Offshore Wind-farm Protection Philosophy

There are three types of protection philosophies which will ensure the reliable operation of wind farm and the connected network namely: -

·?????? System Level Protection Philosophy

·?????? Segment/Zone Level Protection Philosophy

·?????? Component Level Protection Philosophy

Regional and IEC standard 61400-3-1 outlines the design requirements for fixed offshore wind turbines, emphasizing the need for redundancy in the electrical infrastructure to ensure reliability and prevent total shutdowns during faults. These documents along with other relevant literature will be the key to design a reliable protection philosophy for all three levels as enlisted above.

System Level Protection Philosophy

System-level protection philosophy for an offshore wind farm involves a comprehensive approach to ensure the safety, reliability, and efficiency of the entire wind farm operation and the connected network. This philosophy encompasses multiple layers of protection and strategies to mitigate risks associated with electrical faults, environmental conditions, and operational failures. Implementing a robust system-level protection philosophy involves a combination of technical, operational, and safety measures to ensure the efficient and safe operation of an offshore wind farm. Here are the key components:

Design and Layout Considerations

Redundancy and Reliability:

?Designing the electrical infrastructure with redundancy to ensure that failure in one part does not lead to a complete shutdown. This includes multiple pathways for power flow and redundant communication systems.s.

Zoning and Segmentation:

?Dividing the wind farm into manageable zones or segments to localize faults and prevent cascading failures as shown in the Figure 1 of a typical offshore wind farm.

Electrical Protection Systems

Over current & Earth Fault Protection:

Implementing the over current & Earth fault protection relays (ANSI 50/51) to safeguard against excessive currents due to short circuits or overloads. These relays will be coordinated to downstream as well as upstream network to isolate the faulty section only. System level coordination scheme is shown in the Figure 3 &4 . The t1, t2,t3,t4 and ΔT are showing the coordination time interval for definite time operations and time coordination interval (ΔT) respectively. These time delays (t1, t2,t3 & t4) will include fault detection, relay inherent & circuit breaker operating time delays as well as the coordination and safety margins

Differential Protection:

Differential protection will be utilized for key components like transformers, cables, reactors and filters to detect and isolate internal faults quickly. The main concept of the differential protection is based on the Kirchhoff’s current law and is shown in the Figure 2.

Figure 2: Differential Protection with Variable-Percentage Characteristic

Distance Protection:

Distance protection relays will be implemented to protect WTG array cables to detect and clear faults based on the distance from the relay location. This will be utilized in segment-A of the offshore wind farm as shown in the Figure 1. Figure 3 & 4 is showing the relay location and associated zones for distance protection. Here t1, t2 and t3 are representing the Zone1, Zone2 and Zone3 operating times respectively.

Overcurrent Earth Fault Protection

o?? Instantaneous over current protection and timed over current (ANSI 50/51) provides fast tripping for high-magnitude faults and overload protection respectively. They provide simple and reliable backup protection. The t1 will be time of operation for this protection and will be properly coordinated with the downstream protection (WTG Over current & Earth Fault protection) and upstream protection having t2 time of operation with coordination time interval ΔT.

o?? Negative sequence over current element can also be applied for additional protection and to satisfy specialized device coordination requirements [5].

o?? Monitoring features like cold load pick and disturbance recorder will also be enabled to cope with inrush current which may cause maloperation/mis-judgment during energization time.

o?? During fault current calculations and associated relay operating time calculations, minimum fault current will be used to ensure proper coordination during summer season. The setting calculations will be as follows:

???? Three- and Two-Phase Fault Pick up settings:

o?? The pickup/plug settings is used to define the pickup current of the relay and the fault current seen by the relay are expressed as multiples of this. This value is usually referred as plug setting multiplier (PSM). The pickup setting will be determined by allowing a margin for overload above the nominal current, as in the following expression :

?

Where

OLF??????? = overload factor that depends upon the element being protected. For wind farm collector feeder ≥1.3 is used.

Inom??????? = nominal circuit current rating

CTR?????? = CT ratio???????????

???? Single Phase Fault Pick up settings:

o?? Pick up settings for earth fault relays will be determined by considering the maximum unbalance that would exist in the system during normal operating conditions. A typical unbalanced allowance is 20% so the expression will become

o?? It will be ensured the pickup setting, at least, greater than or equal to relay minimum detection limits.

o?? A time discrimination margin between two successive (WTG transformer HV & offshore substation transformer LV) time/current characteristics should be used to avoid losing selectivity due to one or more of the following: -

·?????? Breaker opening time

·?????? Relay overturn time after fault clearance

·?????? Variations in the fault level, deviations from the characteristic curves of the relays (Manufacturing tolerances and errors in the current transformers).

o?? In numerical relays, the margin as shown in the ?Figure 5 , could be chosen as low as 200 ms .

Figure 5: Over current inverse time relay curves associated with two breakers on the same feeder

Circuit Breaker Failure Monitoring:

o?? Circuit breaker failure protection is an essential aspect of ensuring the reliability and safety of electrical power systems. Circuit breakers are crucial for interrupting fault currents and isolating faulty sections of the system to prevent damage to equipment and maintain service continuity. However, if a circuit breaker fails to operate, it can lead to severe consequences, including extended outages and equipment damage. Therefore, implementing a robust protection scheme to handle circuit breaker failure is vital.

o?? A breaker failure relay monitors the operation of the circuit breaker. If the breaker fails to open within a specified time after a trip command is issued, it initiates further protective actions after a pre-set time.