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The Best Applications For VFDs

The Best Applications For VFDs

The most commonly used motor in building HVAC applications is the three-phase, induction motor, although some smaller applications may use a single-phase induction motor. VFDs can be applied to both.

While VFD controllers can be used with a range of applications, the ones that will produce the most significant benefits are those that require variable speed operation. For example, the flow rate produced by pumps serving building HVAC systems can be matched to the building load by using a VFD to vary the flow rate. Similarly, in systems that require a constant pressure be maintained regardless of the flow rate, such as in domestic hot and cold water systems, a VFD controlled by a pressure setpoint can maintain the pressure over most demand levels.

The majority of commercial and institutional HVAC systems use variable volume fan systems to distribute conditioned air. Most are controlled by a system of variable inlet vanes in the fan system and variable air volume boxes. As the load on the system decreases, the variable air volume boxes close down, increasing the static pressure in the system. The fan’s controller senses this increase and closes down its inlet vanes. While using this type of control system will reduce system fan energy requirements, it is not as efficient or as accurate as a VFD-based system.

Another candidate for VFD use is a variable refrigerant flow systems. Variable refrigerant flow systems connect one or more compressors to a common refrigerant supply system that feeds multiple evaporators. By piping refrigerant instead of using air ducts, the distribution energy requirements are greatly reduced. Because the load on the compressor is constantly changing based on the demand from the evaporators, a VFD can be used to control the operating speed of the compressor to match the load, reducing energy requirements under part-load conditions.

Additional VFD Applications

While the primary benefit of both of these VFD applications is energy savings, VFDs are well suited for use in other applications where energy conservation is of secondary importance. For example, VFDs can provide precise speed or torque control in some commercial applications.

Some specialized applications use dual fans or pumps. VFDs, with their precise speed control, can ensure that the two units are operated at the desired speed and do not end up fighting each other or having one unit carry more than its design load level.

Advances in technology have increased the number of loads that can be driven by the units. Today, units are available with voltage and current ratings that can match the majority of three-phase induction motors found in buildings. With 500 horsepower units or higher available, facility executives have installed them on large capacity centrifugal chillers where very large energy savings can be achieved.

One of the most significant changes that has taken place recently is that with the widespread acceptance of the units and the recognition of the energy and maintenance benefits, manufacturers are including VFD controls as part of their system in a number of applications. For example, manufacturers of centrifugal chillers offer VFD controls as an option on a number of their units. Similarly, manufacturers of domestic water booster pump systems also offer the controls as part of their system, providing users with better control strategies while reducing energy and maintenance costs.

A Few Cautions

When evaluating the installation of a VFD, facility executives should take into consideration a number of factors related to the specifics of the application. For example, most VFDs emit a series of pulses that are rapidly switched. These pulses can be reflected back from the motor terminals into the cable that connects the VFD to the motor. In applications where there is a long run between the motor and the VFD, these reflected pulses can produce voltages that exceed the line voltage, causing stresses in the cable and motor windings that could lead to insulation failure. While this effect is not very significant in motors that operate at 230 volts or less, it is a concern for those that operate at 480 volts or higher. For those applications, minimize the distance between the VFD and the motor, use cabling specifically designed for use with VFDs, and consider installing a filter specifically designed to reduce the impact of the reflected pulses.

Another factor to consider is the impact the VFD may have on the motor’s bearings. The pulses produced by the VFD can generate a voltage differential between the motor shaft and its casing. If this voltage is high enough, it can generate sparks in the bearings that erode their surfaces. This condition can also be avoided by using a cable designed specifically for use with VFDs.

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SOURCE: facilitiesnet

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The Benefits of VFDs In HVAC Systems

The Benefits of VFDs In HVAC Systems

One of the most successful energy management tools ever applied to building HVAC systems is the variable frequency drive (VFD). For more than 20 years, VFDs have successfully been installed on fan and pump motors in a range of variable load applications. Energy savings vary from 35 to 50 percent over conventional constant speed applications, resulting in a return on investment of six months to two years.

While the number of applications suitable for early generation drives was limited based on the horsepower of the motor, today’s drives can be installed in practically any HVAC application found in commercial and institutional buildings. Systems can be operated at higher voltages than those used by earlier generations, resulting in off the shelf systems for motors up to 500 horsepower.

Early generation systems also suffered from low power factor. Low power factor robs the facility of electrical distribution capacity and can result in cost penalties imposed by electrical utility companies. Today’s systems operate at a nearly constant power factor over the entire speed range of the motor.

Another problem that has been corrected by today’s systems is operational noise. As the output frequency of the drives decreased in response to the load, vibrations induced in the motor laminations generated noise that was easily transmitted through the motor mounts to the building interior. Today’s drives operate at higher frequencies, resulting in the associated noise being above the audible range.

And VFDs continue to evolve. From numerous system benefits to an increasing range of available applications, VFDs are proving to be ever more useful and powerful.

The Heart of VFDs

Most conventional building HVAC applications are designed to operate fans and pumps at a constant speed. Building loads, however, are anything but constant. In a conventional system, some form of mechanical throttling can be used to reduce water or air flow in the system. The drive motor, however, continues to operate at full speed, using nearly the same amount of energy regardless of the heating or cooling load on the system. While mechanical throttling can provide a good level of control, it is not very efficient. VFDs offer an effective and efficient alternative.

Three factors work together to improve operating efficiency with VFDs:

1. Operating at less than full load. Building systems are sized for peak load conditions. In typical applications, peak load conditions occur between 1 and 5 percent of the annual operating hours. This means that pump and fan motors are using more energy than necessary 95 to 99 percent of their operating hours.

2. Oversized system designs. Designing for peak load oversizes the system for most operating hours. This condition is further compounded by the practice of oversizing the system design to allow for underestimated and unexpected loads as well as future loads that might result from changes in how the building space is used.

3. Motor energy use is a function of speed. The most commonly used motor in building HVAC systems is the induction motor. With induction motors, the power drawn by the motor varies with the cube of the motor’s speed. This means that if the motor can be slowed by 25 percent of its normal operating speed, its energy use is reduced by nearly 60 percent. At a 50 percent reduction in speed, energy use is reduced by nearly 90 percent.

The installation of a VFD in an HVAC application addresses the inefficiencies introduced by the first two factors, while producing the energy savings made possible by the third. The VFD accomplishes this by converting 60 cycle line current to direct current, then to an output that varies in voltage and frequency based on the load placed on the system. As the system load decreases, the VFD’s controller reduces the motor’s operating speed so that the flow rate through the system meets but does not exceed the load requirements.

VFD Benefits

The most significant benefit to using a VFD is energy savings. By matching system capacity to the actual load throughout the entire year, major savings in system motor energy use are achieved.

Another benefit of the units is reduced wear and tear on the motors. When an induction motor is started, it draws a much higher current than during normal operation. This inrush current can be three to ten times the full-load operating current for the motor, generating both heat and stress in the motor’s windings and other components. In motors that start and stop frequently, this contributes to early motor failures.

In contrast, when a motor connected to a VFD is started, the VFD applies a very low frequency and low voltage to the motor. Both are gradually ramped up at a controlled rate to normal operating conditions, extending motor life.

VFDs also provide more precise levels of control of applications. For example, high-rise buildings use a booster pump system on the domestic water supply to maintain adequate water pressure at all levels within the building. Conventional pump controls in this type of application can maintain the pressure within a certain range, but a VFD-based system can maintain more precise control over a wider range of flow rates, while reducing energy requirements and pump wear.

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SOURCE: facilitiesnet

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Siemens technical publication | Loss Of Vacuum

Siemens technical publication | Loss Of Vacuum

If a vacuum interrupter should lose vacuum, several operating situations should be considered:

1. With contacts open
2. When closing
3. When closed and operating normally
4. When opening and interrupting normal current
5. When opening and interrupting a fault.

Cases 1, 2 and 3 are relatively straightforward. Generally, the system sees no impact from loss of vacuum in such a situation. Cases 4 and 5, however, require further discussion. Suppose there is a feeder circuit breaker with a vacuum interrupter on phase 3 that has lost vacuum. If the load being served by the failed interrupter is a deltaconnected (ungrounded) load, a switching operation would not result in a failure. Essentially, nothing would happen. The two good phases (phase 1 and phase 2, in this example) would be able to clear the circuit, and current in the failed interrupter (phase 3) would cease.

The alternative case of a grounded load is a different situation. In this case, interruption in the two good phases (phase 1 and phase 2) would not cause current to stop flowing in phase 3, and the arc would continue to exist in phase 3. With nothing to stop it, this current would continue until some backup protection operated. The result, of course, would be destruction of the interrupter.

Since the predominant usage of circuit breakers in the 5-15 kV range is on grounded circuits, we investigated the impact of a failed interrupter some years ago in the test lab. We intentionally caused an interrupter to lose vacuum by opening the tube to the atmosphere. We then subjected the circuit breaker to a full short circuit interruption. As predicted,
the “flat” interrupter did not successfully clear the affected phase, and the “flat” interrupter was destroyed. The laboratory backup breaker cleared the fault. Following the test, the circuit breaker was removed from the switchgear cell. It was very sooty, but mechanically intact. The soot was cleaned from the circuit breaker and the switchgear cell, the faulty interrupter was replaced, and the circuit breaker was re-inserted in the cell. Further short circuit interruption tests were conducted the same day on the circuit breaker.

Field experience in the years since that test was conducted supports the information gained in the laboratory experiment. One of our customers, a large chemical operation, encountered separate failures (one with an air magnetic circuit breaker and one with a vacuum circuit breaker) on a particular circuit configuration. Two different installations, in different countries, were involved. They shared a common circuit configuration and failure mode. The circuit configuration, a tie circuit in which the sources on each side of the circuit
breaker were not in synchronism, imposed approximately double rated voltage across the contact gap, which caused the circuit breaker to fail. Since these failures resulted from application in violation of the guidelines of the ANSI standards, and greatly in excess of the design ratings of the circuit breakers, they are not indicative of a design
problem with the equipment.

However, the damage that resulted from the failures is of interest. In the case of the air magnetic circuit breaker, the unit housing the failed circuit breaker was destroyed, and the adjacent switchgear units on either side were damaged extensively, requiring significant rebuilding. The air magnetic circuit breaker was a total loss. In the case of the vacuum circuit breaker, the failure was considerably less violent. The vacuum interrupters were replaced, and the arc by-products (soot) cleaned from both the circuit breaker and the compartment. The unit was put back into service. Our test experience in the laboratory, where we routinely explore the limits of interrupter performance, also supports these results.

More recently, several tests were performed in our high-power test laboratory to compare the results of attempted interruptions with “leaky” vacuum interrupters. A small hole (approximately 1/8” diameter) was drilled in the interrupter housing, to simulate a vacuum interrupter that had lost vacuum.

The results of these tests were very interesting:

  1. One pole of a vacuum circuit breaker was subjected to an attempted interruption of 1310 A (rated continuous current = 1250 A). The current was allowed to flow in the “failed” interrupter for 2.06 seconds, at which point the laboratory breaker interrupted. No parts of the “failed” circuit breaker or the interrupter flew off, nor did the circuit breaker explode. The paint on the exterior of the interrupter arcing chamber peeled off. The remainder of the circuit breaker was undamaged.
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  2. A second pole of the same vacuum circuit breaker was subjected to an attempted interruption of 25 kA (rated interrupting current = 25 kA), for an arc-duration of 0.60 seconds, with the laboratory breaker interrupting the current at that time. The arc burned a hole in the side of the arc chamber. The circuit breaker did not explode, nor did parts of the circuit breaker fly off. Glowing particles were ejected from the hole in the arcing chamber. None of the mechanical components or other interrupters were damaged. Essentially, all damage was confined to the failed interrupter.

Our experience suggests rather strongly that the effects of a vacuum interrupter failure on the equipment are very minor, compared to the impact of failures with alternative interruption technologies. But the real question is not what the results of a failure might be, but rather, what is the likelihood of a failure? The failure rate of Siemens vacuum interrupters is so low that loss of vacuum is no longer a significant concern. In the early 1960s with early vacuum interrupters, it was a big problem. A vacuum interrupter is constructed with all connections between dissimilar materials made by brazing or welding. No organic materials are used. In the early years, many hand-production techniques were used, especially when borosilicate glass was used for the insulating envelope, as it could not tolerate high temperatures. Today, machine welding and batch induction furnace brazing are employed with extremely tight process control. The only moving part inside the interrupter is the copper contact, which is connected to the interrupter end plate with a welded stainless steel bellows. Since the bellows is welded to both the contact and the interrupter end plate, the failure rate of this moving connection is extremely low. This accounts for the
extremely high reliability of Siemens vacuum interrupters today.

In fact, the MTTF (mean time to failure) of Siemens power vacuum interrupters has now reached 24,000 years (as of October 1991). Questions raised by customers regarding loss of vacuum were legitimate concerns in the 1960s, when the use of vacuum interrupters for power applications was in its infancy. At that time, vacuum interrupters suffered from frequent leaks, and surges were a problem. There was only one firm that offered vacuum circuit breakers then, and reports suggest that they had many problems. We entered the vacuum circuit breaker market in 1974, using Allis-Chalmers’ technology and copper-bismuth contact materials. In the early 1980′s, after becoming part of the worldwide Siemens organization, we were able to convert our vacuum designs to use Siemens vacuum interrupters, which had been introduced in Europe in the mid-1970s. Thus, when we adopted the Siemens vacuum interrupters in the U.S., they already had a very well established field performance record.

The principle conceptual differences in the modern Siemens vacuum interrupters from the early 1960s designs lies in contact material and process control. Surge phenomena are more difficult to deal with when copper-bismuth contacts are used than with today’s chromecopper contacts. Similarly, leaks were harder to control with vacuum interrupters built largely by hand than with today’s units. Today, great attention is paid to process control and elimination of the human factor (variability) in manufacture. The result is that the Siemens vacuum interrupters today can be expected to have a long service life and to impose dielectric stress on load equipment that is not significantly different from the stresses associated with traditional air magnetic or oil circuit breakers.

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Published by: SIEMENS AG

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IEC 61850 Standard In Details (part 2)

IEC 61850 Standard In Details (part 2)

Continued from first part of article IEC 61850 Standard In Details (1)

Services provided by ACSI include querying object set, getting/setting data values, controlling system objects, report manipulation, log manipulation, and other services like file upload/download. Table 3 gives a list of ACSI services defined in the IEC 61850 standard.

All ACSI services are requested by applications and responded by servers. In order to request a service in a server, an application must first establish a valid two-party application association (TPAA) with the server. The TPAA maintains the session states and provides a virtual view of the server to the application.

A typical interaction procedure between an application A and a server S goes as follows:

  1. A establishes a TCP connection with S;
  2. A “logs in” to S by requesting the Associate service from S, providing authentication related information as parameters;
  3. S validates the information provided by A and creates a TPAA object, which provides a virtual view of S to A;
  4. A requests subsequent services while S processes the requests and responses with appropriate responses defined in the IEC 61850 standard;
  5. A issues a Release request to S ; 6. S reclaims the TPAA of A and ends the session.

The virtual view of server provided by a TPAA enforces the access control policies set forth by the server. This virtual view defines which objects in the server are visible and accessible to the application and what kinds of service of those objects are accessible from the application. The concept of virtual view is very flexible and the IEC 61850 standard does not place any restriction on the access control policies of the server. One possible and relatively simple access control is the world-group-owner access control for files used in many UNIX systems.

Table 3: ACSI services

Table 3: ACSI services

Reporting and logging

The IEC 61850 standard provides an efficient mechanism called reporting for applications to track changes to the subscribed system objects. Instead of polling the data attribute values periodically, applications can group the interesting data attributes into a data set, and require the logical node hosting this data set report any changes to the members of this data set. Theoretically a data set can contain data objects/data attributes from different logical nodes, but data sets for reporting usually contain only the data objects/data attributes in the same logical node.

The procedure of report generation and transmission is under the control of an information block called report control block (RCB). A RCB maintains the necessary information to generate a report like which fields should be included in the report, on what events a report should be generated, the sequence number of the current report, whether this RCB is enabled, etc..

A typical report generation and transmission procedure is described as follows:

  1. client application creates a data set containing all the data objects and data attributes it concerns;
  2. client sets the parameters in a RCB, specifies the aforementioned data set as the source of the reports (this step is called to “subscribe to a data set”) and enables this RCB;
  3. on any change to any member of the data set, the logical node tests this change against the event list of the RCB, and issues an internal event if any event in the list gets matched;
  4. on receiving the internal event, the RCB stores this event for later sending;
  5. if the condition of sending the report is satisfied, the logical node collects necessary data, generates the report and sends it out to the client via the association under the direction of the parameters in the RCB.

Reports can be sent by either two-party application association (TPAA) or multi-party application association (MPAA). TPAA is the association that can serve only one client while MPAA can serve more than one client simultaneously. Reports generated on the request of the Report service is sent by TPAA, whereas other reports are sent by MPAA. Reporting uses a publisher/subscriber mechanism: for every client, the server must assign an individual RCB to handle the report generation and transmission.

Logging is a mechanism to record the device events. Logs are stored in the server and hosted by the corresponding logical device. Unlike reporting, every time a device event is triggered, the logical device merely saves a log entry into the log database for later inquiries.

Generic substation event

Besides reporting, the IEC 61850 standard defines generic substation event (GSE) as another means for applications to monitor changes to the data objects/data attributes. GSE is designed for fast delivering notification of system object changes. There are two kinds of GSE, generic object-oriented substation events (GOOSE) and generic substation state event (GSSE). GOOSE is used to exchange a wide range of common data while GSSE is used to convey state change information. The GSE mechanism shares a lot of similarities with reporting, with the major difference that GSE is designed for fast information exchange inside a substation while reporting is mainly used for sending notification from the server side to remote control centers or browsers.

Since real-time performance is critical for GSE messages, the message format and communication stack for GSE transmission is very different from those for reporting. GSE messages are transmitted in binary format, which provides shorter message body and higher message encoding/decoding speed. In stead of using TCP or UDP as the underlying transport layer, GSSE uses its specific transport layer while GOOSE messages are sent to the Ethernet link layer directly without going through any transport layer or network layer.
GSE also utilizes a publisher/subscriber mechanism to transmit the messages. This mechanism is implemented by the Ethernet multicast feature: the publisher sends the GSE message to a specific multi-cast MAC address and the subscribers pick up messages sent to this address, put them into their local buffer for the local applications to consume.

Communication network

The IEC 61850 standard defines a distributed system consisting of interacting logical nodes, which are connected by logical connections. However, in order for this distributed system to work correctly and intelligently, there must be some intelligent components inside this network. It is not hard to see that applications play this role. An interesting question is how to integrate applications into the interacting logical node network. We try to answer this question by starting from the simple ACSI server-application network.

Figure 5 shows such a server-application network. Clearly, each server may serve several applications and vice versa. The dotted lines in Figure 5 refer to the communication channels for reports and GSE messages. Although there are no restrictions about the relationship between report subscribers and report publishers, it is not true for GSE messages. The design of GSE has physical concerns. GSE messages are sent from one IED to another, thus the subscribers and the publishers should not reside in the same IED.

To make things more clear, let us transform Figure 5 into Figure 6, which shows the communication between applications and logical nodes (we hid the communication to server and logical devices). At this point, we can clearly see that logical connections between logical nodes are actually a mixture of several kinds of connections: when a logical node sends a message to another logical node, it is virtually sending reports/GSE messages to the relevant applications; after necessary processing of the reports/GSE messages, the applications issue relevant requests to the other logical node, and vice versa.

Thus we can deploy the application logic to the relevant logical nodes and get Figure 7 by abstracting away the applications. Being aware of this abstraction is important for both implementation and simulation of the IEC 61850 protocol.

Figure 5: Server-Application network

Figure 5: Server-Application network

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Figure 6: Logical node-application network

Figure 6: Logical node-application network

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Figure 7: Logical node network

Figure 7: Logical node network

Simulating the IEC 61850

In this section, we talk about the design and implementation of a simulator of the IEC 61850 standard. Due to the challenges listed in Section 2.1, simulating the entire protocol is very difficult. Since the goal of our simulation is to inspect possible security vulnerabilities in the protocol, we refined the protocol to a version containing only data gathering/setting related ACSI services and reporting services. Furthermore we simplified the data model by abstracting various data attribute types to string and discarding the concept of functional constraint. Currently, features of the IEC 61850 protocol supported by our simulation model include two-party application association, data attribute, data object, data set, logical node, logical device, server, ACSI services, reporting and unbuffered report control block.

Design and implementation of the simulator

In order to inspect the IEC 61850 standard, we need a tool to simulate the protocol. Demo software of the protocol exists [10]. However, they are mostly provided by the IED manufacturer and are not freely available for research purposes. Kostic et al. proposed an implementation of ACSI [6]. The work of Kostic et al. focuses on implementing a set of application programming interface for device and application development, while we would like to build our own simulation tool for further study on the IEC 61850, especially on network security issues.
Our simulator consists of about 3,500 lines of Java code. Its components are divided into three major categories: the data model, internal message representation and the service model. Figure 8 shows the architecture of the simulator.

Figure 8: The IEC 61850 simulator architecture

Figure 8: The IEC 61850 simulator architecture

Data model

The internal data model in the simulator is the same as the one defined in the IEC 61850 standard. All the IEC 61850 objects are instances of the I6Object class and are organized as a tree as shown in Figure 9. I6Object class provides a collection of services to manage the objects.

Figure 9: The IEC 61850 simulator data model

Figure 9: The IEC 61850 simulator data model

Table 4 lists some frequently used services.

Table 4: Major services of I6Object

Table 4: Major services of I6Object

The lock and unlock services are necessary because the IEC 61850 standard allows multiple accesses to the same object simultaneously. One would resolve this problem by sequentializing all the received requests, but since reports could be generated at the time when the related data objects are being updated, synchronization mechanism is still needed. One could implement lock as locking the whole object tree, but this would lead to severe performance problems, even dead locks. Our solution is to lock the whole subtree dominated by the locked object and mark all the ancestor nodes of that object as partly locked so that these ancestor nodes cannot be locked by other Associations.

Each kind of the IEC 61850 objects are implemented as a subclass of I6Object.    These subclasses provide templates to define the desired specialized classes. For example, in order to create a specialized logical device class, one just need to derive a subclass from the LDevice class and use the declareLN method to declare the member logical nodes in the configure function.

Using the IEC 61850 data model as the internal data model has two advantages: on the one hand, there is no need to map the internal data representation to the IEC 61850 data model in such an approach; on the other hand, we can deploy the implementation of the ACSI services to the relevant object classes thus achieve a simple object-oriented approach. However, our simulation experience shows such a simple internal data representation might not be a good choice because of the following two reasons:

  1. The data model defined by the IEC 61850 is unnecessarily structured as a tree. Functional constraints present another view of data attributes in a logical node, breaking the tree structure maintained by data objects.
  2. Some ACSI services are not easy to deploy to the specific object classes. One example is the CreateDataSet service, which should be implemented by the LogicalNode class or the Association class instead of the DataSet class.

To maintain the tree structure, one can implement functional constraints as implicit data sets, and redirect the service requests to the appropriate object class using a dispatcher in the Association class.

Figure 10: Internal message of the simulator

Figure 10: Internal message of the simulator

Instead of maintaining an internal representation, we recommend an other approach: using a lightweight database system as the backend storage support. This approach provides several benefits:

  1. A database system provides the most flexible and easiest way to maintain the appropriate tables and views. A database system incorporates all the necessary services to operate on the object tables, reducing the complexity of maintaining an internal data representation.
  2. A database system supports exclusive object accesses, so the engineer does not need to explicitly perform lock and unlock operations.
  3. Database systems are usually optimized for data storage and access, thus the database system approach can give comparable real-time performance against the internal data representation approach.

Message representation

Messages are used in two cases: as internal events and as requests/responses of ACSI services. Although the IEC 61850 standard defines manufacturing message specification (MMS) as its representation format for information exchange, we would like to use another information representation format because on the one hand, MMS is relatively complicated and on the other hand, the details about the MMS standard is not freely available.

Messages in our simulator are represented as attributed trees. There are three kinds of tree nodes (Figure 10): SimpleMsg, ComplexMsg and ListMsg. A SimpleMsg node is a leaf node containing a string value, which is used to represent a value. A ComplexMsg node is a node containing a collection of named attributes, the values of which are also nodes.

Figure 11: Implementation of the ACSI service

Figure 11: Implementation of the ACSI service

A ListMsg node is a node that represents a list, with each member also a node. Our message tree is virtually a simplified version of a XML DOM tree. Using such kind of message representation, we can easily pass the internal events and service parameters/responses among different internal objects.

Service model

Figure 11 shows the internal architecture of the implementation of the ACSI services on the server side. Each active association is equipped with an instance of AssocAPI. AssocAPI provides the basic ACSI interface for an association instance. On receiving the application requests, the association instance invokes relevant services provided by AssocAPI to complete the tasks. AssocAPI is supported both by the underlying kernel services which provides the basic functions to complete the ACSI service requests, and by the communication utilities, which talks to J-Sim [11], a network simulator.

Instead of delivering the reports using multi-party application associations, we deliver them to the applications using the corresponding two-party application associations. The reporting procedure is rather straight-forward: when the value of a data attribute is changed or updated, we check whether this operation satisfies the trigger condition. If the trigger condition is met, the data attribute issues an internal event to all the data sets marking it as a member and those data sets will forward the internal event to the active report control blocks, which generate the reports and send them to the subscribing applications. For the sake of simplification, we assume the members of a data set are all data attributes in our simulation.

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SOURCE:

  • Understanding and Simulating the IEC 61850 Standard by Yingyi Liang & Roy H. Campbell, Department of Computer Science University of Illinois at Urbana-Champaign

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Busbar Technical Specification

Busbar Technical Specification

Copper busbars are normally part of a larger generation or transmission system. The continuous rating of the main components such as generators, transformers, rectifiers, etc., therefore determine the nominal current carried by the busbars but in most power systems a one to four second short-circuit current has to be accommodated.

The value of these currents is calculated from the inductive reactances of the power system components and gives rise to different maximum short-circuit currents in the various system sections.

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Performance under Short-circuit Conditions

Busbar trunking systems to BS EN 60439-2 are designed to withstand the effects of short-circuit currents resulting from a fault at any load point in the system, e.g. at a tap off point or at the end of a feeder run.
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Rating under Short-circuit Conditions

The withstand ability will be expressed in one or more of the following ways:

  1. short-time withstand rating (current and time)
  2. peak current withstand rating
  3. conditional short-circuit rating when protected by a short-circuit protective device (s.c.p.d.)

These ratings are explained in more detail:

1. Short-time Withstand Rating

This is an expression of the value of rms current that the system can withstand for a specified period of time without being adversely affected such as to prevent further service. Typically the period of time associated with a short-circuit fault current will be 1 second, however, other time periods may be applicable.

The rated value of current may be anywhere from about 10kA up to 50kA or more according to the construction and thermal rating of the system.

2. Peak Current Withstand Rating

This defines the peak current, occurring virtually instantaneously, that the system can withstand, this being the value that exerts the maximum stress on the supporting insulation.

In an A.C. system rated in terms of short-time withstand current the peak current rating must be at least equal to the peak current produced by the natural asymmetry occurring at the initiation of a fault current in an inductive circuit. This peak is dependent on the power-factor of the circuit under fault conditions and can exceed the value of the steady state fault current by a factor of up to 2.2 times.

3. Conditional Short-circuit Rating

Short-circuit protective devices (s.c.p.ds) are commonly current-limiting devices; that is they are able to respond to a fault current within the first few milliseconds and prevent the current rising to its prospective peak value. This applies to HRC fuses and many circuit breakers in the instantaneous tripping mode. Advantage is taken of these current limiting properties in the rating of busbar trunking for high prospective fault levels. The condition is that the specified s.c.p.d. (fuse or circuit breaker) is installed up stream of the trunking. Each of the ratings above takes into account the two major effects of a fault current, these being heat and electromagnetic force.

The heating effect needs to be limited to avoid damage to supporting insulation. The electromagnetic effect produces forces between the busbars which stress the supporting mechanical structure, including vibrational forces on A.C. The only way to verify the quoted ratings satisfactorily is by means of type tests to the British Standard.
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Type Testing

Busbar trunking systems are tested in accordance with BS EN 60439-2 to establish one or more of the short circuit withstand ratings defined above. In the case of short-time rating the specified current is applied for the quoted time. A separate test may be required to establish the peak withstand current if the quoted value is not obtained during the short-time test. In the case of a conditional rating with a specified s.c.p.d. the test is conducted with the full prospective current value at the trunking feeder unit and not less than 105% rated voltage, since the s.c.p.d. (fuse or circuit breaker) will be voltage dependent in terms of let through energy.
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Application

It is necessary for the system designer to determine the prospective fault current at every relevant point in the installation by calculation, measurement or based on information provided e.g. by the supply authority. The method for this is well established, in general terms being the source voltage divided by the circuit impedance to each point. The designer will then select protective devices at each point where a circuit change occurs e.g. between a feeder and a distribution run of a lower current rating. The device selected must operate within the limits of the busbar trunking short-circuit withstand.

The time delay settings of any circuit breaker must be within the specified short time quoted for the prospective fault current. Any s.c.p.d. used against a conditional short-circuit rating must have energy limitation not exceeding that of the quoted s.c.p.d. For preference the s.c.p.d. recommended by the trunking manufacturer should be used.
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Voltage Drop

The requirements for voltage-drop are given in BS 7671: Regulation 525-01-02. For busbar trunking systems the method of calculating voltage drop is given in BS EN 60439-2 from which the following guidance notes have been prepared.

Voltage Drop

Figures for voltage drop for busbar trunking systems are given in the manufacturer’s literature.

The figures are expressed in volts or milli-volts per metre or 100 metres, allowing a simple calculation for a given length of run.

The figures are usually given as line-to-line voltage drop for a 3 phase balanced load.

The figures take into account resistance to joints and temperature of conductors and assume the system is fully loaded.

Standard Data

BS EN 60439-2 requires the manufacturer to provide the following data for the purposes of calculation, where necessary:

R20 the mean ohmic resistance of the system, unloaded, at 20ºC per metre per phase

X the mean reactance of the system, per metre per phase

For systems rated over 630A:

RT the mean ohmic resistance when loaded at rated current per metre per phase

Application

In general the voltage drop figures provided by the manufacturer are used directly to establish the total voltage drop on a given system; however this will give a pessimistic result in the majority of cases.

Where a more precise calculation is required (e.g. for a very long run or where the voltage level is more critical) advantage may be taken of the basic data to obtain a more exact figure.

  1. Resistance – the actual current is usually lower than the rated current and hence the resistance of the conductors will be lower due to the reduced operating temperature.
    .
    Rx = R20 [1+0.004(Tc - 20)] ohms/metre and Tc is approximately Ta + Tr

    where Rx is the actual conductor resistance

    Ta is the ambient temperature

    Tr is the full load temperature rise in ºC (assume say 55ºC)

  2. Power factor – the load power factor will influence the voltage drop according to the resistance and reactance of the busbar trunking itself.
    The voltage drop line-to-line ( Δv) is calculated as follows:

    Δv = √ 3 I (R x cos Φ + X sin Φ) volts/metre

    where I is the load current

    Rx is the actual conductor resistance (Ω/m)

    X is the conductor reactance (Ω/m)

    Cos Φ is the load power factor

    sin Φ = sin (cos-1 Φ )

  3. Distributed Load – where the load is tapped off the busbar trunking along its length this may also be taken into account by calculating the voltage drop for each section. As a rule of thumb the full load voltage drop may be divided by 2 to give the approximate voltage drop at the end of a system with distributed load.
    .
  4. Frequency – the manufacturers data will generally give reactance (X) at 50Hz for mains supply in the UK. At any other frequency the reactance should be re-calculated.
    .
    Xf = x F/50
    .
    where Xf is the reactance at frequency F in Hz

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Source: Siemens Barduct Busbar Specification

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Dry-Type disc wound transformers in MV applications

Dry-Type disc wound transformers in MV applications

Medium voltage, dry-type transformers may have their high voltage windings constructed using either the layer winding technique or the disc winding technique.

Both winding techniques provide the same result in terms of electrical performance parameters, i.e. turns ratio, impedance etc.

However, the use of transformers employing disc wound high voltage windings can result in increased reliability and therefore reduced downtime.

Introduction

The basic purpose of a transformer is to convert electricity at one voltage to electricity at another voltage, either of higher or lower value. In order to achieve this voltage conversion, coils are wound on a laminated silicon steel core which provides a path for the magnetic flux. The coils comprise a number of turns of conductor, either copper or aluminum, wound as two electrically separate windings, called the primary winding and the secondary winding. The primary winding is connected to the source of voltage while the secondary winding is connected to the load. The ratio of primary to secondary turns is the same as the required ratio of primary to secondary voltages.

The turns of conductor forming the primary and secondary windings must be insulated from one another, while the primary winding must be insulated from the secondary winding and both the primary and secondary windings must be insulated from ground. The insulation of turns and windings is collectively called the insulation system of the transformer. The insulation system must be designed to withstand the effects of lightning strikes and switching surges to which the transformer is subjected, in addition to the normal operating voltages. A further requirement of the insulation system is that it must withstand the environmental conditions to which it is exposed, such as moisture, dust etc. A variety of techniques and materials are employed to achieve the necessary performance characteristics of the insulation system.

Layer winding

Fig.1 Layer winding

Fig.1 Layer winding

For low voltage, i.e. 600 Volt class windings, the winding technique used almost exclusively is the layer winding technique, also sometimes called helical winding or barrel winding. In this technique, the turns required for the winding are wound in one or more concentric layers connected in series, with the turns of each layer being wound side by side along the axial length of the coil until the layer is full. The conductors of the winding are insulated and so between turns there will be a minimum of two thicknesses of insulation. Between each pair of layers there will be layers of insulation material and/or an air duct.

Low voltage windings will generally be wound top to bottom, bottom to top etc. using a continuous conductor, until all layers are complete. High voltage windings, i.e. above 600 Volt class, may be wound in the same way, provided the voltage between layers is not too great.

To reduce the voltage stress between layers, high voltage windings are often wound in only one direction, for example, top to bottom. When the first layer of winding is complete, the winding conductor is laid across the completed layer from bottom to top and then the next layer is wound, again from top to bottom. In this way, the voltage stress between layers is halved.

The conductor must, of course, have additional insulation where it crosses the winding from bottom to top.

Fig.2 Transformer with layer wound coils

Fig.2 Transformer with layer wound coils

Disc winding

In the disc winding, the required number of turns are wound in a number of horizontal discs spaced along the axial length of the coil. The conductor is usually rectangular in cross-section and the turns are wound in a radial direction, one on top of the other i.e. one turn per layer, until the required number of turns per disc has been wound.

Fig.3 Disc winding

Fig.3 Disc winding

The conductor is then moved to the next disc and the process repeated until all turns have been wound. There is an air space, or duct, between each pair of discs. The disc winding requires insulation only on the conductor itself, no additional insulation is required between layers, as in the layer winding.

The disc wound high voltage winding is usually wound in two halves, in order that the required voltage adjustment taps may be positioned at the electrical center of the winding. In this way the magnetic, or effective length of the winding is maintained, irrespective of which tap is used, and therefore the magnetic balance between primary and secondary windings is always close to its optimum.

This is essential to maintain the short circuit strength of the winding, and reduces the axial electromagnetic forces which arise when the windings are not perfectly balanced.

Fig.4 Transformer with disc wound coils

Fig.4 Transformer with disc wound coils

Characteristics of Layer wound coils

As stated previously, the layer wound coil requires insulation between layers, in addition to the conductor insulation. The thickness of insulation required will depend upon the voltage stress between layers, and comprises one or more thicknesses of the appropriate insulation material. In practice, due to the nature of the construction of a layer wound coil, the finished coil will have several unavoidable small air pockets between turns and between layers. Many of these air pockets will become filled with resin during vacuum pressure impregnation of the coil.

Fig. 5 Equivalent circuit for Impulse voltage distribution

Fig. 5 Equivalent circuit for Impulse voltage distribution

However, it sometimes happens that some air pockets remain and it is in these air pockets that partial discharges can occur, greatly increasing the possibility of premature aging of the insulation and eventual failure.

Catastrophic failure can occur within a few months of energization. Under short circuit conditions, the electromagnetic forces developed cause transformer windings to attempt to telescope. At the same time the coil end blocking is trying to prevent movement. The result is often that the turns of the winding have a tendency to slip over one another, causing turn-to turn failure, due to abrasion of the insulation as the turns rub together. A further disadvantage of the layer wound coil is its poor impulse voltage distribution between the first few turns of the winding, due to the high ground capacitance and the low series capacitance.

A transformer winding forms a complex network of resistance, inductance and capacitance. As far as the impulse voltage distribution is concerned, the resistance can be ignored and at the instant of application of the impulse wave, when very high frequencies are predominant, the inductive elements become effectively infinite impedances. The whole structure therefore reduces to a capacitive network (see fig.5). Each turn of a transformer winding is insulated with a dielectric material and can be thought of as one plate of a multiple plate capacitor. In addition, the combination of dielectric material and air between each turn and ground forms further capacitive elements.

Characteristics of Disc wound coils

The major advantage of the disc wound coil lies in its open construction and relative lack of insulation. For a 15kV class transformer employing a disc wound primary winding, the number of discs will typically be in the range 36 to 48, resulting in a relatively low voltage per disc. Since each disc is separated from the next by an air space, the voltage stress between discs can easily be handled by the combination of conductor insulation and air, no additional insulation being necessary.

Each disc comprises a number of turns with each turn occupying one layer, i.e. one turn per layer: the voltage stress between layers is therefore the same as the voltage stress between turns and again, can easily be handled by the conductor insulation. The turns of each disc, being wound tightly together provide almost no possibility of air pockets being present within the disc.

Due to the open construction of the discs, any small air pockets which may be present are readily filled with resin during vacuum pressure impregnation of the coil. A properly designed and manufactured dry-type transformer disc winding therefore displays very low values of partial discharge, typically in the range 10 to 20 picocoulombs.

Unlike the layer wound coil, the disc wound coil provides good impulse voltage distribution, due to its inherently low value of ground capacitance and high series capacitance. The disc wound coil also displays excellent short circuit strength. Each disc by itself is mechanically very strong and the complete assembly of discs are held very securely in place. While the electromagnetic forces resulting from a short circuit result in a tendency, for the windings to telescope, the high voltage turns usually remain intact relative to each other. Instead, the complete disc has a tendency to distort as an assembly, with all the turns distorting by the same amount. The transformer can often continue to function, despite the distortion, until a convenient time arises for repair.

Losses/heat

The flow of electric current through the turns of a transformer winding causes power losses which manifest themselves in the form of heat. These losses are called ‘’load losses’’ and are proportional to the square of the current. Obviously, it is necessary to dissipate this heat, to prevent overheating of the transformer, and in a dry-type transformer, this is achieved by the use of air spaces, or ducts, within the winding. The layer wound coil relies on vertical air ducts between layers and between windings, for cooling. Cool air enters the air ducts at the bottom of the coil and by natural convection, rises through the ducts, collecting heat on its way, then exits the coil at the top. It is essential for proper operation of the transformer that these air ducts are kept clear at all times.

The insulation required between the layers of a layer wound coil has a tendency to thermally lag the winding, impeding the dissipation of heat. The greater the operating voltage of the winding, the greater is the amount of insulation required and the greater is the lagging effect of the insulation. Some radiation also takes place from the outer surfaces of the coils. The open nature of the disc wound coil greatly improves the transfer of heat from the winding to the surrounding air. The thermal lagging effect of insulation is removed and the multiple horizontal air spaces between discs provide a large surface area for cooling by both radiation and convection.

Conclusions

The combination of layer wound low voltage winding, disc wound high voltage winding, NOMEX insulation and vacuum pressure impregnation of the windings with a solventless epoxy resin, results in a very reliable transformer with a long life expectancy. Transformers constructed in this way will be relatively free from partial discharge and will provide excellent impulse strength and short circuit strength, vital requirements for reliable operation in the most demanding of applications.

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Author: Derek Foster, Olsun Electrics Corporation

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Maintenance Of High Voltage Circuit Breakers

Maintenance Of High Voltage Circuit Breakers

Most manufacturers recommend com­plete inspections, external and internal, at intervals of from 6 to 12 months.

Ex­perience has shown that a considerable expense is involved, some of which may be unnecessary, in adhering to the manufacturer’s recommendations of in­ ternal inspections at 6 to 12 month intervals. With proper external checks, part of the expense, delay, and labor of internal inspections may be avoided without sacrifice of dependability.
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Inspection schedule for new breakers

A temporary schedule of frequent inspections is necessary after the erection of new equipment, the modification or modernization of old equipment, or the replication of old equipment under different condi­ tions.

The temporary schedule is required to Correct internal defects which ordinarily appear in the first year of service and to correlate external check procedures with internal conditions as a basis for more conservative maintenance program thereafter. Assuming that a circuit breaker shows no serious defects at the early complete inspections and no heavy interrupting duty is imposed, the following inspection schedule is recommended:

.6 months after erection.Complete inspection and adjustment
.12 months after .previous inspection.Complete inspection and adjustment
.12 months after .previous inspection.Complete inspection and adjustment
.12 months after .previous inspection.External checks and inspection; if checks are .satisfactory, no internal inspection
.12 months after .previous inspection.Complete inspection and adjustment

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Inspection schedule for existing breakers

The inspection schedule should be based by the interrupting duty imposed on the breaker. It is advisable to make a complete internal inspection after the first severe fault interruption. If internal conditions are satisfactory, progressively more fault interruptions may be allowed before an internal inspection is made. Average experience indicates that up to five fault interruptions are allowable between inspections on 230 kV and above circuit breakers, and up to 10 fault interruptions are allowable on circuit breakers rated under 230 kV.

Normally, no more than 2 years should elapse between external in­ spections or 4 years between internal inspections.

External Inspection Guide

The following items should be included in an external inspection of a high-voltage breaker.

  1. Visually inspect PCB externals and operating mechanism. The tripping latches should be examined with spe­ cial care since small errors in adjustments and clearances and roughness of the latching surfaces may cause the breaker to fail to latch properly or increase the force neces­ sary to trip the breaker to such an extent that electrical tripping will not always be successful, especially if the tripping voltage is low. Excessive “opening” spring pressure can cause excessive friction at the tripping latch and should be avoided. Also, some extra pressure against the tripping latch may be caused by the electro­ magnetic forces due to flow of heavy short-circuit currents through the breaker.
    .
    Lubrication of the bearing surfaces of the operating mechanism should be made as recommended in the manufacturer’s instruction book, but excessive lubrication should be avoided as oily surfaces collect dust and grit and get stiff in cold weather, resulting in excessive friction.
    .
  2. Check oil dielectric strength and color for oil breakers. The dielectric strength must be maintained to pre vent internal breakdown under voltage surges and to enable the interrupter to function properly since its action depends upon changing the internal arc path from a fair conductor to a good insulator in the short interval while the current is passing through zero. Manufacturer’s instructions state the lowest allowable dielectric strength for the various circuit break­ ers. It is advisable to maintain the dielectric strength above 20 kV even though some manufacturer’s instructions allow 16 kV.
    .
    If the oil is carbonized, filtering may remove the suspended particles, but the interrupters, bushings, etc., must be wiped clean. If the dielectric strength is lowered by moisture, an inspection of the fiber and wood parts is advisable and the source of the moisture should be corrected. For these reasons, it is rarely worthwhile to filter the oil in a circuit breaker while it is in service.
    .
  3. Observe breaker operation under load.
    .
  4. Operate breaker manually and electrically and observe for malfunc­ tion. The presence of excessive friction in the tripping mechanism and the margin of safety in the tripping function should be determined by making a test of the minimum voltage required to trip the breaker. This can be accomplished by connecting a switch and rheostat in series in the trip-coil circuit at the breaker (across the terminals to the remote control switch) and a voltmeter across the trip coil. Staring with not over 50 percent of rated trip-coil voltage, gradually in­ crease the voltage until the trip-coil plunger picks up and successfully trips the breaker and record the mini­ mum tripping voltage. Most breakers should trip at about 56 percent of rated trip-coil voltage.
    .
    The trip-coil re­ sistance should be measured and compared with the factor test value to disclose shorted turns.
    .
    Most modern breakers have trip coils which will overheat or burn out if left energized for more than a short pe­ riod. An auxiliary switch is used in series with the coil to open the circuit as soon as the breaker has closed. The auxiliary switch must be properly adjusted and successfully break the arc without damage to the contacts.
    .
    Tests should also be made to deter­ mine the minimum voltage which will close the breaker and the closing coil resistance.
    .
  5. Trip breaker from protective relays.
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  6. Check operating mechanism adjustments. Measurements of the mechanical clearances of the operat­ing mechanism associated with the tank or pole should be made. Appre­ ciable variation between the value found and the setting when erected or after the last maintenance overhaul is erected or after the last maintenance overhaul is usually an indication of mechanical trouble. Temperature and difference of temperature between different parts of the mechanism effect the clearances some. The manufacturers’ recommended tolerances usually allow for these effects.
    .
  7. Doble test bushings and breaker.
    .
  8. Measure contact resistance. As long as no foreign material is present, the contact resistance of high-pres- sure, butt-type contacts is practically independent of surface condition. Nevertheless, measurement of the electrical resistance between external bushing terminals of each pole may be regarded as the final “proof of the pudding.” Any abnormal increase in the resistance of this circuit may be an indication of foreign material in contacts, contact loose in support, loose jumper, or loose bushing connection. Any one of these may cause localized heating and deterioration.
    .
    The amount of heat above normal may be readily calculated from the increase in resistance and the current.Resistance of the main contact cir­ cuits can be most conveniently measured with a portable double bridge (Kelvin) or a “Ducter.” The breaker contacts should not be opened during this test because of possible damage to the test equip­ment.
    .
    Table 1
    gives maximum contact resistances for typical classes of breakers.
    .
    .Table 1 | Maximum contact resistances for typical classes of breakers
    .
  9. Make time-travel or motion-analyzer records. Circuit breaker motion an­ alyzers are portable devices designed to monitor the operation of power circuit breakers which permit mechanical coupling of the motion an­ alyzer to the circuit breaker operating rod. These include high-voltage and extra- high-voltage dead tank and SF6 breakers and low-voltage air and vac­ uum circuit breakers.
    .
    Motion analyzers can provide graphic records of close or open initiation signals, contact closing or opening time with respect to initiation signals, contact movement and velocity, and contact bounce or rebound. The records obtained not only indicated when mechanical difficulties are present but also help isolate the cause of the difficulties. It is preferable to obtain a motion-analyzer record on a breaker when it is first installed. This will provide a master record which can be filed and used for comparison with future maintenance checks.
    .
    Tripping and closing voltages should be re­ corded on the master record so subsequent tests can be performed under comparable conditions. Time-travel records are taken on the pole nearest the operating mecha­ nism to avoid the inconsistencies due to linkage vibration and slack in the remote phases..
    .

Internal Inpection Guide – Lines

An internal inspection should include all items listed for an external inspection, plus the breaker tanks or contact heads should be opened and the contacts nd interrupting parts should be inspected. These guidelines are not intended to be a complete list of breaker maintenance but are intended to provide an idea of the scope of each inspection.
A specific checklist should be developed in the field for each type of inspection for each circuit breaker maintained.
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Typical Internal Breaker Problems

The following difficulties should be looked for during internal breaker inspections:

  • Tendency for keys, bolts (espe- cially fiber), cotter pins, etc, to come loose.
  • Tendency for wood operating rods, supports, or guides to come loose from clamps or mountings.
  • Tendency for carbon or sludge to form and accumulate in interrupter or on bushings.
  • Tendency for interrupter to flash over and rupture static shield or resis­ tor.
  • Tendency for interrupter parts or barriers to burn or erode.
  • Tendency for bushing gaskets to leak moisture into breaker insulating material.

Fortunately, these difficulties are most likely to appear early in the use of a breaker and would be disclosed by the early internal inspections. As unsatis­ factory internal conditions are corrected and after one or two inspections show the internal conditions to be satisfactory, the frequency of internal inspections may safely be decreased.

Influence Of Duty Imposed

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Influence of light duty

Internal inspection of a circuit breaker which has had no interruption duty or switching since the previous inspection will not be particularly beneficial although it will not be a total loss. If the breaker has been energized, but open, erosion in the form or irregular grooves (called tracking) on the inner surface of the interrupter or shields may appear due to electrostatic charging current. This is usually aggravated by a deposit of carbon sludge which has previously been generated by some interrupting operation.
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If the breaker has remained closed and carrying current, evidence of heating of the contacts may be found if the contact surfaces were not clean, have oxidized, or if the contact pressure was improper. Any shrinkage and loosening of wood or fiber parts (due to loss of absorbed moisture into the dry oil) will take place following erection, whether the breaker is operated or not. Mechanical operation, however, will make any loosening more evident. It is worthwhile to deliberately impose several switching operations on the breaker before inspection if possible. If this is impossible, some additional information may be gained by operating the breaker several times after it is deenergized, measuring the contact resistance of each pole initially and after each operation.
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Influence of normal duty

The relative severity of duty imposed by load switching, line dropping, and fault interruptions depends upon the type of circuit breaker involved. In circuit breakers which employ an oil blast generated by the power arc, the interruption of light faults or the interruption of line charging current may cause more deterioration than the interruption of heavy faults within the rating of the breaker because of low oil pressure. In some designs using this basic principle of interruption, distress at light interrupting duty is minimized by multiple breaks, rapid contact travel, and turbulence of the oil caused by movement of the contact and mech­ anism.

In designs employing a mechanically driven piston to supple­ ment the arc-driven oil blast, the performance is more uniform. Still more uniform performance is usually yielded by designs which depend for arc interruption upon an oil blast driven by mechanical means. In the latter types, erosion of the contacts may appear only with heavy interruptions. The mechanical stresses which accompany heavy interruptions are always more severe.

These variations of characteristic performance among various designs must be considered when judging the need for maintenance from the service records and when judging the performance of a breaker from evidence on inspection. Because of these variations, the practice of evaluating each fault interruption as equivalent to 100 no-load operations, employed by some companies, is necessarily very approximate although it may be a useful guide in the absence of any other information.
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Influence of severe duty

Erosion of the contacts and damage from severe mechanical stresses may occur during large fault interruption. The most reliable indication of the stress to which a circuit breaker is subjected during fault interruptions is afforded by automatic oscillograph records. Deterioration of the circuit breaker may be assumed to be proportional to the energy dissipated in the breaker during the interruption.

The energy dissipated is approximately proportional to the current and the duration of arcing; that is, the time from parting of the contacts to interruption of the current. However, the parting of contacts is not always evident on the oscillograms, and it is sometimes necessary to determine this from indicated relay time and the known time for breaker contacts to part. Where automatic oscillograph records are available, they may be as useful in guiding oil circuit breaker maintenance as in showing relay and system performance.

Where automatic oscillographs are not available, a very approximate, but nevertheless useful, indication of fault duty imposed on the circuit breakers may be obtained from relay operation targets and accompanying system conditions. All such data should be tabulated in the circuit breaker maintenance file.
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SOURCE: HYDROELECTRIC RESEARCH AND TECHNICAL SERVICES GROUP

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