{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

PUSystem

# PUSystem - ECSE 486 Power Laboratory The Per-Unit System 1...

This preview shows pages 1–2. Sign up to view the full content.

ECSE 486 Power Laboratory The Per-Unit System 1 Introduction and Motivation Power system quantities like power, voltage, current, and impedance are often given in per unit or percent of specified base values. For instance, if a base voltage of 735 kV is specified then the voltage 765 kV is 765 / 735 = 1 . 04 per unit or 104%. Advantages of the per-unit system are many. First, by specifying appropriate base quantities, the transformer equivalent circuit can be simplified by eliminating the ideal transformer winding. As a result, voltages, currents, external impedances and admittances expressed in per-unit do not change when referred either to the primary or the secondary winding. This is a significant advantage when analyzing power systems of even moderate size, where hundreds of transformers can be found. The per-unit system avoids us the possibility of making many serious calculation errors when referring from one transformer side to the other. Another advantage of the per-unit system is that the per-unit impedances of electrical equipment of similar type usually lie within a narrow numerical range when the equipment ratings are used as base values. Thus, per-unit impedance data can be checked rapidly for gross errors by someone familiar with the per-unit quantities. Moreover, manufacturers normally specify the impedances of machines and transformers in per-unit or percent of nameplate rating. Per-unit quantities are calculated as: Per-unit quantity = Actual quantity Base value of quantity , (1) where the actual quantity is the value of the quantity in the actual (SI) units. The base value has the same units as the actual quantity, therefore making the per-unit quantity dimensionless. In addition, the base value is always a real number. Thus, the angle of the per-unit quantity is identical to the angle of the actual quantity. 2 Per-Unit System for Single-Phase Circuits Two independent base values can be arbitrarily selected at one point of a power system. Usually for single-phase systems the base voltage, E base , and the base single-phase complex power, S base , is selected. Then, in order for electrical laws to be valid in the per-unit system, the next relationships must be used for other base values: P base = Q base = S base , (2) I base = S base E base , (3) Z base = R base = X base = E base I base = E 2 base S base , (4) Y base = G base = B base = 1 Z base . (5) By convention, we adopt the following two rules for base quantities: 1. The value of S base is the same for the entire power system being studied.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern