Retrofit Design of High Voltage Motor Frequency Converter

“For the safety of high-voltage inverters with motors in energy vegetation at current, the standard phasor differential methodology utilizing Fourier algorithm isn't appropriate, and the differential safety algorithm of sampled worth can be utilized to appreciate differential safety; the present two transformers might be mixed The capabilities of the motor safety machine and the motor safety machine are built-in into one machine to appreciate backup safety, which not solely meets the necessities of motor safety, but in addition saves prices.

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Summary: With the implementation of the nationwide vitality coverage, increasingly energy vegetation use high-voltage frequency converters to drive electrical motors. This paper focuses on the introduction of a brand new inverter motor differential safety machine developed by a know-how firm, which adopts the sampling worth differential precept to appreciate the differential safety of the inverter motor; the transformer and motor safety capabilities are built-in into one machine. The exhausting strain plate switched by the bypass swap is forged and withdrawn to appreciate the operate of backup safety.

In keeping with the necessities of the nationwide vitality coverage, vitality conservation and emission discount work has been carried out in an all-round means, and in massive thermal energy vegetation, it's crucial to scale back the facility consumption charge of the plant. For top-voltage motors, which account for a lot of the energy consumption of the plant, an necessary technical measure within the subject of vitality saving is the applying of high-voltage frequency conversion know-how. With the event of electrical energy, frequency converters have been broadly utilized in energy vegetation. At current, in newly constructed energy vegetation, necessary auxiliary machines equivalent to followers and water pumps are typically required to be pushed by frequency converters. After the high-voltage motor is pushed by a frequency converter, configure the motor safety to make sure the secure and dependable operation of the unit has turn out to be a priority of energy vegetation, design institutes and safety producers.

1 Conventional motor safety configuration

The faults of asynchronous motors embody stator winding interphase short-circuit fault, winding inter-turn short-circuit fault and single-phase grounding fault; irregular working states primarily embody overload, locked rotor, too lengthy beginning time, unbalanced three-phase energy provide or phase-off operation, Irregular voltage, and so on. Due to this fact, for high-voltage motors, in response to the laws, differential safety or present quick-break safety is the primary safety, and overload safety, overcurrent safety, detrimental sequence safety, zero sequence safety and low voltage safety are used as backup safety.

2 Present inverter motor safety configuration

To be able to make sure the reliability of the system in energy vegetation, high-voltage motors typically use frequency converters with energy frequency bypass, in order that even when the frequency converter is overhauled, the facility frequency bypass might be handed to make sure the conventional operation of the motor. Determine 1 is a schematic diagram of the on-site high-voltage motor inverter transformation. The switches K1 and K2 be certain that there isn't any contact level with the primary circuit when the inverter is overhauled. At the moment, the swap K3 is closed and the motor runs by way of the bypass.

When the motor runs by way of the bypass, the motor is instantly pushed by the facility frequency voltage of the high-voltage busbar of the manufacturing unit energy. The safety objects of the safety machine on the incoming swap QF are the swap outlet and the motor physique. Due to this fact, at the moment, motor safety ought to be configured in response to the necessities of typical motor safety. If there's a differential safety requirement, it's essential to configure motor differential safety.

When the bypass swap K3 is disconnected and the motor is pushed by the inverter, the safety objects of the safety machine on the incoming swap QF are the swap outlet and the inverter. As a result of the frequency converters utilized in energy vegetation are typically composed of rectifier transformers, management cupboards and different elements, that's, the safety objects of the safety machine on the incoming swap QF are the swap outgoing wires and the rectifier transformer. At the moment, the motor turns into the load of the high-voltage inverter after being remoted from the manufacturing unit energy bus, so the safety of the motor ought to be realized by the controller of the high-voltage inverter system. For six-10 kV rectifier transformers, it's typically outfitted with typical transformer backup safety, which is barely completely different from typical transformers when setting. At the moment, the standard differential safety of the motor can't carry out differential safety as a result of the present on the swap is inconsistent with the present frequency of the impartial facet of the motor, and may solely be withdrawn.

At current, the final inverter motor safety configuration contains: motor safety measurement and management machine, motor differential safety machine, and transformer safety measurement and management machine. The operate of the motor safety machine and the transformer safety machine is switched on and off by way of the bypass swap: that's, the bypass swap is disconnected, and the inverter drives the motor at the moment, the transformer safety machine is switched on, and the motor safety machine and the motor differential safety machine are withdrawn; When the bypass swap is closed, the facility frequency grid instantly drives the motor, the motor safety machine and the motor differential safety machine are put in, and the transformer safety machine is out.

At current, there are two primary issues with this safety configuration methodology:

(1) For motors above 2 000 kW, differential safety must be configured. Due to this fact, when the inverter drives the motor, the differential safety of the motor is withdrawn, and the reliability of the safety is affected.

(2) At any time, just one transformer safety machine and motor safety machine is put into use, which reduces the use effectivity of the machine.

3 Inverter motor differential safety

Within the case of utilizing a frequency converter to drive the motor, the rationale why the standard motor differential safety can't be used is: the CT on the motor finish is the present frequency of CT1 on the swap cupboard in Determine 1 and the CT on the impartial facet of the motor, that's, CT3. Not the identical. The literature proposes to make use of magnetic stability differential safety to realize, however there are a number of issues in follow:

(1) At current, the motors utilized in energy vegetation are mainly unable to offer the impartial facet cable leads required for the magnetic stability differential.

(2) The present of the magnetic stability differential is under the inverter and isn't an influence frequency present. For microcomputer safety, the setting worth set in response to the facility frequency 50 Hz isn't relevant to the non-power frequency scenario.

As a result of the present on either side of the differential safety should be the present on the identical frequency. Contemplate putting in a gaggle of CTs beneath the inverter and above the motor, specifically CT2. This group of CTs might be put in within the inverter cupboard, and the differential safety is fashioned by the currents of CT2 and CT3.

The traditional differential safety is phasor differential. The precept is to make use of the Fourier algorithm to calculate the actual and imaginary elements of the incoming and outgoing currents in response to the sampling level of a cycle, after which calculate the amplitude and section of the differential and braking currents. The criterion is constructed by the use of phasor comparability. For the reason that present isn't the 50 Hz energy frequency, it's vital to trace the frequency to make sure the correctness of the calculation consequence when performing the Fourier calculation. Since no voltage is launched beneath the inverter, it can't be achieved by the standard voltage monitoring frequency methodology. Some producers suggest to make use of the present monitoring frequency, however as a result of massive error of the present monitoring frequency, it's simple to trigger malfunction and refusal of the safety, so it's not utilized in follow.

For the sampling worth differential used within the differential safety, all channels within the microcomputer safety are sampled because the instantaneous worth of the present on the identical time: when the protected tools has no lateral inside fault, the sum of the sampled present values ​​is zero; When a fault happens, the sum of every sampled present worth isn't zero. Sampling worth differential safety is constituted by the sum of sampling worth present in response to a sure motion criterion.

In contrast with the standard phasor differential safety, the sampling worth differential has the traits of quick motion pace and fewer calculation quantity. The sampling worth differential doesn't contain Fourier calculation, and the harmonics introduced by the inverter won't have an effect on its calculation diploma. Due to this fact, for the high-voltage variable frequency motor working at 25 ~ 50 Hz, the differential safety might be realized by this algorithm. .

The inverter motor differential safety developed by Jiangsu Jinzhi Know-how Co., Ltd. primarily based on the sampling worth differential precept has been efficiently put into operation on web site. After operating for a time frame, there was no safety malfunction or refusal to function, indicating that the sampling worth differential It may be utilized to differential safety of inverter motors. The present enter of the differential safety machine has 3 units of CTs, that are CT1 on the swap facet, CT2 added above the motor under the inverter, and CT3 on the impartial facet. On the identical time, the facility frequency bypass swap contacts are launched. When the contact of the bypass swap is closed, it's a typical phasor differential, utilizing the Fourier algorithm, and the differential present is the present of CT1 and CT3; Sampling worth differential algorithm, the differential present is CT2 and CT3 present.

4 Inverter motor backup safety

At current, the final inverter motor is provided with a motor safety measurement and management machine and a transformer safety measurement and management machine, and the bypass swap is used to change between the 2 units. Since solely one of many two units is put into use at any time, the use effectivity of the machine is diminished.

The principle capabilities of transformer safety embody overcurrent safety, detrimental sequence safety, grounding safety, overload safety, and so on.; motor safety capabilities primarily embody overload safety, overcurrent safety, detrimental sequence safety, zero sequence safety and low voltage safety.

On this foundation, Jiangsu Jinzhi Know-how Co., Ltd. has developed a brand-new inverter motor backup safety machine. The machine contains transformer safety and motor safety capabilities. The transformer safety and motor safety capabilities are switched on and off by way of the bypass swap. The exhausting urgent plate fashioned by the contact is used for management, in order that the capabilities of the unique two units are built-in into one machine, and the utilization charge of the sector machine is improved. This backup safety machine has been broadly utilized in many websites and has been unanimously acknowledged by customers.

5 Conclusion

For the safety of high-voltage inverters with motors in energy vegetation at current, the standard phasor differential methodology utilizing Fourier algorithm isn't appropriate, and the differential safety algorithm of sampled worth can be utilized to appreciate differential safety; the present two transformers might be mixed The capabilities of the motor safety machine and the motor safety machine are built-in into one machine to appreciate backup safety, which not solely meets the necessities of motor safety, but in addition saves prices.