1. Characteristics of power system of offshore platform

Safe and reliable electrical energy is an important power guarantee for the development of offshore oil and gas fields. The power system of the offshore oil platform is affected by the marine environment and service industry, and its power system composition, voltage level, total installed capacity of the power station, and the type and nature of electrical equipment and loads are very different from those on land. The power system of the offshore platform mainly has the following characteristics: (1) It is an isolated power grid that integrates power generation, substation, transmission and distribution; (2) The single unit capacity and total installed capacity are relatively small, far smaller than the land grid; (3) ) The power supply voltage is low, and the transmission distance is short; (4) The generator outlet is directly connected to the busbar, and multiple generators operate on the same busbar; (5) The power transmission is mainly transmitted through cables, and the ground capacitance of the transmission line is large; ( 6) Compared with the main generator, the capacity of the motor and the transformer is larger, the starting voltage drop of the motor is large, and the excitation inrush current is high when the transformer is air-dropped.

2. Problems existing in the safety of power supply for offshore platforms

With the implementation of Offshore Oil’s “deepwater strategy” and “second leap”, the pace of offshore oil and gas field development has been accelerated, and the power system of offshore platforms has also developed by leaps and bounds. The larger power network integrating electricity, transmission and distribution has gradually developed from an isolated power station of a single platform in the past to a more complex power network that interconnects multiple platforms and even the entire oilfield group network. At present, the single unit capacity and total power station capacity of the power system in offshore oil and gas fields are constantly increasing, the power of substation transformers is increasing, the voltage level is getting higher and higher, the transmission distance is getting longer and farther, and the system topology structure is becoming more and more complex. Therefore, there are some new situations and new problems that affect the safety and stability of the power supply of the platform. For example, the high DC component of the short-circuit current makes it difficult to break the circuit breaker during short-circuit; when the large-capacity transformer is closed at no-load, a huge inrush current is generated, which will affect the system. cause serious impact. These problems have a serious impact on the power supply security and stable operation of the offshore platform power system. Once a node in the system fails, it will not only directly affect the power supply security of the platform, but may even interrupt the power supply of the entire area where the network operates. This will cause large-scale power outages and production stoppages in oilfields, and may even lead to serious safety accidents, resulting in major property and economic losses.

3. Power supply security analysis and solutions

3.1 High short-circuit current DC component

With the continuous improvement of the single unit capacity and the total installed capacity of the power system of the offshore platform, the ratio of the equivalent inductance and resistance of the medium voltage system from the short-circuit point increases, and the attenuation time of the DC component is prolonged, causing the medium voltage system circuit breaker to open when The high DC component of short-circuit current makes it difficult to break, which will seriously affect the safe and stable operation of the power system. The influence of the DC component of the short-circuit current on the normal breaking capacity of the circuit breaker is mainly determined by the following two aspects: (1) The longer the duration of the DC component is, the greater the full current peak value of the first short-circuit half-cycle, and the longer the duration of the DC component will be The energy released by the arc is also larger, which makes it difficult for the circuit breaker to break; (2) the excessive DC component will cause the zero-crossing drift. Through calculation, on some branches, the short-circuit current is small, but the existence of the DC component may cause the short-circuit current to drift at the zero-crossing point. In more serious cases, the arc extinguishing time becomes longer, and even the arc is not easy to extinguish. Studies have shown that when the proportion of the DC component to the amplitude of the AC component of the short-circuit current is less than 1/5, when the circuit breaker is designed and selected, only the rms value of the AC component of the short-circuit current can be verified; when the DC component accounts for the amplitude of the AC component of the short-circuit current When the proportion of the circuit breaker is greater than 20%, the design and selection of the circuit breaker should not only verify the rms value of the AC component, but also consider the influence of the percentage of the DC component. When designing and selecting a specific power system, at present, there are three types of circuit breakers suitable for generator outlet: ordinary vacuum circuit breaker, generator outlet special circuit breaker and fast current limiter. The advantages and disadvantages of the three circuit breakers are different. For example, as shown in Table 1. In the actual selection, the technical feasibility and economy should be considered as a whole according to the grid topology and the actual short-circuit calculation value, and the short-circuit breaking equipment to be used should be judged. A power station of an offshore project is equipped with three large-capacity generator sets. When the generator outlet is short-circuited in three phases, the DC component is as high as 60%, and ordinary circuit breakers cannot be interrupted at all. To solve this problem, the design adopts the “3+2” system topology for the first time, that is, the generator outlet bus is divided into three sections, each powered by a generator, and the three sections of the bus are connected by two fast current limiters. After using the fast current limiter, when a short-circuit fault occurs, the fast current limiter can act within 10ms to quickly divide the power system into three sub-networks (de-column operation), thereby reducing the short-circuit current. After the action of the fast current limiter, the AC and DC components of the short-circuit current are greatly reduced, and the DC component is reduced to 19.6%, and the circuit breaker can operate reliably.

3.2 Transformer no-load closing excitation inrush current is large

The power system of the offshore platform is an isolated system. When the main generator of the platform starts in black, the emergency generator is firstly turned on to supply power to the auxiliary panel of the main generator; Turn on the distribution transformer to switch the auxiliary power supply from the emergency generator to the main generator. At this time, the system only has a single generator running, and the capacity is small. When the transformer is put into operation at no-load, it will generate a large excitation inrush current, which is 6 to 10 times the rated current of the transformer. Excessive excitation inrush current will cause a great impact on the system, and even directly cause the breaker at the outlet of the main generator to trip. Therefore, the excitation inrush current of the transformer must be limited. There are two main inrush current suppression methods commonly used in offshore engineering projects: (1) The inrush current suppressor is used to limit the inrush current of the transformer. By monitoring the magnetic flux of the transformer, the opening and closing angle of the circuit breaker is controlled, so as to suppress the transformer excitation inrush current and avoid the circuit breaker tripping due to the excitation inrush current. In addition, the inrush current suppressor also has a control compensation function, which can dynamically determine the opening and closing points according to the factors affecting the accuracy of the opening and closing angle of the circuit breaker to ensure the effect of inrush current suppression. By using the inrush current suppressor, the success rate of transformer airdrop is greatly improved, and the transient stability of the power system is also improved. In a recent project, an inrush current suppression device based on the three-phase linkage closing technology was used. According to the feedback from the operator, the application effect was good, and the inrush current could be suppressed to a lower level. (2) A solid-state thyristor soft starter is used to start the large-capacity transformer. This technology mainly uses the principle that the solid-state thyristor soft starter can control the voltage output, uses the voltage ramp function of the soft starter to slowly increase the voltage on the primary side of the transformer, and magnetizes the transformer through the linear increase of the primary side voltage to establish a working magnetic field. , during the start-up process, it can be started based on zero voltage, so there is almost no inrush current. The method has the characteristics of simple principle, fast calculation, good practicability, etc. It solves the problem that the transformer fails to close due to excessive excitation inrush current when the transformer is closed at no load, and reduces the adverse effect of shortening the service life of the equipment caused by the large current impact, extending the normal operating hours of the device.

3.3 The problem of the main generator advancing phase operation

The power system of the offshore platform mainly adopts the centralized power supply method. The main power station usually sets several generator sets with the same capacity, and supplies power to other nearby wellhead platforms through submarine cables or trestle cables. When the main power station of the offshore oil and gas field platform transmits power to other wellhead platforms for a long distance through the submarine cable, due to the long submarine cable line and high voltage level, the capacitive reactive power of the line will be very large, causing the main generator to run in phase advance. This will cause a series of problems such as the decrease of static stability, the rise of the temperature of the stator end, the decrease of the voltage of the machine end, and the increase of the stator current, which seriously affects the safety of the generator and the stability of the entire power system. In order to solve the problem of generator phase advance operation caused by the increase of capacitive reactive power of long-distance submarine cable transmission lines, after extensive technical analysis and comparison, the dynamic reactive power compensation device SVG (StaticVarGenerator) was introduced into the power system of the offshore platform. SVG is installed in the power system on-site, and can dynamically adjust and compensate the reactive power of the grid quickly. The function of SVG is equivalent to a reactive power source, it can be automatically adjusted, and automatically compensate and adjust with the profit and loss of reactive load. The reactive power required in the system makes the power factor of the system reach an ideal state, which fundamentally solves the problem of the generator’s advanced phase operation. SVG has the characteristics of fast response, wide operating range, strong ability to suppress voltage flicker, diversified compensation functions, low harmonic components, and small footprint. The use of this device solves the problem of excessive capacitive reactive power in the system and improves the stability of the entire offshore power grid.

4. Conclusion

This paper summarizes various power supply safety problems of offshore platforms: high DC component of short-circuit current, large excitation inrush current of transformer no-load input, and main generator running in advanced phase. The scheme provides a reference and reference experience for the scheme design of the power system in the future offshore oil and gas field development projects.