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Effects on Protection Systems of Distribution Network due to Presence of Solar PV

Introduction

Use of solar PV energy is increasing gradually for more than one decade. Solar Power is a step towards clean energy, reducing the impact of climate change and energy security in long term. One kW of solar energy reduces one ton of carbon emission in one year. It means planting 20 trees every year. India has reduced carbon emission, which is responsible for climate change, by 38 million tons in the last 5 years by using renewable energy. India’s rank has improved from 31 to 25 in the climate change index among 58 worst affected countries by using solar/renewable energy. Global warming and climate change are the most serious threats to humanity at present time. India receives 5000 trillion units of solar energy per year. India spends huge expenditure of foreign exchange for import of oil and gas. Coal is fast depleting for future use. In view of above facts, Indian Government has planned 100 GW of solar energy by 2022 according to National Solar Mission.

As solar PV continues to increase its penetration in Indian Energy Profile, it becomes necessary to understand technical issues and system modifications to be implemented in existing protection systems to maintain reliability and stability of the national grid due to the variable and distributed nature of solar power. To technically penetrate the solar power into the existing national grid, it has to be made compatible with national grid parameters under normal and off normal conditions of operation for the purpose of making it a source from utility scale to small scale power rating. A typical solar power station is shown in fig -1.

Present national grid, which is centralized in nature, is required to be transformed into one that is functional at centralized as well as distributed level. This is due to the fact that Distributed Energy Resources (DER) like solar PV, as the name suggests, are distributed in nature.

Effects of penetration of solar PV into grid

It has both positive and negative effects. Major positive effects are: improvement in the voltage profile, improvement in power quality and support towards voltage stability. So that the system can withstand higher loading situations.

For our purpose of discussion we shall consider a power grid of radial configuration, which has a direction of current flow in one direction i.e. from the generation or substation end to the consumer end. Relay coordination for overcurrent, earth fault, distance and system protection for such power system is based upon direction of flow in one direction. When solar PV is penetrated into the power system, the current flows from generator/substation end to customer end as well as from solar PV end to customer end. This is shown in fig 2 and 3. Hence, now the fault current doesn’t flow from one direction but from multi directions and the radial nature of the power grid is lost. The fault current flow from solar PV end results in increasing/reducing fault current flows through protection relays and adds more directions to the fault current flows. This multi directional current flows due to penetration of solar PV into the existing power grid and increase/decrease in fault current distorts the original protection relay coordination. This is the negative effect of the presence of solar PV in the power grid. Other negative effects are unwanted tripping of feeders, blinding tripping and unwanted islanding.

Limitations of existing circuit breakers due to presence of solar PV

The future concern due to presence of solar PV at distribution level is that the fault current levels shall increase and shall exceed the breaking capacity of existing circuit breakers (CBs). Hence, the existing breaking capacity of CBs shall no longer be capable to interrupt the increased fault currents in the aforesaid scenario. 

As the replacement of existing CBs is not economical and feasible, the most suitable, economical, easiest and feasible way is to use fault current limiters (FCLs) in distributed solar PV generating systems. FCL is described later in the article. Fault Current Limiters are capable of limiting flow of fault currents from distributed solar PV. In this way, with the use of fault current limiters, existing circuit breakers are capable of interrupting the fault currents.

Functions of protection system

Function of a protection system is to detect an abnormal state of electrical parameters in a power system and isolate the part of the power system with abnormal parameters to prevent any damage to personal or property.  Properties of a well-designed protection system are sensitivity, selectivity, speed, reliability and cost.

Types of protection

The protection systems can be divided into two types. 1) Short circuit protection, 2) System protection. Short circuit protection can be classified as overcurrent protection and earth fault protection. 

As the name suggests, over current protection protects power system from line to line faults. 

Phase to ground faults result in flow of earth fault current. The value of this current is much lower than the line to line fault current due to the higher fault impedance for earth faults. 

For system protection, the over and under frequency protection systems are employed. 

Overcurrent/Earth fault Relays 

Two types of overcurrent/earth fault relays exist according to their operating characteristics

  1. Instantaneous overcurrent: 
  2. Inverse Definite Minimum Time overcurrent (IDMT) 
Overcurrent Protection Relay Coordination for Radial Power Distribution System. 

A simplified radial distribution system is shown in fig 1(a). In this figure, the generator is the source of power. 4 Nos of CBs are located at different stages of the distribution system.  Purpose of the protection system is coordination among relays responsible for tripping of these CBs. Relay coordination is selecting time current characteristics of different relays for intended operation. In this way, we control the sequence of operation of relays. In the figure, a fault occurs downstream to CB4. Relay coordination scheme is designed in such a way that the minimum part of the power system is isolated to interrupt the fault current. Hence, the relay coordination scheme selects the CB closets to the fault to trip first. If this CB fails to trip then other upstream CBs trip in sequential order. For this purpose, the relay operating timing is increased from downstream CB to upstream CBs in sequence. Hence, in this distribution system first of all CB4 shall trip. After that CB3, CB2, or CB1 shall trip in a sequence.

Nature of fault current in radial distribution system with solar PV 

Basically, the fault current contribution of solar PV is different from  conventional energy resources in two ways

  • In the radial distribution system, fault current flows from one direction. But in the case of solar PV, fault current is distributed in nature and flows from many directions from all solar PV stations. This can result in phenomena of “blinding” or “sympathetic tripping”. 
  • Solar PV is coupled to the power system via inverters. The short circuit breaking capacity of these inverters is usually not much higher than the overload or surge current breaking capacity intended to protect the inverter. In view of this, the short circuit fault current of power grids with high penetration of inverters is considerably lower than that of grids with conventional generators of the same rating. 

For smooth operation of protection systems, it is required that solar PV is immediately disconnected under faulty grid conditions so that the designed protection schemes function as per requirement. 

It is possible that some solar PV generation connected to low voltage levels may be undisclosed to DISCOM, i.e. not officially declared by the users. If the amount of undisclosed LV solar PV is high, its impact to LV level protection may be unsafe.

Impacts of solar PV to the Distribution System Protection

The conventional distribution system is radial in nature, which is made-up of a single generating source, power transformer, transmission line, distribution transformer and connected loads. In this conventional distribution system, there is a sequential relay coordination system from downstream end to upstream end. With the presence of solar PV, the protection system may lose its integrity and functioning of protection coordination may fail. This may result in Blinding and Sympathetic tripping in addition to other effects, described as below.

 Increased Fault Current 

Fig 2 shows a power system with conventional synchronous generators on the upstream of the R3 and a downstream fault beyond R3. The fault current through R3 is a sum of the generator fault current and the solar PV system fault current. Hence, the resultant fault current through R3 is higher than the fault current which passes through R3 when there is no solar PV system. In this condition, an already designed conventional protection system is not suitable. In this case, fault current has increased as detected by R3. In this case, designed relay coordination between R3 and upstream relays is lost.

Reduced fault Current 

For the radial distribution system as shown in fig 2, consider the relay R1. The fault current through R1 may reduce due to presence of solar PV. This may happen when the solar PV unit is feeding a high fault current. In this case, the relay R1 may fail to operate if the fault current falls below the relay fault current settings. 

Blinding tripping

Fig 3 shows a radial distribution system. The figure shows relay R1 at upstream of solar PV. Relay R2 is located in the solar PV network. A fault occurs downstream to R1. Fault current through R1 may reduce if the fault current flow from the solar PV is high. In this case, R1 may not detect a downstream fault, if the fault current is below the pickup value of R1. This is called Blinding Tripping. 

Sympathetic tripping 

In fig 3, if the fault current fed by solar PV through R2 is high enough to operate the relay, this is called sympathetic tripping. Though, in this case, there is no downstream fault associated with R2. This type of tripping by R2 is not desired. 

Proposed Solutions 

Design analysis is going on to find proven solutions to the above mentioned scenario evolved due to integration of solar PV systems in distribution systems. Various designs have been innovated to solve the problems. Fig 4 shows the modified design of a radial distribution system to resolve aforesaid problems due to presence of solar PV.

The summary of the proposed solutions is given as below. 

Increased Fault Currents 

Increased current through R3 can be prevented by using a fault current limiter (FCL) in series with solar PV, as shown in fig 4. This will limit the effect made by solar PV on functioning of relay coordination during a fault. A fault limiter limits the solar PV current during a fault and allows normal current flow when there is no fault. Fault limiters are devices that have low impedance and produce no action during normal operation. When a fault occurs, the FCL device acts within 20 ms to insert a high impedance in series with the solar PV. 

Reduced Fault Currents 

This effect can be reduced by using FCL as described above. The FCL limits the fault current that the solar PV unit feeds to the fault. This causes the upstream relay R1 to function as per design intent of relay coordination.

Blinding tripping 

This can be solved by installing FCL and employing under voltage protection. In addition to this, new relay coordination design can be employed using computer-based methods. As we know that fault current through R1 is reduced due to high fault current flow from solar PV. In this case, we can change the relay setting of R1 so that the reduced fault current is capable of operating R1.

Sympathetic tripping 

This type of tripping can be avoided by replacing R2 by directional type relay.

Fault Current Limiters

A Fault Current Limiter (FCL) is a device which limits the designed fault current when a fault occurs in a power system. It reduces fault currents to a lower level and enables the use of devices with lower breaking capacity. Recent trend of use of distributed energy resources (DER) like solar PV has enhanced the use and importance of FCL 

Benefits of Fault Current Limiter

Addition in generation capacity, say solar PV in present discussion, results in an increase in fault current level in the distribution system. This needs replacing circuit breakers of lower breaking capacity by higher breaking capacity circuit breakers. This is a highly cost effective affair. This replacement can be avoided by using FCL. Hence use of FCL results in considerable savings. FCLs are installed in each phase of the line, and it insert a series impedance to limit the fault current in the distribution system. Use of FCL redistributes the fault currents in the power system, which was changed due to addition of solar PV and hence facilitating proper functioning of relay coordination scheme. Other advantages are reduced or no wide-area blackouts, reduced local    interruptions, and increased restoration time when interruptions occur.

IDEAL FAULT CURRENT LIMITER

A high quality fault current limiter should have following properties:-

  • Inactive during normal system operation i.e. insert minimum impedance in the system when there is no fault in the system.
  • When fault takes place in the distribution system, FCL must insert impedance as designed. 
  • Time current characteristics of the reactor of FCL must be designed such that the current in the FCL should reach the peak value within the first cycle of the fault current.
  • As we know, FCL inserts a high value of impedance during persistence of fault in the system. This impedance value of the FCL should come back to its normal value within one cycle after the current in the distribution system returns to the normal value. 
  • Must have high endurance value for repeated operations.
  • It should be of small size and cost effective.

August 13, 2020
by 
Anupam Rastogi
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