SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 14 of 27
Positive and zero sequence impedance of MAY transformers in parallel:
=
(0.47 + j8.99)
= (0.24 + j4.5)
2
Overall minimum MAY positive sequence source impedance:
= (3.04 + j16.04) in parallel (0.24 + j4.5)
= (0.287 + j3.52) = 3.53
/85.3° ohms
Overall minimum MAY zero sequence source impedance:
= (11.7 + j36.7) in parallel (0.24 + j4.5)
= (0.32 + j4.05) = 4.06
/85.5° ohms
b) Maximum source impedance at MAY 60kV is when both 50MVA transformers
at MAY are switched out, the 50MVA transformer at APRIL is switched in and
only one of the parallel lines between APRIL and MAY is switched in.
Maximum MAY positive sequence source impedance:
= (0.47 + j8.99) + (5.13 + j14.1)
= (5.6 + j23.1) = 23.8
/76.4° ohms
Maximum MAY zero sequence source impedance:
= (0.47 + j8.99) + (22.5 + j55.5)
= (22.97 + j64.5) = 68.5
/70.4° ohms
19.4 Selecting Zone 1 reach
Required Zone 1 reach is to be 80% of the MAY – JUNE line impedance.
Ratio of secondary impedance to primary impedance:
=
110
x
200
= 0.073
5 60 x 10
3
Required Zone 1 reach:
= 0.8 x 10 (0.171 + j0.47) ohms primary
= 4.0
/70° ohms primary
= 0.292
/70° ohms secondary
The relay coarse reach (Z
Ph
) should be set to the nearest value below the required
Zone 1 reach. It is important to set Z
Ph
as high as possible since the relay current
level detector sensitivities are inversely proportional to Z
Ph
and it is best to have the
most sensitive settings possible.
Z
Ph
=
K1 + K2
θ
Ph
in this case rated current I
n
= 5A
I
n
Select θ
Ph
= 70°
K1 + K2
≤ 0.292 K1 + K2 ≤ 1.46
5
Select K1 = 1
K2 = 0.4
Actual Z
Ph
= 0.28 /70°
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