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INL/CON-14-33199PREPRINT

Significance of Dynamic and Transient Analysis in the Design and Operation of Hybrid Energy Systems

9th International Topical Meeting on Nuclear Plant Instrumentation, Control, and Human Machine Interface Technologies

Mayank Panwar, Manish Mohanpurkar, Julian D. Osorio, Rob Hovsapian

February 2015

1

SIGNIFICANCE OF DYNAMIC AND TRANSIENT ANALYSIS IN THE DESIGN AND OPERATION OF HYBRID ENERGY SYSTEMS

Mayank Panwar, Manish Mohanpurkar, Julian D. Osorio, Rob Hovsapian

Idaho National Laboratory 750 University Boulevard, Idaho Falls, ID-83415-3810

mayank.panwar@inl.gov; manish.mohanpurkar@inl.gov; julian.d.osorio.r@gmail.com; rob.hovsapian@inl.gov

ABSTRACT

The significance of dynamic and transient analysis is an established concept in realm of electrical power systems. The electric grid infrastructure is evolving with the integration of non-dispatchable Renewable Energy Sources (RES) forming a Hybrid Energy System (HES) which differ from the conventional energy systems. It is essential to understand the dynamic and transient interaction of HES with the electric grid for de-risked integration and overall enhanced operational reliability. It is a challenge to accurately model and simulate the large integrated systems for validating design and operation of the future grid. This paper presents some methods to address the challenges in the modeling and simulation of dynamics and transients occurring in the electric grid. A test case of a regional electric grid is considered based on real world system configurations and modeled in a real-time simulation platform – RTDS-RSCAD®. Physics based models are used for electrical-mechanical-thermal co-simulations to accurately represent the dynamic and transient response of the system in a real time environment. Real time simulations for a base case and a future case with average expected load growth and increased penetration of non-dispatchable RES in HES are presented. The severity of system level impact of dynamic and transient system events in future is shown in contrast to the base case. The concluding results establish the need and significance of dynamic and transient analysis as a co-simulation in a real time environment, in context of electric power grid integrated with HES in future.

Key Words: Concentrated solar thermal, hybrid energy, power system transients, real-time digital simulator, wind energy

1 INTRODUCTION

The large-scale penetration of Hybrid Energy Systems (HES) is increasing rapidly [1]. This requires suitable planning and design of the future electric grid for de-risked integration of the new technologies and HES with the existing infrastructure. In context of electric power systems, HES are power systems containing numerous generation technologies i.e., thermal, photovoltaic, wind, nuclear etc. Long-term planning of the operation of HES is performed based on feasibility studies considering time span of few decades. Such studies are critical for an informed decision making, but majority of these studies generally consider the economic aspects. The technical feasibility is based on steady-state analysis of the system for such long term analyses. Such analyses do not include the real-time dynamic and transient operation of the system in different potential scenarios. Although the operational challenges due to non-dispatchability of renewable energy sources (RES) in HES may be mitigated effectively through local operational controls in case of low penetration of HES, the system level impact can be larger when the penetration of RES in HES increases in the electric grid. This might make the system planning and design sub-optimal

This work was supported by Energy Systems & Technologies Division (EEST) and Nuclear Energy (NE) divisions of Idaho National Laboratory (INL), Idaho Falls, Idaho, U.S.A.

due to higher costs of mitigating the operational challenges presented by the high penetration of RES.

This paper examines the role and establishes the significance of dynamic and transient analysis in system level operation of electric grid with high penetration of RES in HES. Such an analysis can be performed effectively through simulations that replicate the real world conditions and capture the response with high accuracy. Higher accuracy of simulations can be achieved through high fidelity models which represent and capture the response of power systems under potential dynamic conditions. Different systems in an integrated hybrid energy environment have inherently different characteristics based on inertia – electrical, thermal, mechanical, chemical etc., which affects the time-scales of simulations. This in-turn also imposes challenges on the integration of various models, platforms, and synchronized simulations considering the differences in features of the involved sub-systems. To address these challenges, physics-based models are used here to accurately capture the dynamic and transient interaction and the system-level impact. However, a balance needs to be achieved between detail and size of the system that can be modeled and simulated in a real time environment.

Accurate representation of the system is a crucial aspect of real time co-simulation. . In context of electrical engineering, the electromagnetic transient (EMT) type solvers are used for high frequency time-domain simulations [3-5]. Some EMT programs are capable of simulations only in non-real time but are used extensively. Real-time simulation tools can run simulations at time step lesser or equal to the time it would take for a phenomenon to actually happen in a real world power system. The criterion for choosing the simulation tools is based on the type of phenomenon to be analyzed and the involvement of hardware-in-the-loop. The simulation platform used in this work is RTDS-RSCAD® [6]. RTDS is a real time digital simulator which uses RISC processors optimized for power system simulations. RSCAD is the graphical user interface (GUI) which is used for modeling. RSCAD has inbuilt library of various power and control system components. RSCAD uses RunTime environment for custom display of simulation results, manual controls, and scripting features for batch files. RTDS® uses a suitable combination of hardware and software platforms and is capable of real-time simulations. The typical time step used is 50µs and is sufficient to capture the effects of most power system transients. High switching frequency elements like power electronics can be simulated using 2µs time steps. The additional advantage provided by RTDS® is that it can be used for including physical hardware models in the simulation loop and performing hardware-in-the-loop (HIL) simulations [7, 8]. This provides the ability to test an actual hardware prototype and provides fidelity against modeling errors where a highly detailed model is required for sub-system representation. Highly detailed EMT simulations can provide an insight into the issues and characteristics of electric grid with HES.

1.1 Motivation and scope The main motivation for this work comes from the need to highlight the role and establish the

significance of the dynamic and transient co-simulations for analysis of integrated HES in the electric grid design and operation. The electric power systems and HES individually are well researched areas, but the research and related literature available in public domain on the topic of integrated analysis of these two systems from a grid level perspective is very scarce.

The scope of the paper spans the presentation of the impact and relevance of the dynamic and transient simulation as an analysis technique. The work here focuses on physics based modeling, use of appropriate EMT tools, HES model integration, and methods of performing accurate simulations in a real time environment. The work does not aim to optimize the performance of the integrated system or mitigate the problems investigated as part of some example simulation tests presented here. The optimum design and operation is a broad topic of future research and is out of scope of the work presented here.

2 TECHNICAL DETAILS

2.1 System Modeling This section describes the approaches, technical data details of the models, and the assumptions

used. The type of model used for each component is presented with relevant model data and configuration.

2.1.1 Electric power system modeling The power system model used is a practical case of a regional electric grid. The model is based on a

real world power system configuration and modeled in a real-time modeling and simulation platform – RTDS-RSCAD®. The transmission network is modeled considering extra high voltage (EHV) lines, i.e., transmission lines of 500 kV, and the tri-bundled ACSR Chukar conductor is used. The EHV lines of the transmission network in Arizona are considered and substations are modeled as the connection points for EHV lines and aggregated loads in the system. Information and data for Arizona Power Service Company (APS) available in public domain and APS renewable energy portfolio standards are used to derive the approximate size of generation and loads in the system [9-12]. The single line diagram of the transmission system modeled is shown in Fig. 1 below.

Legend:

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Bus (from) Bus (to) Length (mi)1 2 701 3 3001 4 3002 3 2302 4 3003 4 2503 31 554 41 280

Line data

Figure 1. Single line diagram of the regional 500 kV transmission network

The total load and generation capacity modeled is approximately based on the APS system data in [9-11] but is not exact representation. A close representation of capacity is considered for a practical case. The thermal generators are modeled as synchronous generators with standard IEEE excitation systems, governors, and Power System Stabilizers (PSS). The concentrated solar thermal (CSP) plant also has a synchronous generator connected to the tandem-compounded steam turbines with IEEE excitation system and PSS. The governor is replaced by a multi-mass model in RSCAD and consists of three steam turbines. The multi-mass model captures the mechanical inertia of the rotating turbine masses and its effect on the dynamics of the electrical output. The input to the multi-mass model comes as mechanical torque from a thermal model of a CSP plant running in MATLAB®. The wind power plant (WPP) is modeled as a large equivalent induction generator with internal bus (bus-41WT) which is a sufficiently accurate approximation for bulk-grid level dynamic simulations [13] and for simulating worst case scenarios [14]. A more detailed representation with appropriate aggregation can be adopted depending on

the phenomenon of interest and available simulation resources as done in [15]. The loads are modeled as balanced three-phase constant power loads in RSCAD. The other details of transmission lines and aggregated loads at the substation buses are presented in Table I below.

Table I. Generation and load details for the 500 kV system modeled in RSCAD

Bus no. Generation Load Name Type Capacity (MW) P (MW) Q (MVAr)

1 G0 Slack – 2000 200 2 G3 Thermal 2000 2000 200 3 CSP Solar thermal 250 – – 4 – – – 1500 300 31 G1, G2 Thermal 2000, 2000 – – 41 WPP Wind 99 – –

2.1.2 Hybrid energy system modeling Wind and CSP are chosen as the RES to form HES in the simulations. Since the case considered is

based on a region similar to Arizona, solar and wind resources are the most appropriate mix for grid level simulations. Wind energy is highly variable and the variations occur at time-scales of a few milliseconds to minutes. The aggregated value of variability is smoothened due to spatial diversity of the wind power plant, but the range of variations might be higher for higher penetrations and can aggravate power system stability under certain scenarios as in [16].

The thermal modeling of the CSP is done in MATLAB and considers the dynamic effects solar energy variations. The CSP plant uses three steam turbines, thermal energy storage, and auxiliary components with thermodynamic variations modeled in detail to include the dynamics due to thermal inertia of CSP. The process uses water as the working fluid and is based on Rankine cycle. This also makes the CSP plant quasi-dispatchable and is considered a preferred choice to be collocated with wind energy in regions with good wind and solar profiles [17, 18].

2.2 Dynamic and Transient Simulations This section presents the simulation setup, methods of integrating various models, simulation tests

performed, and the observations. The analysis can be done effectively using a variety of simulation tests under different scenarios. Several scenarios such as normal and abnormal operating conditions, peak and off-peak loadings, present, and future power system must be considered. Even the most well designed systems may become unstable under certain conditions of operations [19]. Hence, it is important to investigate, understand, and develop measures to mitigate these issues. The basic simulation setup is shown in Fig. 2.

The CSP model runs in MATLAB and generates power output considering the dynamics due to variation of the solar irradiance and air temperature as the input parameters. The two way communication between the HES model and power system simulation is required for accurately simulating the dynamics. Hence, interfacing the models is important [20]. The data communication latency amongst integrated models also plays an important role in performance of the simulation. The allowable latency depends on the phenomenon of interest to be simulated and must be able to capture the effect at each time step. Performance can be measured by accuracy of the results and stability of the simulation. Integrating models with different response times, sometimes in the order of magnitude equal to hundreds or more can be a challenge. For the simulation setup considered here, the EMT simulation time-step is set to 100µs in RTDS without losing generality and for including power electronic based devices built using small-time step modeling technique. Small-time step models are interfaced to the larger time-step power system

chosen, it being on the bus with WPP and no other generation source. The load set point is stepped down from 100% to 1% instantaneously. Fig. 3 shows the response of the system.

Figure 3. Instantaneous step load decrease at bus-4 from 100% to 1%, i.e. 1500 MW/ 300 MVAr to 15 MW/ 3 MVAr. The 3-phase rms voltage magnitudes for each bus are shown.

It may be observed that the bus voltages reach a stable value in spite of large load loss. This is due to the action of the grid generators ramping down for attaining a new steady state of operation. The only control action used here is that due to standard IEEE PSS for each of the synchronous generators. An optimal response can be obtained by netter tuning of the generators and control systems. The per unit (p.u.) voltage is also slightly higher than earlier. This can be regulated by effective voltage control using capacitor switching or other supporting devices. This is not discussed here as it is beyond the scope of the presented work.

It can be seen again in Fig. 4 that the system attains steady-state quickly. The reactive power imbalance is compensated largely by generator G0 which is also the representative of the interconnection to rest of the system. The generators at bus-31, G1 and G2 are also impacted by the load drop and swing together. The large induction machine representative of the wind power plant (WPP) also takes a slight dip in reactive power consumption but is not significant compared to reduced value of nearly 300MVar at bus-4 due to load drop. The speed fluctuation of the CSP generator at bus-4 increases due to the event and can be seen in the speed plots. The generator however has already been oscillating slightly even before the event. The CSP generator is running in free speed, multi-mass mode and is a much accurate representative of actual effects. The small steady state fluctuation is due to continual correction of PSS and variation of system voltages due to presence of dynamic loads in the system. The test was also simulated to step down from Load-4 set point from 100% to 50% instantaneously. The results and trends are similar except the dips in voltage levels and the transients during recovery are not as severe as in the previous case. The waveforms are not shown for brevity.

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Figure 4. Power and speed variation of generators across the system due to load decrease on bus-4

The final test for load demand variation is a fast ramping with increment from 50% to 100% in steps of 10% every 2 second. This is an example of a longer term simulation and helps in looking at dynamics and transients of the system over a time range of a few seconds, instead of typically a few hundred microseconds. This can be useful in understanding interaction of systems with inertial response which differs significantly when analyzing temporally. This case is simulated with step load increment of 10% and the system performs sufficiently well with bus voltages recovering to normal operating ranges within 100 ms. The plots for this case are not shown here for brevity.

WPP disconnection: WPPs are typically designed for operation under system faults and voltage abnormalities. These capabilities are referred to as low-voltage ride through (LVRT) or zero-VRT (ZVRT). The WPP only disconnects from the system in case of severe voltage abnormality or fault persistence due non-clearing of faults. Another issue governing the WPPs is sudden drop in wind speed which can impact large wind installations and impact the grid stability. Such an incident has happened in Texas on February 26, 2008 [13]. Similar scenarios are simulated here assuming a drop in wind velocity, and WPP going offline due to a non-cleared fault in the system. The dynamics are observed, recorded and analyzed.

Simulation was also done for a case when WPP goes offline at off-peak load demand, i.e. loads are at 50% and WPP output = 0.8 p.u. The results are not significantly different and are not shown here for brevity. A scenario simulating WPP output drop from 0.7 p.u. to 0.1 p.u. is also simulated. The drop occurs in two steps of 0.7 p.u. to 0.5 p.u. and further to 0.1 p.u. separated by 5s. This is done to consider the WPP outputs to be affected by wind variation in two stages which is more practical. Although mechanical inertia for the machine is considered in the simulation, a multi-mass body representation of turbine considering the turbine, stator, and rotor inertias for all the turbines in the WPP would be most accurate representation mechanically. The electrical representation used here however is sufficient for bulk grid level phenomenon as given in [13]. Here, we use a single large induction machine as equivalent representation of a WPP. This test for base case does not impact system voltages severely and system remains stable. The plot below shows impact ad steady state after the second step drop of 0.5 pu-0.1 pu.

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The reason may be smaller capacity WPP. This may change in with higher wind penetrations and system impact needs to be simulated.

Figure 5. WPP goes offline at peak load demand, i.e. loads are at 100% and WPP output = 0.7 p.u.

Other tests simulated are asymmetrical single line to ground fault at bus-4 for 100 ms, line tripping of line 3-4, CSP disconnection for low penetration case. Reactive power fluctuations up to 0.03 pu were observed in case of line tripping. For high penetration case, two selected tests were simulated for comparison – the single line to ground fault and WPP disconnection. The WPP disconnection portion is shown in Fig. 6 and can be compared with Fig.5 for low penetration case. In Fig. 6, it may be observed that the peak rms voltage for bus-41WT is almost 1.3 p.u. which in contrast to Fig. 5 is almost 20% higher while the dip in previous case is more. It is to be mentioned that no external control action was taken for the system to settle down to its steady-state, except those by individual generator PSS and slack generator representing the external system. The system recovers quicker in low penetration case, indicating that there might be sustained power oscillations in some cases of higher HES penetration with same outage. These are some of the observations made based on the system response in terms of the most crucial parameters of power system operation at a grid level such as bus voltages, generator speeds, load angles, real and reactive power etc. The stabilizing action of PSS is to be considered and can improve the system response if better tuned. However, the observations made above would still be valid without loss of generality. The CSP generators were run in free-speed mode to account for multi-mass inertia of the turbine-generator system. Switching on and off of the PSS at certain times can be used to simulate loss of control signals and can be helpful in study of wide area control system and its impact in case of electric grid with high penetration of RES in HES. All the observations and inferences presented here help us conclude that certain phenomenon can be aggravating to the system performance when left unconsidered. Since long term planning based only on economic and steady-state analysis fails to capture these dynamic and transient events at grid level, it is essential to look at these for better design and operation of future grid.

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3 CONCLUSIONS

The paper presented the modeling requirements, simulation platforms, and interfacing techniques for co-simulation of electrical and thermal systems in context of the analysis required for de-risked integration of HES with high RES penetration in the future electric grid. An example case for a real world regional power system was chosen and modeled to closely represent the APS system in WECC. Two scenarios, with about 5.5%and 19.5% penetration of RES in HES are presented. The interfacing of a dynamic thermal model of CSP was done with RTDS as a co-simulation. The tests for various abnormal conditions on the grid were performed and relevant cases were presented. The observations and results from the simulations show that effect of dynamic and transient abnormalities and contingencies in future grid with more HES can be severe. Mitigative measures must be taken and it is essential to understand the impact through accurate co-simulations of large-scale electric grid under different scenarios. These conclusions established the significance of dynamic and transient simulations for design and operation of HES integrated electric grid in future.

4 FUTURE WORK

The future work would extend the presented cases of co-simulation of electrical, thermal, and mechanical sub-systems of HES by including other existing tools for nuclear thermal simulation such as RELAP-5, and integrating those with RTDS® for EMT type dynamic and transient co-simulations.

5 ACKNOWLEDGMENTS

This work was funded by the U.S. Department of Energy’s (DOE) Office of Energy Efficiency and Renewable Energy (EERE) and the Office of Nuclear Energy (NE) – Advanced Reactor Technologies Office (ATR). Acknowledgements are also extended to Dr. Siddharth Suryanarayanan of Electrical and Computer Engineering Department at Colorado State University, Fort Collins, CO for his support through technical discussions and suggestions throughout this work.

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