Design and Simulation of Reliability Test System of a Charging Generator

The DC voltage pulsation factor of twelve-phase rectification is much smaller than that of three- and six-phase rectification. Twelve-phase rectifier charging generators are often used where a high quality DC power supply is required. The probability of failure of charging generator is gradually increasing due to its complex structure and function. The reliability test of charging generator can provide a scientific basis for the operation condition, maintenance and determination of the repair cycle of this type of motor. The article combines the basic theory of reliability test, analyses the charging generator constant stress accelerated reliability test scheme, designs the energy feedback test drag system with the generator output as the power source of DC motor, and carries out the dynamic performance simulation and analysis of the reliability test system based on PSCAD/EMTDC. The simulation results show that the energy feedback reliability test system has fast response and good stability.


Introduction
The DC voltage pulsation factor of twelve-phase rectification is much smaller than that of three-and six-phase rectification.Twelve-phase rectifier charging generator is often used to provide high-quality, high-power DC power supply in places where higher quality DC power supply is required [1].The probability of failure of this type of motor is gradually increasing due to its complex structure and function.The failure of a component in the motor may often lead to the failure of the whole machine.A reliability test study of the motor is conducted to determine its reliability characteristics initially, which can provide a scientific basis for the operation, monitoring, maintenance and servicing of this type of motor as well as determining the repair intervals [2][3][4][5][6].
The paper addresses the research object, combines the basic theory of reliability test, analyses and gives the charging generator constant stress ACLERATED LIFE TEST scheme, designs the generator output as the power supply of DC motor energy feedback test drag system, as well as carries out the dynamic performance simulation and analysis research of the reliability test system based on PSCAD/EMTDC.

Types and Methods of Reliability Test
Reliability is the ability of a product to fulfil a specified function under specified conditions and within a specified time.Reliability test refers to all kinds of tests carried out for the purpose of investigating, analyzing and evaluating the reliability of a product, the purpose of which is to assess the reliability level of a product through the analysis of the test results and the analysis of the failure mechanism, to find out the weakness of the reliability of the product, and to put forward targeted recommendations for improvement, so as to improve the reliability of the product.
There are many types of reliability tests and the division of types varies.By the purpose of the test, it can be classified as engineering test and statistical test.By the division of the test place, it can be divided into the use of field tests and laboratory tests, etc.
Life test is a reliability test that evaluates the life characteristics of a product, which is the most important and basic test in reliability test.The life test can be classified into complete life test, truncated life test and accelerated life test according to the sample failure [7].

Reliability Model of Charging Generator
The reliability model is based on the reliability block diagram of the system units, and then the system is derived from the model equations of the units in series and parallel.This charging generator has a complex structure, which is composed of a rotating field synchronous generator body, rectifier, AC exciter, excitation regulator, and built-in temperature sensor.According to its structure and function principle, the reliability block diagram of the charging generator is shown in figure 1.The generator body, AC exciter and excitation regulation system are series models.Both the generator body and the exciter can be equated to series models of their respective stator and rotor windings.It can be seen that a failure in just one part of these stator and rotor windings will cause a failure in this generator system.

Small Sample Accelerated Life Test Programmed for Charging Generator
This charging generator is a special, complex, expensive, and highly reliable product, which is usually subjected to reliability testing under small prototype conditions.In order to study the reliability of a small prototype, a common measure is to subject the product to a reliability test under more severe conditions.Literature [8] proposes to subject the product to severe conditions (increasing the stress to which the product is subjected) for reliability testing.
In order to minimize the test time and to ensure the credibility of the life assessment under small sub-sample conditions, the accelerated life test method is considered.Under the condition of not changing the failure mechanism of charging generator, the failure of the motor is accelerated by increasing the stress it is subjected to.Since the constant stress accelerated life test method is more mature, its accuracy is also higher and the operation is simple, so the accelerated stress should be kept constant during the life test.
Through the analysis of the failure mechanism of this type of motor [9], temperature stress and electric stress are selected as the accelerating stress, and a moderate increase in the temperature of the environment in which the motor is located and an increase in the load of the motor are adopted, as well as a combination of the two is used to increase the experimental stress and to accelerate the aging of the insulating materials of the motor windings.The number of no-load starting and stopping of the motor and the number of sudden loading and unloading of the load are selected to be increased appropriately, and the deformation of the rotor excitation winding is accelerated.Depending on the location and use of the charging generator, a specific environmental simulation and an accelerated life test of the motor operation are developed.

Resistive Load Test System for Charging Generator
In practice, charging generators use turbines or diesel engines as prime movers.In the laboratory, it is obviously not possible to conduct the test in this way.Generally an electric motor is used as the prime mover to drag the charging generator for reliability test.The schematic diagram of charging generator with resistive load test system is shown in figure 2. In figure 2, M is the DC motor as the prime mover, G is the charging generator, and R is the load.
The rated power of this charging generator is about 900 KW.If the test cycle is 5,000 hours, the efficiency of the charging generator is 95%, and the efficiency of the motor is 92%.The total electricity consumption for the test is about 5 million kWh, and the cost of electricity is 0.8 yuan/kWh, which means that the cost of electricity for the test unit alone will be about RMB 4 million.In figure 3, T is the three-phase synchronous motor, M is the DC motor and G is the charging generator.The synchronous motor and DC motor are connected coaxially, and the other end of the DC motor is connected to the charging generator through the gear box.The three-phase synchronous motor is directly supplied by the high-voltage AC power grid (10KV).Another starting power supply (690V) is used to start the DC motor with a three-phase controllable rectifier bridge starter cabinet.

Energy Feedback Test System for Charging Generator
As shown in figure 3, the rectifier bridge drives the DC motor (M) to the rated speed is turned on, and the excitation current of the three-phase synchronous motor (T) is adjusted to establish a voltage of 10kV.The three-phase synchronous motor and the 10kV high-voltage AC grid are connected to the grid, the rectifier bridge is cut off, and the unit (G) is dragged by the three-phase synchronous motor for no-load operation.The charging generator no-load voltage is adjusted to simulate tests such as slow loading and sudden loading; The generator output switch is closed to enable the generator to output electrical power.
After the system has stabilized, the charging generator (G) loads only the DC motor armature circuit.The output power of the generator is the input power of the DC motor.Therefore, the system is a total energy feedback system.The output power can be changed only by adjusting the excitation current of the charging generator.The energy flow of the charging generator energy feedback reliability test system is shown in figure 4. The output power of the generator (POUT) is fed back to the DC motor, and the output of the synchronous motor (P2) only compensates for the loss of power of the whole test system.The power consumption (PIN) of the energy feedback test system is estimated to be about 15% of that of the resistive load test system, which greatly saves the test cost [10].

Simulation Model of the Test System
PSCAD/EMTDC is a power system electromagnetic transient simulation software.It is widely used by domestic and foreign research institutes, schools and electrical engineers because of its large calculation capacity, complete component model library, friendly interface and good openness.
The mathematical and simulation models of twelve-phase (four Y shifted by 15°) synchronous generator are given in the literature, as well as the simulation models of three-phase synchronous motor and DC motor can be directly called from the PSCAD component model library.
The energy feedback reliability test system is composed of three coaxial motors.The rotor equation of motion of the system is different from that of a single motor, and the rotor is subjected to the electromagnetic torque of the three motors at the same time.The system rotor equation of motion is shown in equation (1).
Where esyn T is the input electromagnetic torque of the synchronous generator, edc T is the input electromagnetic torque of the DC motor, e T is the converted output electromagnetic torque of the generator,  T is the total mechanical loss torque of the system, and J is the rotational inertia of the whole unit.
The simulation model of rotor motion equation is built by using the basic operation module in PSCAD component library as shown in figure 5.

Dynamic Simulation of Sudden Loading and Unloading of the Test System
Description of simulation: the test system is running stably without load, the system unit is dragged by a three-phase synchronous motor, the charging generator is running at rated speed, and the switch between the charging generator and the DC motor is in the disconnected state.At 21s, the switch between the charging generator and the DC motor is closed, and the generator is suddenly loaded, and the test system unit transitions from the no-load state to the load state.At 27s, the switch between the charging generator and the DC motor is disconnected, and the test system unit transitions from the load state to the no-load state.As shown in figure 6 and 7, the charging generator of the test system operates stably with no load before 21s, and the load is suddenly added at 21s.After about 2s (23s), the system runs stably with load, and the load is suddenly unloaded at 27s.After about 4s (31s), the system resumes no-load stable operation.The simulation results show that the energy feedback reliability test system has fast response and good stability.

Conclusion
The reliability test study can provide a scientific basis for the operation, maintenance and determination of repair intervals of the charging generator.The design of a reasonable reliability test system is the focus of the test, while the simulation of the test system is the prerequisite for the actual test.This thesis combines the basic theory of reliability test, analyses and designs the reliability test system of charging generator, and carries out the dynamic performance simulation analysis of the reliability test system.The simulation results show that the energy feedback reliability test system has fast response and good stability.

Figure 1 .
Figure 1.Reliability Block Diagram of the charging generator.

Figure 3 .
Figure 3. Schematic diagram of energy feedback reliability test system of charging generator.

Figure 4 .
Figure 4. Energy flow chart of charging generator energy feedback reliability test system.

Figure 5 .
Figure 5. Simulation block diagram of rotor motion equations.

Figure 6 .
Figure 6.Waveforms of electromagnetic torque during sudden loading and unloading.The simulated waveforms of electromagnetic torque of three-phase synchronous motor, DC motor and charging generator during the sudden loading and unloading process are shown in figure6.The simulated waveforms of charging generator output voltage and current are shown in figure7.

Figure 7 .
Figure 7. Waveforms of charging generator output voltage and current during sudden loading and unloading.