Research on Asynchronous Starting Characteristics of Synchronous Motors Based on TSC Reactive Power Compensator

The asynchronous starting current of synchronous motor is very large, which will cause a sharp decline of power grid voltage. Therefore, TSC reactive power compensator was used to provide the reactive components of the starting current to maintain grid voltage stability. TSC reactive power compensator provides is in accordance with 8:4:2:1 unequal capacity grouping mode and the reactive power control mode, using thermistor switches to put in or cut out respective capacitors in real time based on reactive power of the synchronous motor needed to asynchronous starting. The asynchronous starting process were simulated with using MATLAB/ Simulink. The feasibility of reducing system voltage drop with using reactive power compensation in asynchronous starting process was verified by the simulation. On the other hand, Simulation results showed that reactive power compensation can effectively improve the starting speed and reduce starting time. To ensure the quality of the grid voltage, the asynchronous starting with using reactive power compensation should be used for synchronous motor, compared with the direct asynchronous starting mode.


Introduction
Synchronous motors have the advantages of high overload capacity and operational stability, its rotor speed completely depends on the power frequency, does not change with the load in steady state operation.It has become the main power of industrial and mining enterprises.The disadvantage is that the synchronous motor cannot directly start by its own.Therefore, it's important to research on the start-up mode of synchronous motors.
When using auto transformer to reduce voltage to start large synchronous motors [1], although the starting current is reduced [2], but starting torque decreases with the square of the voltage [3], leading to start time increased more.Literature [4] studied soft start technology principle and control system of frequency conversion starting.However, due to the long starting time, the technical defects of harmonic interference and the high cost of soft start equipment, the use of frequency conversion starting still subject to certain restrictions.Literature [5] considered the line impedance, the mathematical model of asynchronous full-voltage starting were established, but the measures to stabilize the voltage of the network were not presented.
In order to solve the starting problem of synchronous motor, this paper proposes an asynchronous start mode based on TSC reactive power compensator.In this start mode, according to the monitored reactive power required to start the motor, the TSC reactive power compensator send a switching signal, then the corresponding capacitors are connected to the grid to provide reactive power and compensate starting current, so as to maintain grid voltage stability.Finally, the feasibility of this start mode was verified by the simulation example.

Asynchronous starting principle based on reactive power compensation
Asynchronous starting principle of synchronous motor based on reactive power compensation is shown in Figure 1.Before starting the motor, all switches are open.To start the motor, firstly, the bidirectional switch K1 of the rotor winding access the additional resistance, whose value is 10 times of the value of the field winding resistance.Then switch K2 is closed, that is synchronous motor stator via the transformer is put into power grid.Synchronous motors rely on asynchronous starting torque to accelerate rotation of the rotor until its speed is close to synchronous speed.When the rotor speed reaches 90% of synchronous speed, throw the bidirectional switch K1 to the excitation power, then the rotor will be pulled in synchronization relying on the synchronizing torque.Be noted that the switch K3 should be closed ahead of the switch K2.Put some capacitors in advance to ensure that the system voltage does not exceed 5% of rated voltage.By doing that, avoid pulling voltage down and resulting in low reactive power compensation value in initial period of starting.Then the dynamic reactive power compensation device by analyzing the detected reactive power put in the appropriate number of capacitor groups to compensate the inductive current whose value is several times of the rated current during the motor startup process.
Thus synchronous motors use the start mode without obtaining excessive reactive power from the system, this can improve the power grid voltage, reduce line losses, improve the transmission capacity of the power system, and improve utilization of other devices in power system.

Effect of reactive power compensation on voltage drop
When the circuit is calculated, generally use per unit form, take each bus rated voltage as the reference voltage.The equivalent reactance of the infinite system is S is short-circuit capacity at the exit of the infinite grid system; B S is reference capacity.The equivalent impedance of the motor access system is net is the equivalent reactance of transformer; L Z is equivalent impedance of transmission lines.net Z is a complex number, which can be decomposed into real and imaginary parts.net R , the real part, is the equivalent resistance of the motor access system, generally is small; net X , the imaginary part, is the equivalent reactance of the motor access system.The voltage drop from the outlet side of the motor to the grid side is shown in (1).
When the synchronous motor is directly started, if the reactive power compensation device is not set up, that is the switch K2 in Figure 1 is not closed, the voltage drop of the motor outlet to the power network side is shown in (2).
When synchronous motors is started asynchronously based on reactive power compensation, the dynamic reactive power compensation device put in the appropriate number of capacitor groups to compensate reactive power generated by synchronous motor.Thus capacitive current generated by capacitor groups and inductive components of starting current of synchronous motor offset each other, that is the current at the motor outlet is purely resistive component, the voltage drop of the motor outlet to the power network side is shown in (3).
The relationship between the above voltage and current is shown in Figure 2. Figure 2 shows that, the gap of voltage drop before and after reactive power compensation is obvious.With the line impedance increasing, especially in long distance power supply of large synchronous motors, voltage drop caused by the direct asynchronously starting also increased, causing severe voltage fluctuations of other electrical equipment in the power line.besides with the grid voltage reducing, motor starting process will be extended [5].Therefore, it is necessary to set reactive power compensation according to the network parameters during starting asynchronously in order to stable grid voltage and reduce power loss.

Grouping and switching control of TSC reactive power compensator
Thyristor switched capacitor (TSC) cannot continuously adjust reactive power, but TSC does not produce harmonics, and generate a smaller loss and noise [6,7], therefore TSC has been more widely used in the power system.TSC has the function of phase separation.Each phase chooses different input time, to ensure that each phase does not generate impulse current, reducing interference on the power grid, increasing the life of the compensating device [8].In addition, TSC device uses star wiring, the phases were separately sampled.TSC reactive power compensator compensates by putting different capacitors separately based on each phase reactive power, without causing overcompensation and making three-phase voltage is more stable.TSC is a kind of reactive power compensation device which can export intermittent adjustable reactive power.If the number of the capacitor is sufficiently refined, the continuous adjustment can be achieved basically.There are isochoric capacity grouping and unequal capacity grouping.In this paper, according to the capacity ranging grouping and the principle of 8:4:2:1, setting four capacitors, 16 kinds of different combinations can be achieved.The four capacitors are connected in parallel, the capacity of which respectively were Q is the single stage compensation capacity, whose theoretical value is about one-fifteenth of the maximum reactive power demand during asynchronous start of synchronous motor.The actual setting can be further adjusted to avoid excessive compensation.Each capacitor in series with bidirectional thyristor is connected to the grid, where the thyristor acts as a switch.Its single phase compensation principle is shown in Figure 3.In Figure 3, Q is setting to avoid switching frequently the thyristors in the steady running state with small disturbance, whose value is the reactive power demand of the motor in the steady running state.In Figure 3, the waveform is the assumption of the single phase reactive power demand of the synchronous motor asynchronously starting in the absence of reactive power compensator.The single stage compensation capacity 0 C Q is determined by (4).The level of reactive power demand int Q is determined by ( 5), the function INT can make a numeric rounded down to the nearest integer.Four signals K 8 , K 4 , K 2 , K 1 correspond to the switching signal of four sets of capacitors in Figure 3, using 1 and 0 to indicate input and removal of the corresponding capacitor, K 8 , K 4 , K 2 , K 1 are determined by (6).
When the thyristor receives switching signals, in order to make the impact of current is zero, the control strategy of thyristor is triggered by zero voltage moment.The trigger pulse corresponding to the reactive power demand are obtained by Switching signal and zero-voltage pulse signal through the logic AND gate.The trigger pulse control putting corresponding capacitors into system to compensate reactive power.When ( n K n = is 0, the trigger pulse of the reactive power demand is always 0, so the corresponding capacitors are removed from the system.When ( 8, 4, 2,1) n K n = is 1, the zero-voltage pulses work, the corresponding capacitors are access to the system.

The analysis of simulation
In the MATLAB/Simulink environment, the power frequency 50Hz system is established.The simulation parameters are shown in Table 1.Infinite system power grid through the 3km long line supply power to the 100kW load and synchronous motor.In order to improve the load terminal voltage, TSC reactive power compensator is added between the load and the synchronous motor.Without TSC reactive power compensator, motor at the time of 4.2s or so ended the asynchronous starting process, at the time of 5s, motor was gradually pulled into synchronization.With TSC reactive power compensator, motor at the time of 3.3s or so ended the asynchronous starting process, at the time of 4.2s,motor was gradually pulled into synchronization.So the start speed can be improved significantly by TSC compensation.In the course of starting process and steady state process, the voltage effective value and voltage offset of the motor outlet are shown in Table 2.As shown in Table 2, without reactive power compensator, the voltage offset during the startup process is too large to meet the requirement of power system.However, with reactive power compensator, the voltage offsets during the startup process and steady state process are in line with the requirements.Therefore, reactive power compensation can improve and stabilize the voltage of the motor outlet.Through the analysis, we can draw: when synchronous motor is started asynchronously, TSC reactive power compensator can improve and stabilize terminal voltage, and can increase the startup speed.

Conclusion
This paper puts forward a kind of asynchronous starting mode of synchronous motors based on TSC reactive power compensator.Simulation results show that the asynchronous starting of synchronous motor when using TSC reactive power compensator with the advantages in two aspects: compensating the start current can effectively improve and stabilize the system voltage; due to the stability of the voltage of the motor outlet, the startup speed of the synchronous motor can be significantly improved.

Figure 1 .
Figure 1.Asynchronous starting principle of synchronous motor based on reactive power compensator

Figure 2 .
Figure 2. Phasor diagram of Voltage and current before and after reactive power compensation

MaximumFigure 3 .
Figure 3. Switching schematic diagram of Single-phase capacitors  in (2) is the starting current, its value is usually several times the rated current, can be divided into resistive components and inductive components, that is Unauthenticated Download Date | 6/22/19 6:44 AM start I

Table 1 .
Simulation Parameters Table

Table 2 .
Simulation Results Table