Startup control strategies for pre-fabricated synchronous condensers using static frequency converters

In ultra-high voltage transmission systems, a considerable number of dynamic reactive power compensation devices must be equipped. Synchronous condensers are capable of providing substantial amounts of dynamic reactive power. Therefore, distributed prefabricated synchronous condensers can be deployed in ultra-high voltage transmission systems to supply reactive power support to the power grid. The start-up of synchronous condensers can only be achieved through external means. Currently, there are multiple start-up methods available for small-capacity prefabricated synchronous condensers. With the rapid advancements in frequency conversion technology and power electronics, static frequency converters (SFCs) can be utilized to initiate synchronous condensers. This paper investigates the principles of SFC start-up and coordinated control while also configuring protective devices for SFCs based on their structural characteristics and operational principles. A validation platform consisting of one SFC starting two synchronous condensers was established to analyze their start-up time, yielding results demonstrating the ability of SFC-driven distributed small-capacity synchronous condensers to rapidly adjust the power quality of the power grid.


Introduction
The western region of China is abundant in renewable energy resources, where large-scale wind farms and solar power plants have been established [1].The establishment of high voltage direct current (HVDC) transmission networks enables the "west-to-east" and "north-to-south" transfer of electricity, facilitating nationwide energy allocation.HVDC technology offers advantages such as high efficiency, low line losses, and reduced line construction costs [2].However, HVDC technology can only transmit active power to the grid and does not provide dynamic reactive power support.Insufficient dynamic reactive power compensation in the power grid can degrade the quality of electrical energy and compromise voltage stability within the grid [3].
Compared to static reactive power compensators and static synchronous compensators, synchronous condensers offer advantages such as high voltage withstand capability, low voltage ride-through capability, and high overload capacity.Furthermore, synchronous condensers have the ability to provide a substantial output of reactive power, both capacitive and inductive [4] [5].Therefore, the utilization of synchronous condensers as reactive power compensation devices is essential in HVDC systems [6] [7].The synchronous condenser system primarily consists of the controller itself, excitation system, booster transformer, starting system, cooling system, oil system, and protection system [8].The key technologies involved in its operation include grid connection startup technology, excitation control technology, monitoring technology, and protection technology [9] [10].Among these, the startup techniques for the synchronous condenser can be categorized into three types: auxiliary motor startup, asynchronous startup, variable-frequency startup, and hydraulic coupling startup.
Compared to other startup methods, the utilization of SFC for startup offers advantages such as low cost, high capacity, adjustable capacity, the ability to start multiple synchronous condensers, ease of operation, convenient maintenance, and high reliability [11].As early as the 1970s, SFC products emerged for starting pump storage units and synchronous condensers.The research on SFC control strategies has mainly focused on areas such as rotor position detection for synchronous condensers and commutation for inverters, achieving certain achievements [12].However, currently, most SFC control systems employ independent control for SFC and startup excitation systems [13].In this study, a coordinated control strategy is adopted where speed information is fed forward to the rectifier control system within the SFC system, enhancing the capacity, startup efficiency, and reliability of the SFC.

Topology of SFC
The basic topology of SFC consists of a rectifier and an inverter forming a "back-to-back" system, where the rectifier and inverter can adopt forms such as 18-pulse, 12-pulse, or 6-pulse.Among these, the 12pulse topology is illustrated in Figure 1(a), while the 6-pulse form is shown in Figure 1(b).In the case of meeting harmonic requirements, the 6-6 pulse form can be chosen.

Principles of Operation for SFC
The process of starting up a synchronous condenser using SFC can be divided into three stages: speed adjustment, voltage building, and grid connection.In the speed adjustment stage, the utility grid voltage is stepped down by a voltage transformer and rectified into DC voltage through the rectifier bridge.The DC current then passes through a smoothing reactor to output a smooth DC bus current.Next, based on the rotor position of the synchronous condenser, the inverter thyristors are controlled to conduct for specific durations, continuously increasing the speed of the synchronous condenser until it reaches 105% of its rated speed.At this point, the synchronous condenser exits the SFC system.During the start-up period, the excitation system of the synchronous condenser adopts the startup excitation system.The synchronous condenser operates in an unloaded state.Upon the exit of the SFC system, the synchronous condenser transitions into an idle deceleration state.During this idle deceleration period, the excitation system of the synchronous condenser is switched, disengaging the start-up excitation system and engaging the main excitation system to commence voltage building.An automatic quasisynchronization device monitors the frequency, magnitude, and phase of the voltage.It regulates the output voltage magnitude, frequency, and phase of the synchronous condenser by adjusting the main N:1 excitation system.Once the automatic-throwing-switch device detects that the frequency, magnitude, and phase of the synchronous condenser's output voltage are compliant with the grid requirements, the circuit breaker is closed, subsequently integrating the synchronous condenser into the power grid.

3.1Coordinated Control of SFC
During the startup process of the synchronous condenser by SFC, it is necessary to adjust the speed of the synchronous condenser according to the given speed and control the excitation system to output the desired excitation current.In other words, during the synchronous condenser startup, the stator current, rotor current, and rotor speed need to be closely coordinated.When using an independent control strategy to individually control the rectifier, inverter, and rectifier bridge in the startup excitation system of SFC, the coordination response between the stator current and rotor speed is relatively slow.Therefore, a coordinated control strategy can be adopted to improve the dynamic response speed and suppress fluctuations in the input on the rectifier side.
The coordinated control strategy diagram for the rectifier, inverter, and excitation rectifier in the excitation system of SFC is illustrated in Figure 2.  of the synchronous condenser, ensuring its smooth acceleration to 105% of the rated speed.The control process involves the following steps: the difference between the set speed and the measured speed is adjusted by a PI controller to generate a DC current setpoint.Then, the difference between the DC current setpoint and the measured grid current information is regulated by another PI controller to determine the triggering angle of the thyristors.
(2) Control Strategy for SFC Inverter The control strategy utilized for the SFC inverter involves fixed-angle triggering.The inverter employs thyristors, which are a type of semi-controlled device that can be controlled for conduction but can only be turned off by applying a reverse voltage.
During the synchronous condenser startup process in SFC, the inverter commutation stage can be divided into two parts: forced commutation and natural commutation.At low speeds, the terminal voltage of the synchronous condenser is relatively low.After triggering the thyristors in the inverter, it is impossible to obtain the current needed to turn off the thyristors, resulting in a failure to commute smoothly.Therefore, forced commutation is required, which involves turning off one pair of thyristors by interrupting the rectifier bridge and then triggering another pair of thyristors.Once a certain speed is reached, the terminal voltage of the synchronous condenser can directly trigger the thyristors.
This study employs the 0° fixed-angle triggering technique to achieve a higher power factor and torque.Moreover, the utilization of 0° fixed-angle triggering ensures reliable commutation of the inverter and enhances the startup success rate of the synchronous condenser. (

3) Excitation System Inverter Control Strategy
The control of the excitation system for the synchronous condenser is divided into two stages: lowspeed stage and high-speed stage.Both stages employ a PI controller.In the low-speed stage, the control variable is the excitation current, while in the high-speed stage, it is the unit magnetic flux.The output of the PI controller is either the excitation current reference value or the excitation voltage reference value.This reference value is then adjusted by the PI controller to determine the triggering angle of the thyristors.

3.2Logical Control of SFC
Figure 3 illustrates the main start-stop process of SFC.Prior to initiating SFC, the power supply status needs to be checked, followed by the issuance of the "SFC ready" signal.Upon receiving the "SFC ready" signal from the distributed control system, a closing signal is sent to the high-voltage isolation switch, and the SFC confirms the synchronous condenser to be started based on the received signal.After closing the switching device for the selected synchronous condenser, SFC issues a command to close the input circuit breaker.Under normal feedback information from the input circuit breaker, SFC controls the excitation system and the "back-to-back" converter to accelerate the synchronous condenser until its speed reaches 1.05 times the rated speed.SFC issues an instruction when the desired speed is reached and then waits to receive a shutdown instruction.Once SFC receives the shutdown instruction, it commands the excitation system to stop, opens the high-voltage isolation switch, and performs the transition between the main excitation system and the startup excitation system.SFC awaits further instructions, which can be categorized into three scenarios: "fast restart," "start the next unit," or "shutdown of the SFC system." Figure 4(a) shows the flowchart of SFC after receiving the "fast restart" instruction.In contrast, Figure 4(b) shows the flowchart of SFC after receiving the "start the next unit" instruction.Figure 4(c) shows the flowchart of SFC after receiving the "shutdown of the SFC system" instruction.

Experimental validation
In order to validate the effectiveness of the methodology employed in this study, experimental validation was conducted.The schematic diagram of the system is depicted in Figure 5.The working status during the start-up of the synchronous condenser is depicted in Figure 6.As observed from Figure 6, upon receiving the start-up command for the synchronous condenser, SFC initiates the excitation system to begin the excitation process.After approximately 10 seconds, when the rotor speed reaches 105% of rated speed, the start-up excitation system switches to the main excitation system.The main excitation system is then activated, and the synchronous condenser starts generating voltage, resulting in a decrease in frequency at the terminal of it.

Main excitation current
According to Figure 6, it can be observed that upon receiving the start-up command for the synchronous condenser, the rotor speed increases from 0 rpm to 3150 rpm, which is approximately 1.05 times the rated speed.At this point, the terminal frequency of the synchronous condenser is 52.5 Hz.When the SFC is disengaged, and the main excitation system is activated, the speed of the synchronous condenser starts decreasing gradually until it reaches 3000 rpm, accompanied by a decrease in the frequency to 50 Hz.
Figure 7 presents the waveform of reactive power output after two synchronous condensers are connected to the grid.From the graph, it can be observed that upon starting the second synchronous condenser using SFC, the fluctuation in the reactive power supply to the grid by the synchronous condenser system is minimal.This indicates that multiple synchronous condensers can be smoothly started using SFC.

Conclusion
This paper presents the topology of the SFC and proposes a coordinated control strategy for the SFC.Additionally, the operational workflow of the SFC system in practical applications is elucidated.The results demonstrate that, under the influence of this coordinated control strategy, the synchronous condenser is capable of rapidly accelerating to 1.05 times the rated speed for the excitation system switching process.

Figure 2 .
Figure 2. Coordinated Control Strategy between SFC and Excitation System.

( 1 )
Control Strategy for SFC RectifierThe rectifier bridge in SFC adopts a dual-loop control strategy, where the outer loop serves as a speed loop and the inner loop functions as a current loop.The ultimate controlled variable is the stator current.The speed loop has a large time constant, resulting in a slow dynamic response but excellent steady-state performance.On the other hand, the current loop has a small time constant, contributing to a fast dynamic response and superior transient performance.The dual-loop system works together to control the speed

Figure 3 .
Figure 3. Main Start-up Process of SFC.

Figure 4 .
Figure 4. Workflow Diagram of SFC.(a) Quick Restart Process of SFC, (b) Process of Starting the Next Unit in SFC, (c) Shutdown Process of SFC System.

Figure 5 .
Figure 5. Schematic Diagram of the System Architecture.

Figure 6 .
Figure 6.Waveform of the Synchronous Condenser during Start-up, (a) Rotor Speed, (b) Terminal Voltage of the Synchronous Condenser, (c) Terminal Frequency of the Synchronous Condenser, (d) Excitation Current Waveform.