Voltage Regulators in Series in Power Distribution System

[Cerkit]

Hi. Can anyone comment on experience with using multiple Voltage Regulators in series on a distribution network, for example in maintain voltage on a long overhead line. Would there be issues with power factor towards the end of the line because of the impedance of the regulators in series? If so how is this overcome, it is my understanding that in the US for example where there are several miles of overhead line distribution they use many of these in succession to maintain the voltage at distribution level.

Thanks

 

[cranky108]

They also install line capacitors for voltage control. Both fixed and switched capacitors.

 

[waross]

The power factor at the end of the line will depend on the power factor of the load at the end of the line, not on series impedance of the line.

distribution they use many of these in succession to maintain the voltage at distribution level.

More accurate to say;
"They may use more than one of these in succession."

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Cerkit]

Hi.

But the impedance of the regulator/s would be present if I had the equivalent model from source to load and via the return path, doesn't the inductance of the regulator (just coils same as a transformer) create a phase shift and hence affect the power factor?

Thanks

 

[davidbeach]

Transformer phase shift affects both current and voltage, has essentially zero affect on power factor.

 

[Cerkit]

Hi.

Interesting you say that. Last I checked a large inductance (ie a transformer) causes the current to lag the voltage.

Thanks

 

[davidbeach]

A transformer and a reactor are rather different things, even if they both have coils of conductor around a core.

 

[crshears]

True enough, David; but as an inductive device a transformer does absorb reactive power, which it will draw from the source in the same manner as any other inductive load.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[davidbeach]

Second or third order effect, sure. Consider a delta-wye transformer, with a phase shift equal to some odd multiple of 30 degrees; apply metering on both sides. The metering will show approximately the same power factor on each side; nothing like a 30 degree change in power angle. The magnetizing branch of the transformer is a shunt load and does have a power factor other than unity. But it will be the load external to the transformer that determines the power factor of the flow through the transformer.

 

[crshears]

Similar to how when placing MW load on a generator excitation has to be increased to compensate for internal drop, otherwise VAR absorption will commence.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[aussiejohn2]

Hi Cerkit,
I have seen three in a row, from distant memory it was a 180 km 22 kV line with a MW or two of total load.
The source power factor is changed only marginally, since the series impedance of regulators is small (zero at nominal tap, often only a percent or two on nameplate base at extreme tap) and the shunt impedance large relative to load impedance.
Utilities often have a rule specifying a maximum number in a row, but nobody has yet explained to me the basis for these rules.
Usually they are time-graded, so the more in a row, the slower the response and in extreme cases lamp flicker or step voltage change limits may impose a constraint on the number in series. Other than this it seems to be based around "That's the rule" with no technical justification.
John.

 

[waross]

Voltage regulators used on long rural lines with predominantly single phase loads may experience a neutral shift. While the phase to neutral voltages are correct, the line to line voltages and the phase angles may become unequal.
This is no problem for single phase loads but reduces the available HP of three phase motors.
In extreme cases the motor current at no load may exceed rated full load current.
When regulators are in series, it is possible for one regulator the correct the neutral shift of another series regulator but the odds against this are overwhelming.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bacon4life]

The series impedance of a regulator is quite small, perhaps equivalent to a few hundred feet of overhead conductor. The losses in a regulator are mostly reactive, so the regulator would have a slight impact on power factor.