Experience Sharing on Island Operation of Self-provided Power Plants

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Based on the design process of the control scheme for an isolated grid system in a self-provided power plant, CHANGHUI Instruments analyzes the issues that need to be considered regarding relay protection and safety and stability control when such a power plant operates in isolated mode. These aspects mainly include the load characteristics of the isolated grid power plant, risk analysis, control measures, and the machine-grid coordination system. By implementing a comprehensive relay protection and control scheme, the requirements of the production process can ultimately be met, ensuring the safe and stable operation of the isolated grid system. A standalone grid is short for an isolated power grid, and generally refers to small-scale grids that are separated from the main power grid. A power grid whose maximum capacity of a single unit is less than 8% of the total grid capacity can be referred to as a large power grid ； Power grids with a machine-to-network capacity ratio of over 8% are collectively referred to as small grids ； A small network operating in isolation is called an isolated network. Operating in isolation: yunrun.com.cn/tech/1875.html. A nickel-iron smelting plant in Indonesia has built two 66MW coal-fired units in its own power plant, which supply power to the facilities at 35 kV and 10 kV. During normal operation, the power plant supplies power to the furnace in the smelter via kV lines connected to the local power grid. During normal operation, the power plant and the local grid are supplied with power by separate heat boilers. During normal operation, the power plant is separated from the local grid and operates as an islanded system; a 10kV diesel generator set is installed on site as a source for black start. The main equipment of the smelter consists of four rotary kilns measuring Φ4.85m×90m, plus four 25.5 MVA round-top fully enclosed submergence arc furnaces. The load balance table for this smelter is shown in Table 1. Table 1: Power load balance sheet of the on-site power plant (2×66MW) at a nickel-iron smelting plant in Indonesia
| Item | Capacity | Percentage of generator capacity | Capacity for plant’s own use |
|------|----------|----------------------------------|----------------------------|
| 15.84MW | 12% | | |
| Electric arc furnace capacity | 91.8MW | 69.5% | |
| Capacity for smelting auxiliary equipment | 21.4MW | 16.2% | |
| Total capacity | 129.04MW | 97.7% | |

1. Load characteristics: The electric arc furnaces in typical metallurgical plants are resistive furnaces. During normal operation, the sulfide ore feedstock is in a semi-molten or fully molten state; slag and ferroalloy melts have significantly different electrical conductivities. As a result, the electrical load fluctuates frequently during normal operation of the arc furnaces, with unbalanced three-phase currents being the norm. The typical load characteristic curve is shown in Figure 1. http://yunrun.com.cn/upload/201803/25/201803251849243252.png Figure 1: Typical electrical load curve of a metallurgical plant (33MVA). The systems of power plants used in metallurgical industries have the following characteristics: ① Due to the limited capacity of the isolated grid, both the kinetic energy stored in the rotational inertia of the generators and the thermal potential energy available in the boilers is low; as a result, the electrical load on the electric arc furnaces fluctuates greatly, causing significant impacts on the frequency and voltage of the isolated grid. ②When the generator set experiences a shutdown fault, the islanding system suffers from a deficit in active power, which affects the normal operation of metallurgical processes. ③The load in electric arc furnaces fluctuates frequently and by large amounts, and there is a high likelihood of these fluctuations occurring simultaneously. This results in frequent, significant load shocks to the steam turbine generators, affecting the safe operation of the isolated power system as well as the lifespan of the main equipment. ④Isolated grids lack the support of large power grids and must be scheduled independently to maintain stability in frequency, voltage, and power angle. Based on the electrical load characteristics of the above-mentioned electric arc furnace, it can be seen that the three-phase electrical load of the furnace fluctuates frequently, and in severe cases, there are situations of sudden loss of high-power loads ； Three-phase current imbalance can lead, in severe cases, to soft or even hard electrode failure; the imbalances in current from multiple electric arc furnaces combine together as well. Moreover, these adverse factors are uncontrollable, and their occurrence is random. 2. Risk analysis of the isolated grid system: Given the load characteristics of the submerged arc furnaces mentioned above, it is evident that during normal operation, the isolated grid operation system of the metallurgical power plant poses risks in terms of system voltage, frequency, and unit lifespan. ①Fluctuations in frequency and voltage: Frequent fluctuations in three-phase loads cause the frequency and voltage in islanded systems to change as well. a. Voltage fluctuations: Due to the high performance of the excitation systems in generators used in small and medium-sized metallurgical power plants, these systems can adjust rapidly, which is generally sufficient to quickly regulate the reactive power of the generators and maintain voltage stability in isolated grid systems. b. Minor frequency fluctuations: Frequent fluctuations in the three-phase load of the submergence arc furnace cause corresponding changes in the frequency of the isolated power grid. As a result, the turbine speed control system’s primary frequency regulation mechanism responds frequently to these fluctuations; consequently, the turbine control valves operate frequently and may even vibrate. c. Sudden load shedding in the electric arc furnace: If the turbine speed control system performs poorly and reacts slowly, it will trigger the over-speed protection mechanism, causing the unit to shut down. In severe cases, this can lead to the collapse of the isolated power grid, resulting in a complete shutdown of the power plant. ②Negative-sequence current in the turbine and power angle oscillations: a) Imbalance in the three-phase load of the electric arc furnace leads to an imbalance in the three-phase currents of the turbogenerator set, resulting in an increase in negative-sequence current. Circulating currents are formed at the ends of the generator rotor, which increases losses and causes problems such as heating and corrosion, thereby affecting the operational life of the generator set. When the three-phase load of the electric arc furnace is severely unbalanced, it will directly cause the generator’s negative sequence protection to activate, resulting in the shutdown of the unit; the islanded operation system collapses, and an outage of the entire auxiliary power plant occurs. b. Reactive power surges and a decline in transient stability lead to power angle oscillations: isolated grids have insufficient reactive power reserves and low short-circuit capacity. If a fault occurs in the excitation system of a generator, it may cause the generator to rely on an isolated grid to supply reactive power, or even enter a leading-phase operating state, thereby posing a risk to voltage stability. ③Generator set lifespan: To cope with the frequent and significant fluctuations in load in metallurgical power plants, the frequency and extent of regulation required of generator sets operating in isolated grids are higher than those of grids connected to the main power grid, and the torsional vibration in the shaft systems of these generator sets also increases. All these factors contribute to an accelerated degradation of the generator set’s lifespan, thereby affecting the safety and economic efficiency of operation in isolated grid conditions. ④With a complete shutdown of the plant power supply, the generator sets in an isolated network are at a higher risk of losing their power supply compared to those connected to the grid. As a result, the equipment designed to ensure the safe shutdown of these generator sets, such as their UPS systems, emergency power supplies, lubrication systems, and systems for supplying water to the steam drums, are put under greater stress. If the relay protection and security and stability control design of a stand-alone system is inadequate, or if it is designed solely based on grid-connected units, it is easy for the aforementioned risks to arise. For example, the on-site power plant of a steel mill in Indonesia that operates under similar conditions to this project has an unstable grid operation (island mode), and frequent imbalances in the three-phase currents cause the two generators to shut down 1–2 times per day. The causes of three-phase current imbalance mainly stem from charging in the nickel-iron furnace, material collapse, or pole breakage. Although the project was later improved through some technical modifications, it remains suboptimal. 3. Control measures for isolated grid systems: Taking into account the load characteristics of the smelting plant as well as the problems encountered during the operation of the existing power plants, the following relay protection and safety/stability control measures were implemented for this smelting plant, focusing on aspects such as electrical load balancing, excitation systems, FCB control, and black-start diesel engines as well as standby diesel engines. ①Electric load balancing devices are necessary because electrical energy cannot be stored; at every moment in an isolated grid, the amount of power generated must balance the amount of power consumed. Otherwise, frequency fluctuations will occur, leading to problems with the dynamic stability of the isolated grid. However, considering factors such as the fatigue and wear of the actuating mechanisms caused by frequent operations of the turbo-generator speed control system, as well as the impact on the sealing performance of the speed control valves, and the severe lag in the adjustment speed of the boiler combustion system, conventional metallurgical captive power plants operating in island mode are unable to meet the changing electrical load requirements of electric arc furnaces and to maintain the safe and stable operation of the islanded system. Therefore, an electrical load balancing device is installed in the power plant; when there are fluctuations in the load of the electric arc furnace (or during sudden load reductions), the activation and deactivation of the energy-consuming resistors installed in the reservoir, via this electrical load balancing device, greatly mitigates the impact on the steam turbine generator set. To minimize wear and tear on generator sets, an electrical load balancing device is used to smooth out sudden load changes during load drops. On the other hand, a load-side increase/decrease allowance signal is established; when the rate of load change exceeds a limit, further changes on the load side are restricted, while load management is used to smooth out sudden changes in active power. To implement this function, it is necessary to establish a well-developed control and protection logic in the secondary system design of the power grid, as well as rapid signal communication with the speed regulation system and excitation system. The schematic diagram of the electrical load balancing device is shown in Figure 2. http://yunrun.com.cn/upload/201803/25/201803251910069285.png Figure 2: Diagram of the electrical load balancing device. ② Generator and its excitation system: The electro-hydraulic regulation system for the generator utilizes a high-performance electro-hydraulic control system designed for steam turbines (including a standalone grid operation module and secondary frequency regulation functionality). The generator excitation system requires a high initial response, fast regulation speed, and reliable operation. It must be capable of automatic excitation regulation and have an additional control unit for the Power System Stabilizer (PSS). It should be able to operate stably over the long term while delivering rated active and reactive power, and it must also remain safe and stable under special operating conditions involving high loads. When the generator’s output voltage drops to 70%, it can maintain the operation of the power grid for 1 second, and the important auxiliary equipment possesses the aforementioned low-voltage ride-through capability. The self-excited stationary excitation system and the brushless excitation system are the two commonly used excitation systems at present; their advantages and disadvantages are shown in Table 2. Table 2 Comparison of Characteristics between Self-excitation Static Excitation Systems and Brushless Excitation Systems. Characteristics of Self-excitation Static Excitation Systems: 1. Mechanical properties – The structure of the generator rotor shaft is retained; the generator shaft length is short, resulting in minimal changes in the dynamic characteristics of the entire rotor, which facilitates maintaining stability. 2. Electrical performance ①Advantages: high initial response, fast adjustment speed ； No rotating components, high reliability ； Full monitoring for easy identification and handling of accidents ； Rectifier devices feature high redundancy, excellent safety, and are easy to maintain ； ②Disadvantages: Brushes need to be replaced regularly, and carbon powder is generated ； It is not suitable for use in environments with explosive gases ； A backup excitation power supply needs to be installed. 3. Economic performance: The elimination of the main and auxiliary exciters reduces the length of the unit, decreases the number of connections at the main shaft, and improves the stability of the shafting. Moreover, for units with the same capacity, this approach significantly reduces the length of the main plant, thereby lowering the construction costs. Characteristics of brushless excitation systems: 1. Mechanical properties – The generator end is connected to the exciter and then to the auxiliary exciter; there are many connection points, the shafting is long, and thus the complexity of the shafting system is high. 2. Electrical performance ① Advantages: Relatively fast response speed ； The control power is low, so the control and protection circuits can be simplified ； No brushes or toner required, resulting in less maintenance work ； No commutation spark issue, suitable for explosion-proof environments ； ②Disadvantage: The generator excitation current cannot be directly measured or monitored ； The demagnetization time is relatively long ； There are rotating devices. 3. Economic performance: Brushless excitation places high demands on rotary diodes. Currently, there are few manufacturers that can supply rotary diodes for use in brushless excitation of generators (imported brands are commonly used), and the cost is higher than that of self-excited or static excitation systems. Although the three-machine brushless excitation method has a higher cost compared to the self-excitation method, it features a longer shafting and a slower adjustment response time than the self-excitation method (though it still meets the requirements). Considering the capacity of the generator set and the characteristics of operating in an isolated grid environment, the three-machine brushless excitation method is more suitable for use in generators of this capacity. ③FCB capability: The steam turbine generator set should possess FCB capability: \"island operation\" with only the plant electricity available. The core of the FCB function consists of the excitation system, the turbine speed control system, and the boiler control system; it should be capable of operating with auxiliary power supplied even when the load is at 100%. If FCB fails, the entire isolated grid will lose power. In such a situation, the switch to the black-start power source should be completed as quickly as possible, and safety devices such as lubrication pumps should be activated to ensure the safe shutdown of the units in the isolated grid. Perform a black start recovery after shutting down. To accommodate operation in island mode, the unit is equipped with two synchronization points: the generator outlet and the circuit breaker on the high-voltage side of the main transformer. ④Black-start diesel generator sets and standby diesel generator sets: When the electric arc furnaces in smelting plants are operating under conditions of high instability, the rapid response capability of diesel generator sets for load adjustment is utilized to stabilize the isolated power system. Install emergency diesel generator sets for accidents to ensure that the turbines and boilers can be shut down safely. Special attention should be paid to the selection of diesel generator sets for use in emergency situations as well as their self-starting capabilities, ensuring compliance with relevant regulations and standards. ⑤Countermeasures for three-phase unbalanced current: The natural power factor of shaft furnaces is generally 0.9, and the unbalanced loads are mainly active loads; single-phase reactive power compensation has little effect. If a single-phase power supply is to be equipped with a balancing device, the investment required is high, the control circuit is complex, and there are no existing implementation examples; moreover, its operating time must be coordinated with the generator’s negative sequence protection. To this end, generators with a high short-circuit ratio (above 0.5) can be selected, and the slot wedges at the rotor ends should be made of materials with high electrical conductivity and high temperature resistance. Special attention should be paid to the three-phase load imbalance in electric arc furnaces, as well as the probability of such imbalanced loads occurring in these furnaces. In addition, a thorough study is needed regarding the issues related to load disconnection when severe three-phase load imbalance occurs in electric arc furnaces. 4. Machine-Grid Coordination System: The machine-grid coordination system serves as a platform for enhancing cooperation and communication between power plants and electrical loads. It is an important means of ensuring the safe, stable, and efficient operation of the power grid, and it helps to improve the quality of electricity supplied by the grid. Power plants and smelters operating in isolated grids should, in line with the functions of the grid’s dispatching system, have their coordination systems equipped with common functions such as automatic generation control (AGC), automatic voltage control (AVC), power angle measurement, special control systems (SPS, also known as stability control systems SCS), under-frequency/under-voltage load shedding (UFLS), and remote terminal units (RTU). It also includes special functions such as unit life optimization, angle/power factor regulation, and the Electric Arc Furnace Load Management System (DMS). Functions of the machine-network coordination system: a. Automatic Generation Control (AGC), in conjunction with DCS and DEH, rapidly allocates active power to maintain the frequency of the isolated grid at 50 Hz. b. Automatic Voltage Control (AVC), in conjunction with the excitation system, distributes reactive power to maintain the voltage in the isolated grid. c. Coordinate reactive power compensation with the active and reactive power loads of the units to maintain balance in power angle/power factor. d. Based on the calculations of active and reactive reserve, issue load increase/decrease permits and interlocked load reduction commands to units under heavy load. e. Issue load shedding and unit shutdown commands based on active and reactive reserve calculations as well as islanding parameter monitoring. f. Implement unified management of power generation and consumption loads to reduce the frequency of adjustments required of the generator sets, thereby minimizing wear and tear on their lifespan. g. Work in coordination with the electrical load balancing control system to reduce the unnecessary power consumption of the load balancing devices ； Use it as a reverse stability control measure for cutting off power and reducing load to improve the stability of isolated grids ； Use an electrical load balancing device to prevent the boiler from venting steam into the air. h. Provide a data platform for dispatchers to monitor power generation and consumption, which can serve as the dispatching platform for power plants. The functions of the machine-network coordinated control system encompass the core functions of various dispatching, control, and protection devices in large power grids, establishing three lines of defense for isolated grids. First line of defense: Utilize rapid balanced distribution of active and reactive power among multiple units to ensure the safe and stable operation of the isolated grid in the face of load shocks equivalent to 5% of the online rated capacity. Second line of defense: By means of a load regulation system and control over the initiation of high loads, the islanded grid can operate safely and stably under the impacts permitted by the capacity of the electrical load balancing devices. Third line of defense: In the face of larger-scale load shocks, the safety and stability control functions are used to disconnect part of the load or units, thereby maintaining the operation of plant power supplies and critical loads and preventing the collapse of the isolated grid. Currently, isolated power grids are mainly found in various privately-owned power plants and power generation facilities in underdeveloped regions abroad. In the past, during the design phase, relay protection measures, safety and stability control strategies, and equipment selection for power plants were primarily determined based on the operating conditions of large power grids. There has been relatively little research conducted on the operation of isolated power grids; consequently, many problems have arisen in numerous such systems in practice. Using actual engineering cases, this paper addresses the load characteristics and operational risks associated with island operation. By employing technical measures such as electrical load balancing devices, advanced excitation systems, turbine FCB control, black-start and standby diesel generators, as well as a comprehensive machine-grid coordination system, it ensures the safe and stable operation of the island system, thereby meeting the requirements of metallurgical processes. Source: Changhui Instrument Network