Technical Improvement of Isolated Grid Operation of Waste Heat Power Generation from 6500t/d Cement Production Line

1. Project Overview

The ACL project is a 6500t/d new dry process cement production line contracted by our company, and a 132kV/6kV total step-down substation (referred to as the "total step-down station") is built in conjunction with it. The single-circuit cable inlet of the total step-down station is led out from the newly added GIS feeder interval on the 132kV primary side.

Two main transformers (SZ1125000/132kV) are installed in the total stepdown station, and the 6.3kV secondary side is a single busbar segmented wiring method. Among them, the I section bus contains loads such as long belt conveyor, limestone crushing, cement mill, and cement packaging, and the II section bus contains loads such as raw material mill, kiln tail, and kiln head. The loads of the production line burning section are all concentrated on the II section bus. At the same time, the 12MW waste heat generator set supporting the production line is also connected to the grid on the 6.3kV secondary side II section bus. The single line (6.3kV part) of the production line distribution system is shown in Figure 1.

2. Problems with the power distribution system

When the production line is operating normally, the two incoming cables corresponding to the two main transformers are in the closed state, and the 6.3kV busbar is in the open state. When the waste heat generator set is connected to the grid, the 6.3kV busbar section needs to be disconnected from the external power grid at the same time. Once the external power grid fails, the waste heat generator set will switch to the isolated grid operation state and take on the load of the 6.3kVⅡ busbar burning section to ensure the continuity and stability of power supply. The current distribution system has the following problems:

(1) The distribution system has two operating modes: gridconnected and isolated grid, but the system cannot automatically determine the change of operating mode, resulting in the waste heat generator set being unable to be in the optimal adjustment mode.

(2) Under the grid-connected condition, the distribution system lacks a self-protection mechanism against external grid faults. Once the external grid fails, the fault can easily spread to the power supply and distribution grid, resulting in damage to the stability of the internal grid and even a complete power outage.

(3) There is a supply gap between the power generation of the current waste heat generator set and the actual power consumption of the distribution system. When the external power grid fails, even if the distribution system can be quickly disconnected from the external grid, the imbalance of power supply and distribution will cause the distribution system to shut down at a low frequency.

(4) In the isolated grid operation mode, the current turbine speed control system lacks the ability to cope with changes in downstream loads. Once load fluctuations occur, the frequency and voltage stability of the distribution system will be affected, thereby reducing the power supply quality.

(5) When the busbar of the distribution system fails, causing all downstream loads to be unloaded, and the waste heat generator set enters the self-contained plant power condition, due to the imbalance between the plant load and mechanical power on the turbine side, the turbine speed will rise rapidly and trigger the OPC solenoid valve to operate. At this time, it is difficult for the traditional turbine speed control system to quickly restore the turbine speed to the normal level of 3000r/min, which can easily cause the turbine to trip.

(6) When the waste heat generator set is in the condition of selfcontained power supply, the turbine valve will quickly reduce the opening, while the steam production of the waste heat boiler will not change suddenly, but will show a trend of gradual decrease. Therefore, there will be a significant imbalance in the load of the turbine and boiler, and the pressure of the waste heat boiler will increase significantly and may exceed the safety limit.

3. Improvement plan for isolated grid control

In view of the problems existing in this project, we have constructed a "power supply, power grid, load" waste heat generator set and external power grid coordination control system (referred to as "machinegrid coordination control system") to meet the stability of the power flow when the waste heat generator set is connected to the grid. At the same time, in the isolated grid operation mode, the waste heat generator set and the power load can operate independently and stably, and the power supply quality meets the various requirements of the power equipment and reaches or approaches the industry standard of the external power grid.

3.1 Functional composition of the machine-grid coordination control system

In this project,when the inplant power grid and the external grid are operated in parallel, the distribution system should have the ability to automatically adjust the power flow of the grid connection line; once entering the isolated grid operation mode, the machinegrid coordination control system needs to be able to quickly identify and automatically switch the in-plant power grid to the isolated grid operation mode. When the grid is isolated, the machine-grid coordination control system needs to monitor and automatically control important parameters such as bus frequency and voltage in real time to ensure its stability; when the external grid fault is restored, the machine-grid coordination control system should control the power grid in the plant to carry load and run in parallel with the external grid. The functional subsystems of the machine-grid coordination control system are as follows:

3.1.1 Automatic network topology identification

The distribution system of this project includes two sections of 6.3kV busbars. After the waste heat generator set is connected to the 6.3kV II section busbar, it is connected to the external grid through the No. 2 main transformer. The machine-grid coordination control system automatically identifies the operation mode of the external grid and the waste heat generator set by collecting the electrical parameters of the grid-connected tie line, the grid-connected switch status, the electrical parameters of each line of the plant grid, the line switch status, the electrical parameters of each bus, the bus switch status, the electrical parameters of the generator outlet, the outlet switch status and other signals, and automatically selects the corresponding operation mode according to the identification results.

3.1.2 Grid-connected flow and waste heat generator set power control

By collecting the PT and CT signals on the grid-connected tie line or the waste heat generator set side, the active and reactive power of the grid-connected line or the waste heat generator set can be obtained. According to the deviation between the set value and actual value of active and reactive power, the load deviation instruction is calculated, and the active and reactive power of the waste heat generator set is quickly adjusted to maintain the stability of the power flow of the grid-connected interconnection line (6.3kV II section bus and No. 2 main transformer incoming line) so that the output of the waste heat generator set is within the set range.

3.1.3 Decoupling protection mechanism

In order to ensure that the burning system load on the 6.3kV II section bus can operate in an isolated network when the external network fails, a special decoupling protection point can be set at the incoming line switch of the No. 2 main transformer of the 6.3kV II section bus. Under this decoupling protection mechanism, the system monitors the key electrical parameters such as the incoming line frequency and voltage in real time. After delaying confirmation and capturing the non-collection fault signal, the decoupling instruction is issued in time to disconnect the power grid in the plant from the external power grid, protect the system from the impact of external network failures, and ensure that the waste heat generator set can bear the isolated network operation of the burning system load.

During the operation of the decoupling protection mechanism, the system will intelligently judge and cut off some non-critical loads according to the operation mode of the power grid before the accident and the cross-sectional flow of the incoming line of the 6.3kV II section bus No. 2 main transformer to reduce the burden on the system. At the same time, the system will send secondary frequency and voltage regulation instructions to the waste heat generator set in real time to accurately regulate the active power and reactive power output of the waste heat generator set to ensure that the core parameters such as the frequency and voltage of the power grid quickly return to stability. In this process, shortterm fluctuations in the main parameters are allowed, but they should not cause the protection action of the internal power grid and the waste heat generator set to ensure that the power generation and power consumption loads are not excessively cut off.

3.1.4 Special control system

The special control system (SPS system) in the isolated grid operation mode can collect the electrical parameter signals of each outlet in real time, including generators, power equipment, grid-connected lines, etc., and compare them with the preset working conditions to understand the actual operating conditions of the system. When a fault occurs, the SPS system can respond quickly and determine the type and degree of the fault. Combined with the power flow data of the power transmission section before and after the fault, it can take measures to quickly cut off the power load or adjust the output of the waste heat generator set to ensure the dynamic balance between the power generation end and the power consumption end, and ensure the quality of the busbar power. 3.1.5 Low-frequency and low-voltage load reduction strategy

The SPS system not only has the function of quickly adjusting the output of the waste heat generator set and cutting off the load,

but also introduces a lowfrequency and lowvoltage load reduction strategy, which is the third line of defense for isolated grid operation.

During isolated grid operation, when the fault causes the frequency or voltage of the distribution system to drop below the threshold, the SPS system will automatically start the low-frequency and low-voltage load reduction function, cut off the corresponding load, and prevent the frequency from further decreasing. In addition,

when the grid-connected to isolated grid operation mode, the switching process of the low-frequency and low-voltage load reduction system should avoid the low-frequency and low-voltage state when the isolated grid is just switched in to prevent the load from being cut off by mistake.

3.1.6 Real-time secondary frequency regulation mechanism of isolated grid bus The real-time secondary frequency

regulation mechanism of isolated grid bus monitors the change of grid frequency in real time, compares it with the set value, and calculates the load deviation instruction of the waste heat generator set based on it. It is connected to the existing speed regulation system of the waste heat generator set through hard wiring to achieve rapid adjustment of the active power of the waste heat generator set and maintain the frequency of the distribution system at the rated value. In addition, when the actual rotating reserve capacity of the power system is less than the required value, the system will issue an alarm signal; when an abnormal situation occurs, the real-time secondary frequency regulation will be automatically stopped and an alarm signal will be issued.

3.1.7 Realtime secondary voltage regulation mechanism of isolated grid bus

Under the condition of isolated grid, the bus voltage fluctuation caused by real-time load change or fault will affect the stability of the distribution system.

The realtime secondary voltage regulation system of isolated grid bus is connected to the existing excitation system of the waste heat generator set through hard wiring. According to the deviation between the actual voltage of the isolated grid and the set value, the load deviation instruction of the waste heat generator set is calculated, and its reactive power is adjusted, so as to stabilize the voltage of the isolated grid at the rated value, meet the demand of the distribution system for reactive power, and improve the power quality.

3.1.8 Fast frequency modulation system

The fast frequency modulation system (FMS system) has a fast primary frequency modulation function, which can realize the coordinated control of power supply, power grid and load. Its primary frequency modulation operation cycle is <20ms, the speed sampling cycle is <10ms, and the control cycle of the entire loop is <50ms. The FMS fast frequency modulation system also has a fast secondary frequency modulation function. The FMS fast frequency modulation system can control the OPC solenoid valve action of the waste heat generator set by adjusting the unit speed, speed acceleration, network electrical parameters, etc., to avoid the waste heat generator set from overspeed shutdown and ensure the stable operation of the isolated grid.

3.1.9 Load synchronous grid-connected operation

When the external grid power supply is restored, the distribution system needs to switch from the isolated grid operation mod to the gridconnected operation mode. This project sets a synchronization point at the incoming switch of the No. 2 main transformer of the 6.3kVⅡ section busbar, and sends the instructions such as synchronous permission conditions, synchronous increase and decrease, and synchronous increase and decrease pressure to the machine-grid coordinated control system to realize the load synchronous grid-connected distribution system. After receiving the synchronous increase/deceleration instruction, the machine-grid coordination control system adjusts the output of the waste heat generator set under the synchronous permission conditions; after receiving the synchronous increase/decrease instruction, under the synchronous permission conditions, it adjusts the excitation system of the waste heat generator set to increase or decrease the excitation, so that the important parameters such as the frequency, voltage, and phase angle of the side to be connected can quickly match the system side, achieve the synchronous grid connection conditions, and realize smooth grid connection operation. The synchronous grid connection process with load is shown in Figure 2.

3.2 Structure of the machine-grid coordination control system

In this project, the machine-grid coordination control system mainly includes three parts: the control center, the waste heat generator set-level control interface, and the load side management interface. As shown in Figure 3, the system fully integrates the core functions of external network control, protection devices, and dispatching.

3.2.1 The control center monitors the real-time secondary frequency regulation and voltage regulation system control operation station to master the operation mode, operation status, and abnormal information of the waste heat generator set and the entire plant power grid. Set the frequency and voltage of the isolated grid,

set the manual/automatic operation status of the waste heat generator set, and manually intervene in the load of the waste heat generator set

on the basis of automatic frequency and voltage regulation.

3.2.2 Waste heat generator unit level control The waste heat generator unit level control interface is a real-time secondary frequency and voltage regulation terminal, which conducts twoway data communication with the waste heat generator

unit control system and excitation device, collects the operation data of the waste heat generator unit,

and issues active and reactive load instructions to it.

3.2.3 Load side management The load side management interface collects the current and voltage signals of the load that can be removed from the downstream load station in real time, calculates the load size, and participates in the formulation of the load removal control strategy of the special control system.

IV. Conclusion As an important power source for the isolated grid operation of the power supply system, the waste heat generator set of the cement production line involves many aspects such as the grid operation characteristics and cement production process. In order to ensure that when the external power grid fails, it can quickly switch to the isolated grid operation mode, issue special control instructions, and automatically implement secondary frequency and voltage regulation control to ensure the stability and reliability of short-term or long-term isolated grid operation, close cooperation between power generation, power consumption, and power distribution is essential. By formulating and implementing a stable and reliable isolated grid operation technical plan, the continuous and stable operation of the cement production line can be effectively ensured, providing a strong guarantee for the smooth progress of cement production.