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White paper | Voltage Regulation & Power Factor Improvement

Voltage Regulation & Power Factor Improvement

Introduction

Depending on its characteristics, a given load absorbs active power and reactive power. The ratio between both powers determines the power factor (cos Φ) at load terminals. An identical power factor would be measured at mains terminals if mains were supplying directly (and exclusively) the considered load. But the insertion of the NO-BREAK KS®, between mains and load, strongly modifies the situation.


In Conditioning Mode, circuit breakers QD1 and QD2 are closed whereas switch QD3 is opened. Therefore the stato-alternator is coupled in parallel on the mains through a choke (reactance X). The stato-alternator excitation is controlled at all times to maintain the rated voltage at terminals of the load connected to the downstream busbar.

 

The following diagram is representative of the situation:

 

Since the electromagnetic clutch is in open position and that the diesel engine is stopped, the synchronous machine cannot deliver any active power (on the contrary, strictly speaking, it absorbs a small amount of energy to compensate the different losses). Therefore the active power absorbed by the load is delivered by mains only.


But the stato-alternator can deliver the reactive power requested by the load. Consequently the power factor (cos Φ) measured at mains terminals is improved significantly and reaches a value close to unity. Basically, the automatic improvement of the power factor measured at mains terminals results from the combination of voltage control – existence of the reactance X.

 

 

Vector diagram and power flow


Electrical quantities can be evaluated by drawing a vector diagram which meets the following conditions:

 



The following diagrams show situations where mains voltage corresponds to 90%, 95%, 100% and 105% of the rated voltage, respectively. The considered load is the rated one, with a power factor (cos Φ) equals to 0.8.

Voltage and current vectors are drawn using rated value as unit length. Voltages are represented by continuous lines while currents are represented by dotted lines.

Beside each vector diagram, the single wire diagram is plotted with active power flow (in blue) and reactive power flow (in green). Arrow width is proportional to the amount of (active or reactive) power flow.

 


 

 


 

 


 

 

These figures clearly show that the stato-alternators delivers all, or at least the major part of, the reactive power requested by the load. Therefore the power factor (cos Φ) measured at mains terminals has been highly improved.

 

  • When mains voltage is equal to the rated one, mains delivers only half of the reactive power for the choke, that is to say less than one sixth of the reactive power requested by the rated load with a power factor (cos Φ) equal to 0.8. The stato-alternator delivers all the reactive power requested by the load as well as half of the reactive power for the choke.
  • When mains voltage decreases and becomes lower than the rated voltage, the synchronous machine delivers more and more reactive power. It continues to supply all the reactive power requested by the load. Moreover it ends up by delivering all the reactive power for the choke, and even by supplying reactive power to the mains. The power factor (cos Φ) measured at mains terminals comes to unity, then current from mains slightly leads voltage. It should be noted that, when mains voltage decreases to 90 % of the rated one, the stato-alternator delivers an amount of reactive power corresponding to 175 % of the one requested by the rated load with a power factor (cos Φ) equals to 0.8. Thanks to the thoughtful design of the NO-BREAK KS® stato-alternator, that constraint can be avoided without any problem.
  • When mains voltage increases and becomes greater than the rated voltage, mains delivers progressively more reactive power. It ends up by supplying all the reactive power for the choke, and even by contributing somewhat to the reactive power requested by the load. Even so, the power factor (cos Φ) measured at mains terminals is improved significantly. Moreover current from mains is equal to only about 4/5 of current flowing to the load.

The following table gives a summary of the different situations (rated load with power factor equals to 0.8).

Analytic relation


Since the voltage at load terminals is continuously controlled and maintained equal to the rated voltage, it can be demonstrated that the power factor (cos Φ) measured at mains terminals is given by the following expession:


This expression shows explicitely that the power factor (cos Φ) measured at mains terminals does not depend on the amount of reactive power requested by the load. The following diagram represents the power factor (cos Φ) measured at mains terminals as a function of the magnitude of mains voltage, for different load levels. It appears clearly that, except when load is low and mains voltage deviates significantly from the rated voltage, the power factor (cos Φ) measured at mains terminals is very close to unity.

Conclusion


Besides its major function as a protection for critical loads against mains faults, the NO-BREAK KS® also plays an important role during operation in Conditioning Mode when mains is present, especially through voltage control associated with the presence of the choke.


This document demonstrates the systematic improvement of the power factor (cos Φ) measured at mains terminals, which avoids the installation of capacitor banks and the phenomena associated to their switching on and off.


Other advantages, in Conditioning Mode (diesel engine stopped), are worth mentioning too:

  • Regulation of voltage at load terminals and compensation of mains voltage deviation from rated voltage (±10% tolerance).
  • Suppression of very short duration voltage dips and spikes.
  • Protection of load against voltage harmonics from mains.
  • Reduction of current harmonics rejected by non linear loads.