MXPA01004199A - A protective relay-based monitoring system of dc power within an electric power substation - Google Patents

Abstract

A DC monitoring system is provided in each protective relay in an electric power substation to detect DC grounds on the DC supply system. The DC monitoring system includes a first portion (62, 64) which indicates the presence of a DC ground. In a second portion of the system (76, 78), the voltages at the contact inputs of the relay are recognized and compared against a standard voltage value range. An indication is provided when the recognized voltage is within said range. The combination of the indications from the first and second portions is useful in determining the location of DC grounds in the substation DC supply system.

Description

MONITORING SYSTEM BASED ON DIRECT CURRENT ENERGY PROTECTOR RELAY INSIDE A SUBSTATION OF ELECTRICAL POWER FIELD OF THE INVENTION This invention relates in general to the detection of unnoticed DC (direct current) earths in the DC supply circuit in a power substation, and more particularly refers to a detection system that includes a DC monitoring circuit in protective relays in the substation in combination with the voltage information in the contact inputs of the protective relays to determine the location of the DC ground.

BACKGROUND OF THE INVENTION The power line protection equipment present in an electrical power substation will typically include a plurality of protective relays, instrument transformers to convert the current voltage values in the line to levels suitable for use in relays. protectors, and circuit breakers sensitive to protective relays, as well as other accompanying elements. Protective relays are used to interrupt the AC (alternating current) energy distributed from the substation, in response to a protective relay that detects a fault in the power lines. The power for the equipment in the substation, including the protective relays, is provided by a DC supply system, the source of which is a battery. Usually a battery charger is also part of the supply system. The DC supply system also typically provides power for communication within the substation. Most substation battery systems operate either 24, 48, 125 or 250 volts DC, although it is common to have two different battery systems in a substation, with different voltage levels, for example, a system can operate at 125 volts while the other operates at 48 volts DC. Typically, DC power systems within a substation are operated without being grounded. The grounding of any part of a DC supply system within this substation in this manner will typically be unintentional, ie, Unnoticed While these unnoticed DC lands are of complete interest, an individual inadvertent land typically will not cause a major failure of the DC supply system, although it may affect the operation of certain equipment within the substation, including possibly the protective relays and the spirals and the spirals of disconnection of derivation or closed spirals that operate the circuit breakers, in particular circumstances A second earth of DC in the system of supply of DC, in the common bar of supply of polarity The opposite will certainly affect the operation of the equipment or will blow the fuses in the system.A typical DC power system within a substation will include long stretches of wire connections, including the wire that extends from the battery source into a room. control to remote locations in the substation, such as to circuit breakers and the apparatus disconnection of the system. These long stretches of wire present significant opportunities for unnoticed DC lands. Inadvertent lands may occur for several different reasons, but typically they are due to a fault in the wire insulation. The damage or failure may be due to aging, weather, poor connections or other "causes." This can occur in any of the wire runs themselves, either short or long, even within the control room, or in connections to relay coils or switches as well as connections to or within the protective relays The DC power system includes common DC bars of positive and negative polarity, as well as extensive equipment connections. less can be detected by conventional DC monitoring systems that are typically located in the vicinity of the battery portion of the system., the DC lands in the rest of the DC supply system are usually very difficult to locate and require a lot of time to locate. In a substation comprising a large plurality of protective relays, transformers, circuit breakers and associated contacts and coils, an indication by the conventional monitoring system of a DC ground will result in an alert to a repair personnel, who has the task, on arrival, if the DC ground is not found to be in the common bar more or less, to subsequently remove from the substation power circuit each protective relay or other piece of equipment to locate the DC ground. This task is complicated by the fact that specialized personnel must approve the temporary disconnection of the circuit breakers in the substation, because the removal of the DC power removes the protective relay equipment and the power source required to operate a switch. during a failure. Additionally, DC ground indications have to occur more frequently in inclement weather conditions, such as rain storms. The repair of these DC lands requires that technicians and operators work within an energized (in operation) rain substation, which can often be confusing to operating personnel, due to obvious hazards. It is important to locate the DC fault as long as the power system is humid, because when the system dries, the DC ground may disappear when the water leaves the breakage in the insulation of the wiring (until the next storm of rain) . Therefore, it is completely desirable to identify the location of unnoticed DC lands within a power substation with greater particularity when it is currently possible.

DESCRIPTION OF THE INVENTION Accordingly, the present invention is a DC monitoring system that is for use in electrical power substations having at least one DC supply system that includes common positive and negative DC supply bars, a substation that includes at least one protective relay that is energized by the DC supply, the monitoring system comprises: a first portion that is sensitive to the voltage between a common supply bar and ground and the voltage between two common supply bars to determine the presence of a DC ground in the DC supply system and if the DC ground is closer to one line of the common supply bar than the other and that provides an indication of it; and a second portion including means for measuring the voltage at the selected contact input connections of the protective relay; means for comparing contact input voltages against a selected range of normal voltage values; and a means providing indication when the measured voltage is within the range, wherein the combination of the indications of the first portion and the second portion is useful in locating the origin of the DC ground in the DC supply system.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified diagram illustrating a portion of a substation protection system, which includes the DC supply for the same. Figure 2 is a block diagram of a portion of the DC monitoring system of the present invention. Figure 3 is a diagram showing another portion of the DC monitoring system of the present invention. Figure 4 is a table showing how the information provided by the system of the present invention can be used to locate an inadvertent DC ground within an energy substation.

BEST MODE FOR CARRYING OUT THE INVENTION As indicated above, the electric power substation, which serves a particular geographic region with electric power, will include a protective system comprising a duality of equipment, all of which require power for operation and communication . This power is typically provided by a centralized DC station battery system that generally includes a battery charger to maintain battery output within a specified range. As indicated above, the battery output voltage may differ from installation to installation. DC power can be provided in some cases by a distributed DC system in which separate batteries are used for each piece of equipment, ie, protective relays, or small groups of equipment. A typical, centralized system uses a 125-volt DC battery, and the following description assumes that voltage level. A typical voltage range for a nominal 125 VDC system is from 108 VDC (low voltage) to 150 VDC (maximum voltage). This last voltage level is often referred to as an equalizing voltage and is commonly used with lead-acid batteries. Some systems can have two DC supplies, with different voltage outputs. It should be understood that the present invention is also applicable to substation DC power systems operating at outputs other than 125 volts. Typically, a 125-volt DC system will comprise two groups of batteries connected in series, each group producing 62.5 volts. Each group of batteries includes 25-30 batteries connected in series, typically lead-acid batteries. An individual battery charger will charge both groups of batteries simultaneously. Most of these DC supply systems include a low-voltage heating alarm that generally indicates malfunction of the charger, as well as a low-low voltage alarm that indicates that the battery is severely degraded in its performance. The portions described above of the DC supply system within a power substation are completely conventional and therefore many are not discussed in detail hereinafter and are shown specifically in the drawings. As indicated above, a power line protection system present in a substation will include a large number of protective relays, several instrument transformers and circuit breakers with related disconnection coils and contacts. For the purposes of explanation of the present invention, a simplified portion of a substation protective system is shown in Figure 1. Figure 1 shows a set of 2 batteries producing 125 VDC and 48 VDC, which will ordinarily include a separate battery charger (not shown) for each battery. The circuit shown in Figure 1 shows typical DC supply connections for a three-pole disconnect and re-close switch application that uses a communication assisted disconnect. In this case, the 125 volt battery 10 is used for disconnecting, closing and controlling the switch, while the 48 volt battery 11 is used for communication, disconnection and assisted control equipment. The present explanation, however, will focus on the 125-volt system. The 125 volt battery 10 includes four lines 12a-12b, 14a-14b, 16a-16b and 18a-18b. From DC common bar plus and minus. Fuses 19-19 are provided to protect against shorts between common positive and negative bars. The battery 10 with its associated system wiring and connections is responsible for energizing the disconnect coils 20 and 22 when the disconnect contacts 24 and 26 close in response to an order from the protective relay. The disconnection spiral 22, the disconnection contact 26 and the associated wiring form a redundant disconnection circuit, for increased safety. In ordinary operation, the circuit breaker of the power system controlled by the protective lifter is closed and the switch contacts 28 and 30 are closed. The closing of the disconnection contacts 24 and 26 by the protective relay thus results in a recurrent path through the associated disconnection coils, which in turn results in the circuit breaker associated with the opening of the coils of disconnection. The closing spiral 32 (common bar lines 18a-18b) is energized by the DC supply system when the resetting circuit of the protective relay signals its associated switch to close upon closing the switch contact 34. The closing of the closing contacts 34 by the protective relay will result in the current flowing through the closing coil 32, resulting in the closing of the circuit breaker of the system after the operation of the switch contacts 35. The DC supply system also provides a communication link between the contacts 37 and the contact input 38 of the protective relay as well as the communication of the closure of the control switch 40 to the contact input 42. The control switch 40 is typically an externally operated, manually operated switch controlled by an operator outside the control panel of the protective relay control terminal. In addition, the DC supply provides power for the power supply of the protective relay, as well as the power to the communication equipment if this equipment is external to the protective relay. As indicated above, the 48 volt battery provides power for the communication of a permissive disconnect signal from a remote line terminal to the protective relay, by virtue of the closing of the control contacts 44, which energizes the input contact 46 of the protective relay. The system of Figure 1 includes conventional DC monitoring systems for each battery 10 and 11. With reference to the battery, the monitoring system includes a series connection of resistors 50 and 52, with a central connection to the substation ground . Resistor 50 and 52 form a typical DC ground detector found in most current substations. Typically, each resistance is 10K ohms. In some cases, two small lamps or voltmeters are used instead of the resistors. The voltage across or the current through each of the resistors 50, 52 is measured to determine the presence of DC unnoticed lands. When there is no DC ground, the voltage across the resistor will be the same and there will be no ground current. In the case of an inadvertent earth, the voltage across one resistor will be greater than the other (in the case of two lamps, one will be brighter). The resistance of the lower voltage indicates the particular side of the DC common bar (more or less) that is associated with the inadvertent earth. As indicated above, however, this system is only able to determine if the DC ground is actually in either the common bar directly or that common bar is closer to the inadvertent ground if the ground is not in a common bar. The monitoring system can not provide any additional information; If the ground is on the equipment, which is more frequent, the operator / technician must carefully remove each piece of equipment in the system in turn to determine the location of the DC ground, which occurs when the DC ground indicator disappears when the grounded circuit is de-energized. The system of the present invention to provide a more particular determination of the location of the DC area is located in Figure 1 to 60 (power supply in 61) and is shown in more detail in Figure 2. It is important Note that the present system does not require additional connections or conductors from the DC battery source or the protective relay or other equipment. The connections to the present system are made directly through the DC power lines to the relay (common plus and minus DC bus lines). In Figure 2, the circuit 60 provides two voltage values CMDC and DMDC at the outputs of the voltage amplifiers 62 and 64. The input to the amplifiers 62 and 64 is through a network of resistances comprising the resistors. 66-69. This r ats, due to its relatively large values (resistances 66 and 69 are 22 mO) compared to the 50 and 50 resistance values. 52, remove very little current from the battery (less than 3 microamps). The DC ground detector of the present invention in this manner can not adversely affect the operation of conventional DC ground detectors that compose the resistor 50 and 52. The amplifier 62 is connected between the common DC bus 12a positive and the earth of Figure 1. Therefore, it is provided. or referred to as a common mode voltage, which is proportional to approximately half the battery voltage, that is, 62.5 volts. Without epbarqo, the amplifier 64 is connected between the common bars 12a and 12b of positive and negative DCs and therefore provides what is referred to as a differential mode voltage, which is proportional to the full voltage of the battery, i.e. voltibs. The difference between the common mode voltage and the differential mode voltage value is a voltage value that is proportional to the voltage between the bar cpmun 12b of negative DC and ground When an unnoticed DC ground occurs in the DC supply system , the nominal voltage value of the amplifier 62 will change significantly. Therefore, the voltage values at the output of the amplifiers 62 and 64 indicate the presence of unnoticed DC ground, either in the common positive or negative DC bars or in the duration of these common bars. The circuit of Figure 2 provides protection against "disturbance" alarms that may be caused by transient conditions or other factors. A dead band of voltage is created to provide this protection.The output of the differential mode 6644 is applied to multipliers 70 and 74. In the multiplier 70, the multiplier value is M / 2, while in the multiplier 74, the multiplier value is 1 / 2M, where M is a selected constant having a range of 1 <M <2. The value of M in the present mode is 1.03, this value provides a reasonable level of security against DC unnoticed, fake lands The output of the multiplier 70 is then applied to the negative input of a comparator 76 while the output of the multiplier 74 is applied to the positive input of a comparator 78. The common mode output of the amplifier 62 it is applied to the positive input of comparator 76 and the negative input of comparator 78. Under conditions of normal operation, the outputs of both comparators will be low, indicating that there is no DC ground. or common mode (positive common ground bar) is greater than the differential mode value multiplied by M / 2, then the output of comparator 76 is going to be high. Conversely, if the value of the differential mode multiplied by 1 / 2M is greater than the value of the common mode, then the output of the comparator 78 is high. The outputs of comparators 76 and 78 are applied to a gate 80 of O, inclusive, a high output from which initiates a timer 82 of collection and exclusion of time delay. Timer 82 is adjusted to prevent erroneous detection of inadvertent DC lands due to transient conditions and provides time discrimination for inadvertent lands detected by voltage monitors around open relay contacts. In the embodiment shown, the exclusion portion retracted in time from the timer is set equal to zero so that the timer output ends when the input to the timer ends. The output of the timer 82 is applied to the gates 84 and 86 of Y. The other inputs of the gates 84 and 86 of Y are the outputs of the comparators 76 and 78, respectively. A high output of the gate 84 of Y is referred to as an NDCG signal, indicative of an inadvertent ground on the negative side (common bar) of the supply, while the output of the gate 86 of Y, referred to as a PDCG signal is indicative of an unnoticed DC ground on the positive side (common bar) of the supply. In Figure 3 shows the effect of the circuit of Figure 2 in relation to the declaration of DC lands. With a differential mode value of 130 volts, the relay will provide an alarm when the common mode voltage is greater than 63.1 volts or less than 66.95. The system of Figure 2, insofar as it is an improvement over conventional systems without virtue of having an adjustable security interval to prevent erroneous alarms, does not yet have the capacity to provide specific information regarding the different DC ground location of that in the common bars of positive or negative DC or closer to one or the other of the common bars. In reality, land in certain locations will still present difficulties for the system in Figure 2 to accurately identify it as an inadvertent DC land. For example, with reference to Figure 1, if the inadmissible earth is presented between the contact input 40 and the control switch 42, with a DC battery voltage of for example 140 volts, the resistor 50 and 52 both being of 10KO, and a contact input resistance of 30KO, the output of the common mode and differential mode amplifiers 62 and 64 will be 80 and 140 volts, respectively. This is a difference of 10 volts for the common mode value of that which is measured in normal operation, that is, without an inadvertent earth. However, if the battery charge is out of line, the low point for the battery that is still within its normal range will be 124.8 volts. In this case, the output of the common mode amplifier will be 71.3 volts, which leaves a small margin slightly greater than 1 volt between a real DC ground condition (when the battery is at 140 volts) and a normal condition when the battery is low (but not low enough to produce • an alarm) . This closed margin could easily result in errors in the recognition of the DC ground. In another example, wherein the DC ground is located between a disconnecting contact 24 and the disconnecting spiral 20, using the previously indicated resistors of a 140 volt battery voltage, the output of the common mode amplifier 62 will be 139.8. DC volts and the output of the differential mode amplifier 64 will be 140 volts DC. This will result in a DC ground indication, but will also result in a switch disconnection if a second unnoticed DC ground appears in the DC common bar 12a positive.

The known information about this sequence is simply that the switch was disconnected, without the relay or control switch that issues a trip command. Additionally, the switch can be re-closed or not. To avoid this possibility, it is particularly important to identify the location of the first unnoticed land. The present invention makes this identification possible. In the present invention, in addition to the common mode and differential mode voltage measurements, which includes the circuit of Figure 2 which provides a determination of NDCG (negative DC ground) and PDCG (positive DC ground) and the associated alarms , the present invention includes input voltage measurements of the relay contact that makes possible a more accurate and accurate determination of the location of the DC inadvertent lands within the substation including the substation's protective equipment. Again, a power substation will have a plurality of equipment, including protective relays. If each of the protective relays has a DC monitoring circuit as shown in Figure 2, an alarm from any of the DC monitors will indicate that there is an unnoticed DC current in the DC system in the common bus. DC more or less, or if it is not in the common bars, that the DC ground is closer to a common bar than the other. The contact input measurements of each of the relays will provide additional information as to which of the relays have the DC ground and the location of the DC ground associated with the relay. In this way, with the combination of the PDCG and NDCG outputs of Figure 2 with the voltage information at the relay contact inputs, the location of DC unnoticed lands becomes more systematic and accurate. In the simplest mode, the voltage at each of the contact inputs of a relay is compared against fixed thresholds. In the embodiment shown, with reference in Figure 1, the contact entries include the contact entry 40, among others. The contact input will have a nominal voltage value for the operation. The nominal or normal value is defined herein as the voltage value measured differentially. When the voltage at each contact input is less than a quarter of the rated voltage, DC alarm is not provided and the contact is not energized. If the contact input voltage is between one quarter and three quarters of the rated voltage, it is indicated that an abnormal condition exists. Either the contact input is malfunctioning, a leak is occurring around an open contact, or a significant resistance exists in a closed contact, or a DC ground exists somewhere between the contact of the relay itself and the contact input . With this interval, a DC alarm is provided. This indicates that the circuit connected to that input probably has a ground to DC. Normally, a closed contact input will be valued when the voltage at the contact input is more than 50 percent rated voltage. However, between 75 percent and the full nominal value, the contact input will be secured and will not also open a DC ground alarm. If the applied voltage is greater than 1.3 times the rated voltage, then the contact input is either faulty, is incorrectly configured or the battery charger is malfunctioning. An alarm is provided and the contact input is secured. Table 4 shows a representative sampling of several possible DC ground locations in relation to a protective relay and the resulting PDCG and NDCG outputs and the alarm condition of the various developer contact inputs. For example, location No. 1 is a ground in the common bar of VDC1, with all the relay contacts in the open relay. In this case there is a PDCG alarm, but no alarm of any of the contact inputs. -This means that the DC ground is not between any of the open relay contacts and the contact inputs shown. If all the 125-volt DC contacts are open (which can be easily ascertained) the inadvertent ground must be one of the common DC bars, that is, the common bars 12, 14, 16 or 18. If all the contacts of DC are open and the DC monitor detects an unnoticed DC ground, and one of the contact inputs also indicates a ground, the actual location of the ground can be pinpointed with greater accuracy. For example where there is a ground between the switch contact 42 and the contact input 40, the PDCG signal will be secured with the open switch contact 42, while when the control switch 42 is opened, the relay logic circuit will ensure the NDCG, as the entry 40 contact. While the system of the present invention does not accurately locate each fault, it is substantially better than existing systems. The contact input voltage can also be compared against adaptive thresholds as opposed to fixed thresholds. In the arrangement of adaptive thresholds, the voltage measured n the contact inputs is compared against a preselected fraction of the real differential mode voltage between the two common DC supply bars (positive and negative). For example, it can be considered that the total interval of. The voltages at the contact input can cover 30 to 150 volts for a nominal voltage value of 125 volts. A voltage at the contact input of less than 30 volts will be considered as a de-energized input, while a voltage of more than 30 volts but less than a selected fraction or percentage of the total differential mode voltage (as measured) will result in result in a DC ground detection alarm. In the modality shown, this selected fraction is 55/100 or 0.55. A range of values is 0.50-0.75, depending on the degree of security desired. For the contact input that is energized without a DC ground alarm, the voltage measured at the contact input will be another fractional value selected from the differential mode voltage, but less than the maximum 150 volts. Above 150 volts there will be an indication of malfunction of the charger. In the present embodiment, the second fraction value is 56/100 or 0.56. The second fraction will always be slightly larger than the first fraction. For a measured voltage value of 140 volts differential mode, the voltage range that will result in a DC alarm is 30-77 volts. The adaptive threshold provides a more reliable location of a DC ground, given the relatively wide range of nominal voltages that may be available at the contact input. Therefore, a system for detecting DC earth in a DC supply system for a power system substation is described, which results in an increased capacity to reliably identify the DC ground location within the substation with a uniquely greater than what has been possible so far. Again, while the above description refers to a single or dual battery system for a complete substation, which serves a plurality of individual equipment, including numerous protective relays, it should be understood that the present DC ground monitoring system could work with a "distributed" DC supply arrangement, in which there is a plurality of DC supply systems within a substation, for DC supply that serves a few even one piece of equipment, such as an individual protective relay. Although a preferred embodiment of the invention has been described herein for purposes of illustration, it should be understood that various changes, modifications and substitutions may be incorporated in this embodiment without departing from the spirit of the invention, which are defined by the claims as follows .

Claims (11)

1. CLAIMS: 1. A DC monitoring system for use in electrical power substations that has at least one DC supply system, which includes common positive and negative supply bars, the substation that includes at least one protective relay that is energized by the DC supply, the monitoring system comprises: a first portion that is sensitive to a first voltage between a common supply bar and ground and a second voltage between two common supply bars to determine the presence of a ground DC in the DC supply system, and if the DC ground is closer to one line of the common supply bar than the other and provide indication of it; a second portion including a means for measuring the voltage at selected contact input d-inlets and protective relay; means for comparing contact input voltages against a selected range of normal voltage values; and a means providing an indication when the measured voltage is within the selected range, wherein the combination of the indications of the first portion and the second portion is useful in the location of the origin of the DC soils in the supply system of DC.
2. 2. A system according to claim 1, wherein the selected voltage range is below a nominal value to energize the contact input.
3. A system according to claim 1, wherein the DC supply energizes a plurality of protective relays within the substation and wherein the DC monitoring system is included in each of the protective relays.
4. 4. A system according to claim 1, wherein the selected range is variable depending on a measured value of the second voltage between the common supply bars.
5. 5. A system according to claim 4, wherein the selected range has an upper limit which is a selected portion of the measured voltage value.
6. 6. A system according to claim 5, wherein the selected portion is within a range of 50-75%.
7. 7. A system according to claim 1, wherein the first portion includes a means for determining a common mode voltage value between ground and one of the common supply bars and a differential voltage value between the two common supply bars. , where a DC ground is indicated when the common mode voltage is approximately zero and the differential mode voltage is approximately the full value of the DC voltage.
8. A system according to claim 1, wherein the first portion includes a means for determining a common mode voltage value between ground and one of the common supply bars and a differential voltage value between the two common supply bars, where a DC ground is indicated when the common mode voltage is approximately equal to the differential mode voltage.
9. A system according to claim 1, wherein the first portion includes a means for determining a common mode voltage value between ground and one of the common supply bars and a differential voltage value between the two common supply bars, which includes a means for increasing the sensitivity of the first portion by indicating a DC ground when the common mode voltage is at least equal to the differential mode voltage multiplied by M / 2 where M is a selected scalar value and also indicating a ground of DC when the common mode voltage is less than the differential mode voltage divided by 2M.
10. 10. A system according to claim 9, wherein the selected scalar value has a range of 1-2. A system according to claim 1, including a plurality of DC supply systems, each DC supply system including at least one protective relay containing a DC monitoring system.