13-03-2014, 03:23 PM
Power Quality Basics: Fixing Low Voltage & Undervoltage Problems
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INTRODUCTION
Chronic low voltage levels are a common problem that can affect any electric customer. The causes of this
power quality problem are relatively easy to identify but potential energy saving measures undertaken by
the operators of the electric grid could make chronic low voltage levels a significant problem for many.
This paper looks at causes, symptoms and solutions for chronic low voltage problems.
Chronic low voltage or undervoltage
The term of art for chronic low voltage is “undervoltage” and
undervoltage is defined as a voltage level 90% or less of the
nominal voltage that exists for one minute or more. The key
factor in this definition is that of the duration of one minute or
more as this distinguishes undervoltage from shorter duration
problems such as voltage sags.
Causes of chronic low voltage – undervoltage
Undervoltage is most commonly attributable to reduced voltage levels on the utility’s transmission and distribution
system. In areas with low electrical load density, such as suburban and rural locations, voltage drop on electrical
conductors is a significant issue. Conductor impedance driven by load demand serves to decrease the voltage along
the length of a conductor. The electric utility uses on-load tap changing voltage regulators (OLTCs) at substations
and line drop compensating voltage regulators (LDCs) along the length of a conductor to boost (raise) or buck
(lower) voltage levels within a proper range for delivery to customers. Customers nearest to an OLTC or LDC will
see the highest voltage levels while those farthest away will see the lowest voltage levels.
Utility and utilization voltage levels
No discussion of undervoltage would be complete without an examination of the regulations and definitions
concerning voltage levels. The first point to be noted is that there is no national (U.S.) standard for the voltage
levels. Individual states usually point to ANSI standard C84 or some variation of the standard to define the voltage
levels that must be supplied. ANSI C84 suggests that the voltage level normally supplied at a customer’s meter
(the “service voltage”) should be within ±5% of the nominal voltage. However, the standard goes on to recognize
that the voltage level may “infrequently” be -10% to +6% of nominal. No specific definition of “infrequently” is
provided however the standard does say the utility-supplied voltage level may be worse than -10 to +6% for “brief”
periods due to unexpected operational issues. And, this is not the end of the voltage level story.