Elegant Ground Fault Protection Systems considers Zero Phase Sequence Currents in Three Wire Power Systems By David L. Swindler P. E.

Transcription

1 Elegant round Fault Protection Systems considers ero Phase Sequence Currents in Three Wire Power Systems By avid L. Swindler P. E. Introduction: Power Systems associated with data centers have become very complex in an effort to maintain power system reliability. Such systems typically employ multiple sources of power involving multiple grounds. In an effort to reduce the complexity of the power system, three-wire, three-phase, low voltage power systems are often employed in place of the usual wire systems typically found at 80 Volts. It is generally felt that by getting rid of the system neutral, one will make the round Fault Protective System much simpler and easier to design, install, and test. This might be true except for the possibility of ero Phase Sequence Currents that may flow on a three-wire system due to the existence of multiple bonds to ground. This paper discusses how ero Phase Sequence Currents can exist and then how they are accounted for in an Elegant round Fault Protective System. Background: n Elegant system is one that has ideal characteristics in the face of multiple sources and grounds in very complex power systems. Such a protective system permits the power system to be arrange in zones where the protective observes the current flow into and out of the zone and then is arranged to trip all breakers that can supply power to that zone in the event of a round Fault. round Fault current may be considered as a ero Phase Sequence Current as is passes through the system enrout to the location of the round Fault. This ero Phase Sequence Current may be supplied from several sources and then delivered to the particular zone involving the round Fault. The round Fault Protective system knows the location of the fault and responds by opening all breakers that can supply power to the zone involved in the fault. t the same time one can also have a ero Phase Sequence Currents for a number of other reasons. One might be because of an imbalance in impedances and sources within the power system that has multiple grounding. nother might be harmonic current. These currents would not necessarily be involved in a round Fault. Under these conditions, we would not want the round Fault Protective system to cause false tripping. In addition we would not want a ero Phase Sequence Current flowing through an unaffected zone on its way to some other zone involved in a round Fault to cause unnecessary tripping activity in the unaffected zone. ero Phase Sequence Current as a result of System imbalance: Let us consider two grounded power sources connected in parallel where V 1a. and V a are different while the other two are equal. p Ia = (V 1a - V a )/ p V 1a V a I 0 p I b = 0 V 1c V1b V c V b p I c = 0 I 0 = (I a + I b + I c ) In the case above, ero Phase Sequence Current (x3) is equal to the difference in phase voltages divided by the phase impedance. 1

2 The figure shown on the previous page demonstrates the flow of ero Phase Sequence currents that might flow in a three wire system having two paralleled sources, each having its own separate bond to ground. In this illustration we assume that the potentials of two of the three phases are exactly equal but the potentials of one phase are not. In this case we assume the impedances are equal. In this illustration it can clearly be seen that there will be a current that would flow in the unbalanced phase and cause current to circulate in the ground path. en. practical Case emonstrated: Lets suppose we have a rather simple power system as shown involving a co-generation system. In this illustration we are showing a ero Phase Sequence Current (PS) flowing into the service entrance bond connected to the neutral of the service transformer. s a function of the origin of this current, it may divide and flow in each of the three conductors or simply flow in one conductor as illustrated. Such an occurrence would be caused if the voltage in that phase were not matched between the two sources with the others matched. Note that this PS (ero Phase Sequence) current flows out of the generator Neutral and into ground where it has an opportunity to return to the service bond. Note that in the secondary of the sense circuits for the Elegant round Fault Protective System that this primary ero Phase Sequence current has a path to flow that does not flow through either of the two breaker round Fault Trip functions. Thus this circuit is immune to all such circulating currents. This illustration is quite simple involving just two zones of protection. In this case the ones are defined as the between the two breakers and then the enerator Circuit. For Elegant systems, each of these zones are protected as follows: Elegant round Fault Protection by ones: iven the circuit as shown above, the system may be operated with either the ain or the enerator breaker closed or as a Co-eneration system with both breakers closed. The design goal is to keep power on the as much as possible and also to minimize damage to the system in the event of a ground fault. ssuming both breakers close, if there is a ground fault on the we would want both breakers to trip removing power to the round Fault. If, however, the round Fault occurred on the generator cables or within the generator then we would only want to trip the enerator Breaker to isolate the fault from the balance of the system permitting the ain breaker to remain closed and supply power to the Feeder Bus.

3 en. s shown to the left, we have a ground fault having a magnitude of two per unit on the Phase of the. The utility service at the top and the generator power at the bottom support this ground fault current with each supplying one per unit of current each. The ground fault current returns to the system by means of the bonds at both the utility service entrance and at the generator neutral bond to ground. lso shown is the same ero Phase Sequence current as discussed above. When analyzed on the secondary side of the three groups of sensors, it will be noted that in some cases the ground fault current tends to add to the PS currents while in others they tend to cancel. s will be determined by detailed analysis, only the round Fault current will pass through the breaker round Fault Trip Function. In this case a per unit current will pass from left to right through the ain trip function and then on through the ux. CT. The ux. CT secondary per unit current will then pass through the enerator round Fault Trip Function. The PS component of the Sensor secondary current will simply circulate in the circuit without passing through either of the two round Fault Trip Functions. en. Now in the case to the left we are showing a per unit ground fault within the generator on Phase. This current is supplied by the Utility Service and the enerator power. The direction of fault current flow in the generator breaker sensors reverses. In this case, the secondary circuit is arranged such that the summation of the round Fault currents flows through the enerator Breaker round Fault Trip Function only and not through the ain Breaker round Fault Trip Function. gain the PS component of the Sensor secondary current will simply circulate in the circuit without passing through either of the two round Fault Trip Functions. In summary, the Elegant round Fault Protective circuit not only causes the proper breaker to trip under given conditions, the circuit also does not trip with circulating ero Phase Sequence currents flowing through the system nor is the trip current adversely modified by the PS Currents. 3

4 Conventional round Fault Systems: The above discussion can be best illustrated in contrast by comparing the performance with a conventional round Fault Protective system commonly used. To the left is a diagram of a typical simple system with round Fault protection on both the ain and enerator breakers. Here we are illustrating the flow of ero Phase Sequence Currents as we have for the Elegant system. Note that in both the ain and enerator Breaker round Fault Circuits the PS currents are reflected into the ain and enerator Trip Functions. It is obvious that if the PS current becomes too high then false tripping may occur. For this reason the pick-up setting is often set higher than desired in an effort to precluded unnecessary round Fault Tripping. en. s shown to the left and below, we are illustrating a two per unit round Fault current on the Feeder Bus as in the case above. One per unit of fault current is being supplied by the Utility Service and a second by the enerator. detailed analysis will show that in this case the ain round Fault Trip Function only sees that round Fault Current that passes through the ain breaker. In the Elegant circuit above, the ain Breaker saw a two per unit current, that current that was flowing in the actual ground fault. Since the fault level is two per unit and the breaker is seeing only one per unit, the conventional type of circuit is basically less sensitive to actual round Faults. Looking at the ain Breaker Trip Function, it appears that in this case the PS current is aiding in the tripping of the ain Breaker round Fault Trip Function. en. In the case of the enerator round Fault Trip Function, we see that it only recognizes that ground fault current that happens to flow through the enerator Breaker and not the full two per unit current that is flowing in the ground fault. lso we note that under the conditions illustrated, the PS current tend to cancel the ground fault currents that would flow in the enerator round Fault Trip Function. In this case we see that the proper operation of the ground fault protection system is not sure and is a function of the nature of the PS currents.

5 In the case to the left, we have moved the round Fault to the enerator winding. gain we have assumed a two per unit fault current with one being supplied by the Utility Service and the other by the generator. s in the last case, the breakers trip as a function of what current it is able to see through its internal sensors. The ain Breaker sees the contribution to the ground fault from the Utility Service and also any PS sequence currents that may be flowing. In this case we are assuming the two currents happen to be adding to cause the breaker to trip, however, the opposite could also be true. en. In the case of the generator breaker, it is not seeing the contribution of current to the fault from the generator. The current it sees is that contribution made by the Utility Service to the fault that is passing through the generator breaker. Comparing this circuit with the Elegant circuit, the enerator breaker trips as a function of the actual fault current flowing into the ground and not the contribution through the generator breaker from some other source if any. gain whether the PS current contributes or cancels tripping is problematical. Conclusion: For three wire power systems that have multiple sources and multiple grounds, ero Phase Sequence Currents can flow in phase conductors and circulate back through ground paths. Such currents might have a significant affect on the operation of conventional ground fault protective systems. The use of Elegant round fault systems is essential in nullifying the affect of ero Phase Sequence Currents that may be caused by harmonics, imbalances in impedance s, unbalanced loads, and unequal phase voltages between systems. Elegant round Fault Systems are designed to detect and measure only actual round Fault Currents leaving the system and entering the ground. Elegant round Fault Protective Systems become exceedingly important in large, complex, ata Processing power systems where the quality of operation is essential. ttached is a sketch of a typical ata center. This system has six sources of power each having its own bond to ground. In addition the UPS Sources are also bonded to ground. The general arrangement of this power system is a three-wire three-phase system. The Elegant round Fault Protective System (as shown) is divided into independent zones. Each color represents a different zone.

6 6 (Editor s note: The above illustration is not acceptable for a number of graphic reasons. The graphic needs to be moved to a faster computer for editing. It is acceptable to communicate the relative size and complexity of typical power systems.) C C 3 en Utility Service Utility Service 1 Loa Loa