Permissible Voltage Drop

Introduction

Voltage drops occur throughout the distribution network. However, it is necessary to regulate the voltage in order to ensure that these drops stay within a permissible range. High voltage drops, below the permissible level, can have many negative consequences. These result in an increase in the system maintenance cost and a decrease in the safety and performance of the network. Operating electrical equipment below its rated voltage can be dangerous as well as reducing the expected lifetime of the equipment. When inductive loads are operated below their rated voltage, they tend to overheat and consume more power. Resistive loads which are operated at a too low voltage will not be able to produce the desired output. Reducing the voltage by 10% will reduce the power output by 19%, since power output correlates to the voltage squared. A reduction in the network voltage can cause lights to flicker when other appliances are turned on.

Permissible Voltage Drop

The table below shows the maximum voltage regulation as any point of the distribution line:

Part of Distribution System	Urban Area (%)	Suburban Area (%)	Rural Area (%)
Up to Transformer	2.5	2,5	2.5
Up to Service Main	3	2	0.0
Up to Service Drop	0.5	0.5	0.5
Total	6.0	5.0	3.0

Voltage variations in 33 kV and 11kV feeders should not exceed the following limits at the farthest end under peak load conditions and normal system operation regime.

• Above 33kV (-) 12.5% to (+) 10%.
• Up to 33kV (-) 9.0% to (+) 6.0%.
• Low voltage (-) 6.0% to (+) 6.0%

Sometimes it can be difficult to achieve the required voltage, especially in rural areas . Part of the reason for the high voltage drop in rural areas it due to the fact that these areas are usually further was from the electricity supplies. Therefore, the power lines have to be extended over long distances, resulting in higher line losses. Power theft is a further factor which can contribute to voltage drops in rural areas. In these cases, 11/0.433 kV rather than the usual 11/0.4 kV distribution transformers can be used. 

Calculating a Voltage Drop

The figurel below shows the basic formula for calculating the voltage drop in an electric circuit.

More detailed information on how to calculate the voltage drop in a distribution network can be found on the electrical construction and maintenance website, under the following article: Calculating Voltage Drop in Power Distribution Systems, on engineers edge under Voltage Drop Calculations or on the electrical engineering portal under the article: How to calculate voltage regulation of distribution line

Furthermore, the electrical engineering portal’s website provides a spread sheet for voltage drop calculation.

Permissible Voltage Drop in India

Official Guidelines from the National Electrical Code, published by the Bureau of Indian Standards in 2011

 

 “4.2.10 Voltage Drop in Consumers’ Installations

4.2.10.1 Acceptable values of voltage drop

Under normal service conditions the voltage at the terminals of any fixed current-using equipment shall be greater than the lower limit corresponding to the Indian Standard relevant to the equipment.

Where the fixed current-using equipment concerned is not the subject of Indian Standard the voltage at the terminals shall be such as not to impair the safe functioning of the equipment.

The requirements are deemed to be satisfied for a supply given if the voltage drop between the origin of the installation (usually the supply terminal) and the fixed current-using equipment does not exceed 5 percent of the normal voltage of the supply.

A greater voltage drop may be accepted for a motor during starting periods and for other equipment with high in-rush currents provided it is verified that the voltage variations are within the limits specified in the relevant Indian Standards for the equipment or, in the absence of an Indian Standard, in accordance with the manufacturer’s recommendations.

…

 4.8 Voltage Drop in Consumer’s Installations

Under normal service conditions the voltage at the terminals of any fixed current-using equipment shall be greater than the lower limit corresponding to the Indian Standard relevant to the equipment wherever existing. In the absence of such a standard, then the Voltage at the terminals shall be such as not to impair the safe functioning of the equipment.

The voltage drop between the origin of the installation (usually the supply terminal) and the fixed current-using equipment should not exceed 4 percent of the normal voltage of the supply.

A greater voltage drop maybe accepted for a motor during starting periods and for other equipment with high inrush currents provided it is verified that the voltage variations are within the limits specified in the relevant Indian Standards for the equipment or, in the absence of a Indian Standard, in accordance with the manufacturer’s recommendations. Temporary conditions such as voltage transients and voltage variation due to abnormal operation may be disregarded.”

Training notes Published by the Bangalore Electricity Supply company

While in theory the permissible voltage variation should be within +/-6%, in practice the supply voltage in many distribution systems varies by 10% or more. A voltage drops of 10% results in a 10-15% increase in the load current drawn by induction motors, a decrease in the starting torque of nearly 19% and an increase in the line losses in the distributer increase by 20%. 'Since most of the load in rural areas consists of induction motors, the line losses in the concerned distribution system might actually reach as high as 20'%. In an article published in the International Electrical Engineering Journal This situation can be addresses by “operating an ‘on-load-tap changer’ in the power transformer situated at 66/11 kV high voltage sub-stations and 33/11 kV sub-stations and by providing a combination of switched capacitors and automatic voltage regulators on the 11kV feeders.” 

Article Published in the International Electrical Engineering Journal

The article titled “Voltage Profile Improvement of Rural Distribution Network by Conductor Replacement” lists yet another value for the voltage drop. The paper claims that during peak load times in some cases the voltage drop at tail end sometimes reaches 36%. The article goes on the suggest that this voltage drop “can be reduced by taking measures like implementing High Voltage Distribution System (HVDS), ABC, and taking anti-theft measures. Without 100% metering, energy accounting and audit, significant reduction in the Technical and commercial losses cannot be achieved.”

 

Further Information

• Energypedia – Technical Aspects of Grid Interconnection
• Voltage Profile Improvement of Rural Distribution Network by Conductor Replacement
• Electrical Energy Losses in Rural Distribution Feeders - A Case Study
• Calculating Voltage Drop in Power Distribution Systems
• Voltage Drop Calculations
• How to calculate voltage regulation of distribution line
  
• Spread sheet for voltage drop calculation