Ground Fault protection on Delta-Delta system?

[RodLechelt]

We had this question come up and we delt with it one way, using a 750 relay, but I am wondering how any one else solved this problem. How do you protect against ground faults on a delta delta system?

 

[busbar]

This has been exhaustively discussed in Eng-Tips.

Chapter 6 of Donald Beeman, Industrial Power Systems Handbook, McGraw-Hill, 1955

Bridger, Baldwin, Jr, Choosing Grounding Options For Electrical Power Systems Electrical Construction & Maintenance, Feb 1, 1995

Nelson and Sen, High-Resistance Grounding of Low-Voltage Systems: A Standard for the Petroleum and Chemical Industry, IEEE Transactions On Industry Applications, V35N4, July/August 1999

 

[busbar]

Also Schweitzer Engineering Lab bulletin AG2002-06: Applying the SEL-351 Relay to an Ungrounded System

I don’t have a clue what is meant by “a 750 relay.”

 

[RodLechelt]

Busbar,

The 750 is a multifunction microprocessor feeder protection relay built by GE Multilin.

 

[HydroInspector]

Ground Fault Protection Rule 14-102

Ontario Electrical Safety Code

Rule 14-102 requires ground fault protection be provided, to de-energize all normally ungrounded conductors of a
circuit that faults to ground, where one of the following circuit characteristics exists in solidly grounded systems:

a. 2000 Amp or more and rated 150 volts or less to ground; and

b. 1000 Amp or more and more than 150 volts-to-ground, but less than 750 volts phase-to-phase.
Items to note are:

i. Rule 14-102 applies to only solidly grounded systems.

ii. Diagram 3 of The Ontario Electrical Safety Code shows a variety of ultimate points of conductor deenergization
in the event of a ground fault.

iii. Ground fault relays are usually factory- set at the lowest current and shortest time settings available to ensure against unnecessary equipment damage during early stages of construction. These settings should be adjusted to the intended values prior to final commissioning of the equipment and may be found in the co-ordination study.

iv. In special cases the trip setting can be as high as 1200 amps (See Rule 14-102 (2)) to accommodate multiple
circuit grounding schemes. In these cases, designers should include this data with the plan report submitted to the inspection department, showing that such considerations have been made in the design of the system involved.
There are three basic ground fault detection schemes available:

a. Zero Sequence Sensing

- A single current transformer encircling all of the phase conductors of the circuit including neutral.
-
- Vector sum of the currents flowing through the sensor equals zero under normal conditions.
-
· When a circuit conductor faults to ground, the current returns via the grounded metal enclosure, conduit or other path outside the sensor. This results in a non-zero current sum through the sensor, which in turn, generates an output signal to the relay, and the circuit is opened within milliseconds after the fault occurs.
· All grounding of the neutral must be on the line side of the sensor. This is particularly important where the neutral is grounded both at the switchgear and at the transformer.
· The zero-sequence current transformer may be located on either the load or the line side of the circuit breaker contacts.

A residually connected ground fault protection system is a form of zero-sequence sensing. The difference being this system utilizes a number of current transformers, instead of one. The vectorial sum of the phase currents and the neutral current are monitored using separate current transformers and a ground relay. Again note similarity to zero sequence, the grounding points must be on the supply side (ahead) of the sensors (current transformers)

b) Ground Strap Sensing

· A standard ratio type current transformer senses current flow on the bonding strap that connects the frame or grounding bus of the switchboard to the neutral.
· A ground fault on any branch circuit, feeder or sub-feeder, anywhere in the system, will cause the current to flow back to the neutral through the bonding strap which in turn, generates an output signal to the relay, and the circuit is opened.
· Ground strap sensing is applicable where the system neutral is grounded in the switchgear and isolated from ground at the transformer.
· The transformer neutral may be grounded at the transformer only if the ground strap sensor is located at the transformer as well. In this case the neutral must remain ungrounded at the switchboard.
· Most utilities require grounding of the secondary neutral at the transformer. If the current sensor is installed in the main switchboard, an alternative to ground strap sensing shall be chosen.


c) High Resistance Grounding

Usually consists of five basic parts:
· A star point (neutral)
· A grounding resistance
· A fault detector
· An alarm scheme
· A fault locating scheme
· The system neutral is grounded through a resistance that limits the ground fault current.
· The resistor current is equal to ground fault current.
· High resistance grounding limits transient over-voltages without shutting down grounded equipment
· High resistance grounding cannot be used where 3-phase, 4-wire loads must be served.
· Ground fault protection, as per Rule 14-102, is not required if the transformer is resistance grounded to limit fault current to 5 amperes maximum and if the system neutral is not brought into the main switchboard. A suitable ground detector, installed in series with the current limiting resistor, having either audible or visible annunciation will be an acceptable alternative to circuit interruption. This conductor would be a minimum #8 AWG or minimum #6 if exposed.
· The occurrence of a second ground on a different phase of the system results in a phase-to-phase fault on the system. Therefore it is important to properly bond all equipment as per Table 16.
· The resistance grounded conductor must not be brought into the switchgear assembly, except when the grounding resistor itself is located in the switchgear. (See Figure No. 5.)
· There should be no other connections between neutral and ground (ie Grounding of the XO of the transformer bushing. In this case the requirements of Rule 36-308 (6) do not apply).
· Bonding of all equipment must still be ensured. ie: from transformer to the switchgear requires a bond wire installed and sized as per Table 16.
- Ground Fault indicators would not be required.
-
· Signage should be put on the main board indicating this is a high resistive grounded system and the neutral shall not be used.

Other important points to consider are:
(1) The ampere rating of the circuits referred to in Rule 14-102(1) shall be considered to be:

(a) The rating of the largest fuse that can be installed in a fusible disconnecting device, therefore, a 1000-ampere switch fused at 800 amperes would require ground fault protection.
(b) The ampacity of the main conductor feeding the devices located at points marked with an asterisk in Item 2 of Diagram 3, in the case where no main disconnecting device is provided. An exception to this is where transformer secondary bushings are used as splitters.

Tony Moscioni
Electrical Inspector
Electrical Safety Authority

 

[jbartos]

Suggestion: Visit

http://www.geindustrial.com/cwc/products?pnlid=6&famid=31&catid=216&id=750&type=product+information

http://pm.geindustrial.com/FAQ/Documents/750_760/GER-3182.pdf

etc. for more info

 

[huntercon]

There are many monitors manufactured today which are specifically designed for ground detection & location on ungrounded AC and DC systems. Rather than ground 'detectors' they are now referred to as Insulation Monitoring Devices (IMDs), because they accurately measure the insulation level of the ungrounded system and alarm on loss of that insulation.

We prefer working with Bender, Inc. (

http://www.bender.org),

but there are others. They have over 50 years experience in this specific area.

Multi-function relays can be configured to sense grounds in UG applications, but usually involve adding separate PTs. IMDs can be directly connected to systems up to 600V, and higher with couplers (7200V).

Plus, you can decide to monitor, alrm or trip - depending upon the system requirements.

 

[rhatcher]

hydroinspector has some good info in his post, unfortunately (unless I missed something) it doesn't seem to apply to an ungrounded system.

I glanced at the instructions for this relay (link below) and it appears that the "bus undervoltage" or the "neutral displacement" protection schemes would do what you want.

http://www.geindustrial.com/cwc/products?pnlid=6&famid=31&catid=216&id=750&typeId=3&lang=en_US

Keep in mind that for an ungrounded system that cannot use current flow to detect a ground fault until you get 2 ground faults on different phases. When this happens though, your fault current will be line current (from one phase through ground to the other phase) and it will not be possible to distinguish that from a L-L fault.

The only way I know of to detect the first ground fault is by measuring L-G voltage, unless of course you are using an insulation monitor as suggested by Huntercon. In the old days (and even now in a lot of places..) the ground detection scheme for ungrounded systems consisted of 3 lights connected L-G. When one dims and the other 2 brighten, you have a problem...

 

[huntercon]

In many cases - and locations - 'the old days' are still here (as rhatcher mentioned). Because there is a perception by many supervisors and technicians that ground detection methods have not improved since the 3-lightbulb technique; ungrounded systems have received a bad rep and are continuing to be replaced by grounded and res-grounded configurations.

Ungrounded systems were originally developed to provide on-going and continuous service to industrial manufacturing processes. With the technology available today from manufacturers like Bender, Cutler-Hammer, and Controlab; there is no reason why these systems cannot continue to operate effectively and safely.

 

[jbartos]

Suggestion to the previous posting: I agree that the technologies are available. What is left is:
1. Compare costs
2. Reliability and expected life
3. Operation and Maintenance