10kV 20MW water cooling MV AC drive used in gas compressor station



Compressor is the core equipment of long-distance gas pipeline project, the electric motor drive compressor has the advantages of stable operation, less failure, easy operation, low maintenance cost, high efficiency, and energy saving etc.
China West-east pipeline, Shanjing line, Zhonggui line, China-Myanmar line, China-Russia line have adopted plenty of variable frequency drives to drive the compressors.
 The electric drive compressor used for pipeline has large power, high voltage level, complicated system, and strict requirements for the stability. 
The technical team of Nancal combine the products design and application experience accumulated in the localization development of the first generation 20MW AC drives for electric compressor, and developed the second generation NC HVVF series large capacity MV AC drives. Compared with the first generation, many major improvement and upgrade have been made in the new generation, which has greatly improved the electrical and safety protection performance of the products, and can better meet the requirements of the compressor used for long-distance gas pipeline. This product has been successfully put into operation in Guangzhou station of China west-east pipeline.

2-NC HVVF series new generation 20MV medium voltage AC drives
For the special requirements for pipeline and other applications, considering the products serialization, the basic configurations are decided as follows:
Corresponding 2units power cell connected in series & 4units power cell connected in series for each phase.
36pulse input.
Output phase voltage is 5 level and 9 level.
Power cell voltage levels: 800A-1000A-1450A-1750A &2100A
Output capacity: 8-21MVA @6kV, 13-36MVA@10kV.
Cell series multilevel PWM technology, 3300V/4500V IGBT, water cooled, oil phase-shifting transformer, DSP+FPGA digital control system, High vector control performance.
2.1 Oil immersed phase shifting transformer
NC HVVF series large capacity VFDs choose oil immersed phase shifting transformer. Transformer will be installed outside, the other cabinets will be installed inside. The 36-pulse rectification is formed by phase shifting of the secondary side and phase shifting of the primary side. The current of the transformer grid side only contains 36k±1 and above harmonics, and the effective value of each harmonic is inversely proportional to the harmonic order, the ratio to the effective value of the fundamental wave is the reciprocal of the harmonic order, the harmonic pollution to the power grid is small, and the power factor is high.
The secondary outlet of the oil-immersed transformer passes through the cable tray and is connected to the power cell by cables. The typical layout is shown in Figure 1.

Figure 1: typical layout of oil immersed transformer

The oil transformer has the following technical features:
* With dual body design (bigger than 15MW), low-voltage winding phase shift combined with high-voltage winding overall phase shift, small phase shift angle error (less than 0.6°), can ensure optimal input harmonics
* Complete protections and monitors including oil level, winding temperature, pressure, gas leakage etc.
* The body of the transformer adopts the self-developed fastening technology; the casing, sealing material and structure are all highly customized design and durable.
* The production uses a unique process and high-grade insulation materials, partial discharge is less than 70pc, lower than the international and domestic similar product standards, extending the service life.
2.2 Power cell  
NC HVVF series large capacity MV AC drives adopt Cell series multilevel PWM technology with water cooling system. Compared with the first generation, it has the following features:
Adopt 3300/4500V IGBT as key components. Compared with IGCT & IGET, IGBT has very simply driving circuit and is flexible in different applications, It is widely used and there are plenty suppliers in the market. IGBT has a positive temperature coefficient and natural current sharing characteristics, and is easy to be parallelly expanded. In addition, the IGBT is more suitable for high frequency output.
The Power cell structure is fully upgraded, the power density is larger (about 15% increase), the busbar stray inductance is smaller, and the maximum current specification of the same volume power cell can be 2100A.
IGBT drive circuit has multi-section switching function of gate resistance during switching process, switching loss is reduced by about 10%, output du/dt is reduced by about 10%, dynamic active voltage clamping technology is used to effectively suppress IGBT peak voltage.
IGBT parallel drive technology upgrade, the current imbalance of parallel IGBT modules is reduced to less than 3%.
Composite busbar with high insulation level, resistance to moisture and dust, suitable for high altitude applications.
DC filter uses metallized film capacitors for long life and high ripple current tolerance.
Adopt water cooling system, water and electricity separate, each power unit has only one inlet and outlet pipe, no leakage risk, high reliability.

Figure 2: 1550/1450A Power cell

2.3 High performance vector control

NC HVVF series VFD adopts high performance vector control algorithm, with offline identification and online correction of motor parameters, high speed control precision, fast torque dynamic response.
The diagram of the control algorithm is shown in Figure 3.

Figure 3: Vector control algorithm diagram

2.4 Procon technology

NC HVVF series medium voltage AC drives are equipped with advanced Procon (Process continuous) technology to improve the continuous operation capability of the device in response to fluctuations in the grid voltage, drop and short-term power outage. And protect the equipment from external interference, ensuring continuous production and reducing customer losses.
Procon integrates a variety of technologies, including LVRT (Low voltage Ride Through), Spinning load pick up, intelligent torque limit of quadratic load, etc. Figure 4 is a diagram of the algorithm of LVRT.

Figure 4: LVRT diagram

2.5 TIPM technology
Phase shifting transformer used for VFD is a kind of special transformer which is very different from conventional kind. When the short circuit, grounding and inter-turn short circuit occur in a secondary winding, the current of the primary winding does not become significantly larger. The normal primary-side configuration integrated protection cannot achieve 100% protection for the multi-winding phase-shifting transformer.
NC HVVF series VFD have secondary winding fault protection technology. When the fault happened in secondary winding, software algorithm can diagnose the fault and protect it in the first time to prevent the transformer.
3 The application in Guangzhou station of West-east pipeline.
In Guangzhou station, the 1# electric drive compressor uses new generation 20MW NC HVVF VFD. The driving motor is a high-speed synchronous motor with rated voltage of 10kV, rated power of 18MW, speed of 4800rpm. And asynchronous AC brushless excitation system. Nancal VFD mode is NC HVVF 10/10-22000SLO.
10/10 means rated input voltage is 10kv, rated output voltage is 10kv.
22000 means rated capacity of VFD is 22MW
S means driving synchronous motor
L means liquid cooled power cell                           
O means oil-immersed transformer
The diagram of the system is shown in Figure 5.

Figure 5: Primary System diagram

The input side of the VFD adopts oil-immersed transformer and 36-pulse rectification technology. Each power cell is powered by secondary windings with different angular phase differences of the transformer, which can eliminate harmonics of 35th and below. The harmonics of the grid side meet the IEEE standard. It is not required to configure the input filter. Therefore, there is no incentive to cause grid resonance, which completely eliminates the occurrence of grid resonance. Four 1550V/1450A power cells are connected in series to form 9 level phase voltage, 17 level line voltage output, output waveform is basically standard sine waveform, du/dt is small and no effect to motor and its connected power cable. It is more conducive to long term stable operation of the motor.


Figure 6 compressor and motor on site photo

The new generation VFD is more suitable for the application needs of the gas compressor station. The operation is stable and reliable. The entire commissioning process from power-on to full-load operation is completed with only 30% of the planned time. The successful delivery of the station laid a good foundation.

Figure 7Output waveform at rated load

Figure 8 Harmonic spectrum of input current at rated load

4- Conclusion
The new generation 20MW HVVF series Medium voltage AC drives independently developed by Nancal adopts oil-immersed transformer, high voltage water cooling power cell and high-performance vector control algorithm. Compared with the previous generation, this product has undergone a number of major improvements and upgrades, which greatly improved the electrical performance and safety protection performance of the product, and can better meet the application needs of the electric drive compressor unit of the natural gas long pipeline. The product has been successfully put into operation in Guangzhou compressor station of west-east natural gas pipeline, which plays an important role in improving the overall technical performance of domestic electric drive compressor unit and can be popularized and applied in the construction of compressor station of natural gas transmission pipeline in the future.