Load Management in Smart Grids Considering Harmonic Distortion and Transformer Derating

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Load Management in Smart Grids Considering Harmonic Distortion and Transformer DeratingM. A. S. Masoum, Senior Member, IEEE, P. S. Moses, Student Member, IEEE, and S. Deilami, Student Member, IEEEwith the proposed load management techniques of this paper which concentrates on power quality management of smart grids. So far there has been significant research in integrating customer demand side management into smart grids to improve the system load profile and reduce peak demand [37]. To achieve this, many countries are developing technologies such as smart metering and smart appliances. Italy and Sweden, for example, are approaching 100 percent deployment of smart meters for consumers. Furthermore, smart appliances for households such as air-conditioning, dishwashers, clothes dryers, washing machines, as well as plug-in electric vehicles (PEVs), could talk to the grid and decide how best to operate and automatically schedule their activity at strategic times based on available generation. This paper is proposing to go one step further and incorporate power quality into load scheduling, which has not been previously considered. This paper proposes the novel application of using derating K-Factors for load and power quality management in a smart grid. A harmonic load flow algorithm is used to evaluate the harmonic stresses at the distribution transformer serving nonlinear loads. K-Factor derating is applied to determine the amount of load that must be curtailed or reconfigured to minimize harmonic losses at the transformer. The proposed load management strategy based on power quality is a crucial component for smart grids to achieve the goal of maximizing system reliability and improving overall distribution system efficiency. II. HARMONIC POWER FLOW For the harmonic power-flow calculation, a decoupled approach is employed. This is justified due to the acceptable accuracy of the proposed decoupled harmonic power flow (DHPF) and the fact that industrial distribution systems consist of a large number of linear and nonlinear loads that cause convergence and memory storage problems if the harmonic couplings are considered [8]. At harmonic frequencies, the system is modeled as a combination of passive elements and harmonic current sources. The related admittance matrix is modified according to the harmonic frequency [9], [10], [11]. The general model of linear load as resistance in parallel with a reactance is utilized [12]. Nonlinear loads are modeled as current sources that inject harmonic current into the system. The fundamental and the hth harmonic current of the nonlinear load installed at

Abstract-- This paper addresses the important issue of power quality management for smart grids and proposes a load management strategy based on transformer derating for minimizing harmonic distortion in distribution feeders and transformers. Ongoing development of smart grid technologies such as smart metering and smart appliances are creating new opportunities for improving distribution system performance. One area undergoing study is effective control of demand response through (semi)automated load management practices (e.g., smart appliances). Despite these developments, the impact on power quality has not been taken into consideration from a demand side management point of view. Smart grids provide an excellent opportunity to better manage power quality and reduce harmonic distortions present in power networks. In this paper, it is proposed that the impact of harmonics generated by nonlinear loads should be factored into overall load control strategies of smart appliances. This work focuses on the impact on residential distribution transformers which are adversely impacted by harmonic current distortions. A growing concern is the potentially high penetration of plug-in electric vehicles in smart grids. Load management of electric vehicles is studied for an IEEE 30-bus 23 kV distribution system to demonstrate the benefits of the proposed power quality and load management strategy. This paper proposes computing transformer K-Factor derating to control scheduling of smart appliances/loads to reduce harmonic stresses. Index Terms-- Smart grid, harmonic losses, K-factor, load management and nonlinear transformer.

MART GRID technologies are presently undergoing rapid development in an effort to modernize legacy power grids to cope with increasing energy demands of the future [1]. High speed bi-directional communications networks will provide the framework for real time monitoring and control of transmission, distribution and end-user consumer assets for effective coordination and usage of available energy resources. Furthermore, integration of computer automation into all levels of power network operations, especially at the distribution and consumer level (e.g., smart meters), enables smart grids to rapidly self regulate and heal, improve system reliability and security, and more efficiently manage energy delivery and consumption [2]. These objectives are inlineM. A. S. Masoum, P. S. Moses and S. Deilami are with the Department of Electrical and Computer Engineering, Curtin University of Technology, Perth, WA, 6845, Australia (e-mail: m.masoum@curtin.edu.au; paul.s.moses@gmail.com; s.deilami@postgrad.curtin.edu.au).



978-1-4244-6266-7/10/$26.00 2010 IEEE


bus i with real power P and reactive power Q are modeled as

(2) where C(h) is the ratio of the h harmonic current to its fundamental. The harmonic voltages are computed by solving the following load-flow equation:th

I i1 = [( Pi + jQi ) / Vi1 ] * I ih = C ( h) I i1


transformer harmonic losses and temperature rise. The DHPF (Eqs. 1-7) is used to calculate the current harmonics. The full derivation for K-Factor is given in [8]. The definition of KFactor is as follows,



(I h )2 h21 ( I rms ) 2

h =1


Y h V h = I h.The voltage at bus is defined as

(3)2 1/ 2

where I h is the rms load current for harmonic order h , and1 I rms is the rated rms load current of the transformer. Given

H Vi = Vih h =1


and the related total harmonic distortions of voltage (THDv) and current (THDi) are

the load current harmonic spectra and rated eddy current loss coefficient ( PEC R ), the K-Factor can be used to calculate transformer derating [14]:

H 2 1 / 2 THDv = Vih / Vi1 100% h1 1/ 2 H 2 THDi = I ih / I i1 100% h 1

I derated = 1+ K


14 244 4 3= FHL

H h 2 h =1 ( I )

1 ( I rms ) 2

1 + PEC R

( pu )


(PEC R )

The new (derated) apparent power capability of the transformer can be estimated as follows,

where H is the highest harmonic order considered (H = 49 for this paper). At the hth harmonic frequency, power loss in the line section between buses i and i+1 ish Ploss (i,i +1) = Ri,i +1 Vihi +1 Vih yihi +1 , , 2

kVAderated = kVArated I derated ( pu ) Derating = (1 I derated ( pu ) ) 100

(10) (11)

Finally, the percentage decrease in transformer kVA rating is In this paper, the derating factor (K-Factor) is proposed for load control of smart appliances to minimize harmonic stresses at the substation transformers and voltage/current distortions in distribution feeders. IV. SMART LOAD MANAGEMENT (SLM) BASED ON THDI AND K-FACTOR DERATING The main contribution of this work is the smart load management (SLM) strategy in response to excessive harmonic distortion on distribution feeders and transformers caused by nonlinear loads (e.g., battery chargers for electric vehicles) and smart appliances. Some assumptions are made to expedite the explanation of the proposed SLM concept: The smart grid infrastructure is in place to communicate necessary power quality information and harmonic load data to the utility (in this paper, harmonic load flow is used to extract this information). Bidirectional communication network is available to send and receive control and status signals between the utility and smart appliance loads to modify their operation. For the purpose of not distracting from ensuing explanations, conventional demand-side management based on shaping load profile and reducing peak demand is not directly addressed in this work. It is proposed that when the K-Factor and computed derating of the distribution transformer exceeds a certain limit, the smart grid should act to curtail harmonic rich loads. An SLM algorithm is proposed to scan through the distribution system on a priority and THDi basis and request smart loads (possibly


and the total power loss, including losses at harmonic frequencies, for an bus system isH m 1 h h Ploss = Ploss (i,i +1) h =1 i =0


III. IMPACT OF HARMONICS ON TRANSFORMERS This paper proposes using derating factors for load management in smart grids. Derating is defined as the intentional reduction in load capacity of a transformer operating under nonsinusoidal conditions. Derating of transformers is necessary because of additional fundamental and harmonic losses generated by nonsinusoidal load currents which cause abnormal increases in transformer temperature beyond rated operation. Transformers can suffer age reduction and premature failure due to resulting thermal stresses in the windings and core structure. Therefore, it makes sense that any power quality improvement strategy should be aimed at protecting assets prone to harmonic disturbances. The goal of derating is to reduce the transformer kVA loading such that total transformer losses are limited to rated losses. The main methods for estimating transformer derating are: K-Factor [8,27], Harmonic Loss Factor [8], online harmonic loss measurement [13] and computed harmonic losses [14[16


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