Friday, February 19, 2021

METODOS DE DETECCION DE FALLAS A TIERRA

 Autor: Ing. Hugo E Reyes H.

1. INTRODUCCION

En esta ocasión estaremos analizando dos métodos ampliamente utilizados en la detección de fallas de fase a tierra en los sistemas eléctricos de potencia, los cuales se basan en las componentes de secuencia cero de tensiones y/o corrientes. Los sistemas de potencia normalmente operan bajos condiciones idealmente balanceadas. No obstante, un sistema de potencia puede estar funcionando en un estado de desbalance debido a fallas asimétricas o a cargas desbalanceadas, o alguna otra condición anormal.

Siempre que ocurre una falla a tierra o un desbalance en un sistema de potencia está presente la componente de secuencia cero de corrientes y/o tensiones, por esta razón los métodos de detección de fallas a tierra o desbalances están basados en la medición de la corriente residual a través  de un transformador de corriente (CT) en el neutro del sistema, y/o por una conexión en delta abierta de tres transformadores de tensión (VTs).

2. CONCEPTOS BASICOS DE COMPONENTES SIMETRICAS

El método de las componentes simétricas fue desarrollado en el año 1913 por el Ingeniero Electricista Charles L. Fortescue cuando investigaba el comportamiento de motores de inducción en condiciones desbalanceadas. Posteriormente, los Ingenieros C.F. Wagner y R.D. Evans aplicaron esta metodología al estudio de perturbaciones y cortocircuitos en los sistemas eléctricos de potencia, metodología que se mantiene vigente en los estudios de los sistemas de potencia desbalanceados.


Las componentes simétricas es un modelo matemático que transforma un sistema de corrientes y/o tensiones desbalanceadas, mediante la sumatoria de tres componentes trifásicas balanceadas (para una secuencia de rotación de fases A-B-C), las cuales son:


a) Componentes balanceadas de secuencia positiva de tensiones y/o corrientes,
b) Componentes balanceadas de secuencia cero de tensiones y/o corrientes,
c) Componentes balanceadas de secuencia cero de tensiones y/o corrientes.

Esta situación se puede observar en la figura 1, allí se presenta un sistema de corrientes trifásicas desbalanceadas (Fig.1a). 


                            Figura 1 Descomposición de Corrientes Desbalanceadas.

En un sistema trifásico de corrientes perfectamente balanceadas, las tres corrientes son de igual magnitud y están desplazadas cada una a 120 grados, para una secuencia de rotación de fases dada. En esta condición de corrientes balanceadas la sumatoria vectorial de las corrientes de fases será igual a cero. Es decir:

Ia + Ib + Ic = 0                   Ec. 1

Aplicando componentes simétricas a la Ecuación 1, utilizando el operador “a”, y tomando como componente de referencia la corriente de la fase A : Ia, se obtiene:

Ia + a2. Ia + a . Ia = 0         Ec. 2

Ordenando la Ecuación 2, queda:

( 1 + a2 + a) . Ia = 0

Pero:

Ia 0

Entonces:

1 + a2 + a = 0

Así que el valor del operador a es:


Por otra parte, en un sistema trifásico de corrientes desbalanceadas, la sumatoria de las corrientes de fases es diferente de cero, esto es:

Ia + Ib + Ic 0                       Ec. 3

Resultando así un valor de corriente residual diferente de cero retornando por el neutro del sistema, el cual es proporcional a la corriente de secuencia cero, como detallaremos más adelante.

           3. ECUACIONES BASICAS DE COMPONENTES SIMETRICAS  

        Las ecuaciones básicas para un sistema de corrientes trifásicas en función de                         sus componentes de   secuencia son en forma matricial:

Donde:

I0 = es la componente de corriente de secuencia cero.

I1 = es la componente de corriente de secuencia positiva.

I2 = es la componente de corriente de secuencia negativa. 

Mientras que las ecuaciones básicas para un sistema de tensiones trifásicas en                       funcion de sus componentes de secuencia son en forma matricial:

A continuación se presentan las ecuaciones para la obtención de las corrientes de secuencia cero, positiva y negativa, en función del sistema trifásico de corrientes de fases desbalanceadas.

         Presentamos a continuación las ecuaciones de los voltajes de secuencia cero, positiva y          negativa, en función del sistema trifásico de voltajes desbalanceados.

                .
                   4.      METODO BASADO EN LA MEDICION DE CORRIENTE RESIDUAL (3.I0)

   Como hemos mencionado previamente, la medición de la corriente residual ha sido                  ampliamente utilizada para detectar fallas a tierras en los sistemas de potencia;                       adicionalmente, se utilizan en los relés de protección direccionales (67/67N) como                   magnitud   de corriente de polarización para  determinar la dirección de una falla                      a tierra. Existen tres formas para la medición de corriente residual (3.I0) utilizando                     transformadores  de corrientes (CTs), las cuales se muestran en la Figura 2 siguiente:     

                                      


                
Figura 2 Tres Formas de Medir Corriente Residual (3.I0)

Opción 1: se basa en la medición de la corriente residual a través  de un transformador de  corriente (CT) en el neutro del sistema. De esta forma se obtiene la medición en el primario   del CT del neutro:

  Mientras que la corriente “vista” por el relé de falla a tierra (50N/51N) es:

                            
                

    Donde:
   Ia, Ib, Ic: son las corrientes de fases en Amperios.

  I0: es la corriente de secuencia cero en Amperios.

  CTR: es la relación de vueltas del transformador de corriente (1:N).

Opción 2: se basa en la medición de la corriente residual a través de tres transformadores de corrientes (CTs) conectados en estrella, acá el relé de falla a Tierra (50N/51N) se encuentra   conectado en el neutro del sistema. Al igual que la opción 1, son aplicables las expresiones:

Mientras que la corriente “vista” por el relé de falla a tierra (50N/51N) es:

                    

 Este esquema es tradicionalmente utilizado en los relés electromecánicos de sobre-            corriente. Mientras que en los relés numéricos la corriente residual es obtenida                        mediante algoritmo de cálculo de las componentes simétricas.

Opción 3esta opción es implementada utilizando un transformador de corriente tipo             toroidal, a través de la ventana del CT pasan los conductores del circuito trifásico, los            cuales quedan rodeados por un núcleo magnético común. Este método es más sensible que los de las opciones 1 y 2, ya que el rango de uso está diseñado para manejar desbalances  de corrientes, los cuales por lo general son valores bajos de corrientes, un valor típico del  rango máximo es 10% de la corriente máxima de carga. Mientras que por el contrario, los métodos de las opciones 1 y 2 el rango de corrientes está basado en la máxima corriente de  carga permitida del circuito (100% de la carga). l igual que las opciones 1 y 2, este método utiliza las mismas ecuaciones para el cálculo de la corriente residual de falla a tierra y/o desbalances de corrientes.

            5. METODO BASADO EN LA MEDICION DE VOLTAGE RESIDUAL (3.V0)

Este método para detectar fallas a tierras y desbalances de tensiones en los sistemas de potencia también es utilizado como magnitud de voltaje de polarización para determinar la dirección de  fallas a tierra en los relés de protección direccionales (67/67N). La medición de voltaje residual (3.V0) se obtiene utilizando tres transformadores de tensión (VT) con el primario en estrella y el lado secundario en  conexión delta abierta (ver Figura 3).

Figura 3 Detección de Voltaje Residual (3.V


 De esta forma se obtiene la medición de voltaje residual mediante:

  Mientras que la tensión residual “vista” por el relé de falla a tierra (59N) es:


  Donde,

  Va, Vb, Vc: son las corrientes de fases en Amperios.

  V0: es la corriente de secuencia cero en Amperios.

   VTR: es la relación de vueltas de los transformadores de tensión (1:N).

6. CONCLUSIONES

Las componentes simétricas son un algoritmo matemático, pero a pesar de eso, no es irreal. Por ejemplo, la componente de secuencia positiva de voltajes y corrientes, y por consiguiente la potencia/energía, son generadas, transmitidas y utilizadas por consumidores  finales. La corriente de secuencia cero circula por los neutros, tierras y conexiones en delta. La corriente y voltaje de secuencia negativa no puede ser medida directamente con un amperímetro o voltímetro, pero puede ser obtenida, en forma indirecta, mediante cálculos matemáticos.

 Con respecto a la ocurrencia de fallas o cortocircuitos:

- La componente de secuencia positiva se manifiesta en todo tipo de falla y, desde el punto de vista los sistemas de protecciones eléctricas, puede ser utilizada por los relés de protecciones contra todo tipo de falla. Los elementos de protección de secuencia positiva deben ser ajustados a un valor superior a la máxima corriente de carga esperada, por éste motivo los elementos de protección de secuencia positiva tienen limitaciones de sensibilidad ante fallas de baja intensidad, y por tal motivo son aplicados típicamente para despejar fallas trifásicas, que producen corrientes de alta intensidad. 
- La componente de secuencia negativa se manifiesta ante fallas o cargas desbalanceadas, típicamente los elementos de secuencia negativa se utilizan en las protecciones contra fallas entre líneas o también con fallas de líneas a tierra. Este elemento es más sensible que un elemento de protección de secuencia positiva.

- La componente de secuencia cero se presenta siempre en fallas a tierra de cualquier tipo. Este elemento es utilizado para la protección de fallas a tierra debido a su alta sensibilidad.



Friday, February 12, 2021

CLASSIFICATION OF GROUNDING SYSTEMS

1.    AUTHOR: BSEE Hugo E Reyes H

 1. INTRODUCTION

The system grounding is very important because its main goal is to provide a path to the earth in order to minimize potential transient overvoltages from lightning strikes, line surges, or unintentional contact by higher-voltage lines, and helps to determine the system protection requirements. The system grounding arrangement is determined by the grounding of the power sources, and also determines the types of loads the system can accommodate.

The National Electrical Code (NEC – NFPA 70) article 250.4 (A) (1) defines a grounded systems as follows: “Electrical System that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltages lines and that will stabilize the voltage to earth during normal operation”.

2.      2. TYPES OF SYSTEM GROUNDING

Grounding are classified as follow:

·         SOLID OR EFFECTIVE GROUNDING

·         LOW-IMPEDANCE GROUNDING

·         HIGH-IMPEDANCE GROUNDING

·         UNGROUNDED SYSTEMS

 2.1. SOLID GROUNDING (OR EFFECTIVE GROUNDING)

Solidly grounded systems are in Wye (Y or Star) connection and have all neutrals connected to ground without any intentional impedance between the neutral and ground (Earth). This type of system ground provides excellent protection against overvoltage and ground fault currents. Other important characteristic of solidly-grounded systems is that ground faults may cause high short-circuit current. Thus, the occurrence of a ground fault needs to be cleared as fast as possible.

In order to be classified as solidly grounded, in these system the X0/X1 ratio must be positive and less than 3.0, and the R0/X1 ratio must be positive and less than 1.0 at all point and under all operating conditions, where R0 and X0 are the zero-sequence resistance and reactance, and X1 is the positive-sequence reactance of the power system.

The Figure 1 shows an effective grounding with a single-point grounding. In this application we have one arrangement with three phase and three wires (3Ph/3W) with every load connected phase to phase.  On the other hand, we can get a second arrangement with three phase and four wires (3Ph/4W) with an isolated neutral and every load connected phase-to-neutral. In this second arrangement, the load unbalance current returns through the neutral, but any ground fault returns through the earth to the substation neutral.

EFFECTIVE GROUNDING

The Figure 2 shows an effective grounding with a multiple-point grounding. In this application we have three phase with four wires (3Ph/4W) and phase to neutral loads, the system is grounded at the substation, at every power transformer location along the circuit, and every, 1 000 feet (305 m) or so if there is no power transformer ground. In this type of arrangement, both load unbalance and ground fault currents divide between the neutral conductor and earth. Detecting high-resistance ground faults is difficult because the protective relay measures the high-resistance ground fault combined with the unbalanced current. Most utility companies uses multiple-point grounding, the typical case is to ground the transformer neutral at overhead line poles, creating in this way a multiple-point grounding. Due to a separate grounding conductor is not run with the overhead power line, the resistance of the earth limits the circulating ground currents. Multiple-point grounding in National Electrical Code (NEC) jurisdictions, such as commercials or industrial facilities, are actually not allowed in most cases. Instead, a single-point grounding is preferred.


2.2. LOW IMPEDANCE GROUNDING
This method is similar to the solidly grounding system in that transient over-voltages are not a problem, see Figure 3. The impedance may be a resistor (Rg) or a reactance (Xg). Low impedance grounding is typically used in medium voltage systems, but not on low-voltage system. The impedance is usually a resistor that is selected to limit ground fault current magnitude, but leave the current high enough to be detected by sensitive relays. 
Resistor (resistance) grounding is normally selected as a method to limit ground fault current in the range of 200 A to 500 A and is rated for approximately 10 seconds. Many medium-voltage industrial power systems has a resistance grounding with typical ground fault currents in the range from 100 A to 1000 A. 
In reactance grounding systems, the available ground fault current must be in the range of 25 @ 65 % of the three phase fault current to prevent serious transient over-voltages (X0 ≤ 10.X1), which is higher than the ground fault levels in a resistor grounded systems. Thus, the resistance grounding scheme is preferred, because it allows more reduction of ground fault currents than reactance grounding without risk of transient over-voltage.

LOW-RESISTOR GROUNDING

2.3. HIGH IMPEDANCE GROUNDING
There are two types of  high-impedance grounding systems: a) high-resistance grounding; and, b) resonant grounding.
2.3.1. HIGH-RESISTANCE GROUNDING
In this scheme, the neutral point is grounded to earth using a high-impedance resistor with magnitude equal to or slightly less than the total system reactive capacitance to ground (R0 ≤ XC0) in order to limit transient over-voltages to safe values during ground faults. This scheme limits transient over-voltages to less than 250% the peak value of the nominal phase-to-ground voltage. The resistor is sized typically to allow a ground fault current below 15 A, this system is illustrated in Figure 4.


HIGH-RESISTANCE GROUNDING
                                                    Figure 4 High-Resistance Grounding

High-resistance grounding schemes connect a distribution transformer between neutral point and ground, with a resistor on the transformer secondary. The transformer primary is rated for primary phase-to-phase voltage, and a 240 Volts secondary limits the secondary to a 139 Volts maximum.

This scheme is specially used in medium voltage, permits the utility to continue operating the system during the first sustained ground fault condition until a favorable time for an outage to clear the fault, provided that the cable carrying the fault is rated 173% of the voltage level. If a second ground fault occurs on another phase before the first ground fault is cleared, a phase-to-ground-to-phase fault occurs that is not limited by the neutral grounding resistor. Thus, the second fault may be an arcing fault, whose magnitude is limited by the ground-path impedance to a value high enough to cause severe arcing damage, but too low to activate the overcurrent relays quickly enough to prevent or limit this damage. For this reason, systems of 13.8 kV and higher generate too much heat to justify a delay in tripping. To avoid arcing damage, we need to use two relay levels, the first level is used to issue an alarm on first phase-to-ground fault, and the second level to issue a trip on second fault in time to prevent arcing damages. 

2.3.2. RESONANT GROUNDING
Here, the system is grounded through a high-impedance reactor, the total system capacitive reactance is canceled by an equal inductive reactance connected between the neutral point and ground, illustrated in Figure 5. The variable reactor or reactance is called Petersen Coil, when this neutral reactor is tuned to the total system capacitance, the ground current fault is ideally zero. The circuit is nothing but a parallel resonant circuit, and the very low ground fault current cause minimum fault damage. The low ground fault current require sensitive fault detection devices. If the coil reactance is greater than the system capacitive reactance, the system is overcompensated, and it if is smaller the system is undercompensated. 
Resonant grounding method is used in medium voltage generators connected to a power system through a dedicated step-up power transformer. The reactor magnitude is adjusted to 1/3 the per-phase capacitive reactance to ground. For ground fault currents, the system is in resonance and the fault current is typically below 1 A. However, generator resonant grounding is much less used that high-resistance grounding.
PETERSEN COIL

Figure 5 Resonant Grounding System

2.4. UNGROUNDED SYSTEMS

These system has no intentional connection between the neutral and ground. However, the system is connected to ground through the lines-to- ground capacitances as we can see in Figure 6. However, ungrounded systems must still be grounded. Do it per NEC 250.4 (B) (1) through (4).



                                                        Figure 6 Ungrounded System
As we can see in Figure 6b and 6c, a ground fault in phase “A” shift the system neutral voltage but leave the phase-to-phase voltage triangle intact. Hence, after a single ground fault the ungrounded system is able to remain in service. In the normal balanced system (see Figure 6b) VAN = VAG, VBN = VBG, and VCN = VCG, so N and G has the same potential. When a ground fault occurs in phase “A” the voltage between phase faulted (“A”) and Ground becomes to zero Volts, but the voltage between N and G is now equal to the zero-sequence voltage (V0). The phase-to-ground voltage on the un-faulted phases are 3 (1.73) of their nominal values. This has consequences because the system must have a phase-to-phase insulation level and all power equipment and loads.

On the other hand, the ground fault current has a magnitude equal to the capacitive current by connecting one of the phases to ground. For medium voltage (MV) systems, these currents are in a range from 5 to 15 Amperes, depending on system voltage characteristics. Thus, the ground fault current is very low, such that equipment damage is minimal, and it is not necessarily essential that the faulted area be rapidly cleared. Generally, undergrounded systems are not  recommended., because they may be exposed to high and destructive transient overvoltages, they represent potential hazards to equipment and personnel.

SUMMARY
The main goals of system grounding are provide personnel safety, minimize potential transient overvoltage and thermal stress, reduce communications system interferences, and give assistance in rapid detection and elimination of ground faults.
Each type of grounding systems has advantages and disadvantages, and no one method is generally accepted. We can mention some factors that influence the choice, for instances:

- Voltage levels.
- Transient over-voltage possibilities.
- Required continuity of service.
- Methods used on existing systems.
- Cost of equipment.
- Availability of convenient grounding point.
- Safety, including fire and shock hazard.
- Tolerable fault damage levels.
- Etc.

A summary of various systems grounding characteristics are:

-          Ungrounded: this system is not generally recommended but sometimes is used in industrial and utility stations for high-service continuity. It is not recommended in utility transmission, sub-transmission and distribution. The fault currents in ungrounded systems are very low and easy to detect. There are possibilities of transient over-voltages and ferro-resonance phenomena.

-          High-Impedance Grounding: is recommended in industrial and utility stations in medium voltage due to high-service continuity, but is not recommended in utility transmission, sub-transmission and distribution. The fault currents are low in the range of 1 @ 10 A, they are easy to detect. Transient over-voltages are limited to 250% of the phase-to-neutral voltage.

-          Low-Impedance Grounding: is recommended in industrial and utility stations in medium voltage systems. It is not recommended in utility transmission, sub-transmission and distribution. The fault currents are limited in the range of 50 @ 600 A, and they are easy to detect.

-          Effective or Solid Grounding: this system is recommended in industrial and utility stations, and also is recommended in utility transmission, sub-transmission and distribution. The fault currents are in the range from low to high levels, they are easy to detect.


REFERENCES
1. IEEE STD 142-1991. IEEE RECOMMENDED PRACTICE FOR GROUNDING OF INDUSTRIAL AND COMMERCIAL POWER SYSTEMS.
2. IEEE STD 242-2001. IEEE RECOMMENDED PRACTICE FOR PROTECTION AND COORDINATION OF INDUSTRIAL AND COMMERCIAL POWEWR SYSTEMS.
3. NATIONAL ELECTRICAL CODE (NEC –NFPA 70).
4. PROTECTIVE RELAYING PRINCIPLES AND APPLICATIONS - THIRD EDITION. J. LEWIS BLACKBURN, THOMAS J. DOMIN.
5. PROTECTIVE RELAYING THEORY AND APPLICATIONS – SECOND EDITION. WALTER A. ELMORE.
6. SYSTEM GROUNDIG. BILL BROWN. SQUARE D ENGINEERING SERVICES.

 



UNDERSTANDING THE NEGATIVE-SEQUENCE DIRECTIONAL ELEMENT (32Q)

 Author: BSEE Hugo Reyes Hernández 1. INTRODUCTION Negative-Sequence Directional Elements ( 32Q ) determine the direction of the PHASE-TO-...