Elspec EQUALIZER-ST, Руководство пользователя и инструкция по установке, Страница: 111 / 121

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www.elspec-ltd.com | December 2014 | SMX-0609-0101
User Manual | EQ-ST | V15
In order to avoid resonance hazards, harmonies amplitudes (created at the load) are measured and their damage
potential is assessed before a compensation system is designed. The most common harmonies presented are 5
th
and above or 3
rd
and above. During the design process, an inductor is added in series to capacitors forming a
series LC circuit with a resonance frequency lower than the lowest hazardous harmony possible. This series LC
re the most common values are:
1.
𝑋
𝐿
= 7% ∙ 𝑋
𝐶
on main frequency i.e. 50Hz or 60Hz to avoid 5
th
harmony or above.
2.
𝑋
𝐿
= 14% ∙ 𝑋
𝐶
on main frequency i.e. 50Hz or 60Hz to avoid 3
rd
harmony or above.
This design immunes the facility from potential resonance due to compensation system presence.
Standard balanced systems are 7%, unbalanced, single phase and systems placed at high distortion networks are
14%.
M
OTOR
S
TART
C
OMPENSATION
A motor start demands very high reactive power for a relatively short period of time.
The starting current of an AC motor usually varies from 6 to 8 times the nominal current at steady state. This is
due to the large amount of energy that is required to magnetize the motor, enough to overcome the inertia the
system has at standstill. The high current drawn from the network usually causes problems such as voltage drop,
high transients and, in some cases, uncontrolled shutdowns. The graph bellow demonstrates a typical startup
phenomena for a 500KW motor, with a power factor of 0.89.
Figure 100: Motor Operation Vector
Deciding The Required System Size
Each facility has different conditions producing different constraints.
When the motor has a high demand during the startup in comparison with the infrastructure where it is connected,
the result is a network voltage drop. Utility limits the allowed voltage drop (usually at the substation connection
point). In many cases even if this limit is not exceeded, the impact of the voltage drop on the lower voltage is so
significant that appliances or other devices in the network may restart or malfunction. This is considered as a
customer constraint.