Impact of Islanding and Resynchroniza?on on Distribu?on Systems

Transcription

1 1 Impact of Islanding and Resynchroniza?on on Distribu?on Systems Authors: Dr. Vijay Sood Damanjot Singh Kush Duggal

2 2 Contents Introduction System Description Rural Radial Distribution System Urban Meshed Distribution System System Studies Fault on Source Bus Load Rejection Three phase fault within the Islanded System Grid Resynchronization Single phase temporary fault at the DG Bus Three phase temporary fault at the DG Bus Conclusion

3 3 Introduc?on Electrical grid was designed during the th centuries and it was mostly radial and had centralized generation. Due to environmental compliance, energy conservation and better operational efficiencies, it has become necessary to transform the electric grid into the so-called smart grid. Importance of DG is increasingly accepted by utilities but it brings some serious challenges like load forecasting and stability One positive feature of DG is that it can supply power in islanded mode. (Islanding refers to a condition in which the DG system continues to supply power to some critical loads even when the main electric grid is disconnected from the system)

4 4 According to IEEE Std , intentional or planned, islanding is a topic still under consideration by many utilities. Some utilities do not allow intentional islanding at this time because of the safety hazards imposed by islanding since lines may still be live when they are assumed to have been disconnected Another reason for not allowing islanding is to prevent abnormal voltages and frequency excursions on the system. However, planned islanding if it is used prudently can enhance reliability by serving critical loads when the main grid has been disconnected.

5 5 System Descrip?on Two electric distribution systems modeled in EMTP-RV simulation package: Rural Radial Distribution System Urban Meshed Distribution System Both systems are integrated with a DG using a Synchronous Generator at the Point of Common Coupling (PCC). The DG is assumed to be a synchronous machine with a capacity of 10 MVA.

6 6 Rural Radial Distribu?on System

7 7 33 kv distribution system. Typical radial system that consists of 3 AC loads of 17.5 MVA and a nonlinear DC load of 3 MW (i.e. saw-mill). DG is installed at Bus 4 which is the point of common coupling (PCC). When system suffers a fault at the main feeder, protection will open breaker B1 which will create a micro-grid with DG alone providing the feed. This scenario will create an islanded micro-grid and loads within it will have to be re-adjusted.

8 8 Urban Meshed Distribu?on System

9 9 This system has feeds from two sides besides the internal feed in the form of a DG. The supply to the system loads is based on the concept of an open ring system, in which two different feeders supply the two halves of the ring. The ring can be opened at one point by a disconnecting device. Protection of the power system equipment and safety of personnel poses a major threat due to potential synchronization problems.

10 10 System Studies According to IEEE Std [5], as soon as main source is disconnected from the grid due to a fault, all DGs connected to the electric grid should also be disconnected due to safety hazards and risk of system failure. The simulations show the effect of non compliance of the IEEE Std and out of phase resynchronization of the DGs to the grid. Following test studies were performed only on the radial distribution system. This was due to the fact that without a proper frequency/voltage controller combined with load shedding, the urban meshed distribution system in the islanded mode will not be able to maintain the IEEE Std specified limits.

11 11 Fault On Source Bus Voltage Voltage When a fault occurs at Bus 1, the protection relay disconnects the main source from the system. Then the DG alone will supply power to the loads in the system. Active Power (c) Active Power at Source Bus (MW) Active Power (c) Active Power at DG Bus (MW) Time (sec) The voltage at the DG bus also does not stabilize after the fault as shown in Fig. (b). Active power from the main source bus is reduced to zero when it is disconnected from the system shown in (c).

12 12 System frequency starts to decrease. This is because the DG output (10 MW) is less than the total demand due to which the machine slows down to 0.90 pu, which is out of the allowable limits. Hence the system is dynamically unstable, and the DG should be disconnected to prevent any damage.

13 13 Load Rejec?on When the frequency of the system goes below 0.97 pu a load rejection/ shedding routine is applied to balance the generation and the load. Islanding occurs at 6 s and the frequency of the system starts to drop. The under frequency protection acts and an AC load is disconnected from the system. The frequency of the system is constant after the load shedding and its value is 1.01 pu.

14 14 Three phase fault within the Islanded System. To further test the stability of the system, a 4 cycle three phase fault is applied at 8 s within the islanded system. Voltage at the DG bus reduces to 0.30 pu but comes back to 0.87 pu within 8 cycles. Due to under voltage protection, a load of 2.5 MW is further rejected. Frequency drops down to 0.92 pu but comes back to 1.02 pu.

15 15 Grid Resynchroniza?on Single phase Temporary fault at DG Bus A 6 cycle single phase fault is created at DG Bus at 8 s. Once the fault is cleared, the system becomes stable after a few transients There is no loss of synchronism between the DG and the main source. During the fault, the voltage at the DG bus reduces to 0.78 pu but it comes back to 0.98 pu once the fault is cleared.

16 16 Three phase temporary fault at DG Bus System does not stabilize after the fault which means that there is a loss of synchronism. Loss of synchronism is because of the miss-match between the voltages and/or the frequency of the main and the DG sources.

17 17 After the fault, frequency of the system does not remain constant and goes out of the allowed limits. Also, the voltage at the source bus and the DG bus are in synchronism before the fault but after the fault this synchronism is lost. DG is unable to resynchronize itself to the grid after a 3 phase fault

18 18 Conclusion The ability of DG to form an islanded system upon separation of the main grid may be beneficial to provide continuity of service if suitable measures are taken. The old adage of disconnecting the DG upon separation from the main grid is too protectionist. However, formation of an island requires care and suitable a load shedding and Var compensation strategy may be required. Further care is required when resynchronization back to the main grid is desired.

19 19 Questions?