[time-nuts] Line Voltage - USA

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Tue Jan 3 13:50:17 EST 2017

On 2 January 2017 at 05:15, Jeremy Nichols <jn6wfo at gmail.com> wrote:

> Thank you for the detailed analysis, Bill. The voltage measurements I made
> in my garage laboratory were duplicated by the utility with their meter,
> which was connected at the service entrance.

I have just been chatting to a friend who was a controller at two power
stations in the UK - Darlington (coal) and Bradwell (nuclear). He tells me
that the voltage is likely to be higher in the summer around 2-3 am in the
morning. Now it might seem obvious that the load is smaller in summer than
in the middle of winter, but this is NOT the reason the voltage rises more
in summer. I must admit though, I could still not understand it, and he
admits he could not explain it, but just tells me it is so. But a few
things I did get, which are not all obvious - some are.

1) The real power consumed by the users + losses must balance the power
generated. That's pretty obvious.

2) The reactive power (V*A) must also balance - perhaps less obvious.

3) The voltage generated by a generator when it is not providing any load
is controlled by the current in the field winding.

4) Before connecting a generator to the grid it is necessary to ensure the
voltage and phases are matched.

5) Once the generator is on the grid, there's nothing the generator can do
that has any practical effect on the voltage. Even with a nuclear power
station, the output power it is a small fraction of the overall power being
generated by the all the power stations, so one power station coming on/off
line does not have any significant effect on the voltage of the grid.

6) What the operator can do is

* Generator more power, by increasing the steam that drivers the generator.
* Change the reactive power by changing the field current

7) As soon as the generator is connector, he would increase the steam to
provide at least  5 MW at Bradwell (nuclear, 2 MW at Darlington (coal), as
failing to do so risks the generator going unstable due to disturbances on
the grid.  This could easily result in the generator becoming a motor,
which is not good. So there's a minimum power a generator can practically
provide - in his case 2 or 5 MW.

8) If there were no uses on the grid, so nobody using any electricity, the
capacitance of the cables would make the load capacitive.

9) Users are generally inductive, so in practice the current lags the
voltage, as the reactive power of users is greater than the the grid.

10) The higher power usage in winter means that the power factor is further
from 1.0.

I get the feeling that the voltage might go up more in summer as the
generator are running closer to a point of instability, with small changes
in load causes significantly more change in power factor than in the

As I say, I never really seemed to get to the bottom of fully understanding
this, but he assures me that voltages will be less stable at light load
than at heavy load.

I guess if I do report a problem, I will get them to measure all 3 phases.
That must increase the chances of at least one phase going outside
specification. I am rugulary going over 250 V, but not 10% more which would
be 253 V.


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