S YSTEM R ELIABILITY
3.3 F UNCTIONAL I NTEGRATION E XAMPLES
In this section, we provide a number of examples to illustrate the use and the benefits of functional integration in modern numerical distance relays.
3.3.1 Breaker Failure Protection Applications
3.3.1.1 Communication in Conventional Breaker Failure Protection
Communication is used for breaker failure protection in conventional systems in the case of a bus fault when one of the transmission line breakers fails to trip. The fault can be cleared using remote-end backup protection (Zone 2 trip). However, if high-speed fault clearing is required for dynamic stability or power quality reasons, the breaker failure protection will send a direct or permissive intertrip signal to the remote end using a dedicated signaling device or use the available binary signals in the data message of a multifunctional line protection IED.
When the signal is received, the IED will trip the local line breaker (or breakers in the case of a breaker-and-a-half or ring-bus) to clear the fault, thus achieving faster fault clearing compared to a Zone 2 or time-overcurrent remote backup trip.
IED
Relay A Relay B
Breaker Failure Relay IED
• • • PIT=1 Data Message
• • •
– +
–
IB
F
+
Figure 3.5 Breaker failure protection
3.3.1.2 Distributed Breaker Failure Protection Application
Figure 3.6 shows an example of a distributed breaker failure protection scheme. When the relay detects a fault condition on the protected line, it issues a trip signal in order to clear the fault. This can be a GOOSE message with the trip bit pair indicating the operation of the trip output function of the IED.
The bay controller that implements the distributed breaker failure function subscribes to this message, and, as soon as the GOOSE message is received, the bay controller starts the breaker failure timer. If the breaker fails to trip, the breaker failure relay (BFR) indicates a breaker failure and sends a GOOSE message to the substation LAN to trip adjacent breakers in order to clear the fault.
Relay
Breaker Failure Protection
GOOSE Message
Trip
Relay Relay
In some cases, the BFR function in the bay controller can also send a trip GOOSE message to the network that will attempt to retrip the faulted line breaker through a different physical connection or another breaker IED connected to the substation LAN.
When the breaker failure protection function is built into the distance relay that detects the fault condition, it detects a failure of the breaker and issues a GOOSE message with a bit pair set to indicate the breaker failure condition. All IEDs that are associated with breakers adjacent to the failed breaker will have to be set to subscribe to such a GOOSE message and issue the signal to trip their associated breakers.
Distributed communications-based breaker failure protection can be designed in two different ways:
• As a function in an IED that initiates the breaker failure protection when it sees the trip signal from the relay protecting the faulted power system device
• As a built-in function in the protection IED that detects the fault and issues the trip signal The breaker failure protection (BFP) element can be configured to operate for trips triggered by protection elements within the relay or via an external protection trip. The latter is achieved by allocating one of the relay optoisolated inputs or virtual inputs to “external trip.”
3.3.2 Application of Integrated Sensitive Negative- and Zero-Sequence Overcurrent Functions in Distance Protection
In Figure 3.7, Line 1 in Substation A is protected with an electromechanical distance relay scheme in a permissive underreach mode and with discrete phase and ground electromechanical directional overcurrent relays for backup protection. For the same line in Substations B and C, static type distance relays are used in a permissive underreaching mode and directional ground overcurrent relays are provided for the backup protection function.
According to the topology of the network shown in Figure 3.7 and the short-circuit current contributions at the faulted locations, the quadrilateral distance relay at Substation C may not be able to detect a resistive ground fault (30 ohms) at Location F1. This is due to the limited resistive reach of the distance relay and the load impedance of the system. In this case, only the time-delayed ground directional protection is able to detect and trip the circuit breaker of the faulted line at Substation C.
Unselective tripping can easily result upon failure to fast trip the faulted line by the distance protection at both Substations A and B. For instance, if the distance relay at Substation A fails to receive the remote line end permissive trip (PT) signal to allow Zone 2 to trip, then additional fault clearing delays will be imposed by the starting distance element and may consequently affect the selectivity of the healthy line upon the reversal of current at Substation C. Note that the directional ground overcurrent relays may be impacted at all tapped substations if not set long enough to override the longest fault clearing time of the distance devices at Substations A and B.
NO
NC NO
NO NO
F1
Substation C
Substation B Substation A
161 kV
Line 1
Line 2
Figure 3.7 System single-line diagram
In order to improve the reliability and the security of the discussed power network, modern numerical distance multifunctional relays can be employed. A definite improvement in protection performance is achieved by enabling sensitive directional negative- and zero-sequence current elements in conjunction with the flexible distance protection application provided in the same device. The desired high-speed operation of the line protection by means of relay-to-relay communications and programmable logic is assured even in the event of one-channel loss between any two terminals. The exchange of various protection elements data in Substations A, B, and C, with the use of the relay DTT programmable features, will result in secure and dependable fast protection operation. Finally, adaptive techniques can be considered as an alternative solution by changing relay settings groups according to the prevailing system and load conditions. The approach to adapt new resistive reach settings is possible in today’s modern distance protection devices.
3.3.3 Numerical Distance Relay Settings Group Change
Protection engineers can use innovative and creative applications in both protection and control areas due to the flexibility of multiple inputs and outputs provided in numerical relays. The use of multifunctional distance relays with relay-to-relay communications offers numerous opportunities to enhance control functions and, at the same time, automatically improve the protection performance through a change of settings groups to accommodate the change in network configurations.
21
Detect Breaker Opening and Send Message
O
21 Receive Message and
Change Setting
Relay-to-Relay Input 21
Breaker Status O = Open Relay-to-Relay
Commu nications
Figure 3.8 Relay settings group change
In Figure 3.8, if the circuit breaker is taken out from service for maintenance reasons, then it is possible to automatically change the remote line end protection settings using relay-to-relay communications. The protection is enhanced without any human intervention. Change of settings groups is possible for various conditions listed below:
• Change the integrated protection functions and settings based on load and breaker status in order to enhance sensitivity and to improve protection coordination
• Change in relay settings based on source conditions that affect sequence current and/or voltage contributions to the fault location
• Change in remote distance protection settings based on system configuration
• Change in settings for adaptive circuit breaker reclosing schemes
• Condition the remote line end circuit breaker reclosing