The integration of new energy sources is a relatively sensitive issue. It is also a problem often encountered in the design of power systems. It is as small as a single project access system, and as large as a region of new energy grid consumption, both There is a lot of attention.
The impact of new energy grid-connected on the grid
New energy actually contains a lot of things. Here we mainly discuss wind power, photovoltaic and distributed power. Of course, distributed power sources may contain small photovoltaics, etc.
The impact on the power grid is actually divided into two parts. One is the new energy source that accesses the main network, and the other is the new energy source that is connected to the distribution network. The impact of the two is different. .
1) Accessing the new energy of the main network, taking wind power as an example, mainly has the following effects:
Increase the peaking and frequency adjustment difficulty: wind power is random and intermittent. The fluctuation range is large and the fluctuation frequency is irregular. The anti-peak characteristics of wind power increase the difficulty of peak shaving. According to the statistics of the Northeast, Mengxi and Jilin power grids, the probability of wind power reverse peaking is 60%, 57% and 56% respectively. Due to the access of wind power in Jilin Power Grid, the peak-to-valley difference during the year has reached 210 days. Due to insufficient peaking capacity, Jilin and Mengxi power grids have experienced abandonment of wind during low-load periods. The figure below shows that the wind power output and the grid load show strong anti-regulation characteristics. (North China Power Grid Zhangjiakou Area)
Increasing the difficulty of grid voltage control: Wind farm operation relies too much on system reactive power compensation, which limits the flexibility of grid operation. According to statistics, affected by wind power: the 220 kV bus voltage of the substation along the Ximeng ashengliang wind power base in Mengxi Power Grid is maintained at 1.1 times of the rated voltage throughout the year; when the 500 kV reactive power compensation equipment in the Mengxi Tala area is out of service, 220 The kilovolt system voltage rises to a maximum of 257 kV.
Insufficient local grid access capability: Most wind farms are at the end of the power grid. After large-scale access, a large number of Internet access during the wind power boom period, the power grid transmission trend is increasing, the heavy-duty operation lines are increasing, and the thermal stability problem is becoming increasingly prominent. Since 2007, the wind power and small hydropower in Gansu Jiuquan area have developed rapidly, and the contradiction has been intensified. Despite the use of overload cutting machines and substation split operation to improve the transmission capacity, the wind field abandonment problem still exists for a long time.
Increasing the stability risk of the grid: The intermittent nature of wind power increases the potential risk of stable operation of the grid. First, the trend caused by wind power is changeable, which increases the difficulty of operation control of the transmission section with stable limits. Second, the increase of wind power generation component causes the inertia of the system to decrease and affect the dynamic stability of the grid under the same load level. After the system fails, the wind turbine may not be able to re-establish the terminal voltage and lose stability, which will cause the voltage stability of the regional power grid to be stable.
2) Access to the new energy of the distribution network, taking distributed power as an example, mainly has the following effects:
Protection problem: The change of the current will cause the sensitivity of the line protection to decrease and refuse to move; the second will cause the line protection to malfunction; the third will cause the instantaneous quick-break protection of the adjacent line to malfunction and lose selectivity; the four times will cause coincidence The gate was not successful.
Countermeasures: First, limit the grid-connected capacity of the distributed power supply; second, increase the impedance of the distributed power supply isolation transformer; third, increase the direction power protection of the distributed power supply outlet; the most critical point is that before the distributed power supply is connected to the network, it must be fully Based on investigating the impedance of the power generation equipment, calculate the increase in short-circuit current that may be caused by the distributed power source to determine the grid-connected capacity of the distributed power source.
Voltage problems: First, the impact of distributed power supply start and stop, and second, the intermittent impact of distributed power supply.
Countermeasures: First, from the perspective of voltage support, distributed power sources have a significant positive effect. This depends on the reasonable choice of access location, reasonable selection of capacity and proper scheduling; second, under normal circumstances, distributed power supply should be more active and less reactive, maintain high power factor operation; reduce line voltage dependence on distributed generation; For distributed power access locations, appropriate reactive voltage support equipment should be installed and put into operation when the distributed power supply is out of operation.
Power quality problem: distributed power generation is connected to the grid through power electronic inverters, which is easy to generate harmonics and three-phase voltage/current imbalance. The randomness of output power is easy to cause grid voltage fluctuation and flicker; distributed power supply is directly connected to the user. Into the grid, power quality issues directly affect the safety of users' electrical equipment.
Countermeasures: First, the problem of distributed power supply and DC injection, in the grid-connected mode, through the distributed power control function, the isolation transformer is installed at the outlet of the larger-capacity inverter-type distributed power unit. The second is the voltage imbalance problem. For a distributed power supply unit with single-phase grid connection, it can be connected to a phase with more load, thus reducing the voltage imbalance.
There are still many problems in accessing the distribution network: short-circuit current problem, communication measurement problem, island problem, etc., but the fundamental lies in: distributed power supply access distribution network, power flow distribution change, power distribution system from radial to multi-power structure .
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