Simulation of an Electrochemical Energy Storage in a Reactive Power Source Mode

Alexander FedotovEngineering center Kazan State Power Engineering University, Kazan, Russia, fed.ai@mail.ru

Kamil BakhteevEngineering center Kazan State Power Engineering University, Kazan, Russia, kam.rav@bk.ru

Rinat MisbakhovEngineering center Kazan State Power Engineering University, Kazan, Russia, erdex@bk.ru

Rustem AbdullazyanovRelay protection and automation of power systems Kazan State Power, Engineering University, Kazan, Russia, kafedrarza@mail.ru

Abstract  The speed of the processes, the complexity of the equipment, the creation of micro-­networks tighten the requirements for the reliability and quality of the supplied electricity. Thanks to a breakthrough in the technologies of industrial energy storage and a sharp reduction in the cost of their production, one of the key technical solutions in realizing the task of improving the quality of power supply can be the use of high power energy storage systems (ESS). This paper presents the results of a simulation of a typical power supply system for industrial enterprises using electric battery (EB), supercapacitors (SC), an EB together with SC as part of a hybrid energy storage system, confirming the effectiveness of using ESS to maintain the voltage at a load during short-term power outages (STPO)b.

Keywords: mathematical modeling, electrical grids, supercapacitor, battery, power quality, voltage dip

© The Authors, published by CULTURAL-EDUCATIONAL CENTER, LLC, 2020

This work is licensed under Attribution-NonCommercial 4.0 International

I. Introduction

The basis for economic growth and improving industrial levels of enterprise productivity is the quality of electrical energy. In an industrial economy, the quality of electric energy attributed to power outages was numerically measured by the number of interruptions and estimated an average of 2–3 outages per year [1–3]. In the new digital economy, this definition does not correspond to modern equipment and sensitive processes. The operation of low-voltage electric motors for oil pump drives, fans, and similar mechanisms included in technological protections of technological processes, microprocessor technology, telecommunications and process control systems, expensive medical equipment, standard units of new digital technologies and the Internet are often interrupted by short failures (over milliseconds) and overloads supply voltage, which occur 20–30 times a year and lead to an expensive economic loss.

II. Research Methods

The existing market for solutions to improve the quality of electric energy is focused on the old system of views and design standards for protecting enterprises from 2–3 power outages per year, although up to 10–40 occur in different regions [4–7]. Thus, according to the data of the company Organic Synthesis PJSC, a technological process violation due to an STPO, which causes voltage drops, reaches 15 accidents per year, which brings an average of 13 million rubles of damage on one STPO.

These figures show that despite the high price of ESS, their use for high-speed short-term compensation of the effects of STPO can be quite economically justified. Moreover, in these cases, the STPO can be of high power, but relatively low energy intensity.

In this paper, using simulation modeling, we consider aspects of the use of ESS to increase the level of residual voltage on a load during a three-­phase fault (TPF) in a supply grid of a higher rated voltage. The developed methodology fully extends to STPO occurring in the electric grid of the same voltage class as ESS. In these cases, it is not the TPF current on the substation’s buses that varies, but the distance to the TPF location, which simplifies the calculations.

As a basic model, a typical scheme of an industrial power supply system is adopted, Fig. 1.

Figure 1. Schematic diagram of the power supply system:
SG — generator, M — internal combustion engine,
Q — liquid or gas fuel.

In order to track the effect of a sharp increase in load on a battery energy storage system (BESS), the actual battery parameters of the Russian company Liotech LLC, which is a design company of RUSNANO JSC, were taken [8–10]. In their product line, the maximum capacity of one battery is 700 Ah. The ESS consists of 147 batteries connected in series with a total voltage of 470 V, the initial charge level of the battery is 95% [10; 12]. In Fig. 2 shows the corresponding simulation model compiled in the Matlab Simulink software product.

Figure 2. Battery simulation model.

Assumptions accepted in the model:

• The internal resistance of the battery is considered constant during charge and discharge cycles and does not change with the amplitude of the current.

• The parameters of the model are obtained from the characteristics of the discharge, are considered the same for charging.

• Battery self-discharge not shown. It can be imagined by adding a lot of resistance parallel to the battery terminals.

• The battery has no memory effect.

• The battery capacity does not change with the amplitude of the current (Peckert effect).

• The load is simplified as a constant current source.

In Fig. 3 presents the simulation results. At a time from 0 to 5 s, the BESS operates in nominal mode with a current of 300 A for a load of 180 kW. At the moment of time 5 s, a pulse generator switches from the nominal mode to the emergency mode with a current of 2100 A, where the operation of an industrial consumer with a total power consumption of 1 MW is simulated [10; 12].

Figure 3. ABBM discharge with a sharp load surge.

As can be seen from Fig. 3, after 5 seconds of operation of the BESS on a load of 1 MW, the BESS was discharged by 1%. If we recalculate in Ah for a duration of 5 seconds in the event of an STPO, then we obtain a battery capacity consumption of 2.91 Ah. Before reaching the minimum voltage, the duration of the possible operation is 11 minutes, Fig. 4.

Figure 4. Changing the duration of the BEES during discharge currents from 0.5C to 3C.

Therefore, the conditions for choosing a battery with a sharp surge in load are dictated, first of all, by the value of the discharge current and practically do not discharge the batteries due to the TPF of their operation in this mode.

In Fig. 4 the upper graph shows the nominal discharge characteristic of the battery. As you can see, with a nominal operation of BESS, its capacity is enough for 2.2 hours of operation. The bottom graph shows that when discharged by a current of 3C, the BESS capacity will be enough to supply an industrial consumer of 1 MW for 11 minutes. Therefore, the discharge time of the BESS is inversely proportional to the increase in load.

However, in the case of a longer discharge with an increased current of 3C, which is not typical for the battery, the duration of the discharge and the number of discharge/charge cycles are reduced. The initial battery level in the two experiments was 100%.

Figure 5. BESS capacity change during discharge by currents from 0.5С to 3С.

In Fig. 5 shows that the capacity of BESS due to discharge with a high current of 3C decreased compared with a discharge of 2C by 22%, from 1C by 28%, from 0.5C by 32%.

One of the features of the battery is that the greater the discharge current, the lower the voltage to which the battery can be discharged. This is due to the fact that with a fast discharge with large currents (from 1400 A and above), the battery capacity is significantly reduced, because the electrolyte does not have time to mix, and the discharged layer accumulates around the plates, as a result of which the voltage on the battery drops, and the discharge current and battery discharge time become not proportional to each other. However, after several tens of minutes, the electrolyte is mixed, and the capacity and voltage of the BESS increase.

Given the characteristics of the processes of charge/discharge of lithium-ion batteries in accordance with Fig. 1, a simulation model of a typical industrial consumer was developed, which is presented in Fig. 6, with a total consumed a load of 1 MVA, with a BESS connected through a stabilizer, inverter and step-up transformer to 10 kV buses, to identify the BESS ability to limit the voltage dip in the absence of consumer own generation.

It is worth noting that the connection of the stabilizer allows you to maintain the output voltage and current stably throughout the entire discharge interval BESS. The voltage to the stabilizer is supplied as follows: minus is applied to the anode, plus, respectively, to the cathode. With this inclusion, a reverse current from the rectifier flows through it.

Figure 6. Simulation model of the power supply system
with ESS.

The voltage from the output of the BESS will change, and the reverse current will accordingly change, while the voltage and current on the stabilizer and, accordingly, on the load will remain unchanged, that is, stable. The ballast resistance is 0.003 Ohms and is calculated by the formula (1): where U0 is the input voltage of the stabilizer, UST is voltage stabilization, IST is the output current equal to the load current IL.

III. Modeling Transition Processes

In the simulation model, Fig. 6, the TPF was reproduced on the outgoing overhead line during the operation of emergency control systems. In Fig. 7 presents the results of modeling the ESS operation mode.

Figure 7. Voltage and load current. Full backup protection response time, BESS response time 20 ms.

As can be seen from the oscillograms, Fig. 7, in the event of a voltage dip occur, the load is switched using a fast-automatic transfer switch (FATS), which is presented in the model simplified by the breaker unit, Fig. 6. The time of the full cycle of operation of the backup protection is 45–60 ms. This time consists of the reaction of the automatics to the emergency situation (5–10 ms), the operation of the sectional switch (20–25 ms), and the opening of the quick-­start circuit breaker on the damaged section (20–25 ms). As noted above, in the absence of self-generation, the entire load power is covered by ESS.

As can be seen from the oscillogram, Fig. 7, at a time instant of 0.1 s, a voltage dip occurs up to 40% of the rated voltage. After 50 ms, it switches to BESS and the voltage on the industrial consumer buses increases to 107% of the nominal value in 2 ms and stabilizes to the nominal value in 10 ms.

Emergency operation continues until the TPF is turned off. Then, when the backup protection is turned off, the consumer continues to receive power from the power system, and the battery starts charging. The charging unit is not implemented in the model, the reaction of the battery during discharge and surge is similar to Figure 3, and the discharge characteristics correspond to a current of 3C, Fig. 5 and Fig. 6.

Fig. 8 shows that the operating voltage of BESS is 550 V, while the stabilizer limits it to 470 V, which allows the use of energy to power a load of 1 MW for 11 minutes.

Figure 8. BESS currents and voltages.

The battery efficiency decreases from 68% due to discharge by a current of 3C to 51% and high ballast resistance, because the stabilizer is a simplified prototype, which limits high currents and voltages.

The waveform in Fig. 8 shows that the current does not exceed a value of 2.4 kA over the entire power supply interval of the BESS load of 1 MW. This is due to the fact that losses occur during voltage stabilization, when converting it to alternating voltage in the inverter, in the transformer, in the line and supplying the additional inductive load, because cosφ = 0.9 is given in the model.

Above, an option was presented for the ESS, which is in standby mode and turned on by the emergency control command. In Fig. 9, a simulation model is presented in which the BESS is in a normal mode on a charge, and in an emergency, it starts to produce stored energy.

Figure 9. Simulation model of BESS permanent connection to 10 kV industrial consumer buses.

In this simulation model, the operation of the battery during STPO was studied. As can be seen from the oscillogram, Fig. 10, with a long response of BESS, STPO with a duration of 1–80 ms has time to have a negative impact on the energy system of the enterprise.

Figure 10. Simulation model of BESS permanent connection to 10 kV industrial consumer buses.

The load at real enterprises will also vary when the maximum permissible current 3C is exceeded. For a short time, you can maintain the voltage level close to the nominal, while sacrificing battery life or disconnect part of the load in order to work within the maximum permissible limits. However, in both cases, when discharging with high currents, as noted earlier, the charge/discharge cycles of the BESS are reduced and a significant decrease in the capacity of the BESS occurs.

Such shortcomings are deprived of electrochemical storages based on the supercapacitor energy storage system (SESS). To date, the price per 1 kW of power at SESS is significantly higher than that of a battery, however, to limit a voltage drop of 5 s for a load of 1 MW, a BESS with a margin of 11 minutes will be required, while SC can be assembled for power output within 5–10 s taking into account the feeding place TPF, it all depends on the needs of the enterprise.

The use of a different composition of combined-type SESS is necessary for a short-term discharge of energy into the grid to maintain a sufficient voltage level when the TPF is not disconnected, or to provide guaranteed power for the time it is switched to a backup source. Significant advantages of SESS over BESS, expanding their application in pulsed technologies are presented in [13–18].

To assess the influence of SESS on limiting the depth of voltage dips, a simulation model of SESS is presented, shown in Fig. 11 and consisting of 78 capacitors with a capacity of 500 F and a total voltage of 526 V, the initial SESS charge level is 100%. The SESS operates at a load of 1 MW through a stabilizer, which limits the voltage to 470 V. In order to track the effect of a sharp increase in load on the SESS, the parameters of the Maxwell battery were taken [19].

Figure 11. Simulation model of a SESS discharge with a sharp load surge.

Assumptions accepted in the model:

— Internal resistance and capacitance are assumed to be constant during charge and discharge cycles.

— The model does not take into account the temperature effect.

— The effect of aging is not taken into account.

— Redistribution of charge is the same for all voltage values.

— The block does not simulate cell balancing.

— The current through the capacitors is assumed to be continuous.

— The load is simplified as a constant current source.

In Fig. 12 presents the simulation results.

Figure 12. SESS discharge during a sharp load surge.

Within 30 seconds, a load surge occurs lasting 5 seconds, where a load of an industrial consumer with a total power consumption of 1 MW and load shedding is simulated. As can be seen from Fig. 13, during the first load surge of 1 MW for 5 seconds of operation, the battery discharged from 100% to 95%. In contrast to a similar experiment with BESS, SESS discharge occurs much faster. A fast aperiodic discharge of SESS occurs, in which there are no damped voltage fluctuations on the capacitor plates.

In Fig. 13 presents a simulation model of a power supply system with a hybrid energy system storage (HESS).

Figure 13. HESS simulation model.

The main elements of the HESS simulation model are:

— ESS in the form of BESS and SESS;

— a system for converting current from one kind to another, consisting of an inverter, a voltage stabilizer, and filters;

— control system for this converter;

— The control system of the electrical system as a whole.

The results of the simulation are presented in Fig. 14.

Figure 14. Voltage and load current during TPF
in the supply grid.

As can be seen from the oscillogram, Fig. 14, the level of residual voltage during STPO was 40%. After 10 ms, SESS was turned on, which ensured an almost instantaneous voltage recovery to a residual voltage level of 93%. 100 ms after voltage dip, BESS is connected. For a short time they work in parallel, then SESS is turned off, and work continues on BESS.

IV. Discussion of Results

Simulation of the modes of STPO of the industrial load node in the MATLAB Simulink environment showed that BESS can significantly reduce the effect of reducing the voltage on the load. Supplementing BESS with SESS allows the HESS to respond practically to inertia to external disturbances. The power of the SESS is determined by the desired level of residual voltage and the compensated depth of the voltage dip.

V. Conclusions

Simulation of the modes of STPO of the industrial load node in the MATLAB Simulink environment showed that BESS can significantly reduce the effect of reducing the voltage on the load. Supplementing BESS with SESS allows the HESS to respond practically to inertia to external disturbances. The power of the SESS is determined by the desired level of residual voltage and the compensated depth of the voltage dip.

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b Research is made with financial support of the Ministry of Education and Science of the Russian Federation within implementation of the federal special program “Research and development in the priority directions of scientific and technological complex of Russia for 2014-2020”, the agreement on granting a subsidy № 075-15-2019-057, unique identifier of applied scientific research (project) RFMEFI57418X0188. All research articles should have a funding acknowledgment statement included.