Insulation monitoring in railroad applications

 

Presentation for the Bender Export Rep meeting on 15th/16th March 1999 Sportschule Grünberg

Claus Lange + Matthias Schwabe/Bender Netzschutztechnik GmbH

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table of Contents

 

1 Introduction ………………………………………………….…………………………….… 2

2 The ungrounded power system (IT system) …………………………………………… 2

3 Railroad signaling applications ………………………………………………………..… 4

4 Insulation monitoring in applications
   with rolling stock (locomotives, coaches)
…………………………………………….. 13

5 Off-line monitoring ……………………………………………………………………….… 17

6 Ground fault location methods ………………………………………………………….. 19

7 Reference list ………………………………………………………………………………... 21

8 Literature …………………………………………………………………………………..… 22

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1 Introduction

In recent years traveling by train has become more and more popular. Permanent traffic jams on our roads as well as the introduction of high speed railroad links made the railway one of the best choices in terms of traveling.

A safe and reliable transport system puts high requirements on both the equipment and the electrical power system. Focusing on the electrical installations the control-, signaling-and power systems along the track side as well as the locomotives and coaches need to be considered.

National standards dealing with the maintenance of signaling circuits define such circuits as follows:

"A control system circuit carrying information used for the operation of the railway signaling system, electrification control or associated with the safe movement of trains."

The key word in this phrase is the „safe movement of trains". To meet this requirement, the electrical power system supplying the signaling circuit as well as the signaling circuit itself needs to be safe and secure. Therefore, design engineers around the globe have decided to utilize the ungrounded system (IT system) for signaling circuits.

Similar approaches have taken place in the design of locomotives and coaches. Both the AC and DC systems supplying the traction power systems and the emergency lighting circuits (another DC consumer is the air conditioning) respectively are designed on an ungrounded basis. Section 2 describes the basics on the ungrounded system, section 3 the specialties of the signaling circuits, section 4 deals with the rolling stock, section 5 with off-line monitoring and section 6 with ground fault location methods. A current reference list is given in section 7.

 

  1. The ungrounded power system (IT system)

The specialty on the ungrounded system is, of course, that none of the power conductors is connected to ground as shown in figure 2.1. There is only capacitive coupling between the conductors and ground - better known as leakage capacitance. The leakage capacitance represented by CE1 and CE2 (Figure 2.1) is the natural capacitive leakage of the wires/cables and also the presence of EMI/RFI filters.

Figure 2.1 The ungrounded system (IT system)

 

 

The resistor RF in figure 2.1 is to represent the insulation resistance of the ungrounded power system. The insulation resistance of an ungrounded power system can not be compared with an ohm resistor available at an electronic store, it is a product of several high resistance faults between the power lines and ground. The following influences the insulation resistance:

In comparison with the grounded system, the ungrounded system does have the following benefits:

Operational Safety

A solid ground fault can be present in the system without disturbing the operation. This is due to the small ground fault currents flowing in the event of a ground fault (ground fault current is limited by the fault resistance and the impedance of the leakage capacitance). The first fault needs to be removed before a second one occurs.

Accident Prevention due to limited touch voltages

Personnel shock hazard in ungrounded systems is reduced since the ground leakage currents and in consequence the highest possible touch voltages can be kept at a low level.

Fire Safety

The fire hazard in ungrounded systems is very limited due to the small ground fault currents and eliminated risk of arcing in the event of short circuits (first fault condition).

 

In order to enjoy all of the benefits of the ungrounded system, it needs to be monitored on a continuous basis. In comparison to the grounded system where the imbalance or residual currents of the power lines are monitored, ungrounded power systems require the monitoring of the insulation resistance between the power lines and ground. For reference, please refer to the IT system description in the IEC Standards IEC364-3 and IEC364-4-41.

 

 

 

 

 

 

 

 

 

 

 

3 Railroad signaling applications

The following section shows design examples from the United States, England and Germany of power systems for signaling, track switches and track heaters. The block diagrams of the American and the English designs are shown in figure 3.1. The German design requires a more detailed view, refer to figure 3.2.

a)

b)

 

Figure 3.1 Block diagram of the electrical power systems supplying signaling circuits,

track switches and track heater

  a) American Design b) English Design

 

The German Railroad Authorities require the negative power line (L-) to be connected to the star point of the three phase system as well as the power line L2 of the single phase ac system. This is due to certain monitoring schemes (continuity of the power lines, connections to the load). Another specialty is that the frames of the switchgear do have their own ground (not equal to electrical ground).

Figure 3.2 Block diagram of the electrical power systems supplying signaling circuits,
          track switches and track heaters, German Design

As indicated in the paragraph „operational safety" a first fault is not going to disturb the function of an ungrounded system. The situation is getting crucial in the event of multiple ground faults- of course- depending on their location. As a result, contacts may be bridged and relays are energized or relays may not reset. The effect of those malfunction may be disastrous. Incorrect set track switches, signals etc. An example is shown in figure 3.3.

Figure 3.3 Holding effect of control relays

The fault is represented by multiple fault the resistance RF1+, RF2+, RF3+ and RF4+ on the positive power line L+. Please note that those faults are in parallel and also multiple ground faults of high resistant nature will create a low resistant path to ground. A second solid ground fault (zero ohms, RF-) between the switch „S" and the relay „K1" keeps the relay „K1" energized even the switch „S1" is open. Of course, the total sum of the ground fault resistance RF1+, RF2+, RF3+, RF4+ needs to be sufficiently low (RF- is without effect since it is zero ohms) to generate a current causing the „holding effect" of relay „K1". Relays used for signaling applications e.g. the type K50 can be tripped at a total ground fault resistance (resistance between the points „1" and „2" in figure 3.3) below 30kW. The cradle relays in similar applications can be tripped already at a ground fault levels below 50kW.

As a result, an insulation monitor shall be installed fulfilling or exceeding the following requirements:

Example: AC/DC Converter supplied by an ungrounded AC supply, no isolation between the inputs and outputs of the converter

In order to provide a proper maintenance on the ungrounded systems supplying track switches, signaling equipment etc. various standards were established. A non exhaustive list of railway standards including the summary of requirements regarding the insulation monitoring is given in table 3.1 (page 8).

As shown in table 3.1, certain standards recommend the use of a passive measuring method, other the use of an active one. The following paragraphs are intended to discuss and compare those methods. The voltmeter method (based on the „three lamp method") shown in figure 3.4 belongs to the category of the passive methods.

 

 

Figure 3.4 The Voltmeter Method

 

In the ungrounded system shown in figure 3.4, there is a reduction in the voltage across the voltmeter associated with the faulted phase.

In theory, the three voltmeter method depicted from figure 3.4 will detect a fault between one phase and earth. For a meter with a 50mA movement set to 500V, one might expect that a fault at an insulation level of 1MW or lower could be clearly detected by observing the voltmeter readings. In practice, however, one has to contend with the reality that every system has a charging current. Its magnitude depends on the size of the system and the breadth of its physical coverage. A system with a total charging current of 1A would be considered small. For the purpose of this exercise, let us assume that this is the situation. The line-to-earth capacitance would for this case be 3.3mF. For corresponding impedance at 60Hz would be 804W. The charging current flowing through this impedance will be 1/3A [277V/804W] if we assume that the three phase line-to-line voltage is 460V. This means that an insulation fault in this system will not be detected until it is considerably below 800W, say 200W. It is likely that the fault by the time it reaches this level will be near 0W. Thus, there is the risk that by the time it is detected, a 2nd fault could occur with disastrous consequences.

Both, the lamp and the voltmeter method have the following drawbacks:

A more sophisticated passive method is the asymmetry measuring method. The block diagram is shown in figure 3.5. The coupling resistors RP and RN together with the ground fault resistance RF+ and RF- form a bridge circuitry. The bridge is considered „balanced" as long as the ratio of RP to RN is equal the ration of RF+ to RF-. As a result there will be no current flowing through the measuring resistor RM. By measuring the voltages UN, UP and U the ground fault resistance RF+ as well as RF- can be determined. Unfortunately this method will not detect symmetrical ground faults (RF+ = RF-). Another drawback is the effect of load changes (fluctuation of the system voltage) as well as the charging and de-charging effects of the leakage capacitance CE+ and CE on the measuring method.

Note. The passive method is not recognized for use alone in IT systems by IEC61557-8 1997 and IEC364, theses only accept active IMDs alone or Passive plus Active systems.

Figure 3.5 Block Diagram of an Insulation Monitoring Device (IMD) utilizing the asymmetry measuring method

Table 1/1 Railway standards for signaling applications world-wide and there requirements

Country

Standards (Title etc.)

Measuring Method

Alarm Set-Point

Additional Information, Comments

USA

Department of Transportation - Rules and regulations governing railroad signal and train control systems

  • Part 236: Rules

standards and instructions governing the installation, inspection and repair of signal and train control systems devices and appliances

N/A

§ 236.552 Insulation resistance, requirement

  • Inductive Automatic Cab Signal System, Automatic Train Control System, Automatic Train Stop System: 250kW (min. Insulation resistance)
  • Intermittent Inductive Automatic Train Stop System: 20kW (min. Insulation resistance)

§ 236.527 Roadway element insulation resistance

  • Insulation resistance between roadway inductor and ground: 10kW (min. Insulation resistance)
 

USA

Philadelphia Transit Authorities (SEPTA)

Active 2)

§ 2.3/2.5 Ground Detectors

  • Ungrounded DC Circuits: 7kW (min. Insulation resistance)
  • Ungrounded AC Circuits: 10kW (min. Insulation resistance)

The specification requires the following features:

  • Indication of Fault Location [AC busses, DC busses (pos. or neg. power line)]
  • Fault Memory
  • Test-Function (simulation of Ground Fault)
  • Ground detector to be „immune" against leakage capacitance.
  • Programmable auxiliary alarm contacts

Table 1/2

Country

Standards (Title etc.)

Measuring Method

Alarm Set-Point

Additional Information, Comments

USA

General Railway

Signal (GRS) 1)

Active 2)

Section 3.25

§ 2.1 Basic Design Requirements

  • Ungrounded DC Circuits (12V, 28V): 10kW (min. Insulation resistance)
  • Ungrounded AC Circuits (120V, 1Ph.): 2kW (min. Insulation resistance)

The specification requires the following features:

  • Indication of Fault Location [AC busses, DC busses (BX, CX indication respectively positive or negative power line)]
  • Fault Memory
  • Cross Fault Detection 3)
  • Test-Function (simulation of Ground fault)
  • Monitoring of the Connection to the system being monitored and ground
  • Ground detector to be „immune" against leakage capacitance.
  • Programmable auxiliary alarm contacts

United

Kingdom (UK)

Railway Group Standard GK/RT0171 - Maintenance of Signaling Circuit Insulation

Passive

Requirements

  • Ungrounded DC Circuits (12V, 24V, 50V, 120V): Pre-Alarm at 50kW,

Main Alarm at 22kW

  • Ungrounded AC Circuits (110V, 1Ph.): Pre-Alarm at 40kW,

Main Alarm at 11kW

  • Ungrounded AC Circuits (650V, 3Ph.): Pre-Alarm at 1MW,

Main Alarm at 0.5MW

The standard considers two types of systems:

  • Continuously monitored Systems equipped with Earth Leakage Detectors (ELDs, utilizing asymmetry measuring method)
  • Systems without ELDs monitoring (system voltages below 110V ac, 120V dc). In those systems voltage measurements are taken on a frequent basis and compared with tables. Discrepancies to indicate malfunction (ground fault) of the system.

Table 1/3

Country

Standards (Title etc.)

Measuring Method

Alarm Set-Point

Additional Information, Comments

Germany

DIN VDE 0831

Active

Based on practical experience:

"Normal" Insulation Level: 100kW

"Critical" Insulation Level: (Main Alarm) 30kW

The specification requires the following features:

  • Programmable auxiliary alarm contacts
  • Ability to detect ground faults between the power lines and ground (electrical ground) and between the power lines and the frames of the switch gear (frames are isolated from the "electrical" ground). See the description of the German Railway Signaling design for details.

1) Specification of a manufacturer of railway signaling equipment

2) The standard/specification does not require an active measuring method of the Insulation Monitoring Device (IMD). Based

on the requirements given in the standard/specification can only be met by and active measuring method.

3) A cross fault is a fault between two normally isolated (separate) IT systems. In this instance, the IMDs must recognize each

other to avoid interference.

 

 

 

 

 

 

 

 

 

 

 

The Insulation Monitoring Device (IMD) utilizing an active measuring method is described in the IEC Standard IEC61557-8 4) (Title: „Insulation Monitoring Devices for IT a.c. Systems, for IT a.c. systems with galvanically connected d.c. circuits and for IT d.c. systems") and the European CENELEC standard EN61557-8 4). Figure 3.6 shows a block diagram of the IMD.

Figure 3.6 Block Diagram of an Insulation Monitoring Device (IMD) utilizing the active
measuring method

The coupling unit which connects the IMD to the power lines is in series with the evaluation circuitry and the generator for the measuring voltage. The other side of the generator is connected to ground. The IMD monitors continuously the insulation resistance between the power lines and ground by superimposing a measuring voltage to the lines which returns via the insulation fault and the ground path into the IMD. The IMD evaluates the measuring signal returning via the ground path and displays the total insulation resistance of the IT system. In comparison to the asymmetry method, load changes or the leakage capacitance will not affect the active method as long as the IMD uses the adaptive measuring pulse (AMP 5)) method. The AMP method automatically adjusts to the changing environmental conditions, such as a change in leakage capacitance. Furthermore, IMDs using the AMP method can be used in both AC and DC systems, connected on the AC side of a battery charger 6) the IMD monitors the AC as well as the DC side of the charger for ground faults. As a result only one IMD is required per IT system.

All those arguments have led the IEC committees to allow the passive measuring method only in conjunction with an IMD using an active measuring method.

On top of all those benefits, the IMD provides the user with an early detection of developing ground faults. Especially this feature helps the user to analyze their system and to do trend analysis by evaluating the data stream from the interface of the IMD.

4) The IT system is described in the IEC Standard IEC364, which also requires the use of an Insulation Monitoring Device (IMD)

5) The measuring method of the adaptive measuring pulse (AMP) was developed by

Bender, the patent is pending.

6) Note: The battery charger needs to have a galvanical connection between the inputs and outputs. Both, the AC side and the DC side of the battery charger must be ungrounded.

Most IMDs provide two alarm set-points providing the user with a pre-and a main-alarm. The time between the pre- and the main-alarm can be used to initiate the maintenance on the system. The representative Bender product is the IRDH265../365.. Series and is shown in the appendix.

Figure 3.7 shows the location and connection of the IMD for the German design. As indicated before, the German design is special for the following reasons:

  • All three electrical systems (signaling, track switches and control circuits) are tied together as shown below (one common point)
  • The frames of the control racks/cabinets are partially ungrounded (isolated from ground)

As a result two IMDs are required in each application. IMD No. 1 monitors the insulation resistance between the live conductors and ground. Therefore, the ground faults labeled as RF1, RF2 and RF3 are monitored by IMD No. 1. An insulation breakdown between a live conductor and the frame of the cabinet (RF4) is detected by IMD No. 2. The equipotential bonding between the cabinets is monitored via IMD No. 2 (E-KE). A broken equipotential conductor causes the IMD No. 2 to alarm (Alarm „E-KE").

Please note, that the frames (metal work) of the control cabinets are grounded in most instances. Therefore only one IMD is required per ungrounded system.

Figure 3.7 Connection Diagram of the IMD in a German railroad signaling application

4 Insulation monitoring in applications with rolling stock (locomotives, coaches)

Similar to the approaches for the signaling circuits, most of the electrical circuits on locomotives and coaches are designed on an ungrounded basis. One of the reasons is the requirement for a high operational safety of those systems. Imagine, a train breaks down on a track because of a ground fault. A scenario which must be avoided. In addition to the benefits of the ungrounded system, modern insulation monitoring methods ensure that ground faults are already detected in the developing stage.

The following section lists the standards regulating the electrical installations in locomotives and coaches, describes the block diagrams of locomotives and coaches (simplified) and the methods of insulation monitoring used in those applications.

There are several standards describing the construction and the protective measures to be taken when designing the electrical systems in locomotives and coaches. One of those standards is the German VDE (Verein Deutscher Ingenieure) standard DIN VDE0115. Below a list of the most important sections of the DIN VDE0115:

Part 1: General

Part 200 7): "Elektronische Einrichtungen auf Schienenfahrzeugen"

(Electronic Installations onboard rolling stock)

This part was adopted by CENELEC (European standard), EN Standard EN50155. The standard deals with the requirements for equipment installed in applications with rolling stock.

Part 403: "Drehende elektrische Maschinen" [electrical machines (turning)]

Part 410: "Elektronische Stromrichter auf Bahnfahrzeugen"

(Electronic Inverters onboard rolling stock)

For regulations regarding the required insulation resistance onboard of rolling stock, DIN VDE0115 references to the following standards:

DIN VDE 0510 (1986-07): Required insulation resistance for batteries

DIN VDE 0105-1 (1983-07): Required insulation resistance in single- and three phase AC systems

Figure 4.1 shows an example for a train consisting of a locomotive powered by a diesel and coaches. The voltage generated by the generator-set (diesel engine plus generator) is converted into a DC voltage. The inverters for the traction control, the inverter for the auxiliary circuits are connected to the DC bus of the locomotive. The inverter for the auxiliary circuits is supplying the A/C, battery chargers, control circuits as well as the emergency circuits. Also the power supply (single phase ac voltage) for the coaches is generated via a single phase inverter (with galvanically isolated outputs), connected to the DC bus. One phase of the supply for the coaches is the railroad track, the other one is linked to the coaches via a cable. A transformer in the coaches is reducing the input voltage for the individual coach to a common voltage e.g. 230V ac. At this point it shall be mentioned that the coaches may also be equipped with one or two axle generators providing the power for the coach. In general the distribution in the coach itself is designed as an ungrounded system. The typical loads in the coaches are shown in figure 4.1.

 

7) The content of the BN standard [issued by "Deutsche Bahn"(German Railroad)] BN411002 is similar to DIN VDE 0115-200

Figure 4.1 Example of the electrical installations onboard of a train powered by a diesel locomotive

 

Figure 4.2 Example of the electrical installations onboard of a train powered by a locomotive using power from the overhead lines

Figure 4.2 shows a locomotive using power from the overhead lines. In applications with an AC supply the primary side of the transformer installed in the locomotive is connected between the overhead power lines and the rail which functions as a ground. The windings of the secondary supply the traction control systems, the common supply for the coaches and the auxiliary circuits. Similar to the approach in the diesel locomotives, the ac voltage supplying the traction control systems is converted to dc. The inverters powering the motors are connected to the dc bus. The auxiliary circuits are supplied by an inverter which generates an ac system with a common frequency of 50 or 60Hz (e.g. the German railroad uses a mains frequency of 16 2/3Hz in their overhead ac system).

 

 

 

 

 

This is beneficial since standard industrial equipment can be used. In applications with a dc overhead system, the transformer and the rectifier supplying the dc bus are substituted.

As indicated before, the insulation monitoring of the ungrounded systems is important to maintain the benefits gained by introducing the ungrounded system. The insulation monitoring device (IMD) shall be able to withstand the environmental conditions

(temperature, shock, vibration etc.).

Furthermore, it shall indicate a correct insulation resistance even in systems with high noise levels. Other disturbances (high leakage capacitance) caused by inverters and variable frequency drives shall not cause interference with the measuring method. The IMD shall also able to detect symmetrical and asymmetrical ground faults in every stage (in the ac system, the rectifier, on the dc bus, the inverter and the load side of the inverter and the motor) of the power conversion process. In the case, that all stages are galvanically connected, only one IMD shall be capable to monitor the entire system. For ground faults on the dc systems, the location of the ground fault (positive or negative pole) shall be indicated on the IMD. The minimum insulation resistance to be detected by the IMD is described in the German Standards DIN VDE0510 (Required insulation resistance for batteries; 100W/V) and VDE0105 (Required insulation resistance in single- and three phase AC systems; 50W/V). Applying the above mentioned requirements to the measuring methods described in section 3 such as the voltmeter, lamp or asymmetry method it can be seen that those methods are not suitable for the application onboard of rolling stock.

Before comparing those methods in table 4.1 another one initially designed for applications with dc drives and motors shall be introduced. A typical application for the method later named the "relay method" is:

The auxiliary contacts of the relays K1 and K2 change their state as soon as a current of 3A passes through the coil. Assuming a line to line voltage of 1500V dc, a common grounding resistance RG of 40W and coil resistance of 100W, the resistance of the ground fault RF needs to be in the neighborhood of 360W. Furthermore only ground faults on the positive line can be detected. As a result this method can not provide adequate protection in modern power conversion systems.

This leaves us with the active measuring method of an IMD, described earlier (page11) as AMP method. Trails with this unit in locomotives of the ICE train as well as modern dining cars and coaches have proofed the performance of this method. Table 4.1 shows a detailed comparison of the measuring methods.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 4.1 Comparison of the measuring methods

Requirement/ Feature

Lamp or

Voltmeter

method

Asymmetry method

Relay

method

IMD using

the AMP method

Charging currents or the change of the leakage capacitance interfere with the measuring method?

Yes

Yes

Yes

No

Detection of symmetrical ground faults?

No

No

No

Yes

Detection of asymmetrical ground faults?

Yes

Yes

Yes

Yes

Does the sensitivity of the measuring method depend on the system voltage?

Yes

Yes

Yes

No

Does the measuring method meet the required sensitivity described in DIN VDE0105 (50W/V) or DIN VDE0510 (100W/V)?

Example

System Voltage UN: 1000V dc

Minimum Insulation Resistance RF

(acc. DIN VDE0105): 50kW

No

Yes

No

Yes

Provides auxiliary alarm contacts ?

No

Yes

Yes

Yes

Detects ground faults in every stage of the power conversion process as described in the requirements ?

No

No

No

Yes

The location where Insulation Monitoring Devices (IMD) are installed in rolling stock applications, is shown in figure 4.1 and 4.2. When installing the IMD, the operating voltage shall be derived from the emergency power source (backed up by batteries) to allow the monitoring of the system, even the dc bus is without voltage.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  1. Off-line monitoring

The off-line monitoring addresses the monitoring of idle consumers, to avoid activation in the presence of a ground fault. The block diagram of an off-line IMD is similar to the block diagram showed in figure 3.6. In comparison to the IMD’s using the AMP method, off-line IMD’s superimpose a dc measuring signal to the system being monitored. The available Bender off-line IMD’s are shown in table 5.1. The table also indicates the measuring voltage (max.) of the individual IMD.

Table 5.1 The Off-line IMD’s and their measuring voltages

Bender Type

Measuring Voltage (max.)

IREH1520

500V dc

IR470LY2-60..

40V dc

IREH470Y2-6..

20V dc

IREH470Y2-60..

40V dc

In recent investigations, the impact of the measuring voltage on the accuracy of the measurement was tested. The results showed only a slight variations of the accuracy with a decreasing measuring voltage.

Still, various clients prefer to use the Bender Type IREH1520 with a measuring voltage of 500V dc which is equivalent to the one used in common "Megger" testers. There are various standards addressing this issue. One of them is the American standard ASTM F1134-88 which is dealing with the monitoring of idle consumers in shipboard applications. In this standard the max. allowable measuring voltage is 24V dc. The IEC standard IEC61557-8 (Title: „Insulation Monitoring Devices for IT a.c. Systems, for IT a.c. systems with galvanically connected d.c. circuits and for IT d.c. systems") describes the IMD’s in general, limits the measuring voltage to 120V (peak value).

In the railroad industry, off-line monitors are often used to monitor the heating circuits of track switches. Figure 5.1 shows an example for an off-line IMD Bender Series IREH1520 and IREH470Y2.. . Those IMD’s are connected to the system being monitored via an auxiliary contact of a contactor 8).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8) In application with a grounded supply, the contactor needs to control all power lines including the neutral (if available).

Figure 5.1 Application with an Off-line IMD (Bender Series IREH..), monitoring of the heating circuits of track switches.

 

The Bender Series IR470LY2-60 is a combined On-line/Off-line monitor. Set to the Off-line mode, the unit monitors continuously the system voltage. A voltage below 80V activates the Off-line IMD. An auxiliary contact is not required. A typical application is shown in figure 5.2.

 

 

Figure 5.2 Application with an Off-line IMD (Bender Series IR470LY2-60..), monitoring of the heating circuits of track switches.

 

6 Ground fault location methods

Once an ground fault is detected, most users seek a tool to locate their fault, in the past a very time-consuming and costly task. This problem was also picked up by the IEC committees which developed the IEC standard IEC61557-9 with the title „Equipment for insulation fault location in IT a.c. systems, IT a.c. systems with galvanically connected d.c. systems during operation". Ground fault location systems use the IMD to provide the start signal for the location process as soon as the insulation resistance falls below a the threshold which is enables the location process. This threshold of the insulation resistance depends on the system voltage and the leakage capacitance present in the system and will be e.g. 50k
W in a 3 Phase, 400V system with a leakage capacitance of 1mF. The diagram of a ground fault locating system is shown in figure 6.1.

 

Figure 6.1 wiring diagram of an EDS470 system

 

 

 

 

 

 

 

 

 

 

The Bender ground fault location system EDS470 complies with the above mentioned standard. Described is an active method of earth fault location which consists of a test unit (Bender Type PGH471), an evaluator unit (Bender Type EDS470-12) and current transformers (CT). Once the IMD starts the locating process, the Test Unit generates a positive and a negative pulse in a low frequent sequence. The pulses return via the faulty branch and the earth path back into the test unit. The CTs connected to the Evaluator sense the sequence of the pulse and indicate the faulty branch. In addition the current caused by the test-unit can be limited to 10mA, to prevent false tripping in most instances. In general the evaluator consists of two functional blocks- the scanning logic and the evaluator circuitry. This concept allows to connect up to twelve (12) CTs to one evaluator. During operation, one channel after another will be connected to the evaluator and scanned for the test pulses. As soon as the fault has been located, the portable evaluator unit (Bender Type EDS165) can be used to sense the pulse downstream of the CT, e.g. inside of the consumer. To ease the wiring of the individual units a RS485 bus system is used. This allows networks with a total bus length of 1200m. This is important when central indication of alarms e.g. in a remotely located control room is required. In those applications we recommend the use of a via the control and indicating device Bender Type PRC470. In addition it provides expansion capabilities, as required in large IT systems or in case of modifications. The EDS470 system allows the monitoring of up to 360 branches.

Important Note: The sensitivity of PLC inputs in control circuits may be less than 10mA. To avoid false tripping or interference with the normal operation of the circuits, precaution needs to be taken. This may be the use of ground fault location systems during times without operation, maintenance intervals etc. For more details contact Bender.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

7 Reference list

Table 7.1 shows a non-exhaustive list of customers using Insulation Monitoring Devices in railroad applications. The field of the application is also described in the table below.

Table 7.1 Reference list "Railroad Applications"

Customer

Bender

Application

Product Code

Signaling Circuits

Coaches/ Locomotives

Track switches (Heating)

Power Supply (Train)

Croatian Railroad, Croatia

IRDH265

X

Slovenian Railraod, Slovenia

IRDH265

X

Czech Railroad, Czech Republic

IRDH265

X

Finish Railroad, Finland

IRDH265

X

Daewoo Eng., South Korea

IRDH265

X

Deutsche Bundesbahn, Germany

IRDH265

X

X

Deutsche Bundesbahn, Germany

IR470LY2

X

General Railway Signal, USA

IRDH265

X

Hamburger Hochbahnen, Germany

G200M 9)

X

Hyundia, South Korea

IRDH265

X

London Underground, UK

PR4000

X

London Underground, UK

IRDH265

X

Metro North, USA

IRDH265

X

Netherland Spoorwagen, Netherlands

IR1030

System

X

Safetrain, USA

IRDH265

X

SEPTA, Philadelphia, USA

IRDH265

X

Siemens Heathrow-London Express, UK

IRDH265

X

X

Siemens, Germany

IRDH265

X

X

X

SCNF, France

IRDH265

X

Spanish Railroad, Spain

IRD200M 9)

X

Svenska Jaernwagen, Sweden

IRDH265

X

Swiss Tram Authorities, Switzerland

PNG101S9)

X

Union Switch & Signal, USA

IRDH265

X

Gesellschaft f. elektr. Zugausrüstung (GEZ), Germany

IRE200 9)

IRD200 9)

X

Gesellschaft f. elektr. Zugausrüstung (GEZ), Germany

IRDH265

X

FARGA, Germany

IRDH265

X

ABB, Germany

IRDH265

X

ABB, Sweden

IRDH265

X

ADTRANZ, Germany

IRDH265

X

GEC Signalrail

IRDH265

X

Harmon Industries, USA

IRDH265

X

9) Replaced by the Bender Type IRDH265

8 Literature

[1] "Protective Measures with Insulation Monitoring", Wolfgang Hofheinz, VDE-Verlag

ISBN (German edition): 3-8007-2215-1

ISBN (English edition): 3-8007-1880-4

[2] "Insulation Monitoring on Railways", Technical report, Bender 08.92

[3] "Systemtechnologie, Fahrzeugtechnik für alle Anwendungen", ABB/Henschel, 1991

[4] "Insulation Monitoring in Railroad Signaling Applications", report for the International Union of Railways, congress in Tokyo΄98 , Steffen Langner, Bender


Per ulteriori informazioni, commenti o suggerimenti compilate il modulo di richiesta rapida di informazioni
Ultimo aggiornamento: 5 giugno 2001 - Copyright FANCOS S.p.A. - La riproduzione parziale o totale θ vietata