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Our generator is connected to grid by 33kv line. Due to some reason we supply a fixed amount of power to grid. This is done by a device named Auto Load Limiter Device which looks like

Auto Load Limiter Device

It measures line current. But here we used this device to limit the power. We can set upper limit and lower limit of current. For example, Upper limit=120A, Lower limit=117A, Then it reduces fuel when current is greater than 120A and vice versa for lower than 117A. For a 33kv line 120A means around 6MW power. But the problem is, there is no polarity check for power. I mean it shows 120A(6MW) when we supply to grid. But if 120A(-6MW) comes to generator (it also called reverse power) it doesn't change it's polarity. Suppose in a faulty condition current becomes -20A(-1MW), now it is desired to be 117-120A(6MW), but MT4W increases it to -120A(-6MW) from -20A (-1MW). But our generator trips around -40A (-2MW). How to solve this problem? Is there any setting in MT4W so that it can measure reverse current? What parameters should i change in MT4W? You can say to use power meter which needs ct pt to measure power. power measurement is not my problem. We use this MT4W device to limit the load we supply to grid at 6MW not to measure power. You can ask why i need to limit the load to exact 6MW. The answer is, due to poor condition of transmission line we shouldn't exceed 6MW. This is a 40MW gas turbine and we supply 6MW to national grid and remaining 34MW goes to domestic load. Every power plant can run in pre selected load mode where you can select the load you want to supply. But it is only possible when you are solely connected to grid. But i can't use this feature because i am giving 34 MW (15-34MW, 34 is peak value) domestic load. In grid you can control your power. You can supply as much as you want, also can take from grid as much as you want. But the domestic load is beyond the control.After fulfilling the domestic load, i have to control the load to grid at 6MW (which is done by MT4W). I already read the MT4W datasheet and there is a setting to show display value from -1999 to 9999 that means also negative value. But how to activate? Or is there any other way to limit the load?

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  • \$\begingroup\$ This is something you should take up with the manufacturer of your equipment. \$\endgroup\$ – Barry Nov 11 '15 at 4:09
  • \$\begingroup\$ I already read the datasheet. But it seems ambiguous to me. \$\endgroup\$ – xcoder Nov 11 '15 at 7:40
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The MT4W appears to be a ammeter/voltmeter.

It is not intended to be operated as a power meter.

It is not intended to be used for reverse power protection.

This function would normally be implemented on a protection relay. You should check your existing protection relays to see if they have a power / reverse power element already. If not, you will need some new equipment.

The SEL-751A is an example of a relay including directional power elements (ANSI 32). Budgetary price for this relay is $950 USD so it need not be expensive.

You will require both current and voltage inputs. This means you will need to install VT's if you don't already have them.

Detailed design and installation of this equipment should be done by people who are trained, deemed competent, and authorised to design and work with HV equipment in your country.

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  • \$\begingroup\$ Thanks for your response. But in my question i said that our generator trips at 2MW reverse power. Doesn't this mean that we have reverse power protection? We are using sepam 80 series protection relay. \$\endgroup\$ – xcoder Nov 12 '15 at 4:47
  • \$\begingroup\$ The MT4W appears to be a ammeter/voltmeter. Yes, but it also can regulate power to a specified limit. We are using it exactly for that. \$\endgroup\$ – xcoder Nov 12 '15 at 4:52
  • \$\begingroup\$ @xcoder: I admit I didn't read your entire question. Without paragraph breaks or formatting, it's quite hard to pick out the relevant information from your question. That aside, the reverse power protection would be on the grid feeder, not on the generator. (The 2MW reverse power on the generator is a generator protection function, don't mess with that!!) I will draw a single line diagram later. \$\endgroup\$ – Li-aung Yip Nov 12 '15 at 5:13
  • \$\begingroup\$ Yes you can ask me why there is no reverse power power protection in grid feeder? Actually sometimes we need to take power from grid. For example when we start turbine we take power from grid to run auxiliary loads. Or when turbine is stopped, domestic feeder takes power from grid. \$\endgroup\$ – xcoder Nov 12 '15 at 8:20
  • \$\begingroup\$ I think i should draw a single line diagram. Ok soon I shall upload. \$\endgroup\$ – xcoder Nov 12 '15 at 8:23
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This ain't power meter it measures the current only, right. So, an ampermeter can't deteremine if the power flow is positive or negative as it measures AC current. You should install a power meter, with current and potential transformers, which would require competent personel, at 33kV there are a lot of precautions regarding clearance distances, isolators,...etc
Just another question: what kind of generator is this, and where the negative power goes to.

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  • \$\begingroup\$ We already have power meter. But this device we used for a special purpose that is to limit the load to grid at 6MW. \$\endgroup\$ – xcoder Nov 11 '15 at 15:27
  • \$\begingroup\$ We already have power meter. But this device we used for a special purpose that is to limit the load to grid at 6MW. \$\endgroup\$ – xcoder Nov 11 '15 at 15:28
  • \$\begingroup\$ This is a gas turbo generator of 40MW. \$\endgroup\$ – xcoder Nov 11 '15 at 15:29
  • \$\begingroup\$ Sorry, your description of the problem is too bad to be able to understand what you want, also it seems that you do not understand the relation between current, voltage and power. \$\endgroup\$ – Marko Buršič Nov 11 '15 at 18:24
  • \$\begingroup\$ Marko do you understand my question? You said to use power meter which needs ct pt to measure power. But in my question, did i say that power measurement is my problem? We use this MT4W device to limit the load we supply to grid at 6MW not to measure power. You can ask why i need to limit the load to exact 6MW. The answer is, due to poor condition of transmission line we shouldn't exceed 6MW. This is a 40MW gas turbine and we supply 6MW to national grid and remaining 34MW goes to domestic load. \$\endgroup\$ – xcoder Nov 12 '15 at 1:57

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