The details of 'effective grounding' as far as the utility is concerned are totally different than what electricians normally deal with on the customer side of things. This has to do with how distributed generation interacts with the POCO systems in the event that one of the POCO wires faults. I don't know any more than the reason for why the requirements exist, not how to properly implement them. As you learn more about this particular topic, writing about it would be a great way to return knowledge to the forum.
I can tell you that they don't want to desensitize the relays upstream.
What that actually
means is totally up to you!
Trick: open the pdf, hit ctrl-f, and type in the word ground.
The Company may require that grounding impedance be limited to the highest value
suitable for neutral stabilization or to limit generator ground fault contributions.
Contribution to the faults on the Company’s four-wire distribution EPS can
desensitize the relays upstream of the Customer-Generator’s interconnection.
Hence, it is required that the Customer-Generator provide a means to install a
grounding reactor within their facility to limit the ground fault current to three times
the generator rating and, where possible, to limit the rise of ground fault current at
the point on the high voltage (primary) level nearest the proposed service point by no
more than 10%. The grounding reactor shall not violate the effectively grounded
system requirements. The size of the grounding reactor will be specified by the
Company. If the 10% criteria is not met with the grounding reactor, other methods,
upon Company acceptance may be required by the Customer-Generator to mitigate
the increase in fault current.
• If the Customer-Generator is permitted to interconnect through an un-grounded
source, a “zero-sequence” voltage or “3V0” scheme will be required on the primary
side of the approved delta primary wound transformer (see Section 5.5.3) supplying
the DG system. In cases where the Company’s EPS is an ungrounded circuit, the
3V0 scheme may be waived at the Company’s discretion on a case by case basis.
Refer to Figure 4 for additional information.
5.7.8 Overcurrent Relays
See Sections 5.3.4 and 5.5 for transformer requirements. Overcurrent protection is
required on the high side of the DG Customer’s interface transformer to detect faults on
the Company’s EPS. Voltage controlled overcurrent elements (51C) are required for
both phase and ground. These relays shall utilize voltage sensing via the Yg-Yg VTs on
the DG Customer’s transformer secondary delta winding to detect the single line-toground
faults. The 51C elements shall trip the high side interrupting device.
The DG Customer is responsible for over
voltage detection and the detection of line-to-ground faults on the primary and
secondary sides of the step-up transformer as well as the Company’s EPS. For
wye reactively grounded primary, delta secondary transformers, the DG
Customer is required to install Yg-Yg VTs on their transformer secondary delta
winding which will have the dual purpose to detect the voltage depression on the
faulted phase on low voltage line-to-ground faults to enable the operation of the
51C controlled overcurrent elements when faults occur on the Company EPS, as
well as, detect the over voltage on the unfaulted phases for single phase-toground
faults on the delta side of the step-up transformer. The use of Yg-Open
Delta VTs will be considered by the Company on a case-by-case basis.
http://www.nationalgridus.com/non_html/shared_constr_esb756.pdf