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Cover Story | August 2016

Many PQ events result in end-user problems

Prakash Nayak | Senior Member of the Institution of Engineering and Technology (IET)

What is the importance of Power Quality (QoP) and what will be the impact of inadequacies in maintaining reliable power supply on various equipment used by different categories of users? Explain those that could be sensed immediately and those which are not.

Power Quality is little more complex as it has dependencies on both supplier and consumer. (Definition) Definition of Power Quality (PQ), depends on one´s frame of reference. For example, a utility may define power quality as reliability and show statistics that its system is 98% or 99% reliable. This is the usual criteria established by regulatory agencies. A manufacturer of electrical equipment may define power quality as those characteristics of the power supply that enable the equipment to work properly. These characteristics can be very different for different criteria. Power quality is ultimately a consumer driven issue, and the end user´s point of reference takes precedence. Therefore, the definition of a power quality problem is any power problem manifested in voltage, current, or frequency deviations those results in failure or mal-operation of customer equipment. There are also many misunderstandings regarding the causes of power quality problems. If you conduct a survey of utility personnel and customers and ask them about what causes power quality problems, it clearly illustrates that both tend to blame about two-thirds of the events on natural phenomena, e.g., lightning. And utilities will not accept that they are at fault.

When there is a power problem with a piece of equipment, end users may be quick to complain to the utility of an ´outage´ or ´glitch´ that has caused the problem. It must be realised that there are many events resulting in end-user problems that never show up in the utility failure/ power quality statistics.

One such example is capacitor switching for VAR compensation, which is quite common and normal in the utility system, but can cause transient over voltages that disrupt electrical machinery. Another example is a momentary fault elsewhere in the system that causes the voltage to sag, huge fault current, briefly at the location of the customer in question. This might be because a drive or a distributed generator to trip off, but the utility will have no indication that anything was amiss on the feeder unless it has a power quality monitor or protection relays installed.

In addition to real power quality problems, there are also perceived power quality problems that may actually be related to hardware, software or electronics system malfunctions. Electronic components can degrade over time due to repeated transient voltages and eventually fail. Thus, it is sometimes difficult to associate a failure with a specific cause. This was common with early versions of new computer-controlled load equipments.

In response to this growing concern for power quality, electric utilities have programmes that help them respond to customer concerns in reactive, where the utility responds to customer complaints, to proactive, where the utility is regularly involved in educating the customer and promoting services that can help develop solutions to power quality problems. Since power quality problems often involve interactions between the supply system and the customer facility and equipment, regulators should make sure that there are established guidelines and codes for distribution companies, attach some incentives to work with customers and help customers solve these problems.

The economics involved in solving a power quality problem must also be included in the analysis. It is not always economical to eliminate power quality variations on the supply side. In many cases, the optimal solution to a problem may involve making a particular piece of customer´s sensitive equipment less sensitive to power quality variations. The level of power quality required is that level which will result in proper operation of the equipment at a particular facility.

Power quality, like quality in other goods and services, is difficult to quantify. There is no single accepted definition of quality power. There are standards for voltage and other technical criteria that may be measured, but the ultimate measure of power quality is determined by the performance and productivity of customer equipment. If the electric power is inadequate for those needs, then the´quality´ is lacking.

What are the types of Power Quality (PQ) problems that are prevalent in the power supply system today and what are the measures do you suggest curtailing them? A recent survey of PQ experts indicates that 50% of all power quality problems are related to grounding, ground bonds, and neutral to ground voltages, ground loops, ground current or other ground associated issues and electrically operated or connected equipment is affected by power quality. This is also one of the neglected topics by most of the customers. Determining the exact problems requires sophisticated electronic test equipment.

The following symptoms are indicators of Power Quality problems:

  • Piece of equipment mal-operates at the same time of day.
  • Circuit breakers trip without being overloaded.
  • Equipment fails during a thunderstorm.
  • Automated systems stop for no apparent reason.
  • Electronic systems fail or fail to operate on a frequent basis.
  • Electronic systems work in one location but not in another location.

The commonly used terms those describe the parameters of electrical power that describe or measure power quality are Voltage sags, Voltage variations, Interruptions, Brownouts, Blackouts, Voltage imbalance, Distortion, Harmonics, Harmonic resonance, Inter-harmonics, Notching, Noise, Impulse, Spikes (Voltage), Ground noise, Common mode noise, Critical load, Crest factor, Electromagnetic compatibility, Dropout, Fault, Flicker, Ground, Raw power, Clean ground, Ground loops, Voltage fluctuations, Transient, Dirty power, Momentary interruption, Over voltage, Under voltage, Nonlinear load, THD, Voltage dip, Voltage regulation, Blink, Oscillatory transient etc.

The issue of electric power quality is gaining importance because of several reasons - society is becoming increasingly dependent on the electrical supply. A small power outage has a great economical impact on the industrial consumers. A longer interruption harms practically all operations of a modern society. New equipments are more sensitive to power quality variations, the advent of new power electronic equipment, such as variable speed drives and switched mode power supplies, has brought new disturbances into the supply system. Yet another changed scenario is larger introduction of Renewable Energy (RE) in to the power system grid either as distributed energy source, or centralised RE generation.

What are the latest research and development (R&D) out comes that will help improve power quality?
Electrical power generation is changing dramatically across the world because of the need to reduce greenhouse gas emissions and to introduce mixed energy sources. The power network faces greater challenges in transmission and distribution to meet demand with unpredictable daily and seasonal variations. To maintain the power network stability, the load balance has mainly been managed through fossil fuel power plants. To achieve the target of reducing CO2 emissions, future electricity generation will progress with diminishing reliance on fossil fuels, growing use of renewable energy sources and with a greater respect for the environment. However, most renewable energy sources are intermittent in their nature, which pose a great challenge in energy generation and load balance maintenance to ensure power network stability and reliability. Thus I am addressing one of the Power Quality issues related to complex network with variable energy introduction from both wind and solar. Great efforts have been made in searching for viable solutions, including Electrical Energy Storage (EES), load shifting through demand management, interconnection with external grids, etc. Amongst all the possible solutions, EES has been recognised as one of the most promising approaches and continuous development.

EES technology refers to the process of converting energy from one form (mainly electrical energy) to a storable form and reserving it in various mediums; then the stored energy can be converted back into electrical energy when needed. EES can have multiple attractive value propositions to power network operation and load balancing, like, helping in meeting peak electrical load demands, providing time varying energy management, alleviating the intermittence of renewable source power generation, improving power quality/reliability, meeting remote and vehicle load needs, supporting the realization of smart grids, helping with the management of distributed/standby power generation, reducing electrical energy import during peak demand periods. EES technologies, i.e., batteries, super capacitors, superconducting magnetic pump storage and flywheels etc. provided an insightful review of the advanced materials for several EES technologies. The strategies for developing high-performance hydrogen storage materials and electrochemical lithium-ion battery materials are the ones ongoing research development to increase the life and also commercial viability.

What are the regulatory and non-regulatory measures that are helping the cause of maintaining power quality in India and what is their impact?
From India perspective, following are the utilities and customer key focus areas maintaining the power quality:

  • Reliability of supply
  • Reliability of supplies

Major concern for Indian electrical power system is to maintain reliable power supply. The end customer has become more demanding and conscious about interruption free supply. High DT failure rate - which forms a critical piece of the distribution network, reduces quality and reliability of supply.

AT&C losses (~30%)
Distribution companies are facing huge burden of losses Rs 2400 billion and high average as per ministry´s report of 2013. Distribution companies are exploring and investing in all areas including network improvement, IT to meet their loss reduction targets (See state-wise AT&C loss)

Energy Efficiency
Energy Efficiency emerged as a key policy priority in India´s energy sector since Energy Conservation Act, 2001. Poor EE also affects the environment and the profitability of the utility consequently Indian economy.

Following are the Key PQ issues in electrical network and its measures for improving performance. The growing use of electronic loads in networks - means increased concerns about PQ. Some of the PQ disturbances in network are caused due to following key three factors:

Natural Causes
Faults or lightning strikes on transmission lines or distribution feeders falling of trees or branches on feeders during stormy weather conditions

Transmission
Power electronic Transformer energization or feeder capacitor switching and equipment failure because of poor operation and maintenance.

Non-linear loads on consumer side loads UPS,
Adjustable Speed Drives (ASD), converters, etc.) arc furnaces and induction heating systems Switching on or off of large loads, etc.

It is understood that ´Network Improvement´ improves power quality, reliability of supply which in turn will reduce technical losses and improve Energy Efficiency. Now coming to regulatory process in India on Power Quality, there are various agencies guiding the utilities and customers to improve the power quality. Some states have adopted state regulatory mechanisms and have also laid down the steps forward, we have discussed here about one state, similar and little more specific area of interest is addressed in other States.

To Start with, CEA, Central Electricity Authority and CERC, had laid down the following, these undergo modifications with lessons learned

Central Electricity Authority (CEA): GRID Standards Regulation, 2010 and Technical Standards for Connectivity to the Grid Amendment Regulation, 2013

Central Electricity Regulatory Commission (CERC): Indian Electricity Grid Code Regulations, 2010

State Electricity Regulatory Commission (SERC): State Grid code, State Supply Code and Standard of Performance (SOP) of distribution licensees

Common Power Quality aspects covered under current regulations for distribution licensees:

Reliability

  • System Average Interruption Frequency Index (SAIFI)
  • System Average Interruption Duration Index (SAIDI)
  • Customer Average Interruption Duration Index (CAIDI)

Voltage
  • Voltage variation
  • Voltage unbalance
  • Voltage monitoring and management in some states

Current

  • Current unbalance in few states
  • Harmonics
  • THD limits in majority of states in limited states
  • However, in my opinion there is no well established monitoring and implementation framework for Power Quality in Indian Regulations. Just an example of State of Maharashtra

Maharashtra, PQ Regulation Structure

  • Relevant Regulation
  • State Grid Code.
  • Standard of Performance Regulations.
  • Supply Code.
  • Distribution Open Access Regulations.

Voltage indices

  • Specifies the voltage variation limits for EHT, HT and LT voltages
  • Provision for compensation to be paid by licensee to consumer for failing to maintain standard

Harmonics
Consumer to control harmonics as per IEEE 5191992 within 3 months and as per relevant commission order. Penalty for low /high harmonics as per notification from the Commission issued as and when.

Reliability
System Average Interruption Frequency Index (SAIFI),System Average Interruption Duration Index (SAIDI) and Customer Average Interruption Duration Index (CAIDI) However, as on December 2015, no such penalty or incentive is ever specified so far. No monitoring and implementation framework for voltage variation and harmonics. Consumer is responsible for taking corrective action for harmonics injection.

What are the challenges in ensuring PQ and what kind of measures are necessary to improve the scenario?
Following are the measures by utility operators to improve electrical network system by and large same is adopted in many states, however, they may be in various stages of its implementation.
Flexible AC Transmission System (FACTS) devices increase the ability of transmission capacity of lines, and help control power flow over designated transmission, electronically and statically
Distribution Static Compensator (D-STATCOM) is used for voltage regulation, compensation of reactive power, correction of power factor and elimination of current harmonics
On-Load Tap Changer (OLTC) transformers equipped with Automatic Voltage Control (AVC) are used between multiple voltage levels to regulate and maintain the voltage, which is supplied to customers.
Automatic Voltage Regulators (AVRs) are units that regulate the voltage to ensure electronic units like rectifiers continue to operate during extreme mains voltage variations, without getting damaged.
One of the PQ issue, i.e. harmonics affects electrical network assets too and the same need to be addressed, these are produced by the customers connected to the Utility networks.
Harmonics, one of the growing PQ issue, are caused by the non-linearity of customer loads.
A harmonic current flow upstream from non linear loads, through the impedance of cables and transformers and creates harmonic voltage distortion.
Harmonic currents also create increased heating in electrical cables, leading to premature ageing and overstressing of the electrical insulation, in some cases leading to fire due to burning of cable, wires etc.
Nuisance tripping of protective devices, often dependent on periodic zero crossing of waveform, overheating of conductors (burn off, damage), etc.

Following are the measures to prevent or reduce Harmonics
Use of Active and Passive filters improves power factor thereby reducing high frequency harmonics and controlling output current
Use of capacitor banks or filters in local grid helps reduce reactive power demand or harmonics issues by reactive power compensation
Harmonic Mitigations Transformers (HMTs) have become a leading economical solution nowadays to improve the system reliability. When energized, they provide harmonic treatment and have excellent energy saving characteristics
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