4a Coordination Study Opening Section.pdf

Protective Device Coordination Study

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Introduction

The electrical power system protective device’s primary function is to detect fault currents and isolate the fault from its source of power by activating appropriate circuit interrupting devices. Protective Device Coordination Studies strive to establish the setting for or selection of protective devices that achieve best overall system response to an electrical fault. Commonly there are mutually exclusive objectives in this endeavor. Attention must be given to quick isolation of a fault to minimize its escalation and equipment damage while avoiding a system so sensitive that protective devices operate upon normal system disturbances. Other objectives are to isolate the smallest possible area of the electrical system, protect cables and equipment in overload conditions, reduce arc flash levels and achieve “selectivity” between devices.

Selectivity for a protective system can be defined in general terms as: The inter-related performance of protective devices, whereby a minimum amount of equipment is removed from service when isolating a fault. Stated differently, selectivity requires the closest protective device to the fault to respond to and isolate the fault.

Achieving these objectives is highly desirable. However, it is very common to have to make compromises to meet as many objectives as possible. The term Coordination is sometimes used to describe a reasonable compromise, between the mutually desirable, but competing objectives, of maximum availability and maximum protection. The protective device setting or selection of equipment recommended in this study is based on an exercise of engineering judgment to achieve a balance between these objectives.

Per the National Electrical code Life Safety, Elevators, Equipment and Critical circuits must be selectively coordinated. This is a more stringent condition than just being coordinated. The NEC defines selective coordination as “Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings”. An example would be an electrical fault at one elevator would not cause a feeder breaker serving two or more elevators to open, thus taking more than one elevator out of service.

The objective of selecting and setting protective devices is to minimize equipment damage and to isolate only the faulty circuit while obtaining the least amount of disturbance to the remaining system. To achieve the best system performance requires not only good engineering judgment, but also personal knowledge of local operating conditions. The owner is advised to review any recommended changes in light of their system experiences. The recommendations presented in this section are offered as a guide until such time when operating conditions or experiences indicate changes are warranted.

System changes such as changes in a utility’s available fault current, system capacity, system loads and operating procedures may warrant a review of this report’s recommendations.

This study groups the information into two sections – common or generic information and specific study results. This section includes the generic information such as descriptions of study analysis. The study’s specific results and findings are grouped under individual tabs labeled by the studied system’s source transformer. Each section has a summary page capturing that section’s specific findings and observations.

Findings and Recommendations

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The National Electrical Code requires that essential electrical systems (Article 517.26) shall meet the requirements of Article 700, except as amended by Article 517. Per Article 700.27 and 701.18 Emergency and Legally required standby system(s) and Article 517.26 Equipment and Critical circuits overcurrent devices shall be selectively coordinated with the supply side overcurrent protective devices. Overall these systems do not achieve selective coordination particularly with molded case breakers having instantaneous trip units. It is not possible to make recommendations on how best to correct these conditions without an engineering study. The same is true in estimating the cost to bring the system into compliance.

The non-essential systems also have areas where selective coordination is not achieved. Again, these are mainly related to molded case breakers having instantaneous trip units.

At the end of this section are Time-Current Curve (TCC) comments listed by system section and TCC.

Relaying Fundamentals

Basic philosophy in establishing protective device setting or their selection is:

1. The closest upstream protective device to a fault must clear it as quickly as possible to help assure the remaining system is capable of riding through the disturbance and only the fault is removed.

These protective devices are often referred to as “first-line” devices. High speed devices that provide first-line protection are customarily instantaneous over-current devices and differential relays.

2. The next upstream protective device provides a “back-up” function. To achieve this added protection the device must be able to detect the same fault and operate to clear the fault sometime after the downstream device would normally have cleared the fault. Time-over-current protective devices are commonly used in this role.

A backup device can also service as a “first-line” protective device for an upstream protective zone.

Criteria used in determining time over-current protective device settings can be based on any or all of the following:

1. The protective device must have a low enough setting to detect fault short circuit current within the protective zone.

2. The protective device must have a high enough setting to avoid nuisance tripping due to transformer magnetizing and/or motor starting inrush currents.

3. The protective device should have the ability to provide flexible settings for circuit full load currents and the ability to coordinate with other protective devices under fault current conditions.

4. Comply with ANSI/IEEE and NEC standards for the protection of transformers.

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Protective Device One-Line Diagram

A time-current curve’s (TCC) interpretation is dependent on knowing the physical arrangement of each protective device within the system. The one-line diagram included with each TCC is provided to illustrate this relationship. The one-line also provides information on system equipment influencing relay settings such as motors and their starting curves; transformers and associated inrush and protective curves; and breakers having current limiting fuses.

Coordination Curves

Due to the complex relationship between protective devices, competing objectives in obtaining their coordination, and system restraints, the time-current curve (TCC) evolved as the ideal tool for engineers to perform this work and as an effective means for communication and presentation of the end results. TCC curves use a standardized log-log paper to illustrate the time versus current relationship of each protective device and other equipment. This standardization permits different protective devices, by different manufacturers, and of different technologies to be viewed in a common environment.

On a TCC graph time is the “y-axis” while current is the “x-axis”. The primary objective in coordinating a system is to have time intervals between protective device curves. In an ideal situation the comparison of two curves would have “daylight” between them. ANSI/IEEE Std-242 has been published to standardize these time intervals. As an illustration coordinating two inverse time over-current relays in series a time interval of 0.3 to 0.4 seconds may be prescribed for a five and eight cycle breaker. These time intervals are composed of the following components:

• circuit breaker opening time (5 cycles): 0.08 seconds

• relay over-travel: 0.10 seconds

• safety factor for CT saturation, setting errors, etc.: 0.22 seconds

When coordinating with fuses, the following components require consideration:

• Fuse total clearing time

• Fuse minimum melting time

The time-current curves included in this report illustrate the system conditions and the performance that can be obtained from the equipment installed. In most electrical systems having common equipment types by the same manufacturer, patterns develop so that the creation of TCC for every device is not necessary.

This is the case for this system. The recommended relay settings have been established from the enclosed TCC’s.

Conductor Damage Curves

Electrical conductors are restricted in ampacity due to thermal limits. For continuous or long time loads measured in hours or days, the National Electrical Code mandates a conductor rated current carrying capacity. A conductor’s capacity is governed by ambient temperature, allowable temperature rise, geometry, and installation conditions. For insulated conductors the insulation temperature design limits

Protective Device Coordination Study Page 4 cannot be exceeded or loss of insulation life will result, but not necessarily resulting in instantaneous failure.

For bare conductors mechanical strength will be affected if temperature design limits are exceeded. Again mechanical life is lost, but not necessarily resulting in instantaneous failure.

For long time continuous operations conductor’s temperature rise is influenced by the above mentioned parameters. For fault conditions time restraints are so short that conductor environmental conditions do not factor into a conductors temperature rise. The TCC conductor curves are based on calculations assuming the heat generated by fault currents is absorbed by the conductor metal and no heat is transmitted to the conductor’s insulation.

Calibration and Testing of Protective Devices

The final step in a coordination study is to apply the recommendations. Generally, this requires the studied relay to have the new setting applied and tested. Relay testing is a critical step in this process to validate the breaker and relay are calibrated and working correctly.

It is assumed an electrical system will operate predictably and reliably over long periods of time and its breakers and relays will perform as expected. To achieve this routing maintenance is required. It is absolutely imperative that regular calibration and maintenance be conducted at regular intervals as recommended by the equipment manufacturers.