ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
Where is Everyone?
by luckyshadow - 11/21/21 10:14 AM
It's been an interesting career
by The Watt Doctor - 11/19/21 09:56 AM
Well I am back to stay (nearly 6 years)
by The Watt Doctor - 11/19/21 09:17 AM
Motor Load Relationships Between Fans and Pumps
by The Watt Doctor - 11/18/21 09:24 AM
GFCI's pops in large numbers
by dsk - 11/05/21 06:45 AM
New in the Gallery:
240/208 to a house
240/208 to a house
by wa2ise, October 9
Now you know.
Now you know.
by Tom_Horne, September 7
Who's Online Now
0 members (), 39 guests, and 20 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Joined: Jul 2004
Posts: 783
L
LarryC Offline OP
Member
Folks,

I understand why and where RPR are used. What I do not understand is how they determine the direction of power flow. Can anybody help me?

Thanks

Joined: Jun 2004
Posts: 1,273
T
Member
Voltage drop across a resistor.

Simple.


Tesla
Joined: Jul 2004
Posts: 783
L
LarryC Offline OP
Member
Tesla, that describes a current shunt.

Where I am familiar with reverse power relays is when you have multiple paralelled generators or a generator tied to the grid. If the prime mover slows down, the slower generator turns into a motor and consumes power instead of producing power. This motoring action can then damage the prime mover.

Looking at the tech sheets, there are connections to current transformers and potential transformers, but how do you determine the direction of power flow?

Larry

Joined: Jul 2004
Posts: 9,672
Likes: 7
G
Member
I am guessing here but I bet it is by seeing which one has the higher voltage, by looking at that resistor.
Power will always flow from a higher voltage to a lower voltage and if you compare volts in and out of a resistor you can see which end is higher. We are talking about a pretty small delta.


Greg Fretwell
Joined: Jun 2004
Posts: 1,273
T
Member
LarryC

The grid is an infinite bus.

All big prime movers drive synchronous alternators -- and they stay in lockstep via their excitation.

Frequency control and synchronization were solved by Tesla.



Tesla
Joined: May 2011
Posts: 10
A
Member
Reverse Power Relays are indeed used to protect an alternator from 'motoring' in case of failure of the prime mover.

This can't be done by anything as simple as using a resistor inline - we are dealing with AC here, the direction of the current changes 60 times a second anyway!

These devices do have connections to both CT's & VT's. They use the voltage reference as a 'polarising voltage' ie. the current in L1 phase is polarised by the voltage L2-L3 etc. By analysing the angles between the polarising voltage & current it is possible to work out the direction of the real power (W) flow. This is the basic principle of operation of a reverse power relay & also of a directional overcurrent relay.

Remember also, when paralled against an infinite bus:
Prime-mover Torque = Real Power Output
Excitation = terminal voltage = Reactive Power Output

Either of these may be imported & exported independently on the same machine.

Last edited by AdrianW; 05/23/11 03:25 PM. Reason: fingers ahead of brain!
Joined: Jul 2004
Posts: 783
L
LarryC Offline OP
Member
Thank you AdrianW.


Link Copied to Clipboard
Featured:

Tools for Electricians
Tools for Electricians
 

* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
JoeKP
JoeKP
Berkley, MA
Posts: 144
Joined: March 2008
Top Posters(30 Days)
Trumpy 5
Admin 3
Popular Topics(Views)
286,164 Are you busy
218,600 Re: Forum
204,808 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.5