]. The control components shown in the figure are logically connected to the wind turbine (WT) model and are accessible for control on the RSCAD runtime module. In the same figure, WT1PitchAngle is the signal name to control the change in the pitch angle of the wind turbine, while WindSpd1(km/h) is the input wind speed to the wind turbine. The pitch angle is adjusted using a dial selector switch WT1PitchAngleAdjust with figures or positions 1 to 5. The wind speed is adjusted with a slider labeled WindSpd1Adjust. This logic is the same for all other WTs in the LSWPP model.

Other parameters like Default Air Temperature (DegC), Default Barometric Pressure (millibar), and Default Relative Humidity (%) do not need any additional control components to connect to them logically. Figure 27 shows the setting of these parameters on the WT model as viewed on the RSCAD draft module. By default, these parameters are accessible and can be tuned on the runtime module. Reference [36] can be visited for more information on how the LSWPP was modeled.

3.2.2. Large-Scale Wind Power Plant Simulations

The wind turbines (WTs) in the LSWPP model used in this study have operating temperatures between −20 °C and 45 °C. Nevertheless, WTs can be adjusted to generate the necessary output power almost equivalent to the rated output power as long as the temperature stays within the working range. Therefore, a temperature of 25 °C has been selected and used in this study.

The power coefficient, $C_P$, is also significantly influenced by air pressure and humidity. Humidity and air pressure were set to 30% and 1415 mbar, respectively, for the simulations. The sliders, dial selectors, and switches for these and other parameters in RSCAD runtime are shown in Figure 28.

Simulations were performed for the LSWPP; the results are presented in

Table 12. Mechanical data for wind turbine generator units is monitored at steady-state power flow.

Quantities	Values
Wind speed	14.0000 m/s
Wind power	23.3100 MW
Wind turbine $C_p$	0.1720
Wind turbine power	4.0040 MW
Wind turbine rotor speed	9.9000 rpm
Wind turbine torque	3.8580 MN.m

,

Table 13. Wind turbine generator unit electrical data monitored at steady-state power flow.

Quantities	Values
WTGSU primary current	0.0160 kA
WTGSU secondary current	0.0010 kA
WTGU electrical torque	0.0100 PU
WTGU terminal voltage	1.0800 PU
WTGU exciter reactive power	2.1230 MVAr
WTGU active power (P) output	0.0240 MW
WTGU reactive power (Q) output	−2.0090 MVAr
WTGU reactive power circuit current	0.1600 kA

and

Table 14. Wind power plant sending-end data monitored at steady-state power flow.

Quantities	Values
	Actual	Per Unit
Group 1 collector terminal voltage	26.0200 kV	1.0801 PU
Group 2 collector terminal voltage	26.0200 kV	1.0801 PU
Group 3 collector terminal voltage	26.0200 kV	1.0801 PU
WPP sending-end bus voltage	249.9000 kV	1.0801 PU
WPP receiving-end bus voltage	249.9000 kV	1.0801 PU
WPP sending-end frequency	50.0000 Hz	-
WPP receiving-end frequency	50.0000 Hz	-

.

3.2.3. Verifications of Renewable Energy Grid Code Specifications

The renewable energy grid codes (REGCs) specify the continuous operating per unit voltage level for Category C renewable power plants as 0.9000 (minimum) to 1.0985 (maximum). As shown in Table 14, under normal load flow conditions, the voltage measured on the high-voltage busbar of the modeled large-scale wind power plant (LSWPP) is at 1.0860 per unit, and is therefore acceptable. It is also stated that the RPP can run continuously at the frequency range between 49.0000 Hz and 51.0000 Hz. The frequency of the modeled LSWPP does not violate the continuous operating range specified by the grid codes, as it is at 50.0000 Hz under normal operating conditions. This means that the modeling and simulation (test) of the LSWPP model in this study were successful. This has also been attested in our previous study in reference [36].

4. Large-Scale Wind Power Plant Integration

Wind integration is the collection of all activities related to connecting the WPPs to the power grid. There are three high-level activities of grid integration, as listed in

Table 15. High-level activities of grid integration.

Planning	Physical Connection	System Operations
Power flow, short-circuit, and system stability studies	Build transmission from WPP to substation	Unit commitment
System operations study	Connection at substation	Economic dispatch
Wind power interconnection code	-	Wind energy forecasting

[10].

This study focused on planning, as it uses the simulation-based technique in proposing and (or) setting up a significant method of evaluating the operation of an interconnected system consisting of a LSWPP. Among the activities listed under planning, short-circuit studies are out of the current scope of this study.

The traditional electrical power grid and the LSWPP were studied individually in the previous sections. The LSWPP used has already been modeled with its transmission line for electrical power transmission to the busbar of selection (PCC) on the traditional power grid. Figure 29 illustrates the modern power grid formulated from the two systems studied above: the IEEE 9 Bus Power System and the LSWPP. The components WTGU1 to WTGU10 in the figure representing the first to the eighteenth wind turbine generator units. The LSWPP model and its components were modeled in subsystems (or racks) 3, 4, and 5. It was also shown in Section 3 that the IEEE 9 Bus Power System alone occupied subsystems 1 and 2.

The WTGUs are a combination of wind turbines (WTs), squirrel-cage induction generators (SCIGs), and wind turbine generator substation unit transformers (WTGSUTs). It should be noted that the LSWPP used in this study consists of SCIGs whose output terminal voltage is 4 kV. These WTGSUTs step up to a medium voltage (MV) of 24 kV. This is to say that the voltage generated by each WTGU is considered MV and is distributed through the LSWPP MV circuit. This is the voltage where the high-voltage (HV) step-up power transformers (WPPHVTRFs) step up for transmission over the lines LineGroup1, LineGroup2, and LineGroup3, and then over the transmission line, WPPTransLine, to the grid point of connection (PCC). PCCTerm in the figure is the point of common coupling (PCC) terminal. It connects the LSWPP to the electrical power grid through Bus2, which was discussed in the early sections.

The parameters of the modern power grid (interconnected system) are listed in Appendix B of this article.

After integration, there is an additional transmission line circuit interconnecting the LSWPP, which, according to the renewable energy grid codes (REGCs), depending on the agreement between the power system operator (PSO) and the WPP operator (WPPO), this transmission line can either be operated by the PSO or the WPPO. At this point, attention was only given to monitoring from the sending-end busbar (WFSEBus) and receiving-end busbar (WFSEBus) up to the traditional electrical power grid side.

5. Evaluation of the Grid-Connected Mode of Operation of a Large-Scale Wind Power Plant

Case studies were executed to determine if the electrical quantities in the interconnected system were according to the renewable energy grid codes (REGCs). The case studies first looked at the initial load simulations to check if the busbar voltages were within the acceptable values. After that, the increased load demand was simulated to check if the voltages at the busbars had improved and were settling at acceptable values based on the power grid operating standards.

5.1. Initial Load Demand Simulations for the Modern Power Grid System

In this simulation, we put the load demand at its initial stage and monitored the load and busbar active and reactive power, as tabulated in

Table 16. Active power on load busbars and their loads under initial load demand for the modern power grid.

	Load Active Power (MW)	Busbar Active Power (MW)
DLoad1|Bus2	125.0000	125.0000
DLoad2|Bus3	90.0000	90.0000
DLoad3|Bua5	100.0000	100.0000

and

Table 17. Reactive power on load busbars and their loads under initial load demand for the modern power grid.

	Load Reactive Power (MVAr)	Busbar Reactive Power (MVAr)
DLoad1|Bus2	50.0000	50.0000
DLoad2|Bus3	30.0000	30.0000
DLoad3|Bua5	35.0000	35.0000

. We also recorded busbar voltage values, as shown in

Table 18. Busbar voltage under initial load demand for the modern power grid.

Busbars	Voltage (PU)
Bus1	1.0210
Bus2	0.9928
Bus3	1.0060
Bus4	1.0230
Bus5	1.0130
Bus6	1.0280
WPPSEBus	0.9930
WPPREBus	0.9934

.

5.2. Increased Load Demand Simulations for the Modern Power Grid System

Our analysis in this section is based on Bus2 only since it has already been identified as the critical busbar in previous sections. While the attention was focused on Bus2, the load demand increase simulation was again performed, and the active power and reactive power for both DLoad1 and Bus2, along with voltage results pertinent to this busbar, have been recorded in Figure 30, Figure 31, and Figure 32, respectively.

The voltage Bus2 was again observed during this simulation in response to the load demand increase in DLoad1. With an increase in load demand, the voltage at Bus2 dropped less significantly compared to the standalone traditional power grid.

6. Discussion of Results

When the load demand increase was implemented, a significant improvement in the voltage stability of the electrical power grid system was revealed after integrating the large-scale wind power plant (LSWPP) into this study. This finding is not new. However, the approach to reaching it is new in the following manner. The existing studies, like [19,20,21] rarely consider a step-by-step approach to obtaining to this finding. They did not consider evaluating the renewable energy grid code specifications before integrating the developed systems into the electrical power grid. Unlike the existing studies, this study aligned with the design and operational requirements of WPPs mentioned in [22,23,24]. This is due to its approach of first considering the simulations of the developed LSWPP alone to ensure its operation meets the REGC specifications before integrating it into the considered power grid. Furthermore, some studies on wind power plant (WPP) integration use a reasonable megawatt capacity of a single WTGU and treat it as a LSWPP, for instance, in reference [17]. Considering the models used in [17], and other models in [18,19,20,21], the computation time was saved. The challenge is that the models developed in these studies may not provide adequate validation to align the REGCs, whose significance has been mentioned in the work published in the references [1,3,4,14,26] for LSWPPs in the grid-connected mode of operation.

Lastly, using the Real-Time Digital Simulator (RTDS) devices with their software Real-time Simulation Computer-Aided Design (RSCAD) in this study enabled the simulation of load disturbance while monitoring them in real time. This has made the overall simulations in the study realistic compared to other studies.

7. Conclusions and Future Work

This work evaluated the large-scale wind power plant (LSWPP) grid-connected mode of operation by considering grid code specifications in real-time simulation. The studies associated with this topic have been explored. The interconnected system consisting of a LSWPP considered in this study has been carefully selected and, most importantly, described. Case studies have been conducted in consideration of the specifications of renewable energy grid codes (REGCs). The interconnected system developed and considered in this study effectively evaluates the grid-connected mode of operation of an LSWPP. It can be used for other large-scale renewable power plant (LSRPP) systems like photovoltaic (PV) systems.

It should be noted that in a WPP, not all wind turbine generator units (WTGUs) are operated simultaneously. This study considered the worst-case scenario where all WTGUs were required to operate simultaneously when the contribution of the entire WPP was urgently required. This is the part we will need to improve on when studying the need for WPP integration in the future, and we also look at developing a controller to ensure that the supply power required from the WPP side does not approach to the rated capacity.

Before this work, we had our previous work listed as follows:

(1)

We first looked at the design of wind power plants (WPPs) and modeled a large-scale wind power (LSWPP) in [36].

(2)

We performed a recap on the existing control methods for modern power grids consisting of WPPs.

(3)

We evaluated the power grid voltage instability point using system overloading contingency in real-time simulation.

(4)

Following the work in (3), we recently tested the busbar differential protection system, based on practical considerations, using a real-time digital simulator hardware-in-the-loop (HIL) testing technique.

Future work will consider a cross-comparison case study, applying the model developed in this study to a different international grid code to demonstrate its adaptability and robustness.