Earth Fault Tripping

Dear Friend.

The reason for tripping ad blackout is seems to be not geniun earth leakage, probably the settings have to be looked in .I think you get the settings checked up to suite the requirenebts of ACB/AC3 feeders.

CVS Murthy

1. When Ac8 and AC3 are connected by tie brk(B/C), then there will be definitely circulating current between these two fdrs. As soon as motors started to take starting current, circulating current* increased and AC8 may trip in directional e/f and suddenly AC3 started to share the entire load and thus tripped off.

* current flows from DG neutral-Motor star point-NGFE bus- AC8

2. Due to sudden jerk in the system, Dg sets were out of synchronism with Grid and directional current flows and both AR3 & AR8 fdr tripped. There upper stream breakers tripped off first and lower streams tripped in Upstream Down logic or vice versa.

These may be two reasons.

I gave the 2nd repply. Thanx

The first point seems to be a viable reason for the tripping of the feeders. There are no directional earthfault relays connected to both the feeders. Also, the overcurrent settings of AC3 feeder are well above the normal load range. Infact, both the feeders tripped on sensitive earth fault. Can you please elaborate the first point. Thanks in advance.

In the event of grid reverse power, DOC's connected to TR1A secondary trips the TR1A LT CB. So, I can rule out the possibility of the second point.

SLD would help to visualize the situation,

"Phase angle mismatch also cause this type of problems when connected to grid".

Yes , indeed SLD would be helpful to appreciate situation better. Although a coupler would have helped isolating the problem.

Dear Sir

To understand your system better SLD is required.

Motor capacity which is being started is required.

is this phenomenon of tripping is recent one.

What are the relay settings.

you may send the details to personal email id if you wish.Let me understand and to best of my knowledge I can help.

Electronaut, I would check the Earth leakage current before and after tie-breaker closure. In some cases the leakage current with tie-breaker open is not sufficient to trip the relay but will be enough to trip after closure.

Are the neutrals on the two buses bonded? What would be the effects (relevant to the OP) if they were not bonded and vise-versa? I expect a deviation from the expected behaviour if the neutrals are not bonded.

Load rejection characteristics on the governor side may need to be checked, too. Why did the CPP go to black-out status? Overspeed? Idle run protection? Or are the E/F relays configured to trip the DG breakers as well as the bus breakers? Though if there is an E/F on the bus you would want to isolate ASAP, it would help if we knew what tripped the DG breakers too.

We had done the exercise of closing the tie breaker before with both AC8/AC3 in line. We had even measured the E/F current going into the E/F relay. The E/F current at the moment of closure of the tie CB shot up to considerable high value (I'm not able to recall the value) and immediately settled to the normal value. Fortunately, we had increased the E/F setting of both AC8 and AC3 feeder at CPP, expecting the E/F relay to act during the closure of tie CB.

The bus system both at CPP and distribution substation (NGEF and Voltas) does not have a neutral.

The CPP went for a black out due to tripping of some of the DG sets on reverse power and the remaining on overspeed. 7 DG sets were in line. No grid disturbance like frequency/voltage dip/shoot was observed.

There is an E/F relay for CPP 11kV bus (Residual voltage connection). However, the relay did not register any event of an E/F.

At 25MW you were doing about 50% of capacity? Or what are site capacities for the DGs? With 50% output on each unit I expect the governor-fuel linkage be able to recover from load rejection when the DG CBs open due to relay trip signal. So check this linkage for free movement (the mechanics should do that), especially on the fuel racks. Depending on the final drive of governor output (motor or hydraulic?), you should check each group (governor, actuator, links, racks) individually and then as an assembly. This is done to rectify the case of reverse power or overspeed due to sticky mechanisms.

The E/F relay tripping may not be adequately addressed here without analysing the event sequence (from bus-tie closure to DG tripping) at your plant. As you said you do not have directional E/F protection, so we can not determine if the E/F was at the bus or on the motor. The particular equivalent R-L components for the motor circuit without bus-tie breaker may be different from those with bus-tie. Has it occurred again, is it a repeatable fault?

The site capacity of the DG set is 6 MW (Wartsila 18V32). When both AC8/AC3 CB tripped on E/F, the power plant experienced complete loss of load (Plant load - 25MW) resulting in the DG sets tripping on O/S and RP. After the loss of load, the load on the DG sets will be that of the power plant auxiliaries (approx. 0.6MW). Such scant load on the power plant with 7 m/c's running will naturally lead to tripping of the DG sets on RP and O/S. Am I right?.

If the E/F were to be on the motor, corresponding motor protection relay should have registered at least "Start of E/F". Also, if the E/F were on the busbar, the LT CB of TR1A should have tripped on O/C or E/F after the black out in CPP and subsequent transfer of the load on the grid. But none of these relays have acted/started. It was only the E/F relays of AC3 and AC8 that acted and tripped the CB.

After the occurrence of this event, we have avoided to start the HT motors with the loop CPP Bus - AC3 - Voltas - TR1A - Tie CB - NGEF - AC8 - CPP Bus. It was for the first time in 25 years that we started the motor with this loop.

You said you have 8 units at 6MW that gives 48MW. Unless my arithmetic has gone on leave. Wartsila 32 most likely has electro-hydraulic governor system. If the PID parameters were properly set AND all the mechanical connections on the fuel control system are smooth and proportional, the DG set should recover very smoothly after CB trip even at full load of 6MW. Being a base-load plant you probably run all the units all the time unless on maintenance. The fuel racks tend to pit or scuff near the full load position making them sluggish on recovery (there will be a mechanical lag to the output even if the electrical input is correct). This is what makes the engines go into overspeed (excess fuel) or reverse power (fuel starvation) when there is a CB trip. Corrective action is to work (clean while lubricating) the fuel racks and associated linkages to remove that unplanned-for time lag induced by friction. After doing that you should have an idle run-time shutdown on CB trip but not on overspeed or reverse power. These two trippings are not good for your generator bearings.

In my experience I found that the W32 fuel system has too many transitions resulting in places where the bearings and bushes could accumulate dirt.

An E/F trip indicates that a current went through that path being monitored. Where are the CTs for the E/F relays? The TR1A 11KV side would be connected, right? If that is the case how are the CTs connected to get the E/F current? What condition would force current to flow through that path? Hopefully, by answering these questions you will be closer to a solution.

Thanks for the info.

The earth fault signal for E/F of AC8 and AC3 are derived by connecting 3 CT's in residual current connection. The LT side of TR1A is connected in star (YNyn0D). Primary star point is solidly connected to the ground and the LT star point is connected to the gorund through an NGR (159ohm, 40A, 60s). TR1A Sensitive E/F - derived by connecting 3 CT's in residual current connection.

In a statement, you mentioned that a few DG was tripped off in reverse power indication. It may happen that those DG's may desynchronised with the system and started to run as motor and drew power from sytem. Due to this, upstream breaker of AC3 & AC8 fdr tripped off. As all loads were thrown off suddenly, other DG's tripped off in over speed. Thus CPP became dark. Due to tripping of upstream breakers, lower stream breakers tripped off in upstream breaker trip indication. But NGEF & Voltas bus was alive due to grid power.

Sidhu

Dear friend,

Except for the motors, don't you have any other load to the NGEF or Voltas Bus.?If you have so especially transformers, you can undergo insulation testing on the transformers. Here at Ambuklao Hydro Electric Power Plant, last year we had experienced tripping of our tie breaker w/o any transient loading or unloading. We Tried hiring of expert for protection relays but nothing found on the relays and lastly found that it was the transformers who were having very low insulations.

hope it helps..

kind regards,

Carel

Generator Tripped by Power Reverse Relay.

When CPP bus is paralleled with grid system thru transformer TR1A both voltages are Equal. Fig.1 indicates the system arrangement. Please corrected if it is wrong.

During motor starting, the voltage generated by DG is drop due to slow response from AVR or Governor to maintain CPP bus voltage, while the voltage from grid is fixed and maintain voltage as it is.

Grid sees the Generator as a motor due to generator voltage lower than grid voltage and then power reverse relay operated.

AC3 & AC8 Feeders Tripped.

As stated by post(er) that Sensitive Earthfault relay connection is using residual connection for both feeder protections. Residual connection relay may operate due to ground fault condition, unbalance current on each phase and unbalance saturation on CTs as relay input.

I think the relay operate due to unbalance current on each phase flowing in the primary circuit during motor starting. I try to explain this situation by Fig.1. The unbalance itself occurred by the total impedance on both loop including load impedance connected to CPP bus and NGEF bus slightly different.

The unbalance current in the secondary circuit are shown in Fig.2 and Fig3. By i_N and i_n.

The unbalance current during motor starting is equal zero (I_P + I_Q + I_R = 0) in motor feeder (Fig.1). In many implementations, motor feeder is provided by SER (sensitive earth relay) by using current balance CT or residual connection and Tie CB is not provided with SER.