DCBPU Operating Concept

Directional Comparison Bus Protection employs direction of fault of all connected feeders to detect the bus fault. It performs a proper operation when all of the directional relays have determined the fault direction.

• If the fault direction in all of the circuits is into the bus, an internal bus fault exists (Figure 1a)
• If the fault direction in at least one circuit is away from the bus Di = -1, an external fault exists (Figure 1b)

Different algorithms such as negative or zero sequence based directional elements or superimposed directional method can be used to implement DCBPU.  It should be noted that, there is a difference between fault direction and current direction. In case of a high resistive fault which is very rare in case of bus faults, the current in some feeders could still flow away from the bus while the true fault direction is into the bus. DCBPU requires fault direction to reliably detect the bus fault in all cases.
The proper selection of current source for each feeder and zones of protection can be followed as per “IEEE Guide for Protective Relay Applications to Power System Buses," while the voltage source should be carefully selected.
As investigated in "Suitability analysis of practical directional algorithms for use in directional comparison bus protection based on IEC61850 process bus," bus voltage either measured or calculated out of feeder voltages should be used for superimposed directional techniques while feeder voltages are appropriate for other directional techniques. Considering that the DCBPU excludes the feeder with open breaker from the decision making, the challenge is to ensure that the directional element performs properly in the context of directional comparison bus protection during simultaneous breaker operation and a bus fault.

This can be overcome first by the combined use of current signal and breaker status to reliably determine the breaker status, and second by exclusion of directional elements with small measured current to avoid incorrect directional determination when there is significant subsidence current at the secondary of the saturated current transformer.
As shown in Figure 2, in case of a single bus sectionalized with a tie breaker and current transformers on both sides of the tie breaker, there should be two zones of protection - Bus zone 1 and Bus zone 2 protecting segments S1 and S2 respectively.
For Bus zone 1, four directional elements are required. Let’s assume that the current flows into the bus for Feeders 1 and 2 and exits from the bus to Feeder 3 and the tie breaker branch from left to right. Now if the tie breaker starts opening and at the same time a fault occurs on segment S1, all three feeders, Feeders 1-3, will feed the fault.

Therefore, it is up to the fourth directional element and breaker status to determine the output of DCBPU. Three scenarios can occur as described below:

• The tie breaker is physically open and it has been detected as open. In this case, the directional element associated with the tie breaker branch will be excluded and a bus fault will be detected since all the included directional elements determine fault direction towards the bus
• The tie breaker is physically open and it has not been detected as open yet. In this case, the current measured by CT1 is small and the directional element associated with the tie breaker branch will be excluded. Therefore, a bus fault will be detected as all the included directional elements determine fault direction towards the bus
• The tie breaker is not physically open and it has not been detected as open. In this case, the current measured by CT1 is not small and the directional element associated with the tie breaker branch will be included, but its direction will change from right to left towards the bus fault. Therefore, a bus fault will be detected as all the included directional elements determine fault direction towards the bus S1

Operating principles of directional comparison bus protection unit based on IEC 61850 process-bus are similar to conventional directional comparison bus protection principles. However, in the former one, sampled bus voltage and feeder currents are converted into network packets in MUs and they are transferred through network cables or fiber optics through network switches or directly to the DCBPU based on IEC 61850-9-2. Directional Comparison Bus Protection receives the data packets and performs protection functions to determine the trip signal. Trip signals are sent to breaker IEDs or MUs to trip associated circuit breakers using several GOOSE messages within process bus according to IEC 61850-8-1.